US GEOTRACES GP17-OCE: Evaluating Southern Ocean Control of Global Marine Si Isotope Distribution

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National Science Foundation
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Ivia Closset
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We propose to measure silicon isotopes in silicic acid and in biogenic silica along US GEOTRACES GP17OCE from Tahiti, French Polynesia to Punta Arenas, Chile scheduled for November 2021 to January 2022.  Our main goal is to test the prediction from models that processes in the Southern Ocean dictate silicon isotope distributions throughout the global ocean. That objective will be met by measuring the isotopic signature of key end-member water masses and how they are altered by the biological pump and advection.

Intellectual Merit - The silicon isotope proxy is a powerful tool for assessing the role of diatoms and of silicic acid in ocean productivity and their implications for climate. It is now clear that successful application of the proxy requires a mechanistic understanding of how the silicon isotopic composition of ventilating waters masses varies in time and space. The Southern Ocean is key as it serves as the central hub of the meridional overturning circulation where upwelled abyssal waters are redistributed to low latitudes as mode and deep waters. The region’s meridional counter current circulation both traps inflowing silicic acid within the Southern Ocean and partitions Si isotopes between mode and deep waters. Heavy Si isotopes are distilled out of the Southern Ocean in relatively shallow mode waters while light isotopes ride the northward flow of deep waters. Measurements of Si isotopes in GP17OCE will provide the first ocean section that samples all relevant water masses involved in these processes synoptically. 

The opportunities afforded by GP17OCE include assessment of the isotopic signatures of Antarctic Bottom Water and upper and lower Circumpolar Deep Water. The cruise track also samples a key region involved in the formation of AAIW and SAMW near South America where significant silicic acid depletion should result in a heavy isotopic signature of mode-water end members. Once formed both SAMW and AAIW flow to the northwest intercepting the main meridional section allowing evaluation of their alteration during advection. Fractionation in surface waters should reflect the strong gradient in productivity from the hyper-oligotrophic south Pacific, to the HNLC waters of the Antarctic Circumpolar Current and the productive iceedge zone. Fe limitation in the ACC should lead to a strong decoupling of N and Si use by phytoplankton resulting in contrasting N and Si isotope distributions allowing an empirical test of the ability of Si and N isotopes to predict relative nutrient depletion patterns as assumed in paleo-applications of these proxies. 

Broader Impacts - Our proposed project is part of a larger international effort to understand Si isotope dynamics and distributions in the global ocean. International GEOTRACES sections completed by Canada, Belgium, Great Britain, France, Germany, Sweden and India include δ30Si(OH)4 measurements that compliment those proposed here. All data collected as part of this project will be submitted to the GEOTRACES database to aid this larger global synthesis. In addition, the PI supplies both secondary Si isotope standards as well as reference seawater samples to all PI’s measuring Si isotopes as part of International GEOTRACES.

A post doc will be trained and the research will also involve undergraduates who will be trained to assist in verification of [Si(OH)4] of each sample using colorimetric methods, data entry and data management. We will also create K-12 and undergraduate curricula related to this project, and to GEOTRACES in general, through UCSB’s Research Experience and Education Facility (REEF) that reaches 20,000 grade school and 4000 undergraduates students annually.