After thirty-five years on the Williams College faculty, Markes Johnson is retiring, at least from his formal teaching duties. He, Gudveig Baarli, and David Backus will continue their collaborative work on Baja California and other projects from their new offices in Mather House. Fortunately, we will still be able to offer courses and research opportunities in paleontology because Phoebe Cohen will be joining our department on July 1. Phoebe received her Ph.D. from Harvard University and researched the evolution of Neoproterozoic eukaryotes. She is currently a postdoctoral associate at the Massachusetts Institute of Technology where she divides her time between research and the Education and Public Outreach group at the NASA MIT Astrobiology Advent of Complex Life team.
Another major change in the Geosciences Department is the addition of a technical assistant to help the faculty teach, train students, and conduct research. After an extensive search we were fortunate to hire Steve Albino, a person with extensive GIS experience, a degree in Geology, and a seemingly inexhaustible list of skills.
Assistant Professor Mea Cook, a paleo-oceanographer, continued to do research work on determining the age of sediments from two sites in the Bering Sea. She also continues work on reconstructing ocean circulation during the end of the last ice age.
Professor Rónadh Cox continued her field research on enigmatic boulder ridges on shore-hugging cliffs in the Aran Islands with Leaf Elliot ’13 and Kalle Jahn ’14. She and her students have been using field observations and Geographic Information System analysis to track movement of the boulder ridges since a detailed 1839 Ordnance Survey map was made. She was also appointed a co-editor of the journal Geology.
Prof. David Dethier continued his NSF-funded investigation of weathering and erosion rates in the Colorado Front Range in collaboration with University of Vermont Prof. Paul Bierman ’85 and University of Connecticut Prof. Will Ouimet ’01. Dethier is also the coordinator of data collection for weather, stream flow, and precipitation chemistry in Hopkins Memorial Forest.
Assistant Professor Lisa Gilbert, at Williams-Mystic, began a new project on the permeability of oceanic crust, using innovative techniques to estimate hydrothermal circulation within samples of the seafloor.
Professor Markes Johnson continued his long-term research in Baja California with Lecturer David Backus. They studied Pliocene fan delta deposits on Isla del Carmen. Markes and Research Associate Gudveig Baarli continue their research on the Canary and Cape Verdi Islands.
Professor and Chair Paul Karabinos continued fieldwork in New England and devoted much of his research to improving the three-dimensional visualization of the geology and topography in the Appalachians. He taught a new tutorial called Evolution of and on Volcanic Islands. During the two-week spring break Karabinos led a course field trip to the big island of Hawaii supported by the Freeman Foote Travel Fund for the Sciences and the Geosciences Department. He organized the second biannual Structural Geology and Tectonics Forum, which was held on the Williams College campus June 12-18.
2011-12 was the 45th year of teaching in the Geosciences Dept. for Professor Bud Wobus. He continued to attend the annual meetings of the Geological Society of America and American Geophysical Union, organizing alumni reunions at each one. He also participated as a faculty leader on an alumni trip to Patagonia this past January. He was the thesis advisor this year for Katie Kumamoto ’12 whose thesis involved research through the Keck Geology Consortium on the Hrafnfjörður volcano in Iceland.
The Geosciences Department continued to participate in the Class of 1960 Scholars Program. The lecture series was organized by Paul Karabinos and was integrated with the tutorial Evolution of and on Volcanic Islands.
Geosciences faculty, students, and alumni published widely in scientific journals and presented numerous talks at the National Geological Society of America meeting in Minneapolis, Minnesota, and the American Geophysical Union Meeting in San Francisco, California. Kathryn Kumamoto ’12 gave a research presentation at the Keck Symposium hosted by Amherst College.
The David Major Fund in Geology offered field camp scholarships to three of our majors during the summer of 2011. Caleb Lucy ’11 and James McCarthy ’11 both attended the Indiana University field camp and Lisa Merkhofer ’11 attended the YBRA field camp run by the Univ. of Houston. The Keck Geology Consortium supported field work by Katie Kumamoto ’12 in Iceland. The McAleenan Family Fund in Geology supported James McCarthy’s attendance at the National Meeting of the Geological Society of America in Minneapolis, Minnesota, in October and Nari Miller’s attendance of the American Geophysical Union conference in San Francisco, California in December. Johanna Eldmann ’13 received an NAGT field camp scholarship to attend the Lehigh field camp this summer (2012).
Two honors students presented their research results on May 1, and Katie Kumamoto won the Freeman Foote prize for the best thesis presentation. Katie also won the Mineralogical Society of America prize, and Nari Miller received the David Major Prize in Geology. Katie Kumamoto and Nari Miller were inducted into Sigma Xi. Katie was also a runner-up in the NSF fellowship competition.
Mea Cook gave an invited lecture at the American Geophysical Union fall meeting in San Francisco, California, in a session on Bering Sea paleoceanography. She also attended an Integrated Ocean Drilling Program (IODP) post-cruise meeting in Salamanca, Spain, for participants in Expedition 323 to the Bering Sea. At these two conferences, she presented the work she did over the last two years with nine students determining the age of sediments from two IODP sites in the Bering Sea, and reconstructing ocean circulation during the end of the last ice age using radiocarbon measurements from those sediments. Mea’s honors student, Nari Miller, studied the organic and inorganic geochemistry of sediments from the penultimate ice age to investigate rapid climate and ocean circulation changes in the Bering Sea.
Rónadh Cox had two field seasons in summer of 2011. She spent the month of June measuring boulder movements on the Aran Islands, with Dave Rapp ’13 and Miranda Bona ’13 and then spent a few weeks in Madagascar collecting samples to help evaluate regional erosion rates: river sand samples for cosmogenic isotope analysis, and charcoal for 14C age dating. She was joined in the field by Ny Riavo Voarintsoa, a Malagasy student who spent the 2010-2011 academic year at Williams, taking Geosciences classes and working in Rónadh’s lab on GIS analysis of lavaka erosion. Both field seasons were successful. The Aran Islands group documented movement of a number of large boulders at considerable distances from the sea shore and at elevations of metres to 10s of metres above mean sea level, including a 10-ton block that had moved several metres since June 2010: these observations underscore previous findings that storm waves are moving these large blocks on a regular basis, in spite of their size and their distance above sea level. The sample set from the Madagascar expedition is currently being analysed in the UVM lab of Paul Bierman ’85, with the assistantship of James McCarthy ’11.
In January, Rónadh became a Science Editor for the journal Geology. The journal has 5 science editors, who between them handle more than 1000 manuscript submissions per year, covering the whole range of geological topics. Reading such large numbers of papers on lots of different subjects is fascinating, if time consuming!
Plans for summer 2012 include another season in the west of Ireland. In addition to returning to the Aran Islands to the boulder-monitoring sites, Rónadh will work with two students, Leaf Elliot ’13 and Kalle Jahn ’14 at two new sites: Annagh Head, in Co. Mayo (on the mainland), and the Inishkea Islands (off the coast of Mayo). They will be joined by Physics prof. Ward Lopes, who will lend his expertise to the group’s attempts to understand the mechanisms by which storm waves can move large boulders. Back at Williams later in the summer, Johanna Eidmann ’14 will work in Rónadh’s lab pushing forward the GIS analysis of lavaka evolution in Madagascar, trying to quantify rates of lavaka formation and evolution by comparing old air photos with recent orthoimagery.
David Dethier continued his NSF-sponsored research in the Colorado Front Range, focused mainly on the measurement of geomorphic and geochemical processes in the Boulder Creek “critical zone” (CZO), which includes the mantle of soil and weathered material above fresh bedrock. In cooperation with the NSF CZO project, during July and August he supervised (with Will Ouimet ’01, University of Connecticut) a Keck Geology Consortium project involving 4 students in the Boulder Creek area. There were no Williams students on the project this year. Paul Bierman ’85 (University of Vermont), Will Ouimet and Dethier continued investigations of Front Range weathering and erosion rates using meteoric and in-situ cosmogenic 10Be techniques. Dethier continued to work in the City of Boulder watershed and adjacent areas with Matthias Leopold and other colleagues from the Technical University of Munich, using seismic refraction, resistivity and ground-penetrating radar to non-destructively image the shallow subsurface in a suite of mainly alpine study areas.
Dethier helps to coordinate ongoing collection of weather, streamflow, precipitation chemistry and other environmental data from Hopkins Memorial Forest and their analysis in the Environmental Science Lab in the Morley Science Center. Real-time weather and groundwater data and archived weather data from 27 years of monitoring are available at http://oit.williams.edu/weather/; archived watershed data (streamflow and temperature, stream chemistry and bulk precipitation chemistry) are at: http://oit.williams.edu/weather/watershed/.
Lisa Gilbert continues her studies of seafloor formation and evolution. Gilbert traveled to the Annual Meeting of the Geological Society of America in Minneapolis to present work she has been doing with colleague Matt Salisbury (Geological Survey of Canada) on the velocity structure of oceanic crust. Their work appeared in the journal Geochemistry, Geophysics, Geosystems. This year she began a new project on the permeability of oceanic crust, using innovative techniques to estimate hydrothermal circulation within samples of the seafloor. This spring, Grace LaPier ’13 and Connor Dempsey ’13 assisted with laboratory measurements and this summer Miranda Bona ’13 has begun her honors thesis related to this project. Bryce Mitsunaga ’13 is also working with her on the properties of seafloor rocks; he is using samples from seamounts of different ages to understand the physical evolution of potential upflow zones for hydrothermal circulation in old oceanic crust.
