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

Abstract

The cycling and transport of dissolved silicon (Si) in the ocean may be traced by its stable isotope composition, δ³⁰Si. We present a dataset of δ³⁰Si 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 δ³⁰Si values (up to +3.2‰) in the summer mixed layer. High δ³⁰Si 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 δ³⁰Si 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 δ³⁰Si values in the well-ventilated eastern South Pacific. Values of δ³⁰Si 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 δ³⁰Si 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, δ³⁰Si 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 δ³⁰Si value of deep waters as they traverse the deep Pacific Ocean.