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2019
Aarons, SM, Aciego SM, McConnell JR, Delmonte B, Baccolo G.  2019.  Dust transport to the Taylor Glacier, Antarctica, during the last interglacial. Geophysical Research Letters. 46:2261-2270.   10.1029/2018gl081887   AbstractWebsite

Abstract Changes in the composition of dust trapped in ice provide evidence of past atmospheric circulation and earth surface conditions. Investigations of dust provenance in Antarctic ice during glacial and interglacial periods indicate that South America is the primary dust source during both climate regimes. Here, we present results from a new ice core dust archive extracted from the Taylor Glacier in coastal East Antarctica during the deglacial transition from Marine Isotope Stage 6 to 5e. Radiogenic strontium and neodymium isotopes indicate that last interglacial dust is young and volcanic, in contrast to the observed preindustrial and Holocene (Marine Isotope Stage 1) dust composition. The dust composition differences from the last interglacial and current interglacial period at the site require a profound difference in atmospheric transport and environmental conditions. We consider several potential causes for enhanced transport of volcanic material to the site, including increased availability of volcanic material and large-scale atmospheric circulation changes.

2017
Arendt, CA, Aciego SM, Sims KWW, Aarons SM.  2017.  Seasonal progression of uranium series isotopes in subglacial meltwater: Implications for subglacial storage time. Chemical Geology. 467:42-52.   10.1016/j.chemgeo.2017.07.007   AbstractWebsite

The residence time of subglacial meltwater impacts aquifer recharge, nutrient production, and chemical signals that reflect underlying bedrock/substrate, but is inaccessible to direct observation. Here we report the seasonal evolution of subglacial meltwater chemistry from the 2011 melt season at the terminus of the Athabasca Glacier, Canada. We measured major and trace analytes and U-series isotopes for twenty-nine bulk meltwater samples collected over the duration of the melt season. This dataset, which is the longest time-series record of (U-234/U-238) isotopes in a glacial meltwater system, provides insight into the hydrologic evolution of the subglacial system during active melting. Meltwater samples, measured from the outflow, were analyzed for (U-238), (Rn-222) and U-(234/U-238)(activity), conductivity, alkalinity, pH and major cations. Subglacial meltwater varied in [U-238] and (Rn-222) from 23 to 832 ppt and 9 to 171 pCi/L, respectively. Activity ratios of (U-234/U-238) ranged from 1.003 to 1.040, with the highest (U-238), (Rn-222) and (U-234/U-238)(activity) values occurring in early May when delayed-flow basal meltwater composed a significant portion of the bulk melt. From the chemical evolution of the meltwater, we posit that the relative subglacial water residence times decrease over the course of the melt season. This decrease in qualitative residence time during active melt is consistent with prior field studies and model-predicted channel switching from a delayed, distributed network to a fast, channelized network flow. As such, our study provides support for linking U-series isotopes to storage lengths of meltwater beneath glacial systems as subglacial hydrologic networks evolve with increased melting and channel network efficiency.

Aarons, SM, Blakowski MA, Aciego SM, Stevenson EI, Sims KWW, Scott SR, Aarons C.  2017.  Geochemical characterization of critical dust source regions in the American West. Geochimica Et Cosmochimica Acta. 215:141-161.   10.1016/j.gca.2017.07.024   AbstractWebsite