She also traveled to the Northeast Section Meeting of the Geological Society of America in Hartford with two of her fall Williams-Mystic students, Zara Currimjee ’13 and Nuria Clodius ’13 (Mt Holyoke). The students presented results of an on-going collaborative study at Barn Island Marsh in CT. Anna Szymanski ’12 and Abigail Martin ’11 were among their co-authors.
Markes Johnson was declared the Charles L. MacMillan Emeritus Professor of Natural Science at the conclusion of the 2011-2012 academic year, bringing to a close 35 years of service to the Geosciences Department and the College. Although he will no longer be active in the classroom, Prof. Johnson expects to remain busy working on research articles and book manuscripts from a campus office in Mather House. The Iberian research group, to which he and spouse Gudveig Baarli (Williams Research Scientist) belong, has another field season ahead in 2013 under a grant awarded from the Spanish Ministry of Science and Technology for research on rocky shores in the Canary Islands (Spain) and Cape Verde Islands (former possession of Portugal). Markes and Gudveig participated in fieldwork with the group on three islands (Santiago, Maio, and São Nicolau) during Spring Break in March 2012.
Earlier, during the January Winter Study Period 2012, Research Scientist David Backus and Markes Johnson collaborated on studies related to a Pliocene fan delta preserved on Isla del Carmen in comparison with the modern delta at Loreto, Baja California Sur, Mexico. Preparation of research materials related to ongoing projects in Mexico and the Cape Verde Islands promises to keep Markes, Gudveig, and David involved with geological studies for quite some time into the future.
Academically, the highlight of the year came with publication of the Springer book, Earth and Life, with a lengthy (65-page) chapter by Markes and Gudveig on the “Development of Intertidal Biotas through Phanerozoic Time.”
During the course of the academic year, Prof. Johnson wrote peer reviews on manuscripts submitted to Palaeogeography, Palaeoclimatology, Palaeoecololgy; Journal of Coastal Research; Sedimentology, and Naturwissenschaten. He also performed reviews on grant proposals for the Natural Sciences and Engineering Research Council of Canada and the DFG (German Research Foundation).
Professor and Chair of Geosciences Paul Karabinos continued research on a three-year $144,000 grant from the National Science Foundation to support an educational initiative Visualizing strain in rocks with interactive computer programs. This project, in collaboration with Chris Warren from the Office of Information Technology, aims to create new computer programs written in Java, and accompanying modules for classroom and laboratory use, to enhance student learning of fundamental concepts of strain analysis in rocks.
Karabinos was awarded a National Science Foundation grant: “Structural Geology and Tectonics Forum in Williamstown, MA” for $35,100. The conference lasted seven days from June 12 to 18, and included three days of technical sessions and four days of field trips, short courses, and workshops. It was attended by 115 structural geologists from the United States, Canada, and Europe. Approximately one third of the participants were graduate students. Karabinos also gave a presentation at the meeting on how to use Google SketchUp for teaching structural geology. He also led a field trip to the Berkshire massif in western Massachusetts.
Karabinos attended the New England Intercollegiate Geologic Conference in Middlebury, Vermont, in October. He led a field trip to the Chester dome in southeastern Vermont.
Karabinos attended the National Meeting of the Geological Society of America in Minneapolis, Minnesota, in October, 2010, where he gave an invited presentation at a theme session titled Virtual Reality in Geoscience Education I (Digital Posters). He gave another presentation in a theme session called Multidisciplinary Studies of Fault System Deformation.
At the same meeting in Minneapolis, Karabinos was elected a Fellow of the Geological Society of America.
During his 45th year of teaching at Williams, Prof. Bud Wobus attended the annual meeting of the Geological Society of America in Minneapolis in October. While at GSA he represented the department at the semi-annual board meeting of the Keck Geology Consortium and organized a reunion of some 25-30 alumni and faculty at the meeting. Later in the fall he led his annual hike on the geology of Stone Hill for the Williamstown Rural Lands Foundation, with about 30 participants. In December he hosted a reunion of alumni attending the American Geophysical Union meeting in San Francisco, including several new alums working in the Bay Area.
During January he was the faculty leader of an 18-day trip for alumni to Argentine and Chilean Patagonia, giving several evening lectures and “on-the-road” seminars using the geological guidebook he prepared for the trip. Just after spring vacation he participated in the 25th annual Keck Geology Symposium at Amherst College, where he gave a talk at the Silver Anniversary banquet recalling the founding and early years of the Consortium. His honors student, Katie Kumamoto ’12, presented a short talk and poster and contributed a paper to the Symposium proceedings about her Keck-sponsored field and lab research studying the Hrafnfjörður volcano in the West fjords region of Iceland.
For alumni reunion in June he inaugurated a new campus tour, “Williams Rocks – What Williams is Built In, On, and Of” to introduce walkers to the geological history of the area as illustrated by campus outcrops and building materials. This summer he will visit a Keck project in the Powder River Volcanic Field of NE Oregon where his next honors student, Johnny Ray Hirojosa ’13, will be working.
Class of 1960 Scholars in Geology
Miranda L. Bona
Kathryn M. Kumamoto
Peter L. Clement
Johanna S. Eldmann
Ian M. Nesbitt
Christopher M. Elliott
Sarah E. Rowe
Thomas J. Gaidus
Gordon P. Smith
Kalle L. Jahn
Oona G. Watkins
Dr. Keith Klepeis, University of Vermont
Sperry Lecture Speaker and Geosciences Class of 1960 Scholar Speaker
“The Patagonian Andes: Formation of a Collisional-Style Mountain Range in a Back-Arc Setting”
“What Lies Beneath? A View of New Zealand’s Mountain Ranges from Deep Beneath the Earth’s Surface”
Dr. Michael Williams, Univ. of Massachusetts, Amherst
Geosciences Class of 1960 Scholar Speaker
“Structural Processes in the Lower Crust: New Tools and New Interpretations from North America’s Largest Sample of Continental Lower Crust”
Dr. Mea Cook, Williams College
“The Ocean’s Role in Past and Future Climate Changes”
“Reconstructing Ocean Circulation and Greenhouse Gases Using Deep-Sea Sediments”
Dr. Rónadh Cox, Williams College
“Giant Boulders on Irish Cliffs: How Did They Get There?”
“Boulder Ideas: Storm Waves can Move Megagravel on Cliff Tops of the Aran Islands, Ireland”
Dr. Daniel Ksepka, North Carolina State University
“March of the Fossil Penguins: Applying New Methods to Avian Paleontology”
Dr. Phoebe Cohen, MIT
“Tracing the History of Eukaryotes in Precambrian Seas”
Dr. Paul Harnik, The National Evolutionary Synthesis Center
“The Ecology of Extinction in Ancient and Modern Seas”
Dr. Markes E. Johnson, Williams College
“Charles Darwin, Pleistocene Rhodoliths, and the Cape Verde Islands”
Geosciences Student Colloquia
Kathryn M. Kumamoto ’12
“Magmatic Processes of the Hrafnfjörður Central Volcano, Northwest Iceland
Nari V. Miller ’12
Evidence for Methane Release from Laminated Bering Sea Sediments During the Penultimate Glaciation”
“Understanding Climate Change: Tracing Productivity and Ocean Circulation in the NW Pacific Using Benthic Formaminifera and Radiocarbon”
Williams-Mystic, Mystic Seaport
“Tracing Productivity and Ocean Circulation in the NW Pacific Using Benthic Foraminifera and Radiocarbon”
“Methane and Climate changes During the Last Glaciation”
“The Ocean’s Role in Past and Future Climate Changes”
Thomas Jefferson High School for Science and Technology
Dr. Rónadh Cox
Museum Institute for Teaching Science, Summer Institute
“Regional Erosion Rates in Madagascar via Cosmogenic Isotope Analysis”
University of Witwatersrand, Johannesburg, South Africa
“Boulder Ideas: Wave-Emplaced Megagravel on the Aran Islands, Ireland”
“Comets Cause Chaos, by Jupiter! Crust-Penetrating Impacts at Europa”
University of Illinois, Champaign-Urbana
“Storm Waves Move Large Boulder Ridges on the Aran Islands, Ireland
Geological Society of America Annual Meeting, Minneapolis, MN
“Crust-Breaching Impacts at Europa: Hydrocode Models and Geomorphologic Constraints on Ice Thickness”
Geological Society of America Annual Meeting, Minneapolis, MN
Postgraduate Plans of Geosciences Majors
|Erik M. Anderson||Unknown|
|Kathryn M. Kumamoto||Ph.D. program in Geology at Stanford University|
|Nari V. Miller||Geosciences field camp; travel for one year; graduate school|
Magmatic Processes at the Hrafnfjörður Central Volcano, Northwest Iceland
Kathryn M. Kumamoto
The Hrafnfjörður central volcano is a 350 km2 volcano located in the Northwest Fjords of Iceland. The volcano is part of the abandoned Snaefellsnes-Skagi rift complex (active 15-7 Ma) and with an age of 14 Myr is the oldest central volcano on Iceland. The ﬂows from Hrafnfjörður are stratiﬁed, with basalts and basaltic andesites generally on the bottom close to the shore of the fjord and more silicic rocks forming cliﬀs above. Above the silicic cliﬀs is another package of extensive plateau basalts, but these basalts are believed to have originated elsewhere.
