Geochemistry of mineral dust in the McMurdo Dry Valleys Region, Antarctica

The transport and deposition of windblown materials are major processes in the ice-free areas of polar regions. The deposition of aeolian material provides connectivity within the ecosystems of these regions and is integral in understanding geochemical balances and exchanges between landscape units. We have analyzed materials deposited on glacier and permanent lake-ice surfaces as well as geomorphological features formed by aeolian processes in the largest ice-free area in Antarctica, the McMurdo Dry Valleys (~78 °S) in order to determine the source of this sediment. This presentation will focus on the materials collected from the glacier and lake surfaces. The bulk of sediment movement occurs during foehn events in the austral winter that redistribute material throughout the region. The majority of these samples were sand size (\u3e80 %) by weight. Samples containing the highest silt size were from the glaciers in the eastern portion of the Taylor Valley which is the most downwind position. Major rock-forming elements were analyzed using Standard XRF techniques. The alkali metals were depleted with respect to the Upper Continental Crust (UCC), in both the sand and silt fractions, while the alkaline earths were enriched. The TiO2, Fe2O3 and Al2O3 in the sands are similar to UCC values. The major element geochemistry of the aeolian material suggests that it is a mix of the four major rock types in the Valley itself: PreCambrian basement complex, Beacon Sandstone, Ferrar Dolerite and McMurdo Volcanics. Sr isotopic measurements of the fine grained materials from the glacier surfaces indicate the material is similar to the soils from their respective glacier/lake basins. Nd isotope values of this material lie intermediate to the rock values, indicating multiple sources of the aeolian material. The Sr and Nd isotopic data do not plot within the fields of dust from either Vostok or Dome C ice cores which has been interpreted as coming primarily from South America. All of our data suggest a local source of the majority of aeolian material deposited with Taylor Valle

INSIGHTS INTO GROUNDWATER FLOW PATHS IN AN INTENSIVELY MANAGED CRITICAL ZONE IN NEBRASKA

Glacier Creek, a groundwater-fed stream located in Glacier Creek Preserve (GCP) near Omaha, Nebraska, flows through restored tallgrass prairie and agricultural land (corn-soy rotation) draining a 4 km2 area. The 1 km wide watershed developed on Peoria Loess that overlies Sangamon age glacial till; Glacier Creek itself flows through the glacial till. Previous work concerning land use impacts on solute fluxes indicated a distinct distribution and flux of solutes through restored prairie and agricultural land. Inputs into the subsurface on agricultural land are slow and more concentrated but are diluted by precipitation along shallow flow paths to the north fork of Glacier Creek. In contrast, subsurface flow paths through the restored prairie are more rapid and deeper, leading to less concentrated water in the south fork of Glacier Creek. However, little is known about the subsurface stratigraphy and hydrogeology of the groundwater that provides year-round flow into Glacier Creek. Here we present the initial interpretation of a series of sediment cores and aquifer tests from the ridgetop, midslope, and foot slope topographic positions of agriculture and restored prairie. Sediment cores from the southern, restored prairie portion of GCP show loess overlying glacial till (identified by the appearance of gravel-sized rock fragments). The stratigraphy of the northern, agricultural portion of GCP is much more complex: while loess does overlie glacial till, there are also a series of sandy outwash deposits that cannot be correlated across the landscape. Under both land uses, the local groundwater table lies within the glacial till as referenced by water depth measurements in monitoring wells and gleyed sediments present in cores. Slug tests conducted in ridgetop and foot slope wells indicate that the saturated hydraulic conductivity of the sediments underlying the agricultural land range from two-fold to an order of magnitude greater than those underlying restored prairie, consistent with the presence of sandy layers that conduct water at a quicker rate. Furthermore, the higher flow rates explain why the north fork of Glacier Creek (draining agriculture) produces more water despite being a smaller portion of the watershed. Given these new findings, we modify our conceptual model of subsurface flow at GCP

