Silicon stable isotope distribution traces Southern Ocean export of Si to the eastern South Pacific thermocline

de Souza, G. F.; Reynolds, B. C.; Johnson, G. C.; Bullister, J. L.; Bourdon, B.

The cycling and transport of dissolved silicon (Si) in the ocean may be traced by its stable isotope composition, δ 30Si. We present a dataset of δ 30Si values along 103° W in the eastern South Pacific Ocean, ranging from the Antarctic Zone of the Southern Ocean (62° S) to the equatorial Pacific (12° S). At high southern latitudes, the uptake and associated isotope fractionation of Si by diatoms results in highly elevated δ 30Si values (up to +3.2‰) in the summer mixed layer. High δ 30Si values (+2‰) are also preserved in the high-latitude fossil winter mixed layer, documenting the efficient export of diatom opal beyond the maximum depth of winter convection. This elevated winter mixed layer δ 30Si signature is introduced into the ocean interior by the subduction of Subantarctic Mode Water (SAMW) and Antarctic Intermediate Water (AAIW), whose northward spreading results in a strong isopycnal control on lower-thermocline and intermediate δ 30Si values in the well-ventilated eastern South Pacific. Values of δ 30Si are strongly conserved along SAMW and AAIW density levels as far north as 26° S, documenting the importance of the export of preformed Si from the surface Southern Ocean to lower latitudes. In contrast, in the equatorial Pacific, depressed δ 30Si values in the mesopelagic ocean are observed, most likely documenting the combined influence of a North Pacific Si source as well as the accumulation of remineralized Si within the eastern equatorial Pacific shadow zone. At depth, δ 30Si values in the South Pacific remain indistinguishable from deep Southern Ocean values of +1.25‰, even within Si-rich and oxygen-poor deep waters returning from the North Pacific. This homogeneity implies that the dissolution of opal plays a negligible role in altering the δ 30Si value of deep waters as they traverse the deep Pacific Ocean.

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de Souza, G. F. / Reynolds, B. C. / Johnson, G. C. / et al: Silicon stable isotope distribution traces Southern Ocean export of Si to the eastern South Pacific thermocline. 2012. Copernicus Publications.

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