Silicon Isotopic Composition of Dry and Wet-Based Glaciers in Antarctica

Jade E Hatton,Catherine Hirst,W Berry Lyons,Elizabeth Bagshaw,Katharine R Hendry,Diane M Mcknight,Michael Staubwasser,Adrián Silva-Busso,Susan A Welch,Sophie Opfergelt,Jemma L Wadham,Susann Henkel

doi:10.3389/feart.2020.00286

Abstract

Glaciers and ice sheets export significant amounts of silicon (Si) to downstream ecosystems, impacting local and potentially global biogeochemical cycles. Recent studies have shown Si in Arctic glacial meltwaters to have an isotopically distinct signature when compared to non-glacial rivers. This is likely linked to subglacial weathering processes and mechanochemical reactions. However, there are currently no silicon isotope (δ30Si) data available from meltwater streams in Antarctica, limiting the current inferences on global glacial silicon isotopic composition and its drivers. To address this gap, we present dissolved silicon (DSi), δ30SiDSi and major ion data from meltwater streams draining a polythermal glacier in the region of the West Antarctic peninsula (King George Island) and a cold-based glacier in East Antarctica (Commonwealth Stream, McMurdo Dry Valleys). These data, alongside other global datasets, improve our understanding of how contrasting glacier thermal regime can impact upon Si cycling and therefore the δ30SiDSi composition. We find a similar δ30SiDSi composition between the two sites, with the streams on King George Island varying between -0.23 and +1.23‰ and the Commonwealth stream varying from -0.40 to +1.14‰. However, meltwater streams in King George Island have higher DSi concentrations, and the two glacial systems exhibit opposite DSi - δ30SiDSi trends. These contrasts likely result from differences in weathering processes, specifically the role of subglacial processes (King George Island) and, supraglacial processes followed by in-stream weathering in hyporheic zones (Commonwealth Stream). These findings are important when considering likely changes in nutrient fluxes from Antarctic glaciers under climatic warming scenarios and consequent shifts in glacial thermal regimes.

Highlights

There is growing evidence that links nutrient fluxes to the ocean with the export of glacial meltwater and icebergs, especially from the Greenland Ice Sheet (Hood et al, 2009; Gerringa et al, 2012; Hawkings et al, 2014, 2015, 2017; Lawson et al, 2014; Hendry et al, 2019)
The similarity in the range of δ30SiDSi composition for both Potter Cove and Commonwealth Streams is surprising given the differences in glacier thermal regime and retreat history
The negative relationship between dissolved silicon (DSi) and δ30SiDSi composition in Commonwealth Stream is consistent with that found in the majority of global rivers and reflects the removal of Si from solution into secondary weathering products or biogenic silica (Sutton et al, 2018)

Summary

INTRODUCTION

There is growing evidence that links nutrient fluxes to the ocean with the export of glacial meltwater and icebergs, especially from the Greenland Ice Sheet (Hood et al, 2009; Gerringa et al, 2012; Hawkings et al, 2014, 2015, 2017; Lawson et al, 2014; Hendry et al, 2019). It is hypothesized that the subglacial weathering processes, fueled by high physical erosion, drive the export of isotopically light glacial silicon due to large amounts of fine grained material enriched in 28Si, high mineral surface areas and elevated rock to water ratios This has implications for understanding of the global silicon cycle over glacial to interglacial timescales when considering oceanic diatom utilization of silicon (Hawkings et al, 2018). Analysis of streams fed by glaciers with contrasting thermal regimes and stream properties allows us to consider the impact differing conditions may have upon Si export and how we may use the δ30SiDSi composition of proglacial streams to infer different weathering processes within different glacial environments This will allow a greater understanding of global glacial silicon cycling. The impact the retreat may have on the silicon cycle has not been studied, but such knowledge could enable us to decipher how other glaciers may behave in the future under climatic warming scenarios where glacial retreat is increasing

MATERIALS AND METHODS

RESULTS

DISCUSSION

DATA AVAILABILITY STATEMENT

CONCLUSION