Silicon isotopes in Arctic and sub-Arctic glacial meltwaters: the role of subglacial weathering in the silicon cycle

Jade E Hatton,Sophie Opfergelt,Jakub D Žárský,Jacob C Yde,Tyler J Kohler,Jemma L Wadham,Marek Stibal,Katharine R Hendry,Jonathan R Hawkings

doi:10.1098/rspa.2019.0098

Abstract

Glacial environments play an important role in high-latitude marine nutrient cycling, potentially contributing significant fluxes of silicon (Si) to the polar oceans, either as dissolved silicon (DSi) or as dissolvable amorphous silica (ASi). Silicon is a key nutrient in promoting marine primary productivity, contributing to atmospheric CO2 removal. We present the current understanding of Si cycling in glacial systems, focusing on the Si isotope (δ30Si) composition of glacial meltwaters. We combine existing glacial δ30Si data with new measurements from 20 sub-Arctic glaciers, showing that glacial meltwaters consistently export isotopically light DSi compared with non-glacial rivers (+0.16‰ versus +1.38‰). Glacial δ30SiASi composition ranges from −0.05‰ to −0.86‰ but exhibits low seasonal variability. Silicon fluxes and δ30Si composition from glacial systems are not commonly included in global Si budgets and isotopic mass balance calculations at present. We discuss outstanding questions, including the formation mechanism of ASi and the export of glacial nutrients from fjords. Finally, we provide a contextual framework for the recent advances in our understanding of subglacial Si cycling and highlight critical research avenues for assessing potential future changes in these environments.

Highlights

Glacial environments play an important role in high-latitude marine nutrient cycling, potentially contributing significant fluxes of silicon (Si) to the polar oceans, either as dissolved silicon (DSi) or as dissolvable amorphous silica (ASi)
We present the current understanding of Si cycling in glacial systems, focusing on the Si isotope (δ30Si) composition of glacial meltwaters
At a time when glacial meltwater fluxes are expected to increase as a result of global climatic change [26,136,137], it is extremely important that we understand the chemical and physical processes occurring, so that we can make robust predictions of downstream biogeochemical response, including biological production in the future

Summary

Review of active processes within the Si cycle

Physical and chemical weathering in subglacial environments results in the export of key nutrients to the ocean that are required for marine primary production [1,2,3,4,5,6,7,8,9,10,11,12,13]. Wadham et al [56] outline a predictive framework of subglacial chemical weathering processes by using data from a range of glacial systems of varying size, to demonstrate the role of ice mass and water residence time on the predominant geochemical reactions, and the chemical composition of exported meltwaters. We expect physical erosion and the dissolution of these isotopically light mineral surfaces to still be important in large systems when the subglacial hydrology is well developed, but we may expect areas of subglacially stored waters with long residence times In these long residence time waters it has been predicted that precipitation of secondary weathering products, such as ASi and potentially clays (discussed further in §3a), may occur as a result of supersaturation with respect to DSi (at least locally).

Methods to characterize Si in glacial meltwaters

Wider impact of glacial Si

Findings

Summary and suggestions for future progress