The generation, transport, and deposition of mineral dust are detectable in paleoclimate records from land, ocean, and ice, providing valuable insight into earth surface conditions and cycles on a range of timescales. Dust deposited in marine and terrestrial ecosystems can provide critical nutrients to nutrient-limited ecosystems, and variations in dust provenance can indicate changes in dust production, sources and transport pathways as a function of climate variability and land use change. Thus, temporal changes in locations of dust source areas and transport pathways have implications for understanding interactions between mineral dust, global climate, and biogeochemical cycles. This work characterizes dust from areas in the American West known for dust events and/or affected by increasing human settlement and livestock grazing during the last 150 years. Dust generation and uplift from these dust source areas depends on climate and land use practices, and the relative contribution of dust has likely changed since the expansion of industrialization and agriculture into the western United States. We present elemental and isotopic analysis of 28 potential dust source area samples analyzed using Thermal Ionization Mass Spectrometry (TIMS) for Sr-87/Sr-86 and Nd-143/Nd-144 composition and Multi-Collector Inductively Coupled Plasma Mass Spectrometer (MC-ICPMS) for Hf-176/Hf-177 composition, and ICPMS for major and trace element concentrations. We find significant variability in the Sr, Nd, and Hf isotope compositions of potential source areas of dust throughout western North America, ranging from Sr-87/Sr-86 = 0.703699 to 0.740236, epsilon(Nd) = -26.6 to 2.4, and epsilon(Hf) = -21.7 to -0.1. We also report differences in the trace metal and phosphorus concentrations in the geologic provinces sampled. This research provides an important resource for the geochemical tracing of dust sources and sinks in western North America, and will aid in modeling the biogeochemical impacts of increased dust generation and deposition caused by higher drought frequency and human activity. (C) 2017 Elsevier Ltd. All rights reserved.

Aciego, SM, Riebe CS, Hart SC, Blakowski MA, Carey CJ, Aarons SM, Dove NC, Botthoff JK, Sims KWW, Aronson EL.  2017.  Dust outpaces bedrock in nutrient supply to montane forest ecosystems. Nature Communications. 8   ARTN 1480010.1038/ncomms14800   AbstractWebsite

Dust provides ecosystem-sustaining nutrients to landscapes underlain by intensively weathered soils. Here we show that dust may also be crucial in montane forest ecosystems, dominating nutrient budgets despite continuous replacement of depleted soils with fresh bedrock via erosion. Strontium and neodymium isotopes in modern dust show that Asian sources contribute 18-45% of dust deposition across our Sierra Nevada, California study sites. The remaining dust originates regionally from the nearby Central Valley. Measured dust fluxes are greater than or equal to modern erosional outputs from hillslopes to channels, and account for 10-20% of estimated millennial-average inputs of bedrock P. Our results demonstrate that exogenic dust can drive the evolution of nutrient budgets in montane ecosystems, with implications for predicting forest response to changes in climate and land use.

Chellman, N, McConnell JR, Arienzo M, Pederson GT, Aarons SM, Csank A.  2017.  Reassessment of the Upper Fremont Glacier Ice-Core Chronologies by Synchronizing of Ice-Core-Water Isotopes to a Nearby Tree-Ring Chronology. Environmental Science & Technology. 51:4230-4238.   10.1021/acs.est.6b06574   AbstractWebsite

The Upper Fremont Glacier (UFG), Wyoming, is one of the few continental glaciers in the contiguous United States known to preserve environmental and climate records spanning recent centuries. A pair of ice cores taken from UFG have been studied extensively to document changes in climate and industrial pollution (most notably, mid-19th century increases in mercury pollution). Fundamental to these studies is the chronology used to map ice-core depth to age. Here, we present a revised chronology for the UFG ice cores based on new measurements and using a novel dating approach of synchronizing continuous water isotope measurements to a nearby tree-ring chronology. While consistent with the few unambiguous age controls underpinning the previous UFG chronologies, the new interpretation suggests a very different time scale for the UFG cores with changes of up to 80 years. Mercury increases previously associated with the mid-19th century Gold Rush now coincide with early-20th century industrial emissions, aligning the UFG record with other North American mercury records from ice and lake sediment cores. Additionally, new UFG records of industrial pollutants parallel changes documented in ice cores from southern Greenland, further validating the new UFG chronologies while documenting the extent of late 19th and early 20th century pollution in remote North America.