The petrographic and geochemical characteristics of the Hrafnfjörður ﬂows were studied in order to determine the petrogenetic mechanisms that took place when the volcano was active. Evidence from bulk chemistry suggests that fractional crystallization was the dominant mechanism. Certain samples, though, do not lie on the fractional crystallization trend. Some of the oﬀ-trend samples contain signs of disequilibrium in the melt, including spongy cellular plagioclase and resorbed plagioclase and clinopyroxene phenocrysts. These were most likely formed through magma mixing. Oﬀ-trend dacites, on the other hand, are enriched in incompatible elements and were most likely formed through partial melting of the crust.
Placing this data in the context of other studies from the Snaefellsnes-Skagi Rift, fractional crystallization appears to be the most important mechanism for the formation of basalt, basaltic andesite, and andesite early in the rift’s life. Partial melting, however, is necessary to form more silicic rocks. This is similar to the behavior of modern Iceland rift volcanoes, though intermediate-silica rocks are very uncommon in the Neovolcanic system. Magma mixing becomes more important as the rift ages.
Evidence for Methane Release from Laminated Bering Sea Sediments During the Penultimate Glacial Periods
Nari V. Miller
There are few high-resolution marine records of oceanographic conditions from Marine Isotope Stage 6, a time of millennial-scale variability in climate and atmosphere greenhouse gasses. The Bering Sea has an active role in climate, as well as being sensitive to climate changes. It experiences annual sea ice formation and high productivity today, and may have had a role in gas exchange and water mass formation. Sedimentation can be high on the continental slopes, preserving millennial-scale features. Large amounts of organic material are buried in the Bering Sea, some of which is anaerobically digested to methane. Early Stage 6 sediments from IODP Site U1345 were characterized by carbon, oxygen, and nitrogen bulk stable isotope analysis; benthic and planktonic foraminifera stable isotopes and Scanning Electron Microscopy, and Electron Dispersive Spectroscopy. Our age model constrains the ages of the Marine Isotope Stage 7-6 interglacial-glacial and the Marine Isotope Stage 5-6 glacial-interglacial transitions by matching a coarse resolution benthic foraminifera record of U1345 to tie points on these transitions in the global stack of benthic foraminifera (Lisiecki and Raymo, 2004). This study focuses on a section of core corresponding to a 10-16 ky interval in early MIS 6, within the age span 154 ka to 188 ka. The sampling resolution is in the range of 0.13 ky/sample to 0.22 ky/sample. The sediments in this study surveys three millennial-scale intervals of laminated sediments.
Laminated intervals indicate suboxia in the water column, which causes denitrification, an isotopically fractionating process, throughout the water column. The bulk sediments show a gradually increasing trend in δ15N upcore from 5.2‰ to 6.7‰, but do not have the expected δ15N enrichment concurrent with the laminated intervals. The stable carbon and oxygen isotopes of benthic and planktonic foraminifera show excursions (N. pachyderma, maximum amplitude, δ13C: 7.61‰, δ18O: 1.06‰) in the laminated intervals.
Authigenic carbonates encrusted foraminifera from some sediment horizons in the core. A foraminifer encrusted with authigenic carbonates was compared to an unencrusted foraminifer (F2). The morphology of the shell is needle-like (width ~1μm) that are encrusted with smaller crystals (<1 μm). F2 had distinctly different morphology, where the calcite crystals (~10 μm width) have a smooth, ridged shape. The isotopic composition of the authigenic carbonate was plotted δ18O vs. δ13C, and fell on a similar mixing line (between a foraminifer composition estimated as δ18Omeas ~ 3.4‰ and δ13Cmeas ~ -0.6‰ and an authigenic carbonate composition δ18Omeas ~ 6.5‰ and δ13Cmeas ~ -22.4‰) to that found by Cook et al. (2010) from MIS 3 sediments on the Umnak Plateau.
The δ18O vs. δ13C of bulk sediments were also plotted; foraminifer calcite and the authigenic carbonate endpoints clustered in the same region, and a third carbonate composition was appeared (estimated at δ18Omeas ~ -8‰ and δ13Cmeas ~ -1‰). The percent of each carbonate end member was estimated by using the M44 peak of the bulk sediment mass spectrometry data as a measure of the total carbonate in each sample. The percent of each end member in a sample was determined using a linear mixing model based on the estimated carbonate end member values of δ18O and δ13C. The background percentage of total carbonate is <1%, but peaks up to 8.5% total carbonate occur with each laminated interval. This study presents evidence of active methane flux during Stage 6 coinciding with three laminated intervals, to region-wide responses to rapid global climate change.
Benthic foraminiferal stable oxygen isotope stratigraphies from IODP Sites U1339 and U1345
Mea Cook, Assistant Professor of Geosciences, A. C. Ravelo, and A. C. Mix
American Geophysical Union, (2012)
We present preliminary age models from two sites drilled during the Integrated Ocean Drilling Program Expedition 323 to the Bering Sea: U1339 at Umnak Plateau and U1345 on the continental slope in north-central Bering Sea. The age models are based on the δ18O measured in benthic foraminifera, then correlated to the global stack of Lisiecki and Raymo (2005). Since no one species was present throughout the core, we measured stable isotopes on five species: Uvigerina peregina, U. senticosa, Elphidium cf. batialis, Nonionella labradorica, and Globobulimina affinis. We corrected the oxygen isotope measurements for the offset between U. peregrina and the other species. Authigenic carbonates appear in sediments at both sites, and affect some of the stable isotope measurements. Bulk sediment stable isotopes measurements from U1339 reveal that there are at least four distinct compositions of authigenic carbonates in those sediments, including ones with anomalously low δ13C, anomalously low δ18O or both, compared to foraminifera.
Intermediate-Depth Ventilation in the Bering Sea from the Last Glacial Maximum to the Holocene
Mea Cook, Assistant Professor of Geosciences, S. A. Schlung, A. C. Ravelo, and T. P. Guilderson
American Geophysical Union, (2011)
The Bering Sea is the northernmost marginal basin in the Pacific Ocean. Though no subsurface water mass forms there today, it has long been regarded as a location where intermediate or deep water may have formed in the past. During the last deglaciation, intermediate-depth sediments from around the North Pacific margin from Baja California to Japan are dysoxic or laminated, including in the Bering Sea. It is not clear what are the relative roles of several mechanisms that can could have contributed to the oxygen depletion: the intensification of local export production, reduced intermediate water ventilation rate, lower preformed oxygen concentration, or change in the source of intermediate water. With sediment cores that range from 1.0 to 2.2 km water depth, we investigate the ventilation history of the Bering Sea using stable oxygen and carbon isotope and radiocarbon measurements on planktonic and benthic foraminifera. During the Bolling warm period, water at intermediate depths in the Bering Sea were enriched in 14C and 13C. This well-ventilated water mass co-existed with a more intense and expanded oxygen minimum zone, implying that the intense surface ocean productivity that occurred in the subarctic Pacific at this time was responsible for the oxygen depletion. Therefore, dysoxia observed around the rest of the North Pacific margin at this time is associated with better intermediate water ventilation, and a decoupling of oxygen concentrations and ventilation rates.
Paleo-Iron Supply to the Western Subarctic Pacific Since the Last Glaciation
P. J. Lam, L. F. Robinson, J. Blusztajn, J. F. McManus, Mea Cook, Assistant Professor of Geosciences, and L. D. Keigwin
American Geophysical Union, (2011)
A strong and pervasive productivity peak has been observed in cores around the North Pacific during the Bølling-Allerød warm period of the last deglaciation. Recently, it has been hypothesized that this peak may have been caused by an influx of iron from the continental shelves as they were flooded during the deglaciation. Here, we examine this hypothesis by reconstructing the flux and sources of detrital material to a sediment core from the Detroit seamount (Vinogradov 19/4 GGC-37, 50.4°N, 167.7°E, 3300m) in the Western Subarctic Pacific since the last glacial maximum (LGM), and compare to several proxies of paleo-the Western Subarctic Pacific since the last glacial maximum (LGM), and compare to several proxies of paleoproductivity. We use 230Th-normalization to reconstruct the flux of biogenic and detrital material, and the neodymium and strontium isotopic compositions to distinguish between volcanic margin and continental loess sources of detrital material. We find that total detrital flux is highest during the last glacial maximum and early deglacial periods, a time of relatively low productivity, with approximately equal contributions from the volcanic margin and from continental loess. Total detrital flux starts to decline around 16 kya, but increases again to 80% of the glacial maximum flux around the time of the Bølling-Allerød productivity peak. The local deglacial maximum in detrital flux coincides with a maximum in authigenic uranium, and immediately precedes maxima in opal flux, carbonate flux, benthic foraminifera abundance, and excess 231Pa/230Th. While the local deglacial maximum in detrital flux is consistent with iron stimulation of productivity, we conclude that iron supply alone is not sufficient to explain the deglacial productivity peak, since glacial times exhibited low productivity despite high detrital flux. Further, the relative and absolute contributions of detrital material of volcanic origin is lower during the deglaciation than during the LGM, suggesting that loess may have contributed more iron during the deglacial productivity peak. To explain the low—high—low pattern of productivity during glacial, deglacial, and Holocene periods, respectively, we suggest that productivity was primarily major nutrient-limited but iron replete during glacial times, replete in both iron and major nutrients during the deglacial productivity peak, and primarily iron limited during the Holocene and modern times. Ironreplete production during the LGM and the deglacial productivity peak is consistent with a higher nutrient utilization suggested by high δ15N during these times.
