The iron isotopic composition of subglacial streams draining the Greenland ice sheet

Stevenson, Emily; Fantle, Matthew S.; Das, Sarah B.; Williams, Helen M.; Aciego, S.

doi:10.1016/j.gca.2017.06.002

Cited by 39 publications

(42 citation statements)

References 87 publications

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“…In contrast, δ 7 Li diss in glacial West Greenland rivers was strongly fractionated, most likely during Li uptake by Fe oxides derived from pyrite oxidation (Wimpenny et al, 2010). The isotopic fractionation of Mg (Wimpenny et al, 2011) and Fe (Stevenson et al, 2017) in Greenland appears to be primarily controlled by incongruent dissolution of primary minerals rather than the precipitation of secondary phases. Similar incongruency might affect riverine Ge/Si diss , δ 30 Si diss , and δ 74 Ge diss of the Watson River, potentially invalidating the modeling results here, given that the model assumes congruent dissolution of primary minerals.…”

Section: Again Excluding Sample Kl09 Neither F Simentioning

confidence: 90%

Ge and Si isotope signatures in rivers: A quantitative multi-proxy approach

Baronas

Torres

West

et al. 2018

Earth and Planetary Science Letters

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Solutes derived from the dissolution of silicate minerals play a key role in Earth's climate via the carbon and other biogeochemical cycles. Silicon (Si) is a unique constituent of silicate minerals and a biologically important nutrient, so tracing its behavior in near-surface environments may provide important insights into weathering processes. However, Si released by weathering is variably incorporated into secondary mineral phases and biota, obscuring signals derived from primary weathering processes. Due to chemical similarities, Germanium (Ge) may help better understand the Si cycle and its relationship to chemical weathering. With this aim, we report new measurements of the concentration and isotopic composition of Ge for both the dissolved and particulate phases of a variety of global rivers. These measurements are combined with analyses of concentration and isotopic ratio of Si on the exact same sample set in order to make direct comparisons of the behavior of these two elements in natural river systems. With this dataset, we develop a new modeling framework describing the full elemental and isotopic systems of these solutes in rivers (i.e., Ge/Si, δ74Ge, and δ30Si). This multi-proxy approach allows us to ascertain the relative importance of biological versus mineral uptake in modulating the fluxes of these elements delivered to the modern ocean. Dissolved δ74Ge composition of rivers studied thus far range from 0.9 to 5.5 ‰ with a discharge-weighted global average of 2.6±0.5 ‰. The Ge isotope composition of riverine suspended and bedload sediments is indistinguishable from silicate source rocks, which is consistent with mass balance expectations. The multi-proxy modeling suggests that, among the watersheds studied here, the isotopic fractionation of Si during secondary mineral phase precipitation (∆30Sisec) ranges from-2.7 to-0.2 ‰, which removes between 19-79% of the initial dissolved Si sequestration, while between 12-54% is incorporated by biota. For Ge, modeling indicates that 79-98% of the dissolved load is incorporated into secondary mineral phases with a ∆74Gesec ranging from-4.9 to-0.3 ‰. The fractionation induced by biological uptake is calculated to range from-2.6 to-1.3 ‰ for ∆30Sibio and-0.7±0.7 ‰ for ∆74Gebio. In addition to improving our understanding of the coupled Ge and Si cycles, our study provides a framework for Please note that this is an author-produced PDF of an article accepted for publication following peer review. The definitive publisher-authenticated version is available on the publisher Web site. using multiple isotopic tracers to elucidate the chemical behavior of solutes in natural waters Highlights ► Ge and Si isotope composition of water and sediments in various rivers is presented. ► Dissolved δ 74 Ge range 0.9-5.5‰, river sediment δ 74 Ge range 0.5-0.7‰. ► Dissolved δ 74 Ge is strongly fractionated due to low chemical Ge mobility. ► Ge/Si-isotope multi-proxy provides quantitative constraints on secondary vs biological fractionation.