GREENLAND REVISITED: LAKE EFFECTS ON COASTAL NUTRIENT FLUXES

Retreat of continental ice sheets exposed ~15% of Earth’s land surface from the Last Glacial Maximum (LGM) to about 6 ka and recent warming has increased glacial melting and meltwater solute fluxes to the oceans. Additional solutes originate from non-glacial streams in landscapes exposed since the LGM. As presented in last year’s pandemic-modified Birdsall-Dreiss lecture, Greenlandic glacial and non-glacial streams have distinct solute concentrations because of differing chemical weathering regimes of comminuted glacial sediment. In this year’s lecture, we evaluate an ~46 km2 non-glacial watershed near Sisimiut, Greenland to assess how lakes may impact non-glacial solute fluxes. Snow accumulates in the watershed from October to freshet in early May, after which discharge responds solely to precipitation events. Three main tributaries provide 92% of flow to the outlet stream and drain sub-watersheds with median slope angles of 16 to 18% and small upland lakes that cover 0.5 and 3.8% of the land area. In contrast, the outlet stream discharges from a landscape with a median slope of ~6% that includes one large and one small lake covering 23% of the area. Streams above and below the outlet lakes show similar variations in solute concentrations through the melt season. However, soon after freshet the outlet stream has major element concentrations ~20% greater than in the tributaries. The excess solute concentrations decrease linearly for ~90 days at which time the tributaries and outlet have similar concentrations. The excess solutes at the outlet may result from over-winter mineral dissolution in lake sediments, cryogenic solute enrichment during lake freeze-in, or dilute runoff in tributaries from snowmelt during and soon after freshet. In contrast, the outlet stream has a 0.6 to 3 times deficit of PO4, NO3, and Si compared with the tributaries, suggesting assimilation within the lake. NH4 concentrations switch from ~60% deficit to ~ 60% excess, reflecting a switch from a lake sink to source. The differences between tributary and outlet stream compositions suggest lake development may have altered coastal nutrient fluxes from non-glacial streams post-LGM. These variations will modify differences in glacial and non-glacial nutrient fluxes to coastal ecosystems, both since the LGM and as melting increases in a future warmer world

THE ROLE OF AEOLIAN DUST IN NUTRIENT AND SOLUTE TRANSPORT IN THE MCMURDO DRY VALLEYS, ANTARCTICA

The McMurdo Dry Valleys (MDV), the largest ice-free expanse in Antarctica, are considered a polar desert with an average annual temperature of -20oC and annual precipitation of \u3c10cm. Despite the extremely arid climate, a hydrologic continuum exists during the austral summer when ephemeral streams formed from glacial meltwater flow into endorheic lakes. Dust is deposited by strong seasonal winds onto the glacier and lake surfaces, as well as in widespread aeolian landforms throughout the MDV. Katabatic winds from the west, probably responsible for the majority of lithogenic dust deposition, dominate during the winter months. Easterly winds from the coast, prominent during the summer, contribute to the dust budget through the addition of salts and marine aerosols. When considered in the context of the unique hydrologic continuum and the climate-sensitivity of the environment, the dissolution of deposited dust may have an impact on salt and nutrient transfer and thus the ecosystem of the MDV. We have simulated this dissolution by conducting a two-step H2O leaching experiment on aeolian sediments collected from select glaciers, lakes, aeolian landforms, and elevated sediment traps. Resulting leachates representing the interaction of 50mL H2O with 25 g of dust sample were analyzed for major ions. NO3- concentrations (leach 1: \u3c1.0-240 µM; leach 2: \u3c1.0-94 µM) generally increase to the west and imply that aeolian deposition is potentially important to the nitrogen cycle in the MDV. Total dissolved solid concentrations (leach 1: 9-544 mg/L; leach 2: 6-150 mg/L), however, do not show any geographic/spatial correlation which is not consistent with previous work and suggests the significance of dust dissolution to the environment. Aliquots of the total dust were also analyzed for total C and N values. All but two samples, Lake Fryxell (0.12% N) and the eastern side of the Commonwealth Glacier (0.09% N), were below detection limit with respect to N (\u3c0.08% N). Both samples are from the Fryxell basin, the youngest of the basins, that is nitrogen limited. C values ranged from below detection (\u3c0.04% C) to 1.27% C. These results attest to the importance of dust as an addition to the ecosystem of the MDV. Further investigation of the dust is planned to constrain its chemical and mineralogical composition

The characterization and role of aeolian deposition on water quality, McMurdo Dry Valleys, Antarctica

The connection of ecosystems by wind-driven transport of material has become a topic of increasing interest and importance. Less than 1% of dust transported worldwide is exported to the Southern Ocean and Antarctic cryosphere; however, aeolian transport on the Antarctic continent is predominantly locally derived from the abrasion of bedrock. The deposition of the aeolian material is integral to nutrient and solute dispersal in the Antarctic ecosystem. This is particularly true in the ice-free areas of Antarctica, such as the McMurdo Dry Valleys (MDV), where aeolian material deposited in the aquatic system is solubilized during the melt season. The material is predominantly locally-derived from the abrasion of bedrock. In this study, a two-step leaching experiment simulates the melt season and we quantify the flux of solutes and nutrients to the aquatic ecosystem. Soluble salts were removed from the aeolian material first during cold water leaching followed by an increase in carbonate and silicate dissolution during freeze–thaw. Major ion fluxes on glaciers and lakes are at least two orders of magnitude greater than nutrient fluxes. However, the fluxes derived from these experiments are less than the estimated flux from streams to lakes and probably represent minima. Aeolian redistribution of local soils is important because they are the only source of new solutes and nutrients to the aquatic ecosystem of the MDV