Enhanced Southern Ocean Ventilation Through the Last Deglaciation
A. C. Elmore, E. L. Sikes, Mea Cook, Assistant Professor of Geosciences, B. Schiraldi, and T. P. Guilderson
American Geophysical Union, (2011)
During the last deglaciation, abrupt changes in Southern Ocean ventilation are linked to corresponding abrupt changes in climate, where ventilation began with a flush of the shallow interior and progressively spread well-ventilated waters deeper as the deglaciation continued. Ventilation changes in the New Zealand region of the Southern Ocean were reconstructed using high-resolution, mono-specific (P. wuellerstorfi), benthic foraminiferal δ13C from 6 cores on the New Zealand Margin, spanning 663 to 3836 m water depth. Age models for these cores were determined using tephrastratigraphy and benthic foraminiferal δ18O. At the Last Glacial Maximum (LGM), the δ13C difference (Δδ13C) between 663 to 2541 m is ~1.7‰. In the Holocene, the deep water δ13C value is higher, and Δδ13C decreases to ~1.0‰, implying that glacial deep waters were less ventilated. In the early deglaciation, during Heinrich Event 1, δ13C at the site of modern-day Antarctic Intemediate Water (1165 m) was significantly higher suggesting a pulse of newly ventilated water emanated from the Southern Ocean. During the Antarctic Cold Reversal, the Δδ13C profile is similar to the LGM, indicating that ventilation slowed, with a return to glacial-like conditions. Enrichment of the water column in δ13C from 1165 to 3836 m during the Younger Dryas suggests increasing ventilation of deeper water masses, whereas at 663 m, δ13C decreases, implying reduced formation of Subantarctic Mode Water.
Depleted Radiocarbon in Deep Water in the Southwest Pacific and Southern Ocean at the Last Glacial Maximum
E. L. Sikes, Mea Cook, Assistant Professor of Geosciences, A. C. Elmore, and T. P. Guilderson
American Geophysical Union, (2011)
The relative 14C ages of surface and deep marine waters reflect the balance between air-sea exchange of 14CO2 in deep-water formation areas, the radioactive decay of 14C during subsurface circulation, and the mixing between adjacent water masses. The Δ14C of the interior ocean is known to vary due to the major reorganization of circulation and CO2 sequestration associated with glacial to interglacial climate changes. Using benthic and planktonic 14C ages from deep-sea sediment cores from the southwest Pacific, east of New Zealand, we have constructed a composite depth profile of 14C ages from the late glaciation through the early deglaciation. By using regionally widespread tepha layers as independent stratigraphic markers we can estimate atmosphere- ocean Δ14C differences. Relative to the atmosphere, our estimates of the Δ14C at intermediate depths (above 1600 m) are broadly consistent with ventilation similar to today. In contrast, the Δ14C of upper and mid-depth deep water (~2000-3800 m) at several points between the late glacial and early deglaciation (24-15 ka) indicate a deep water mass with significantly lower Δ14C than we would expect for a deep circulation similar to today’s. The Δ14C depletions are range from 50-500‰, with the gradient between deep and intermediate waters both shallower and steeper than today. Below 3800 m, deep water Δ14C appear less depleted. Benthic-planktonic Δ14C differences at all depths show greater relative differences than modern, with the largest differences in upper deep waters. The data from these deep Pacific cores may be susceptible to biases from dissolution and/or bioturbation. However, our consistently low Δ14C estimates of deep water during the late glacial support the hypothesis that there was a 14C-depleted CO2 rich water mass in the deep Pacific Ocean during the Last Glacial Maximum that was released to the atmosphere through exchange in the Southern Ocean during the deglaciation.
Boulder Ridges on the Aran Islands (Ireland): Recent Movements Caused by Storm Waves, not Tsunami
Rónadh Cox, Professor of Geosciences, D. B. Zentner ’09, B. J. Kirchner ’13 and M. S. Cook
Journal of Geology, 120 (3), 249-272 (2012)
Ireland’s Aran Island are an excellent place to test whether coastal boulder deposits—including individual rocks weighing several tens of tonnes near sea level, and clasts weighing several tonnes transported at tens of m above sea level—require tsunami for emplacement, or whether storm waves can do this work. Elongate deposits of cobbles, boulders, and megagravel are strung along the Atlantic coasts of the Aran Islands. No tsunami have affected this region in recent centuries, so if these deposits are forming or migrating at the present time, they must be storm-activated. We find a diverse range of evidence for recent ridge activity. First, shells of Hiatella arctica (subtidal rock-boring bivalves preserved in life position within ridge boulders) yield radiocarbon ages from ≈200 AD to modern (post-1950 AD). Second, recent motion is attested to by eye-witness accounts that pin movement of several individual 40-80 tonne blocks to a specific 1991 storm, and by repeat photography over the last few field seasons (2006-2011) that captures movement of boulders (masses up to ≈10.5 tonnes) even in years without exceptionally large storms. Finally, GIS comparison of 19th C Ordnance Survey maps with 21st C orthophotos shows that in several areas the boulder ridges have advanced tens of metres inland since the mid-19th C, overrunning old field walls. These advancing ridges contain boulders with masses up to 78 tonnes at 11 m above high-water mark, so wave energies sufficient to transport those blocks must have occurred since the 1839 survey. Thus there is abundant evidence for ridge activity since the 1839 mapping; and as there have been no tsunami in the northeastern Atlantic in that time period, we conclude that the Aran Islands boulder ridges are built and moved by storm waves.
Hydrodynamic Fractionation of Zircon Age Populations
R. L. Lawrence ’07, Rónadh Cox, Professor of Geosciences, R. W. Mapes, and D. S. Coleman
Geological Society of America Bulletin, 123, 295-305 (2011)
Zircons in transport in the modern Amazon River range from coarse silt to medium sand. Older grains are smaller on average: Mesozoic and Cenozoic grains have average equivalent spherical diameter (ESD) 122 ± 42 µm (lower fine sand), whereas grains >2000 Ma have average ESD 67 ± 14 µm (upper coarse silt). As a full Wentworth size class separates the two values, zircons in these age populations are hydraulically distinct.
Host sand size is correlated with average size of co-transported zircons, implying hydrodynamic fractionation. Zircon size is positively correlated with percent medium sand, and inversely correlated with percent very fine sand (p <0.0001 in both cases). In samples with >50% medium sand, average zircon size is 100 µm, compared with 80 µm in samples with >50% very fine sand. We infer from these data that zircon deposition is not size-blind, and that zircons track with hydraulically comparable sand grains. As different aged grains tend to have different characteristic sizes, this indicates the possibility of hydrodynamic fractionation of age populations.
Five samples representing different hydrodynamic microenvironments of a single dune present significantly different detrital zircon age spectra, apparently the result of hydraulic processes. Peak mismatch (age peaks failing to overlap at 2σ level), is the most common disparity; but age populations present in some samples are missing from other samples. The lack of correspondence among the samples appears to exceed that attributable to random sampling. We conclude that hydrodynamic fractionation of zircons and zircon age populations does occur. Zircon size should therefore be taken into consideration in detrital zircon provenance analysis.
Crust-Breaching Impacts at Europa: Hydrocode Models and Geomorphologic Constraints on Ice Thickness
Rónadh Cox, Professor of Geosciences and A. W. Bauer ’11
Geological Society of America Abstracts with Programs, 43, 75 (2011)
Estimates of Europa’s surface ice thickness range from 1 to 40 km (median estimate is 6 km). For crust thicknesses in this range, some proportion of impactors is likely to fully penetrate through the ice to water beneath. But because inner solar system bodies behave as fully solid targets (having rocky mantles beneath their crusts), our understanding of the dynamics of penetrative impacts is limited. So we have made numerical simulations —using the iSALE hydrocode—of impacts into ice over water, varying impact energy and ice thickness, to examine impact behaviour under Europan conditions.
We find the entire range of proposed Europan crust thickness liable to full impact penetration. For crusts <5 km thick, icy bolides 500 m diameter (impacting at 26.5 km.s-1) will penetrate. In 10 km crust, 500 m objects will form craters but anything larger will penetrate to water (by melting through at smaller diameters or by impact breaching as diameter increases). In 20 km crust, a 1.5 km object will crater, but larger ones will go through. For 30 km crust, a 2 km bolide will crater, but a 3 km one will penetrate. At 40 km, a 3 km bolide will crater, but a 5 km one will punch straight through to liquid beneath.
We may be able to constrain ice thickness at Europa by comparing depth/diameter (d/D) ratios of modeled craters with those measured from Galileo imagery, because shapes of the modeled craters vary as a function of impactor energy and ice thickness. Our data suggest thicknesses <10 km, because large craters produced in thicker crusts have profiles that do not match those of features measured on Europa. For example, the crater produced by a 1.5 km impactor in 20 km crust is 38.5 km in diameter and 1.3 km deep after 50 hours; but actual impact features in the ≈40 km size range on Europa are much shallower (only 50-100 m). We find d/D ratios consistent with Europan observations only for crusts <10 km thick.
