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

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Abstract

Abstract. The cycling and transport of dissolved silicon (Si) in the ocean may be traced by its stable isotope composition, &delta;30Si. We present a dataset of &delta;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 &delta;30Si values (up to +3.2&permil;) in the summer mixed layer. High &delta;30Si values (+2&permil;) 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 &delta;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 &delta;30Si values in the well-ventilated eastern South Pacific. Values of &delta;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 &delta;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, &delta;30Si values in the South Pacific remain indistinguishable from deep Southern Ocean values of +1.25&permil;, 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 &delta;30Si value of deep waters as they traverse the deep Pacific Ocean.