Glacial-interglacial reorganisation of the deep ocean silicon distribution

We present three new records of diatom δ³⁰Si covering the last 30 kyrs in the Indian Sector of the Southern Ocean. The isotopic records are used to understand the source and distribution dissolved silicon (DSi) in the glacial deep waters of the Southern Ocean. The three δ³⁰Si records converge towards light values during the Younger Dryas interval, a period associated with invigorated Southern Ocean overturning. We suggest that these sudden light δ³⁰Si excursions are indicative of an overwhelming supply of DSi to the surface ocean, possibly combined with a lighter δ³⁰Si source signal.

We argue that the pulse of DSi indicated by the δ³⁰Si records was derived from the remnants of glacial deep waters that were stoichiometrically enriched in DSi and isotopically lighter than today. DSi enrichment of glacial bottom waters was likely a response to sluggish deep ocean mixing during glacial periods and compounded by the deep remineralisation profile of biogenic silica. The lighter isotopic composition of these bottom waters would have required partial DSi utilization in the surface ocean, which we ascribe to the reduction in DSi demand by iron-fertilized diatoms.

These data imply a large glacial-interglacial reorganization of the global silicon distribution, driven by changes in deep ocean mixing and iron fertilization. Due to the importance of diatoms in the biological pump, such a restructuring has implications for atmospheric CO₂ drawdown.