Geosciences

 Evolution and Distribution of Permeability in a Complete Section of Upper Oceanic Crust:  IODP Hole 1256D

Miranda L Bona ‘13

 Permeability is the principal hydrologic parameter controlling fluid flow in the Earth’s crust, yet measuring oceanic crust permeability has proven to be a challenge due to limited access and sampling.  To better understand the evolution and distribution of permeability in the upper oceanic crust, we examine permeability of basalt and gabbro core samples recovered from the Integrated Ocean Drilling Program (IODP) Hole 1256D.   We present a new method for determining core-scale permeability, which we call “maximum original permeability”, a two-dimensional value based on the size and shape of micro-fractures analyzed in thin section.  We also determine dry core-sample permeability and seawater-saturated core-sample permeability at confined pressures in the laboratory.  The maximum original permeability of these samples ranges from 10-18 to 10-14 m2, dry laboratory permeability from 10-16 to 10-12 m2, and saturated laboratory permeability from <10-22 to 10-18 m2.  At Hole 1256D, permeability decreases with depth, and the 15 million year old crust preserves multiple generations of hydrothermal mineral precipitates that seal all open pore space.  These fractures, which control or once controlled permeability, also vary in orientation with depth, consistent with models of crustal accretion:  sub-horizontal fractures are found in the lava flows and gabbroic intrusions; sub-vertical fractures in the dikes.  The distribution and evolution of permeability aids our understanding of hydrothermal circulation, the evolution of oceanic crust, the regulation of ocean chemistry, and the distribution of chemosynthetic sub-seafloor microbial communities.

 

 Geochemical Response of Two Adjacent Alpine Basins in Green Lakes Valley, Colorado, in a Low-Snow Year

Claudia R. Corona ‘13

Stream and groundwater geochemistry reflects processes in the uppermost part of the earth’s crust, where rock materials interact with the biosphere, atmosphere, and hydrosphere within the Critical Zone. This study examines the geochemistry of Saddle and Martinelli streams, which flow from two snowmelt-dominated, alpine catchments within the Boulder Creek basin, Front Range, Colorado.

Varying hydrologic inputs and biogeochemical reactions influence spatial variations of basin solute concentrations. The concept of a “representative elementary area” (REA) suggests that the variability of geochemical response to precipitation within a watershed is large when the basins are small. Recent literature suggests basin areas of 0.10-15.0km2, represent the threshold where solute concentrations become fairly constant. Careful sampling of small (0.25 km2) headwater catchments, which encompass zero order ephemeral and intermittent streams, can be used to test whether the REA concept is valid.

Hydrologic, chemical, and spatial data were collected at a site and synoptically upstream and compared to long-term hydrologic, chemical, and meteorological datasets for Saddle and Martinelli basins. My work emphasizes 2012, a summer of low snowmelt and abundant rainfall, and incorporates data from 2010 and 2011. Basin discharge measurements show 2010 was an average water year, 2011 was a wet water year, and 2012 was a low flow year.

Analysis of major ions (Ca2+, Mg2+, Na+ and K+, Cl–, SO42-, HCO3- and NO3-) and dissolved SiO2 in 126 water samples shows near neutral pH, and low solute concentrations; Ca2+ is the dominant cation and HCO3- is the dominant anion. Calculations of total solute flux for the Saddle and Martinelli basins show that cations are derived mainly from weathering and anions are controlled by precipitation and biologic activity. Precipitation accounts for <20% of stream export of K+, Mg2+, and Na+ and 99% of the silica derives from weathering. Precipitation accounts for all of the NO3-, 71% of the SO42- and 24% of the Ca2+ that leaves the basin. As a result of uptake by biologic activity, almost no NO3- leaves the basin. Downstream changes of ion concentration reflect the combined effects of increasing drainage area, longer contact times and groundwater flow. Martinelli and Saddle basin geochemistry does not show a decrease in variability with increasing drainage area, suggesting that REA concept may not apply to small, snowmelt-dominated catchments.