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Section: Again Excluding Sample Kl09 Neither F Simentioning

confidence: 90%

Ge and Si isotope signatures in rivers: A quantitative multi-proxy approach

Baronas

Torres

West

et al. 2018

Earth and Planetary Science Letters

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“…during deglaciation events, Hawkings et al, 2018), present and future. The residence time of subglacial waters will have an influence on the weathering reactions occurring, and therefore the fluxes of key nutrients (Fe, P and Si) exported from glacial systems (Stevenson et al, 2017). More robust estimation of Si fluxes and the 30 Si composition of exported waters from the GrIS requires full consideration and further study of catchment size, hydrological development, weathering regime, and other processes within the complex subglacial system.…”

Section: Implications and Conclusionmentioning

confidence: 99%

“…Glaciation promotes physical and chemical weathering, with dynamic hydrological and microbial systems upon, within and beneath ice (Wadham et al, 2010;Tranter and Wadham, 2014). It is likely that weathering beneath large ice sheets is silicate mineral dominated (Stevenson et al, 2017), due to the enhanced residence times of subglacially stored waters, which result in exhaustion of carbonate minerals or saturation of meltwaters with respect to calcite (Wadham et al, 2010;Hindshaw et al, 2014;Michaud et al, 2016). This is evidenced by the ionic composition of meltwaters (high relative proportions of Na + and K + ) from the bed of the Greenland Ice Sheet (GrIS) (Graly et al, 2014) and the Antarctic Ice Sheet (AIS) (Michaud et al, 2016) compared to Alpine valley glaciers.…”

Section: Introductionmentioning

confidence: 99%

Investigation of subglacial weathering under the Greenland Ice Sheet using silicon isotopes

Hatton

Hendry

Hawkings

et al. 2019

Geochimica et Cosmochimica Acta

100

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Subglacial chemical weathering plays a key role in global silicate weathering budgets, contributing to the cycling of silicon (Si) in terrestrial and marine systems and the potential drawdown of carbon dioxide from the atmosphere. Here, we use data from two Greenland Ice Sheet (GrIS) catchments to demonstrate how Si isotopes from dissolved and amorphous particulate fractions (δ 30 DSi and δ 30 ASi respectively) can be used together with major ion data to assess the degree of secondary silicate weathering product formation and redissolution in subglacial environments. We compare a time-series of summer melt seasons from the two study sites, which differ in catchment size (~600 km 2 for Leverett Glacier (LG) and ~36 km 2 for Kiattuut Sermiat (KS)). Subglacial waters from LG have elevated Na + and K + ions in relation to *Manuscript Ca 2+ and Mg 2+ ions, indicating a predominance of silicate weathering, whilst meltwaters from KS are characterised by carbonate weathering (hydrolysis and carbonation) throughout the melt season. Both catchments have mean δ 30 DSi values substantially lower than average riverine values (KS 0.41‰, LG-0.25‰, versus a global riverine mean of 1.25‰) and display a seasonal decline, which is more pronounced at LG. The δ 30 ASi values (discharge weighted mean values KS-0.44‰, LG-0.22‰) are lighter than the bedrock (mean values KS-0.18±0.12‰, LG 0.00±0.07‰) in both catchments, indicating a secondary weathering product origin or leaching of lighter isotopes during initial weathering of crushed rock. When used in combination, the major ion and silicon isotope data reveal that the extent of silicate weathering and secondary phase redissolution are more pronounced at LG compared to KS. Contrasting weathering regimes and subglacial hydrology between catchments need to be considered when estimating the δ 30 Si composition of silica exported into polar oceans from the GrIS, with larger catchments likely to produce fluxes of lighter δ 30 Si. As larger catchments dominate freshwater export to the ocean, GrIS meltwater is likely to be very light in isotopic composition, and the flux is likely to increase with ice melt as the climate warms.

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“…is constrained by this fractionation factor (Δ free-org ) and the ratio of organically complexed Cu to free Cu (N org /N free ), as described by equation 6 (modified after Stevenson et al, 2017):…”

Section: Isotope Fractionation In the Aqueous Phasementioning

confidence: 99%

Cu and Zn isotope fractionation during extreme chemical weathering

Little

Munson

Prytulak

et al. 2019

Geochimica et Cosmochimica Acta

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The iron isotopic composition of subglacial streams draining the Greenland ice sheet

Abstract: In this study, we present the first measurements of iron (Fe) stable isotopic composition (d 56

Cited by 39 publications

References 87 publications

Ge and Si isotope signatures in rivers: A quantitative multi-proxy approach

Ge and Si isotope signatures in rivers: A quantitative multi-proxy approach

Investigation of subglacial weathering under the Greenland Ice Sheet using silicon isotopes

Cu and Zn isotope fractionation during extreme chemical weathering

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