Abstract
The silicon isotope composition of silicic acid, δ30Si(OH)4, in the deep Arctic Ocean is anomalously heavy compared to all other deep ocean basins. To further evaluate the mechanisms leading to this condition, δ30Si(OH)4 was examined on US GEOTRACES section GN01 from the Bering Strait to the North Pole. Isotope values in the polar mixed layer showed a strong influence of the transpolar drift. Drift waters contained relatively high [Si(OH)4] with heavy δ30Si(OH)4 consistent with the high silicate of riverine source waters and strong biological Si(OH)4 consumption on the Eurasian shelves. The maximum in silicic acid concentration, [Si(OH)4], within the double halocline of the Canada Basin formed a local minimum in δ30Si(OH)4 that extended across the Canada Basin, reflecting the high-[Si(OH)4] Pacific source waters and benthic inputs of Si(OH)4 in the Chukchi Sea. δ30Si(OH)4 became lighter with the increase in [Si(OH)4] in intermediate and deep waters; however, both Canada Basin deep water and Eurasian Basin deep water were heavier than deep waters from other ocean basins. A preliminary isotope budget incorporating all available Arctic δ30Si(OH)4 data confirms the importance of isotopically heavy inflows in creating the anomalous deep Arctic Si isotope signature, but also reveals a surprising similarity in the isotopic composition of the major inflows compared to outflows across the main gateways connecting the Arctic with the Pacific and the Atlantic. This similarity implies a major role of biological productivity and opal burial in removing light isotopes entering the Arctic Ocean from rivers.
Highlights
Silicon is an essential nutrient for the growth of diatoms and other siliceous plankton
Data on the δ30Si of silicic acid obtained during GN01 significantly expands the Si isotope data set for the Arctic Ocean, allowing further assessment of the processes controlling Si isotope distributions in this unique ocean
The light isotopic composition of Pacific waters within the upper halocline layer (UHL) extended across the Canada Basin
Summary
Silicon is an essential nutrient for the growth of diatoms and other siliceous plankton. The isotope models used to infer silica production from the Si isotope composition of diatom frustules in marine sediment or from the water column all depend on knowledge of the isotopic composition of the silicic acid, δ30Si(OH), in upwelling water masses (Varela et al, 2004). Numerical models that combine large-scale ocean circulation with biological fractionation of isotopes of Si in surface waters together with the dissolution of biogenic silica (bSi) in the water column are able to simulate global Si isotope distributions with high skill (Reynolds, 2009; de Souza et al, 2014, 2015; Holzer and Brzezinski, 2015; Gao et al, 2016) suggesting that this relatively simple conceptual framework captures the main relevant processes
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