Investigation of subglacial weathering under the Greenland Ice Sheet using silicon isotopes
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 d...
| Published in: | Geochimica et Cosmochimica Acta |
|---|---|
| Main Authors: | , , , , , , , , |
| Format: | Article in Journal/Newspaper |
| Language: | unknown |
| Published: |
2019
|
| Subjects: | |
| Online Access: | https://zenodo.org/record/3350007 https://doi.org/10.1016/j.gca.2018.12.033 |
| Summary: | 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 (d30DSi and d30ASi 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 sea- sons from the two study sites, which differ in catchment size ($600 km2 for Leverett Glacier (LG) and $36 km2 for Kiattuut Sermiat (KS)). Subglacial waters from LG have elevated Na+ and K+ ions in relation to Ca2+ and Mg2+ 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 d30DSi 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 pro- nounced at LG. The d30ASi 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 com- pared to KS. Contrasting weathering regimes and subglacial hydrology between catchments need to be considered when estimating the d30Si composition of silica exported into polar oceans from the GrIS, with larger catchments likely to produce fluxes of lighter d30Si. As larger catchments dominate freshwater export to the ocean, GrIS meltwater is ... |
|---|