Storm Waves Move Large Boulder Ridges on the Aran Islands, Ireland
Rónadh Cox, Professor of Geosciences, D. B. Zentner ’09, and M.S. Cook
Geological Society of America Abstracts with Programs, 43, 596 (2011)
Boulder ridges on the Atlantic coasts of the Aran Islands are linear or arcuate deposits of cobbles, boulders, and megagravel. The ridge deposits occur at elevations 1-40 m above Higher High Water (HHW), and at horizontal distances that range from a few m to 250 m inland from HHW. Some are perched on top of sheer cliffs, and others are at the back of wide, gently sloping platforms. The boulders come from seaward, eroded from the cliff top or platform surface and transported landward. Clast size is variable: average boulder size decreases at higher elevation, but lower elevation ridges (≈12 m above HHW) incorporate boulders weighing up to 78 tonnes. We present evidence that the ridges are formed and moved by storm waves. First, shells of Hiatella arctica (subtidal rock-boring bivalves preserved in life position within ridge boulders) yield radiocarbon ages from ≈200 AD to modern (post-1950 AD). Second, GIS comparison of 19th C Ordnance Survey maps with 21st C orthophotos shows that in several areas the boulder ridges have advanced 10s of metres inland since the mid-19th C, overrunning old field walls in the process. The advancing ridges include the segment with boulders up to 78 tonnes at 12 m above sea level, so wave energies sufficient to transport those blocks must have occurred since the 1839 survey. Finally, recent motion is attested to by eye-witness accounts that pin movement of several individual 40-80 tonne blocks to a specific 1991 storm, and by repeat photography over the last few field seasons (2006-2011) that captures movement of boulders (masses up to 12 tonnes) even in years without exceptionally large storms. Thus there is abundant evidence for ridge activity since the 1839 mapping. As there have been no tsunami in the northeastern Atlantic in that time period, we conclude that the Aran Islands boulder ridges are built and moved by storm waves.
Geologic map of the Cochiti Dam quadrangle, Sandoval County, New Mexico: U.S. Geological Survey Scientific Investigations Map 3194
David Dethier, Professor of Geosciences, R. A. Thompson, M. R. Hudson, S. A. Minor, and D. A. Sawyer
United States Geological Survey (2011)
The Cochiti Dam quadrangle is located in the southern part of the Española Basin and contains sedimentary and volcanic deposits that record alluvial, colluvial, eolian, tectonic and volcanic processes over the past seventeen million years. The geology was mapped from 1997 to 1999 and modified in 2004 to 2008. The primary mapping responsibilities were as follows: Dethier mapped the surficial deposits, basin-fill sedimentary deposits, Miocene to Quaternary volcanic deposits of the Jemez volcanic field, and a preliminary version of fault distribution. Thompson and Hudson mapped the Pliocene and Quaternary volcanic deposits of the Cerros del Rio volcanic field. Thompson, Minor, and Hudson mapped surface exposures of faults and Hudson conducted paleomagnetic studies for stratigraphic correlations. Thompson prepared the digital compilation of the geologic map.
The mapped distribution of units is based primarily on interpretation of 1:16,000-scale, color aerial photographs taken in 1992, and 1:40,000-scale, black-and-white, aerial photographs taken in 1996. Most of the contacts on the map were transferred from the aerial photographs using a photogrammetric stereo-plotter and subsequently field checked for accuracy and revised based on field determination of allostratigraphic and lithostratigraphic units. Determination of lithostratigraphic units in volcanic deposits was aided by geochemical data, 40Ar/39Ar geochronology, aeromagnetic and paleomagnetic data. Supplemental revision of mapped contacts was based on interpretation of USGS 1-meter orthoimagery.
Characteristics of a Paleosol and its Implication for the Critical Zone Development, Rocky Mountain Front Range of Colorado, USA
Matthias Leopold, Jörg Völkel, David Dethier, Professor of Geosciences, Juliane Huber, and Markus Steffens
Applied Geochemistry, 26, S72-S75 (2011)
Activity and stability phases as well as geomorphic processes within the Critical Zone are well known. Erosion and deposition of sediments represent activity; soils represent geomorphic stability phases. Data are presented from a 4 m deep sediment section that was dated by luminescence techniques. Upslope erosion and resulting sedimentation started in the late Pleistocene around 18 ka until 12 ka. Conditions at the study site then changed, which led to the formation of a well-developed soil. Radiocarbon dating of the organic matter yielded ages between 8552 and 8995 cal. BP. From roughly 6.2 to 5.4 ka another activity phase accompanied by according sediment deposition buried the soil and a new soil, a Cambisol, was formed at the surface. The buried soil is a strongly developed Luvisol. The black colors in the upper part of the buried soil are not the result of pedogenic accumulation of normal organic matter within an A-horizon. Nuclear magnetic resonance spectroscopy clearly documents the high amount of aromatic components (charcoal), which is responsible for the dark color. This indicates severe burning events at the site and the smaller charcoal dust (black carbon) was transported to deeper parts of the profile during the process of clay translocation.
Revisiting Long-Term Dustfall Rates and Chemistry, Colorado Front Range
David Dethier, Professor of Geosciences, James McCarthy ’11, and William Ouimet
Geological Society of America Abstracts with Programs, 43 (5), 336 (2011)
For many decades, geomorphologists and soil scientists have noted the influence of dustfall (clay + silt) on the morphology, texture and chemistry of late Pleistocene and Holocene soils in the Front Range and other upland areas of the S. Rocky Mountains. Contemporary measurements demonstrate the influence of dustfall P and other elements on ecosystem function, but deposition rates over longer time periods are also significant. In addition, recent advances in applying meteoric 10Be to analyze soil age and evolution argue that dust may be an important component of 10Be delivery. Measurements from sites in the Boulder Creek catchment and published values from nearby areas suggest that the long-term, net clay accumulation rate is about 0.04 gm cm-2 kyr-1 in soils on stable, dated surfaces such as Neoglacial, Pinedale and Bull Lake moraines. Clay concentration in parent material is the principal source of measurement uncertainty. Clay+ silt accumulation rates are ~0.1 gm cm-2 kyr-1. Because measured accumulation rates integrate fines generated by weathering and dustfall, they provide an upper limit for dustfall rates. If our values are correct, dustfall should not significantly influence the total amount of met10Be delivered to soil surfaces in the upland Front Range. The chemistry of soil fine fractions does not permit clear separation of pedogenic vs. dustfall origin nor provide simple clues about dust provenance. Trace components such as Zr, Nb and P show strong size fractionation in crushed fresh-rock samples of Boulder Creek granodiorite and other local rock types, complicating elemental-ratio approaches for assessing the origin of fine material. Contemporary dustfall measurements in the upland Front Range suggest that modern rates are much greater than long-term values, perhaps reflecting changes in land use or climate.
Oceanic Crustal Velocities from Laboratory and Logging Measurements of Integrated Ocean Drilling Program Hole 1256D
Lisa Gilbert, Assistant Professor of Geosciences and Marine Sciences and M. H. Salisbury
Geochem. Geophys, Geostyt., 12, 09001 (2011)
Drilling and logging of Integrated Ocean Drilling Program (IODP) Hole 1256D have provided a unique opportunity for systematically studying a fundamental problem in marine geophysics: What influences the seismic structure of oceanic crust, porosity or composition? Compressional wave velocities (Vp) logged in open hole or from regional refraction measurements integrate both the host rock and cracks in the crust. To determine the influence of cracks on Vp at several scales, we first need an accurate ground truth in the form of laboratory Vp on crack-free, or nearly crack-free samples. We measured Vp on 46 water-saturated samples at in situ pressures to determine the baseline velocities of the host rock. These new results match or exceed Vp logs throughout most of the hole, especially in the lower dikes and gabbros, where porosities are low. In contrast, samples measured at sea under ambient laboratory conditions, had consistently lower Vp than the Vp logs, even after correction to in situ pressures. Crack-free Vp calculated from simple models of logging and laboratory porosity data for different lithologies and facies suggest that crustal velocities in the lavas and upper dikes are controlled by porosity. In particular, the models demonstrate significant large-scale porosity in the lavas, especially in the sections identified as fractured flows and breccias. However, crustal velocities in the lower dikes and gabbros are increasingly controlled by petrology as the layer 2-3 boundary is approached.
Geological and Geophysical Observations of Normal Oceanic Crust
Lisa Gilbert, Assistant Professor of Geosciences and Marine Sciences and M. H. Salisbury
Geological Society of America Abstracts with Programs, 43 (5), 278 (2011)
Observations of exposed oceanic crust in tectonic windows and analogies to ophiolites have helped advance our understanding of the geologic nature of oceanic crust. In recent years, the Integrated Ocean Drilling Program (IODP) has succeeded in reaching through lavas and dikes to gabbros for the first time in a section of normal oceanic crust. IODP Hole 1256D provides a unique opportunity to integrate geologic and geophysical data at several scales. Samples and geophysical data logged in the open hole allow us to ground-truth the geophysical layers identified by regional seismic experiments. To determine the influence of cracks on seismic velocity at several scales, we first need an accurate ground-truth, in the form of laboratory velocity of crack-free, or nearly crack-free samples. Hand samples include few cracks since drilling recovery generally excludes cracks except those that have been filled or are small enough to be preserved within the 6 cm diameter core. The influence of cracks on seismic velocity is then determined as the difference between seismic velocities of hand samples and seismic velocities logged in the open hole or from regional experiments. Crack-free velocities calculated from simple models of logging and laboratory porosity data for different lithologies and facies suggest that crustal velocities in the lavas and upper dikes are strongly influenced by porosity. In particular, our models demonstrate significant large-scale porosity in the lavas, especially in the units previously identified as fractured flows and breccias. In the lower dikes and gabbros porosity drops to less than 1% and crustal velocities are controlled by other factors. At this location, seismic velocity and porosity both change noticeably at the transition between lavas and dikes and the seismic layer 2/3 boundary is estimated to be within about 100 m of the bottom of the hole and likely near or coincident with the transition between dikes and gabbros.