Geochemistry and Geothermometry of Mid Miocene to Pliocene Alkalic Rocks of the Powder River Volcanic Field

Johnny R. Hinojosa ‘13

The Neogene Powder River Volcanic Field (PRVF) is surrounded by the regionally dominant Columbia River Basalt Group (CRBG) but contrasts strongly with it compositionally, containing alkaline (highly sodic) rock types ranging from mafic to intermediate composition. The goals of this work are to constrain PRVF magma chamber temperatures based on Fe-Ti geothermometry calculations (Andersen and Lindsley, 1988 and Ghiroso and Evans, 2008) and to relate the sodium-rich lavas to possible contamination by underlying country rocks through geochemistry. Whole-rock and trace element chemistry was done by XRF and ICP-MS, respectively, while precise mineral chemistry was obtained using an electron microprobe. Geothermometry calculations yielded an approximate temperature range of 650˚ – 750˚ C for selected samples of basaltic trachyandesite, which provides a minimum temperature for magma chamber conditions. The temperature range represents the conditions where magnetite and ilmenite exsolved from solid solution. It also allows an estimate of the depth where this occurred, between 17 – 19 km based on an elevated 40˚ C/km geothermal gradient appropriate to an extensional tectonic setting. Samples containing elevated Na2O are more precisely classified as mugearite (trachybasalt) and benmoreite (trachyandesite). Based on elevated Na2O, Sr, and Ba and light rare earth element enrichment, it is likely that a mafic parental magma related to the Yellowstone Hotspot interacted with more siliceous and sodic overlying rock such as the leucotonalite and trondjhemite from the Wallowa batholith.  A future study might focus on a broader range of sampling from the PRVF and units of the Wallowa batholith in order to confirm geochemical relationships.

 

Using Geophysical Techniques in the Critical Zone to Determine the Presence of Permafrost

Gabriel M. Lewis ‘13

 Global warming during the 20th and 21st centuries has increased air temperatures in alpine areas of the southern Rocky Mountains sufficiently to melt large areas of previously frozen ground, referred to as permafrost.  Previous studies used geomorphological, hydrological, and GIS techniques to infer the distribution of frozen ground and ice lenses on Niwot Ridge and in adjacent Green Lakes Valley, Colorado Front Range.  Predictions of permafrost occurrence have not previously been verified in the field by subsurface geophysical measurements, although permafrost, ice lenses, and temperature profiles beneath active gelifluction lobes were documented in several studies during the 1970s along Niwot Ridge.  Electric Resistivity Tomography (ERT) and Ground Penetrating Radar (GPR) are geophysical techniques that have been utilized worldwide to study the evolution of alpine permafrost and ice lenses. Combining these geophysical methods maximizes the accuracy of each method while reducing their inherent ambiguities and limitations. Green Lakes and 4th of July Valleys offer ideal locations to verify the existence of ice masses within rock glaciers, where models predict they exist.

This study reports: (1) interpreted results of 16 ERT and 2 GPR lines totaling 815 m that were collected to test permafrost predictions in alpine zones; (2) soil temperature profiles and morphology in several pits excavated to saprolite along ERT lines; (3) energy modeling of water temperatures for Como Creek, a small alpine creek on Niwot Ridge, compared with measured temperatures for nearby Martinelli Stream; and (4) computer modeling of subsurface temperatures from surface temperatures on Niwot Ridge, Colorado, according to a model calibrated using data from Hopkins Memorial Forest, Williamstown, MA.

