Aciego Sarah M.

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Aciego
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Sarah M.
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  • Preprint
    The iron isotopic composition of subglacial streams draining the Greenland Ice Sheet
    ( 2017-06) Stevenson, Emily I. ; Fantle, Matthew S. ; Das, Sarah B. ; Williams, Helen M. ; Aciego, Sarah M.
    In this study, we present the first measurements of iron (Fe) stable isotopic composition (δ56Fe) of subglacial streams draining the Greenland Ice Sheet (GIS). We measure the δ56Fe values [(δ56Fe, ‰ = (56Fe/54Fe)sample/(56Fe/54Fe)standard-1) x 103] of both dissolved and suspended sediment Fe in subglacial outflows from five distinct land-terminating glaciers. Suspended sediments have δ56Fe values that lie within the crustal array (δ56Fe ~0‰). In contrast, the δ56Fe values of dissolved Fe in subglacial outflows are consistently less than 0‰, reaching a minimum of -2.1‰ in the outflow from the Russell Glacier. The δ56Fe values of dissolved Fe vary geographically and on daily time scales. Major element chemistry and mineral saturation state modeling suggest that incongruent silicate weathering and sulphide oxidation are the likely drivers of subglacial stream Fe chemistry, and that the extent of chemical weathering influences the δ56Fe of dissolved Fe. The largest difference in δ56Fe between dissolved and suspended load is -2.1‰, and occurs in the subglacial system from the Russell glacier (southwest GIS). Major element chemistry indicates this outflow to be the least chemically weathered, while more mature subglacial systems (i.e., that exhibit greater extents of subglacial weathering) have dissolved loads with δ56Fe that are indistinguishable from suspended sediments (Δ56Fesuspended-dissolved ~0‰). Ultimately, the dissolved Fe generated in some subglacial systems from the GIS is a previously unrecognized source of isotopically light Fe into the hydrosphere. The data illustrate that the dissolved Fe supplied by subglacial weathering can have variable δ56Fe values depending on the degree of chemical weathering. Thus, Fe isotopes have potential as a proxy for subglacial chemical weathering intensity or mode. Finally, based on our regional Fe concentration measurements from each glacial outflow, we estimate a flux weighted continental scale dissolved iron export of 2.1 Gg Fe yr-1 to the coastal ocean, which is within the range of previous estimates.
  • Article
    Noble gas signatures in Greenland : tracing glacial meltwater sources
    (John Wiley & Sons, 2015-11-06) Niu, Yi ; Castro, M. Clara ; Aciego, Sarah M. ; Hall, Chris M. ; Stevenson, Emily I. ; Arendt, Carli A. ; Das, Sarah B.
    This study represents the first comprehensive noble gas study in glacial meltwater from the Greenland Ice Sheet. It shows that most samples are in disequilibrium with surface collection conditions. A preliminary Ne and Xe analysis suggests that about half of the samples equilibrated at a temperature of ~0°C and altitudes between 1000 m and 2000 m, with a few samples pointing to lower equilibration altitudes and temperatures between 2°C and 5°C. Two samples suggest an origin as melted ice and complete lack of equilibration with surface conditions. A helium component analysis suggests that this glacial meltwater was isolated from the atmosphere prior to the 1950s, with most samples yielding residence times ≤ 420 years. Most samples represent a mixture between a dominant atmospheric component originating as precipitation and basal meltwater or groundwater, which has accumulated crustal 4He over time.