The Lavas-Dikes Boundary in Superfast Spreading Crust: Inferences from Structure and Geophysical Logs at IODP Hole 1256D (Equatorial Pacific)
P. Tartarotti, M. Crispini, M. Tominaga, Lisa Gilbert, Assistant Professor of Geosciences and Marine Sciences, M. Zucali, and M. Panseri
Geoitalia, the 8th Italian Forum of Earth Sciences (2011)
The boundary between seismic layer 2B (basalt extrusives) and layer 2C (sheeted dike complex) in the oceanic crust created at fast and superfast-spreading ridges is commonly characterized by intense rock fracturing and occurrence of breccias hosting metal sulfides. In ODP-IODP Hole 1256D (East Pacific Rise, Cocos Plate) the layer 2B/2C boundary corresponds to the Transition Zone (TZ) located between the overlying Sheet and Massive Flows and the underlying Sheeted Dike Complex. The TZ is about 60m thick and consists of basaltic sheet and massive flows, a cataclastic unit, and hyaloclastitic breccias cemented by sulfides, quartz, anhydrite, calcite, and minor amphibole. It separates two contrasting metamorphic zones, the shallowest basalts being altered at low temperature conditions, and the deeper basalts being affected by greenschists-facies alteration. The upper and lower boundaries of the TZ have been defined at 1004mbsf and 1061mbsf, respectively. However, due to low recovery rates in Hole 1256D, especially from brecciated intervals and fracture fillings, these two boundary depths do not perfectly fit with those suggested by wireline geophysical logs and by structural data on cores. Namely, steep fractures and cataclasites, likely related to the stress field induced by intrusion of dikes at depth, are concentrated up to ca. 800 mbsf, in accord with FMS and UBI images. The seismic structure of the crust is affected by fracture shape, namely, low aspect ratio (thin fractures) have much more effect on compressional wave velocity than wide open pores, with varying porosity. By integrating structural/microstructural data with physical properties investigations and geophysical logs, the actual thickness of the TZ can be revised and defined more precisely. We used structural data from cores, thin section observation, and X-ray tomography on fractured samples for characterizing the crack porosity and shape, in order to calibrate the geophysical signals along the TZ. The thickness and position of the TZ are crucial for controlling the pattern of crustal permeability and hydrothermal fluid circulation.
Vegetation and Geomorphic Changes in a New England Salt Marsh in the Last 1,000 Years
Lisa Gilbert, Assistant Professor of Geosciences and Marine Sciences, N. Clodius, Z. Currimjee ’13 A.T. M. Martin, R. Neurath, A. Szymanski, and S. J. Bentley
Geological Society of America Abstracts with Programs, 44 (2), 95 (2012)
Headquarters marsh is one of several marshes in the Barn Island Wildlife Management Area in Stonington, CT near the Connecticut-Rhode Island border. Modern coastal marshes in Connecticut generally began forming after the period of rapid post-glacial seal level rise, which ended 3,000 y ago. Headquarters marsh peat records approximately the last 1,000 y of relative sea level rise and subsequent coastal change. We compare peat accumulation rates from Cs-137 derived 1954 horizons, excess Pb-210 dating, and C-14 dating with nearby tide gauge records to trace the growth and landward encroachment of Headquarters marsh. With sea level rise, succession of marsh grasses is preserved in peat as roots or rhizomes. In recent decades, there is a particularly striking reduction in the mid to high marsh species Juncus gerardi and Distichlis spicata. There is also a gradual increase in Spartina spp. on the upper slope of the high marsh. Marsh flora follow a general pattern from low marsh to high marsh to upland, but microtopography, drainage, and other factors such as human modifications influence the mosaic of vegetation visible today. Using data from thirty new cores (up to 7 m in length), we present geomorphic and vegetation changes to the area since the marsh first formed, with some dramatic changes since early studies of this location in the 1940s.
Diverse Macroids and Rhodoliths from the Upper Pleistocene of Baja California Sur, Mexico
B. G. Baarli, A. Santos, C. M. da Silva, J. Ledesma-Vázquez, E. Mayoral, and Markes E. Johnson, Professor of Geosciences
Journal of Coastal Research, 28 (1), 296-305 (2012)
Small multi-taxonomical nodules characterized as rhodoliths, balanuliths, coralliths, bryoliths and nodules composed of vermetids “vermetuliths”, are described from one horizon in carbonate sand from the Upper Pleistocene Mulegé Formation at Playa La Palmita, Baja California Sur, Mexico. Such a diversity of fossil, free-rolling biota is seldom described in the literature. This is the first time vermetuliths are reported in the fossil record, also the coral Astrangia has not been reported to constitute coralliths before. These nodules and their associated firm-ground were generated in a shallow bay near rocky shores. Break up of a firm-ground during a sedimentary hiatus provided fragments of loosely consolidated carbonate sandstone for organic nucleation. Fast growers, like balanids, vermetids and bryozoans settled on these sandstone fragments or on bioclasts. Initial rapid growth of pioneer organisms was succeeded by a period of bioerosion, and finally encrustation with a thin, crustose to lumpy cover of coralline red algae in the climax stage of succession. These were insipient rhodoliths, where the thin cover of coralline red algae reflects a short residence time. Also evident is a rich crypto- and endofauna that lived within and epifauna upon the nodules.
Distribution, Sediment Source, and Coastal Erosion of Fan-Delta Systems on Isla Cerralvo (Lower Gulf of California, Mexico)
D. H. Backus, Markes E. Johnson, Professor of Geosciences, and R. Riosmena-Rodriguez
Journal of Coastal Research, 28 (1), 210-224 (2012)
Located near the tip of the Baja California peninsula, Isla Cerralvo is the sixth largest island in the Gulf of California. Although surrounded by some of the most productive waters in the world, field surveys show that Isla Cerralvo’s shelf is largely devoid of biogenic carbonates, especially rhodolith beds, which are found in abundance elsewhere within the region. In counterpoint, a series of prominent fan deltas extend from the mouths of arroyos on Isla Cerralvo, despite the fact that the island has a granitic core, suggesting that the island’s bedrock is severely weakened. Field observations suggest that the fracture patter (submeter), hydrothermal alteration, as well as the orientations of metamorphic foliation, fracture sets, and fault planes all play a role in the accelerated rate of erosion on the island. The role of hydrothermal alteration is illustrated by a principal components analysis of Advance Spaceborne Thermal Emission and Reflection Radiometer (ASTER) Short Wave Infrared (SWIR) bands, which links heavily eroded areas at the south end of the island to areas with high concentrations of clays. A synthetic drainage system created using a 30-m resolution digital elevation model (DEM) generated from an ASTER image was used to model the Isla Cerralvo drainage basins and stream channel networks. Analyses of basin and stream network information, including basin slope values and channel slope values, were used to identify island-wide differences in basin morphology and erosion characteristics. Stream channel profiles and slope-area data supported by limited uplift data indicate that Isla Cerralvo has not been uplifted as a single block, but it is broken into at least two major structural blocks with different uplift histories. Due to the arid climate and low annual precipitation, we find that sediment removal from the interior of Isla Cerralvo can only be accomplished by episodic, but very short (hours to days), catastrophic rainfall events caused by hurricanes or chubascos (winter storms). Subsequently, the sediment is eroded from fan deltas and transported southward by longshore currents and wind-generated waves, choking carbonate production along Isla Cerralvo’s shores and shelf.
Pliocene Stratigraphy at Paredones Blancos: Significance of a Massive Crushed-Rhodolith Deposit on Isla Cerralvo, Baja California Sur (Mexico)
K.F. Emhoff ’10, Markes E. Johnson, Professor of Geosiences, D. H. Backus, and J. Ledesma-Vázquez
Journal of Coastal Research, 28 (1), 234-243 (2012)
A white blaze across coastal cliffs is the hallmark of Paredones Blancos on Isla Cerralvo. Cliff-forming strata include three roughly 10-m thick units with a lateral coherence of only 0.75 km. The middle unit is a massive deposit of crushed rhodoliths. The other two units consist of matrix-supported conglomerate with cobbles and boulders of granodiorite, basaltic andesite, and hornblende diorite. By volume, the matrix accounts for >80% of those units, mostly grus from weathered granite. Thin sections were studied from samples collected at five levels through the middle rhodolith unit to determine carbonate purity as a ratio between organic CaCO3 and inorganic minerals. The bottom and top margins of the deposit show higher levels of mixed clastics with 37% inorganic mineral content, as compared with 11% toward the middle. Original depositional environments are interpreted as a rhodolith bank adjacent to a large fan delta built seaward from a wide canyon mouth. The stratigraphic sequence records a rise in sea level that brought rhodolith debris to the flooded canyon mouth above the basal conglomerate, and a drop that emplaced another conglomerate above the rhodolith deposit. A Middle Pliocene age is based on co-occurrence of Clypeaster bowersi and Argopecten revellei within the carbonates.
