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The McMurdo Dry Valleys of Antarctica formed by extensive glacial erosion, yet currently exhibit hyperarid polar conditions canonically associated with limited chemical and physical weathering. Efficient chemical weathering occurs when moisture is available, and polythermal subglacial conditions may accommodate ongoing mechanical weathering and valley incision. Taylor Valley, one of the MDV, hosts several Pleistocene glacial drift deposits that record prior expansions of Taylor Glacier and sediment redistribution, if not sediment production. We present U-series isotopics of fine-grained sediments from these drifts to assess the timescales of physical weathering and subsequent chemical alteration. The isotopes ²³⁸U, ²³⁴U, and ²³⁰Th are sensitive to both chemical and physical fractionation processes in sedimentary systems, including the physical fractionation of daughter isotopes by energetic recoil following radioactive decay. By comparing U-series isotopic measurements with models of U-series response to chemical weathering and physical fractionation processes, we show that Pleistocene drift sediments record histories of significant chemical alteration. However, fine-grained sediments entrained in the basal ice of Taylor Glacier record only minor chemical alteration and U-series fractionation, indicating comparatively recent sediment comminution and active incision of upper Taylor Valley by Taylor Glacier over the Pleistocene. In addition, the results of this study emphasize the utility of U-series isotopes as tracers of chemical and physical weathering in sedimentary and pedogenic systems, with particular sensitivity to radionuclide implantation by α-recoil from high-U authigenic phases into lower-U detrital phases. This process has occurred extensively in >200 ka drifts but to a lesser degree in younger deposits. U-series α-recoil implantation is an important physicochemical process with chronometric implications in other hyperarid and saline sedimentary systems, including analogous Martian environments. 

Abstract. Antarctic meltwater is a significant source of iron that fertilizes present-day Southern Ocean ecosystems and may enhance marine carbon burial on geologic timescales. However, it remains uncertain how the nutrient flux from the subglacial system changes through time, particularly in response to climate, due to an absence of geologic records detailing element mobilization beneath ice sheets. In this study, we present a 25 kyr record of aqueous trace metal cycling in subglacial water beneath the David Glacier catchment measured in a subglacial chemical precipitate that formed across glacial termination III (TIII), from 259.5 to 225 ka. The deposition rate and texture of this sample describe a shift in subglacial meltwater flow following the termination. Alternating layers of opal and calcite deposited in the 10 kyr prior to TIII record centennial-scale subglacial flushing events, whereas reduced basal flushing resulted in slower deposition of a trace-metal-rich (Fe, Mn, Mo, Cu) calcite in the 15 kyr after TIII. This sharp increase in calcite metal concentrations following TIII indicates that restricted influx of oxygen from basal ice melt to precipitate-forming waters caused dissolution of redox-sensitive elements from the bedrock substrate. The link between metal concentrations and climate change in this single location across TIII suggests that ice motion may play an important role in subglacial metal mobilization and discharge, whereby heightened basal meltwater flow during terminations supplies oxygen to subglacial waters along the ice sheet periphery, reducing the solubility of redox-sensitive elements. As the climate cools, thinner ice and slower ice flow decrease subglacial meltwater production rates, limiting oxygen delivery and promoting more efficient mobilization of subglacial trace metals. Using a simple model to calculate the concentration of Fe in Antarctic basal water through time, we show that the rate of Antarctic iron discharge to the Southern Ocean is sensitive to this heightened mobility and may therefore increase significantly during cold climate periods. 

Abstract. Antarctic meltwater is a significant source of iron that fertilizes present-day Southern Ocean ecosystems and may enhance marine carbon burial on geologic timescales. However, it remains uncertain how this nutrient flux changes through time, particularly in response to climate, due to an absence of geologic records detailing trace metal mobilization beneath ice sheets. In this study, we present a 25 kyr record of aqueous trace metal cycling beneath the East Antarctic Ice Sheet measured in a subglacial chemical precipitate that formed across glacial termination III (TIII). The deposition rate and texture of this sample describe a shift in basal meltwater flow following the termination. Alternating layers of opal and calcite deposited in the 10 kyr prior to TIII record centennial-scale subglacial flushing events, whereas reduced basal flushing resulted in slower deposition of a trace metal-rich (Fe, Mn, Mo, Cu) calcite in the 15 kyr after TIII. This sharp increase in calcite metal concentrations following TIII indicates that diminished subglacial meltwater flow restricted the influx of oxygen from basal ice melt to precipitate-forming waters, causing dissolution of redox-sensitive trace metals from the bedrock substrate. These results are consistent with a possible feedback between orbital climate cycles and Antarctic subglacial iron discharge to the Southern Ocean, whereby heightened basal meltwater flow during terminations supplies oxygen to subglacial waters along the ice sheet periphery, which reduces the solubility of redox sensitive elements. As the climate cools, thinner ice and slower ice flow reduce basal meltwater production rates, limiting oxygen delivery and promoting more efficient mobilization of subglacial trace metals. Using a simple model to calculate the concentration of Fe in Antarctic basal water through time, we show that the rate of Antarctic iron discharge to the Southern Ocean is highly sensitive to this heightened mobility, and may therefore, increase significantly during cold climate periods. 

