Silicon Isotopes Highlight the Role of Glaciated Fjords in Modifying Coastal Waters

• Published in: Journal of geophysical research. Biogeosciences Vol. 128; no. 7
• Main Authors: Hatton, J. E., Ng, H. C., Meire, L., Woodward, E. M. S., Leng, M. J., Coath, C. D., Stuart‐Lee, A., Wang, T., Annett, A. L., Hendry, K. R.
• Format: Journal Article
• Language: English
• Published: 01.07.2023
• Subjects: fjord nutrient cycling; Greenland Ice Sheet; silicon cycling; silicon isotope geochemistry
• DOI: 10.1029/2022JG007242
• ISSN: 2169-8953

Glaciers and ice sheets are experiencing rapid warming under current climatic change and there is increasing evidence that glacial meltwaters provide key dissolved and dissolvable amorphous nutrients to downstream ecosystems. However, large debate exists around the fate of these nutrients within complex and heterogenous fjord environments, where biogeochemical cycling is still often poorly understood. We combine silicon (Si) concentration data with isotopic compositions to better understand silicon cycling and export in two contrasting fjordic environments in south‐west Greenland. We show that both fjords have isotopically light dissolved silicon (DSi) within surface waters, despite an apparently rapid biological drawdown of DSi with increasing salinity. We hypothesize that such observations cannot be explained by simple water mass mixing processes, and postulate that an isotopically light source of Si, most likely glacially derived amorphous silica (ASi), is responsible for further modifying these coastal waters within the fjords and beyond. Fjord to coastal exchange is likely a relatively slow process (several months), and thus is less impacted by short‐term (<seasonal) changes of glacial meltwater input into the fjord, which has implications when considering the role of glacial meltwaters on nutrient export beyond the shelf break. We highlight the need for isotopic studies combined with dissolved and particulate nutrient concentration analysis to provide a more detailed analysis into the biogeochemical cycles within these highly dynamic fjord environments. Plain Language Summary The Greenland Ice Sheet is melting at a faster rate than the global average as a result of climatic warming. The resulting meltwaters are known to contain essential nutrients, such as silicon, that can stimulate and support the growth of aquatic life in areas downstream to glacier catchments, particularly coastal environments such as fjords. However, the degree to which these glacial nutrients are transported through fjord environments is still highly debated. Furthermore, fjord environments are highly complex with competing biological and physical processes varying across time and space. We investigate two Greenlandic sites to disentangle the processes that govern the fate of glacier‐sourced nutrient silicon in fjords. Our chemical analyses confirm the important role of reactive debris produced from glacier's physical grinding and chemical action on rocks under the ice sheet. We find evidence that the reactive debris continuously release nutrient silicon that is rapidly used by algae in fjords. Downstream transportation may provide a pathway for glacial nutrients to be transferred and used by marine life in the coastal and open ocean. Key Points Fjords have isotopically light dissolved silicon (DSi) in surface waters, despite an apparently rapid biological drawdown of DSi Silicon isotope geochemistry is a vital tool for disentangling complex fjord biogeochemical processes An isotopically light source of Si is responsible for modifying coastal waters within the fjords and beyond