Development of Intertidal Biotas through Phanerozoic Time
Markes E. Johnson, Professor of Geosciences and B. G. Baarli
In: Talent, J.A. (ed), Earth and Life: Global Biodiversity, Extinction Intervals and Biogeographic Perturbations Through Time, Springer Sciences and Media 1,1107 p. (2012)
Changes in the biodiversity and organization of intertidal biotas from rocky, sandy, and muddy shores are summarized on the basis of information in the fossil record for 1,622 extinct and extant species through Phanerozoic strata from the Cambrian to the Pleistocene at 361 localities around the world. To enter the database, each fossil species qualified as intertidal in origin based on sedimentological, geological, and other spatial criteria. Among the study sites documented in the scientific literature, 45% are considered former rocky shorelines. Another 31% represent former muddy shores and 24% are indicative of former sandy shores. Rocky-shore biotas demonstrate the greatest change in biodiversity, with species per Cenozoic study site nearly 2.5 times more than found on average at Paleozoic study sites. Key elements of the modern rocky shore biota were in place by Oligocene time and reflect much the same kind of ecological crowding found in that setting today. Coastal mudflat biotas show only a minor increase in biodiversity based on body fossils, although evidence from trace fossils implies an increase in ecological crowding through time. Sandy-shore biotas are the most conservative and least diverse in their development. The influence on intertidal habitat space by global tectonics, sea-level change, and relationship to other ecosystems is considered.
Rhodoliths, Uniformitarianism, and Darwin: Pleistocene and Recent Carbonate Deposits in the Cape Verde and Canary Archipelagos
Markes E. Johnson, Professor of Geosciences and others
Palaeogeography, Palaeoclimatology, Palaeoecology, 329-330, 83-100 (2012)
Visiting “St. Jago” (Santiago) in the Cape Verde Islands in 1832 and again in 1836 aboard HMS Beagle, Charles Darwin was the first to trace and describe the tri-part sequence of white limestone and sandstone beds stratigraphically located between two levels of basalt exposed almost uninterrupted for 10 km along coastal cliffs. The Pleistocene carbonate sediments dominated by rhodoliths and rhodolith debris accumulated on a basalt shelf and subsequently became buried by subaerial and submarine basalt on the southeast coastline of Santiago. The main goal of this contribution is to re-examine Darwin’s stratigraphic sequence. The secondary goal is to provide a general taphonomical model based on the observation of Recent rhodolith deposits for evaluation of fossil rhodolith assemblages. Environmental uniformitarianism is employed to understand the depositional history of the southern Santiago rhodolith-bearing strata. The mixed clastic-carbonate sequence includes a basalt-derived basal conglomerate with an intertidal to shallow subtidal fossil assemblage mainly denoted by limpets and oysters. Upper layers typically demonstrate swaley and hummocky cross stratification incorporating rhodolith debris further modified by bioturbation. Pillow basalts from 10 – 18 m in thickness succeeded by subaerial flows imply swift burial of the carbonate succession under equivalent water depths. The calcareous nannofossil assemblage was investigated to more precisely date the deposits. Darwin’s paleoshore is reinterpreted to represent two different transgressions occurring between approximately 1.1 and 0.7 Ma. Taphonomic grades from whole rhodoliths to finely crushed rhodolith debris observed under present-day conditions on Maio (Cape Verde Islands) and Fuerteventura (Canary Islands) were used to model rhodolith preservation and to constrain the depositional settings to which rhodoliths may be transported from the offshore banks where they naturally thrive. Coastward transport of rhodoliths commonly ends with deposition in subtidal storm beds, tidal pools, and platform over-wash deposits, as well as beach, berm, hurricane, tsunami, and coastal dune deposits.
Lagoon Microbialites on Isla Angel de la Guarda and Associated Peninsular Shores, Gulf of California, Mexico
Markes E. Johnson, Professor of Geosciences, J. Ledesma-Vásquez, D. H. Backsu, and M. R. Gonzálaz
Sedimentary Geology, 263-264, 76084 (2012)
Examples of two closed lagoons with extensive growth of Recent microbialites showing variable surface morphology and internal structure are found on Isla Angel de la Guarda in the Gulf of California. Comparable lagoonal microbialites also occur ashore from Ensenada El Quemado on the adjacent peninsular mainland of Baja California. The perimeters of all three lagoons feature crusted structures indicative of thrombolites with a knobby surface morphology 2 cm to 3 cm in relief and internal clotting without any sign of laminations. Outward from this zone, thrombolitic construction thins to merge with a white calcified crust below which a soft substratum of dark organic material 4 cm to 6 cm in thickness is concealed. The substratum is laminated and heavily mucilaginous, as observed along the edges of extensive shrinkage cracks in the overlying crust. The thrombolitic crust is anchored to the shore, while the thinner crust and associated stromatolitic mats float on the surface of the lagoons. Laboratory cultures of the dark organic material yielded the solitary cyanobacterium Chroococcidiopsis as the predominant taxon interspersed with filamentous forms. In decreasing order of abundance, other morphotypes present include Phormidium, Oscillatoria, Geitlerinema, Chroococus, and probably Spirulina. The larger of the two island lagoons follows an east-west azimuth and covers 0.225 km2, while the smaller lagoon has a roughly north-south axis and covers only 0.023 km2. The salinity of water in the smaller lagoon was measured as 148 ppt. Pliocene strata along the edge of the smaller modern lagoon include siltstone bearing calcified platelets suggestive of a microbial origin. Dry lagoons abandoned during the later Quaternary occur inland at higher elevations on the island, but retain no fossils except for sporadic white crusts cemented on cobbles around distinct margins. Raised Quaternary lagoons parallel to the big lagoon on Isla Angel de la Guarda are partly obscured by flood damage, but still easily mapped from aerial photos. These features suggest that Isla Angel de la Guarda experienced Quaternary uplift similar in scale to many other gulf islands on which marine terraces are preserved. Closed lagoons around the Gulf of California represent a stable oligotrophic ecosystem affected by extreme aridity and hypersalinity, punctuated episodically by the injection of floodwater from tropical storms. The taxonomic and geographic ranges of microbial communities throughout the larger region remain to be explored.
Rhodolith Stranding Event on a Pliocene Rocky Shore from Isla Cerralvo in the Lower Gulf of California (Mexico)
Markes E. Johnson, Professor of Geosciences, D. M.Perez ’10, and B. G. Baarli
Journal of Coastal Research, 28 (1), 225-233
Controls on present-day sedimentation around Isla Cerralvo in the southern Gulf of California provide a model for restricted Pliocene limestone distribution. The 10.46-km2 island is elongate and roughly parallel to the direction of prevailing north winds. Debris washed from deeply dissected valleys build fan deltas of sand to boulder-size igneous clasts. These are transported south by long-shore currents, but the fans also create leeward zones with less agitated water. Remnants of a large Pliocene fan are exposed during low tide at Los Carillos on the SE side of the island. Adjacent is an unconformity between granite and granite-derived conglomerate with Nerita scabricosta. This extant gastropod is typical of the high intertidal rocky shore. The conglomerate is capped by a sandstone ramp with Argopecten abietus as an offshore facies. Basalt dikes exhumed from the granite formed natural groins that captured sediments and shells in the sand ramp. Whole rhodoliths, mostly 3.5 cm in diameter and 15 cm deep, covered 150 m2 within the ramp, now partly exposed among boulders in the basal conglomerate. Many rhodoliths encrust pea-sized rock cores. Accretion occurred in shallow water protected from extreme agitation by the nearby fan delta. Stranding of rhodoliths on the rocky shore was a storm-induced event.
An Extreme Habitat Adaptation by Boring Bivalves on Volcanically Active Paleoshores from North Atlantic Macaronesia
A. G. Santos, E. Mayoral, Markes E. Johnson, Professor of Geosciences, B. G. Baarli, C. M. da Silva, M. Cachão, and J. Ledesma-Vázquez
Facies, 58, 325-338 (2012)
Extensive bivalve borings are described in detail for the first time from basalt rockgrounds in the North Atlantic volcanic islands of Macraronesia. They occur on a Middle Miocene rocky shore of a small islet of Porto Santo (Madeira Archipelago of Portugal), as well as on Plio-Pleistocene rocky shores on Santiago Island (Cape Verde). A basalt substrate is widely penetrated by clavate-shaped borings belong to the ichnogenus Gastrochaenolites interpreted as dwelling structures of suspension-feeding bivalves. Some of these borings still retain evidence of the alleged trace-makers preserved as body fossils, while others are filled with their casts. The ichnofossil assemblage present on these bioeroded surfaces belongs to the Entobia ichnofacies. Recognition of Gastrochaenolites borings in volcanic rocks provides useful paleoenvironmental information regarding an expanded strategy for hard-substrate colonization. Preliminary results from fieldwork in the Cape Verde Archipelago indicate that such borings are more widespread through Macaronesia than previously thought.