This dataset contains uranium and thorium isotopic compositions (U-234, U-235, U-238, Th-230, Th-232) and major element compositions (Al, K, Na, Ca, Fe, Mn, reported as oxides) for silicate sediments from glaciogenic drifts associated with advances of Taylor Glacier in Taylor Valley, Antarctica. Isotopic measurements were obtained by ID-TIMS in the Keck Isotope Facility at UC Santa Cruz and elemental measurements were obtained by ICP-OES in the Plasma Analytical Laboratory. All measurements include fully propagated analytical and systematic (e.g. isotopic tracer) uncertainties. Chemical index of alteration was calculated from major element data. Prior to measurements, sediments were sieved to ≤125 μm grain sizes, separated into quartz-feldspar-rich and clay-rich aliquots by hydraulic settling, and subjected to sequential chemical extractions ("leaching") prior to silicate digestion. 

Abstract Subglacial calcite precipitation is thought to occur in East Antarctica during periods when warm Southern Ocean waters access the ice sheet margin. Here we present an expanded precipitate archive that includes a continent‐wide compilation of 38 new and previously reported calcite²³⁴U‐²³⁰Th ages with isotopic compositional data. These data are interpreted to record periods when interior meltwaters are exported to the ice sheet margins as a result of ice acceleration and thinning. An assessment of coincidence between²³⁴U‐²³⁰Th dates, ranging from 16 to 256 ka, and peaks in Southern Ocean temperature yields a statistically significant correlation. Additional comparison of precipitate dates and climate data finds that calcite formation and ice acceleration cluster within periods of enhanced millennial scale climate variability as well as high global ice volume. This sensitivity to background climate is consistent with the hypothesis that these factors exert some control on ice sheet response to changes in climate. 

Subglacial mineral precipitates record ocean forcing of Heinrich events and widespread subglacial groundwater connectivity. 

Abstract Ice cores and offshore sedimentary records demonstrate enhanced ice loss along Antarctic coastal margins during millennial-scale warm intervals within the last glacial termination. However, the distal location and short temporal coverage of these records leads to uncertainty in both the spatial footprint of ice loss, and whether millennial-scale ice response occurs outside of glacial terminations. Here we present a >100kyr archive of periodic transitions in subglacial precipitate mineralogy that are synchronous with Late Pleistocene millennial-scale climate cycles. Geochemical and geochronologic data provide evidence for opal formation during cold periods via cryoconcentration of subglacial brine, and calcite formation during warm periods through the addition of subglacial meltwater originating from the ice sheet interior. These freeze-flush cycles represent cyclic changes in subglacial hydrologic-connectivity driven by ice sheet velocity fluctuations. Our findings imply that oscillating Southern Ocean temperatures drive a dynamic response in the Antarctic ice sheet on millennial timescales, regardless of the background climate state. 

Abstract Numerous studies have documented rare-earth element (REE) mobility in hydrothermal and metamorphic fluids, but the processes and timing of REE mobility are rarely well constrained. The Round Top laccolith in the Trans-Pecos magmatic province of west Texas, a REE ore prospect, has crosscutting fractures filled with fluorite and calcite along with a variety of unusual minerals. Most notably among these is an yttrium and heavy rare-earth element (YHREE) carbonate mineral, which is hypothesized to be lokkaite based on elemental analyses. While the Round Top laccolith is dated to 36.2 ± 0.6 Ma based on K/Ar in biotite, U-Pb fluorite and nacrite ages presented here clearly show the mineralization in these veins is younger than 6.2 ± 0.4 Ma (the age of the oldest fluorite). This discrepancy in dates suggests that fluids interacted with the laccolith to mobilize REE more than 30 m.y. after igneous emplacement. The timing of observed REE mobilization overlaps with Rio Grande rift extension, and we suggest that F-bearing fluids associated with extension may be responsible for initial mobilization. A later generation of fluids was able to dissolve fluorite, and we hypothesize this later history involved sulfuric acid. Synchrotron spectroscopy and laser ablation–inductively coupled plasma–mass spectrometry (LA-ICP-MS) U-Pb dating of minerals that record these fluids offer tremendous potential for a more fundamental understanding of processes that are important not only for REE but other ore deposits as well.