Aarons, SM, Aciego SM, Arendt CA, Blakowski MA, Steigmeyer A, Gabrielli P, Sierra-Hernandez MR, Beaudon E, Delmonte B, Baccolo G, May NW, Pratt KA.  2017.  Dust composition changes from Taylor Glacier (East Antarctica) during the last glacial-interglacial transition: A multi-proxy approach. Quaternary Science Reviews. 162:60-71.   10.1016/j.quascirev.2017.03.011   AbstractWebsite

Mineral dust is transported in the atmosphere and deposited in oceans, ice sheets and the terrestrial biosphere. Temporal changes in locations of dust source areas and transport pathways have implications for global climate and biogeochemical cycles. The chemical and physical characterization of the dust record preserved in ice cores is useful for identifying of dust source regions, dust transport, dominant wind direction and storm trajectories. Here, we present a 50,000-year geochemical characterization of mineral dust entrapped in a horizontal ice core from the Taylor Glacier in East Antarctica. Strontium (Sr) and neodymium (Nd) isotopes, grain size distribution, trace and rare earth element (REE) concentrations, and inorganic ion (Cl- and Na+) concentrations were measured in 38 samples, corresponding to a time interval from similar to 46 kyr before present (BP) to present. The Sr and Nd isotope compositions of insoluble dust in the Taylor Glacier ice shows distinct changes between the Last Glacial Period (LGP in this study ranging from similar to 46.7-15.3 kyr BP) the early Holocene (in this study ranging from 14.5-8.7 kyr BP), and zero-age samples. The 87Sr/86Sr isotopic composition of dust in the Taylor Glacier ice ranged from 0.708 to 0.711 during the LGP, while the variability during the early Holocene is higher ranging from 0.707 to 0.714. The eNd composition ranges from 0.1 to 3.9 during the LGP, and is more variable from 1.9 to -8.2 during the early Holocene. The increased isotopic variability during the early Holocene suggests a shift in dust provenance coinciding with the major climate transition from the LGP to the Holocene. The isotopic composition and multiple physical and chemical constraints support previous work attributing Southern South America (SSA) as the main dust source to East Antarctica during the LGP, and a combination of both local Ross Sea Sector dust sources and SSA after the transition into the Holocene. This study provides the first high time resolution data showing variations in dust provenance to East Antarctic ice during a major climate regime shift, and we provide evidence of changes in the atmospheric transport pathways of dust following the last deglaciation. (C) 2017 Elsevier Ltd. All rights reserved.

2016
Aarons, SM, Aciego SM, Gabrielli P, Delmonte B, Koornneef JM, Uglietti C, Wegner A, Blakowski MA, Bouman C.  2016.  Ice core record of dust sources in the western United States over the last 300years. Chemical Geology. 442:160-173.   10.1016/j.chemgeo.2016.09.006   AbstractWebsite

Over the past ~5000years, amplified dust generation and deposition in the American West has been linked to human activity. In recent decades, intensified rates of agriculture and livestock grazing have been correlated with greater dust production detected on seasonal to annual timescales. The combination of land use intensification and climate change (i.e. increased drought frequency) in North America highlights the importance of characterizing the sources of dust both before and after the influence of anthropogenic activity. We apply high-precision geochemical and isotopic (Sr and Nd isotopes) techniques to an ice core from the Upper Fremont Glacier (Wyoming, USA) to produce the first glacial dataset from the American West. Our Sr-Nd isotopic composition data indicates the evolving dust provenance to the Upper Fremont Glacier (UFG) from a long-range transport of mineral dust to a local source. This increasing input of dust from a local source is supported by a rise in average dust particle diameter combined with greater average dust concentration throughout the record. The greater presence of dust particles smaller than 2.5μm in the most recent samples from UFG ice core record support existing satellite and sediment core data regarding the effects of anthropogenic activity upon dust sources and pathways in the American West. Although the Sr-Nd isotope database in North America needs be expanded, our results provide a survey of windborne dust through the past 270years.

Aarons, SM, Aciego SM, Gabrielli P, Delmonte B, Koornneef JM, Wegner A, Blakowski MA.  2016.  The impact of glacier retreat from the Ross Sea on local climate: Characterization of mineral dust in the Taylor Dome ice core, East Antarctica. Earth and Planetary Science Letters. 444:34-44.   10.1016/j.epsl.2016.03.035   AbstractWebsite