Analysis of seven ERT lines from elevations of 3500 to 3900 m on Niwot Ridge demonstrates that my study area lacks permanent ice lenses (resistivity of approximately 200-1000 kΩm) beneath a surface layer of coarse, blocky debris (resistivity of ~ 20 kΩm).  Gelifluction lobes, as well as nearby snow field areas, may contain seasonal ice lenses that are misinterpreted as permafrost features, but ice often melts completely by late summer.  Soil and water temperatures reveal that the subsurface is too warm to permit the development of permafrost, and heat-flow models confirm this hypothesis.  Inactive periglacial deposits within the Boulder Creek Watershed support evidence of a climate through the late Pleistocene that produced and supported permafrost and permanent ice lenses.  During the last glacial maximum, temperature and precipitation values supported gelifluction and permafrost at elevations as low as Gordon Gulch.  Better understanding of the present distribution of permafrost and active periglacial features helps predict changes to alpine landscapes as permafrost disappears and has implications for quantity of runoff in the near future.

 

A Comparative Study of Snowmelt-Driven Water Budgets in Adjacent Alpine Basins, Niwot Ridge, Colorado Front Range

Ian M. Nesbitt ‘13

 The Critical Zone, which extends from the top of the weathered bedrock to the tops of the tallest vegetation in alpine and subalpine headwater areas delivers fresh water to urban corridors near mountainous areas of North America. Snowmelt runoff from alpine basins typically accounts for over 80% of annual flow, but water budgets are not well quantified nor well understood in detail. Redistribution of snow by wind, the difficulty of estimating water losses from sublimation and evapotranspiration, and groundwater gains and losses from outside the basin make streamflow and water budget measurements challenging. I investigated two adjacent 0.25 km2 catchments, Martinelli and Saddle streams, both at ~3500 m, on Niwot Ridge in the Colorado Front Range. Mean annual runoff is ~230 mm (25% of mean annual precipitation) at Saddle basin and ~310 mm (30% of mean annual precipitation) at Martinelli basin, based on 12 and 28 years of gaging records, respectively.

Saddle stream is not fed by a late-lying snowpack, but records indicate that ablation-season discharge is still closely related to snowmelt in the basin. Martinelli basin shelters a ~6 m thick snowpatch in 8 ha of the basin, even in a low snow year. During much of the ablation season, snowpack mass density (ρ) is 0.5 g cm-3 and ablation rates are ~100 mm day-1. Since vegetation is shallow-rooted or nonexistent in Martinelli, evapotranspiration (ET) is probably not a major factor. Saddle basin is more heavily vegetated, but only the lower reaches are wooded; ET is likely < 260 mm annually. Specific runoff measured at the gage during 2012 was ~270 mm at Martinelli and ~35 mm at the Saddle gage. By monitoring snowpack area changes and longitudinal discharge, we were able to demonstrate that at least 30% of annual precipitation in Martinelli basin and 10% in Saddle basin bypasses the gage as subsurface flow. Short-term yield calculations indicate that approximately 2.5% of precipitation discharges from the basin as measurable surface water within a five-day period; the rest recharges groundwater or becomes immeasurable subsurface flow. For comparison, a nearby 2.3 km2 glaciated basin, Green Lake 4, discharges 50% of the water that falls on it within the same five-day period. Measured water yields from small, unglaciated alpine catchments thus should be viewed with caution.

 

Climate-Growth Relationship Divergence in the Hopkins Forest, Williamstown, Massachusetts

Sarah E. Rowe ‘13

 This study analyzes climate and tree growth data from the Hopkins Forest, Williamstown, Massachusetts, to determine what climatic conditions are most favorable for tree growth. Before 1980, red oak growth is positively correlated with wet summers before the growth year and dry springs in the growth year; beech growth is positively correlated with high winter precipitation and dry springs. Sugar maple is positively correlated with wet winters and summers in the growth year, and red maple growth is positively correlated with dry winters, summers, and springs in the growth year. However, after 1980, these relationships change direction; for example, red maple growth after 1980 is positively correlated with dry summers and uncorrelated with the other seasons. This is consistent with evidence of the divergence problem, or breakdown in growth-climate relationships, witnessed globally in the past fifty years. I investigate several hypotheses to explain the divergence problem and find that phase-shift explanations are the most likely to account for this change.