Hydrologic exchange and chemical weathering in a proglacial watershed near Kangerlussuaq, west Greenland

The exchange of proglacial river water with active layer pore water could alter water chemical compositions in glacial outwash plains and oceanic solute fluxes. To evaluate effects of this exchange, we sampled Watson River and adjacent pore water during the 2013 melt season at two sandurs in western Greenland; one in Sandflugtdalen and the other near the confluence with Søndre Strømfjord. We measured temperature, specific conductivity, and head gradients between the river and bank over a week-long period at Sandflugtdalen, as well as sediment hydraulic conductivity and chemical compositions of waters from both sites. Specific conductivity of pore water is four to ten times greater than river water as solutes are concentrated from weathering reactions, cryoconcentration, and evaporation. Pore water compositions are predominantly altered by carbonate dissolution and sulfide mineral oxidation. High concentrations of HCO3 and SO4 result from solute recycling and dissolution of secondary Ca-Mg carbonate/sulfate salts initially formed by near-surface evaporation in the summer and at depth by freeze-in of the active layer and cryoconcentration in the winter. High hydraulic conductivity (10−5 to 10−4 m/s) and diurnal fluctuations of river stage during our study caused exchange of river and pore water immediately adjacent to the river channel, with a net loss of river water to the bank. Pore water \u3e6 m from the river continuously flowed away from the river. Approximately 1–8% of the river discharge through the Sandflugtdalen was lost to the river bank during our 6.75 day study based on calculations using Darcy’s Law. Although not sampled, some of this water should discharge to the river during low river stage early and late in the melt season. Elevated pore water solute concentrations in sandurs and water exchange at diurnal and seasonal frequency should impact fluxes of solutes to the ocean, although understanding the magnitude of this effect will require long-term evaluation throughout the melt season

OCEANIC FLUXES FROM PROGLACIAL AND DEGLACIAL WATERSHEDS IN WESTERN GREENLAND

Weathering in western Greenland occurs in two distinct environments: proglacial watersheds that extend from the margin of the Greenland Ice Sheet (GIS) and derive water from ice melt, and deglacial watersheds that develop on terrains unconnected to the GIS and derive water from annual precipitation. Proglacial and deglacial watersheds currently provide equal amounts of runoff in western Greenland. These watersheds may contribute different solute fluxes to the oceans depending on exposure age, climate, and weathering environment. We test this hypothesis by comparing chemical compositions of streams in four deglacial watersheds (Sisimiut, Nerumaq, Qorlortoq, Kangerlussuaq) and one proglacial watershed (Watson River Akuliarusiarsuup Kuua River; AKR) along a ~160 km transect from the coast to the GIS. Recent work found that weathering reactions in the deglacial watersheds shift from being dominated by carbonate dissolution inland to sulfide oxidation near the coast. Silicate weathering, based on increased Si, Na and K concentrations, is a minor source of solutes to deglacial streams and is less extensive near the GIS than the coast, where older moraines experience greater precipitation. In general, specific conductivity (SpC: 48-301 μS/cm) and pH (7.0-8.2) increase inland as precipitation decreases and fresh mineral surfaces become more common. The AKR, in contrast, has lower average SpC (11.9 uS/cm) and pH (6.86) than the deglacial streams. Low SpC reflects dilution by ice melt and short residence time of water in the subglacial system. Proglacial flow is enriched in Si compared to deglacial flow particularly near headwaters, indicating higher silicate weathering rates in the pro- and sub-glacial systems. Low pH values indicate: 1) equilibration with atmospheric CO2 in the supraglacial system near headwaters, and 2) acid production generated by sulfide oxidation in the hyporheic zone identified by elevated SO4 concentrations. However, Ca, Mg and HCO3 are the dominant ions over the length of the AKR indicating that dissolution of carbonate is the predominant form of weathering. Our results indicate the two types of watersheds provide distinct fluxes of solutes to the oceans that are likely to change as ice sheets retreat and advance with changing climate

MAPPING SEA CLIFFS ON DOMINICA USING PHOTO MOSAICS

Mapping on islands covered by rain forest presents challenges due to the extremely limited exposure of bedrock. In general, exposures are limited to road cuts, quarries, and sea cliffs. While the first two are easily accessible, the last one provides the most reliable series of exposures for most islands, and generally forms the largest exposures. However, these outcrops are frequently difficult to impossible to reach from land, due to a lack of roads and/or strong surf right to the bases of the cliffs. Therefore, in July 2007, we chartered a boat to circumnavigate the island of Dominica in the Lesser Antilles to map and photograph the sea cliffs all around the island. The results provide modifications to the published geological map of the island and hitherto unknown details on the geology of the Miocene, Pliocene, and Pleistocene-to-Recent volcanic centers. For example, an area previously mapped as part of the oldest sequence on the island (Miocene), has been identified as a megabreccia that is part of the Pleistocene sequence of the Grande Soufriere Hills volcanic center, and is now identified as much more extensive than was known from exposures accessible from land. Detailed stratigraphic sections of selected sequences will be presented to illustrate the effectiveness of this technique