Recent declines in ice shelf and sea ice extent experienced in polar regions highlight the importance of evaluating variations in local weather patterns in response to climate change. Airborne mineral particles (dust) transported through the atmosphere and deposited on ice sheets and glaciers in Antarctica and Greenland can provide a robust set of tools for resolving the evolution of climatic systems through time. Here we present the first high time resolution radiogenic isotope (strontium and neodymium) data for Holocene dust in a coastal East Antarctic ice core, accompanied by rare earth element composition, dust concentration, and particle size distribution during the last deglaciation. We aim to use these combined ice core data to determine dust provenance, with variations indicative of shifts in either dust production, sources, and/or transport pathways. We analyzed a series of 17 samples from the Taylor Dome (77°47′47″S, 158°43′26″E) ice core, 113–391 m in depth from 1.1–31.4 ka. Radiogenic isotopic and rare earth element compositions of dust during the last glacial period are in good agreement with previously measured East Antarctic ice core dust records. In contrast, the Holocene dust dataset displays a broad range in isotopic and rare earth element compositions, suggesting a shift from long-range transported dust to a more variable, local input that may be linked to the retreat of the Ross Ice Shelf during the last deglaciation. Observed changes in the dust cycle inferred from a coastal East Antarctic ice core can thus be used to infer an evolving local climate.

2015
Aciego, SM, Aarons SM, Sims KWW.  2015.  The uranium-isotopic composition of Saharan dust collected over the central Atlantic Ocean. Aeolian Research. 17:61-66.   10.1016/j.aeolia.2015.01.003   AbstractWebsite

Uranium isotopic compositions, 234U/238Uactivity, are utilized by earth surface disciplines as chronometers and source tracers, including in soil science where aeolian dust is a significant source to the total nutrient pool. However, the 234U/238Uactivity composition of dust is under characterized due to material and analytical constraints. Here we present new uranium isotope data measured by high precision MC-ICP-MS on ten airborne dust samples collected on the M55 trans-Atlantic cruise in 2002. Two pairs of samples are presented with different size fractions, coarse (1–30μm) and fine (<1μm), and all samples were processed to separate the water soluble component in order to assess the controls on the 234U/238Uactivity of mineral aerosols transported from the Sahara across the Atlantic. Our results indicate 234U/238Uactivity above one for both the water soluble (1.13–1.17) and the residual solid (1.06–1.18) fractions of the dust; no significant correlation is found between isotopic composition and travel distance. Residual solids indicate a slight dependance of 234U/238Uactivity on particle size. Future modeling work that incorporates dust isotopic compositions into mixing or isotopic fractionation models will need to account for the wide variability in dust 234U/238Uactivity.

2013
Aarons, SM, Aciego SM, Gleason JD.  2013.  Variable HfSrNd radiogenic isotopic compositions in a Saharan dust storm over the Atlantic: Implications for dust flux to oceans, ice sheets and the terrestrial biosphere. Chemical Geology. 349-350:18-26.   10.1016/j.chemgeo.2013.04.010   AbstractWebsite

Isotopic characterization of aerosol mineral particles (atmospheric dust) of varying sizes is essential in classifying source areas, and for determining the source of dust deposited over oceans and icesheets. However, the effect of atmospheric transport on radiogenic isotopic compositions is not well constrained, making provenance interpretation difficult. In order to investigate the isotopic variability of 176Hf/177Hf, 87Sr/86Sr and 143Nd/144Nd we analyzed eight airborne dust samples in two size fractions collected along a cross-Atlantic transect through a dust storm originating in the Sahara in late 2002. Past measurements of 176Hf/177Hf, 87Sr/86Sr and 143Nd/144Nd of dust have focused primarily on coarse sized particles (<30μm), whereas far field deposition is primarily finer particles (<2μm). Strontium or neodymium isotopic sorting based on distance is not evident in our dataset; however, the combined isotopic ratios of the dust collected suggest a Saharan origin. Hafnium isotopic compositions show an east to west trend towards more radiogenic compositions across the Atlantic, suggesting grain and mineral sorting during dust transport along the ~4000km transect. Transport models with variable dust particle diameter and wind speed demonstrate the preferential depletion of the high-density mineral zircon during transport of dust from the source area. The transport model combined with a simple two component mixing model show that the Hf isotopic composition changes can be explained by the loss of the mineral zircon during transport. Modeling of this “zircon effect” with distance from the dust source in the Hf isotopic composition of marine, terrestrial and glacial dust deposits may reveal additional information concerning dust transport and sources in the geologic past.