Basalt Mounds and Adjacent Depressions Attract Contrasting Biofacies on a Volcanically Active Middle Miocene Shoreline (Porto Santo, Madeira Archipelago, Portugal)
A. G. Santos, E. Mayoral, Markes E. Johnson, Professor of Geosciences, B. G. Baarli, C. M. da Silva, M. Cachão, and J. Ledesma-Vázquez
Facies, 58, (2012)
Small basalt mounds with encrusting corals and inter-mound carbonate sandy zones with abundant rhodoliths corresponding to an ancient intertidal to shallow-water sea floor are exhumed from overlying volcaniclastic deposits and basalt lava flows at Pedra de Água on Ilhéu de Cima off Porto Santo, one of the islands of the Northeastern Atlantic Madeira Archipelago (Portugal). The mounds rise above the surrounding surface to attain a height of about a half meter. The mounds exhibit an assemblage of in situ hermatypic corals, dominated by Tarbellastrae and Solenastrea. They formed as massive (4.2 × 1.9 m average length), isolated patches in a protected bay close to shore eroded from an uneven basalt substrate dated to the Middle Miocene (14 to 15 Ma). The slightly deeper zones between basalt mounds, which alternate with them over a distance of more than 20 m, are covered mainly by coarse carbonate sand on which rhodoliths up to 14.8 cm in diameter are preserved in situ. Many rhodoliths have grown around a basalt core, which indicates a local, nearshore source for development. Complete burial of the elevated coral settlements and intervening low zones populated by rhodoliths occurred when volcanic lapilli and other tephra catastrophically buried this part of the rocky shore. The rhodoliths and coral assemblages exposed in an area of 12 m2 were canvassed systematically using census quadrats to quantify community relationships.
The Role of Crustose Coralline Algae in Late Pleistocene Reef Development on Isla Cerralvo, Baja California Sur (Mexico)
P. W. Tierney ’11 and Markes E. Johnson, Professor of Geosciences
Journal of Coastal Research, 28 (1), 244-254 (2012)
Crustose coralline algae played a fundamental role in reef establishment during the Late Pleistocene (122,143 ± 175 years before present) on Isla Cerralvo in the southern Gulf of California. Transported cobbles with a generally elongated clast shape (mean sphericity: 0.6) were encrusted by coralline red algae before locking in a north-south alignment (mean: N2°W) and providing a fixed substrate for colonization by Porites and Pocillopora corals. A fringing reef grew on this pavement of clast-encrusting rhodoliths and was succeeded by additional cobble-coral cycles. Out of five stratigraphically repetitive cycles, only the second and third offer sufficient exposure to be quantified with any confidence. Census data, cobble orientations, and measurements of algal rinds were collected to characterize the transition from rhodoliths to corals. Clasts in the second cycle (mean dimensions: 7.4 x 4.6 cm) have rinds that average 5.4 mm (Standard Deviation: 4.2 mm) at their thickest and 0.9 mm (SD: 0.8mm) at their thinnest; overlying corals that average 16.2 cm in height. Clasts within the third cycle (mean dimensions: 7.2 x 4.6 cm) have rinds that average 3.1 mm (SD: 2.5 mm) at their thickest and 0.9 mm (SD: 0.8 mm) at their thinnest; overlying corals that average 15.4 cm in height. Coralline algae helped cement both the underlying cobble pavement and reef corals.
Creating Interactive 3-D Models from Geologic Maps and Cross Sections Using Google SketchUp
Paul Karabinos, Professor of Geosciences
Geological Society of America, Abstracts with Programs, 43, 302 (2011)
The power of geologic maps and cross sections to portray the structure of a region is dramatically enhanced by software that can create interactive 3-D models, which can be rotated, panned, and zoomed by the user. I developed a methodology for creating virtual block diagrams from digital maps and cross-sections primarily using Google SketchUp. It is also essential to have an image-processing program, such as Photoshop, for cropping maps and cross-sections, and GIS software, such as Global Mapper, for preparing digital elevation models (DEM) for SketchUp.
The most effective 3-D models show how the maps and cross-sections connect at the topographic surface. The map can be segmented to give users the ability to ‘turn off’ individual portions of the surface to reveal cross-sections below. Such models help students and non-specialists visualize geologic structures, and provide experienced geologists with a valuable tool for assessing the validity of geologic interpretations.
Once a geologic map is draped on the DEM, SketchUp can aid in the creation of new geologic cross sections using traditional down-plunge and structure contour approaches adapted for a 3-D environment. This approach makes it possible to leverage the geologic information contained in geologic maps from regions with significant topographic relief. It also helps students effectively visualize the process of cross-section construction.
Evidence for Kilometer-Scale Fluid-Controlled Redistribution of Graphite in the Taconic Thrust Bbelt on Mount Greylock, Massachusetts: Implications for Strain Localization and Fault Growth
Paul Karabinos, Professor of Geosciences, R. F. Aronoff ’09, and E. S. Nemser ’98
Geological Society of America, Abstracts with Programs, 43, 324 (2011)
During the Ordovician Taconic orogeny, deep-water deposits from the Laurentian slope and rise were thrust westward over shelf rocks. On Mount Greylock graphite(C)-rich schist is interpreted as a flysch deposit, whereas C-poor schist is assigned to the Taconic thrust sheet by Ratcliffe et al. (1993). According to this interpretation, the major thrust is within the schistose rocks and separates C-rich Ordovician(?) Walloomsac Fm from structurally overlying, C-poor Late Proterozoic Greylock Schist. This interpretation assumes that the present distribution of C preserves primary variations in organic material.
The contact between C-rich and C-poor schist on Mount Greylock is diffuse and not marked by well-defined strain gradients. In contrast the contact between the C-rich schist and the structurally lower marble, is a mélange characterized by intense deformation. Our detailed field mapping indicates that the distinction between C-rich and C-poor rocks is not a reliable stratigraphic tool. Typically C-rich and C-poor rocks are interlayered on scales ranging from 10- to 100-m. Examination of 200 thin sections from 173 locations reveals that 55% of Waloomsac Formation samples are C-rich, but 45% are C-poor. Samples of Greylock Schist are more complex; although only 10% are C-rich; another 55% contain plagioclase porphyroblasts with abundant C inclusions surrounded by a C-poor matrix. This common texture suggests that C was once present in the matrix but was dissolved by aqueous fluids and transported out of the rock.
Thin section evidence suggests that C was formerly widely distributed in the schist on Mount Greylock, but that large-scale redistribution of C occurred during metamorphism and thrusting. We suggest that aqueous fluids dissolved C in large volumes of rock that are now C-poor, as shown by plagioclase porphyroblasts containing abundant C inclusions. Fluids later precipitated C in narrow zones to form C-rich schist. We suggest that a high density of fractures focused fluid flow parallel to thrust faults, and that C precipitated in these zones. C may have weakened the rocks and promoted faulting, thus creating a positive feedback between thrusting, fluid flow, and C precipitation.
Tectonic Thinning in the Mantling Sequence Around the Chester Dome, Vermont: Implications for the Mechanical Decoupling Between Basement and Cover Rocks
Paul Karabinos, Professor of Geosciences
in West, D.P., ed., Guidebook for field trips in Vermont, and adjacent New York, New England Intercollegiate Geological Conference, Middlebury, Vermont, C1-C27 (2011)
The New England Appalachians contain two north-south trending sets of gneiss domes. The western belt contains thirteen domes, including the Chester dome, that expose either 1 Ga Laurentian basement rocks or approximately 475 Ma rocks of the Shelburne Falls arc. The eastern belt contains twenty-one gneiss domes cored by either 600 Ma crust of possible Gondwanan affinity or approximately 450 Ma rocks of the Bronson Hill arc. Domes in both belts are surrounded by Silurian and Early Devonian metasedimentary rocks, which were deposited in two north-south trending basins before the Acadian orogeny.
The Chester dome in southeastern Vermont is a classic example of a mantled gneiss dome. Doll et al. (1961) portrayed the contact between the Mesoproterozoic Mount Holly Complex, in the core of the dome, and Neoproterozoic metasedimentary rocks as an unconformity. They also mapped the contact between Ordovician rocks and Silurian and Devonian metasediments as an unconformity. They recognized that Lower Paleozoic units around the Chester dome are dramatically thinner than they are elsewhere in southern Vermont, but nonetheless interpreted the sequence of rocks as stratigraphic. In contrast, Ratcliffe (2000a, 2000b) explained the stratigraphic omissions around the Chester dome as the result of thrusting. A third explanation for the attenuation and omission of units in the Lower Paleozoic sequence was proposed by Karabinos et al. (2010), who argued that the mantling sequence around the Chester dome preserves a normal-sense shear zone. There is a strong spatial correlation between the highly attenuated mantling units and highly strained, mylonitic rocks. Further, garnet-bearing rocks in the core of the dome record metamorphism during decompression of 2 to 3 kbar, whereas rocks above the high-strain zone were metamorphosed during nearly isobaric conditions. Strain markers and kinematic indicators suggest that extension occurred during northward extrusion of lower to middle crustal wedges of Proterozoic and Ordovician quartz-feldspar-rich gneisses below and up into a thick tectonic cover of Silurian mica-rich metasediments that had been transported westward in large-scale nappes. Electron microprobe dating of monazite and other estimates of the age of peak Acadian metamorphism suggest that extrusion occurred at approximately 380 Ma.