SEASONAL EVOLUTION AND SPATIAL DISTRIBUTION OF WEATHERING IN WESTERN GREENLAND

Through physical weathering, the Greenland Ice Sheet (GIS) produces sediments which are subsequently chemically weathered in three types of watersheds: 1) deglacial watersheds that are physically disconnected from the GIS and drain local precipitation, 2) proglacial watersheds that are hydrologically connected to the GIS, and 3) subglacial watersheds that form beneath the GIS. Chemical weathering in the glacial foreland may be important to atmospheric CO2 drawdown and oceanic fluxes of solutes, yet no holistic study exists that compares solute sources across all types of watersheds and through the melt season. Consequently, we investigated spatiotemporal changes in weathering through the 2013 ablation season from a transect of watersheds spanning the coast to the GIS in western Greenland. We sampled one proglacial (PG) watershed, from which we also assess subglacial (SG) weathering, one inland deglacial (IDG) and one coastal deglacial (CDG) watershed. A simple stoichiometric mass balance quantifies solute sources in each watershed. The principal solute source is trace carbonates in all watersheds; however, IDG has more carbonate (61 vs 36 mol%) and less silicate (3 vs 14 mol%) weathering than CDG. PG has similar carbonate (41 mol%) and silicate weathering (16 mol%) proportions to CDG, despite proximity to IDG. Weathering of biotite decreases from 12 mol% at PG to 3 mol% at CDG along an exposure age gradient, consistent with more radiogenic 87Sr/86Sr in waters at PG (0.73556) than DGC (0.71114). Carbonate weathering decreases and biotite + silicate weathering increases downstream through PG, reflecting increased weathering. Solute sources change little through time or space at IDG, but at PG, silicate weathering increases and carbonate weathering decreases as flow increases through the melt season, consistent with increased contributions of SG waters with long residence times in distributed channels. Thus, the evolution of SG through time and connections between subglacial reservoirs and main flow paths plays an important role in weathering at PG. As the GIS retreats, deglacial watersheds will constitute a greater fraction of the weathering flux and thus increased silicate weathering should alter solute fluxes to the oceans and increase atmospheric CO2 drawdown

WEATHERING, RADIOGENIC ISOTOPES, AND MARINE RECORDS OF GLACIAL DYNAMICS

Glacial advance and retreat is related to numerous climate system feedbacks; yet, this dynamic glacial activity tends to erase its own terrestrial record. As a result, deep-sea sediments may be the best archives for studying past glacial processes. Interpretations of these archives depend on understanding terrestrial sources to the marine sediments. Systematic spatial variations in dissolved riverine and soil leachate Sr, Nd and Pb isotopes across an ~175 km transect from the Greenland Ice Sheet to the coast present an analog for temporal changes during glacial retreat. Specifically, the offset between dissolved (riverine or soil leachates) and bulk sediment (bedload or leached soil) isotopes is highest in young glacial sediments close to the ice sheet and approaches zero in 10 ky old glacial sediments at the coast. This difference is attributed to a transition from preferential chemical weathering of trace minerals and/or radiation damaged sites in freshly comminuted, ice-proximal sediments to predominant weathering of less radiogenic (Sr and Pb) and more radiogenic (Nd) isotopes from bulk major minerals in more extensively weathered coastal material. These isotopes are transported to the ocean where the residence time of Sr is too long to be an effective tracer of local or regional glacial processes; however, the short residence time of Pb makes it an excellent tracer of local chemical weathering processes and the intermediate residence time of Nd allows application to region studies. Data from IODP Sites 1302/3 (3550 m water depth) in the NW Atlantic illustrate that seawater Pb and Nd isotopes preserved in authigenic FeMn-oxide coatings respond dramatically to retreat of the Laurentide Ice Sheet during the penultimate glacial termination (T2; 135-129 ka) and to rapid variations during Dansgaard-Oeschger cycles. These data suggest deep-sea radiogenic isotopes preserve a more detailed record of the long term history of ice sheet dynamics than terrestrial proxies. The systematic variation in chemical weathering linked to ice sheet retreat and reflected in deep-sea isotope records may also help refine estimates of past and future carbon cycling and fluxes of nutrients and isotopes to the ocean associated with high latitude climate change