Silicon stable isotope constraints on the role of the subpolar North Pacific in the marine Si cycle

Recent studies of the stable isotope composition of silicon dissolved in seawater (expressed as δ³⁰Si) have brought attention to the importance of high-latitude processes for the marine cycle of silicon (Si). The last decade has consolidated our understanding of the large-scale influence of the North Atlantic and Southern Ocean on the global Si and δ³⁰Si distributions [1-6]. However, the North Pacific Ocean has not been returned to since early studies of the region [7,8] that documented exceptionally low δ³⁰Si values (< +1‰) in the deep, Si-rich North Pacific interior. This isotopic feature, not predicted by ocean models of a range of complexity [5,6,9], might reflect the influence of subarctic Pacific biological cycling on the marine Si cycle, or external inputs of Si to the deep North Pacific via hydrothermal activity, but extant data have been insufficient to assess these hypotheses. We present water-column δ³⁰Si data spanning the subpolar North Pacific at ~47°N (GEOTRACES transect GP02), including waters from the Cascadia Basin bordering North America, which has been suggested as an external source of Si to the North Pacific [10]. Our data do not document the presence of δ³⁰Si values below +1‰ in the deep subpolar North Pacific, although some deep ocean δ³⁰Si variability is associated with the high Si concentrations in and near the Cascadia Basin. We will discuss the implications of these data for large-scale Si cycling in the light of the global ocean δ³⁰Si systematics.

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