Poster Australian Marine Sciences Association 2022

Silicon Isotopes in Spring East Australian Current Phytoplankton Bloom (#528)

Riteshma Devi 1 , Michael Ellwood 1
  1. Research School of Earth Sciences, The Australian National University, Acton, Canberra, ACT, Australia

Ocean cycling of silicon (Si) isotopes was investigated in an annual phytoplankton bloom in the austral spring of 2018 in the East Australian Current (EAC) which mixes with subantarctic waters in the region of the subtropical front. During this GEOTRACES process study, low concentration of silicic acid (Si(OH)4) (~2 µmol L-1) and higher nitrate (~5 µmol L-1) and particulate iron (Fe) concentrations (~0.54 nmol L-1) were determined in the surface waters in this region. In this study, silicon isotopic signatures of biogenic silica (δ30SibSi) were investigated in the upper 300 m. The isotopic composition of biogenic silica (diatoms) was measured by MC-ICP-MS. Results are expressed as δ30Si relative to the NBS28 standard. We present the first δ30SibSi data from the EAC. The δ30SibSi signatures in the surface waters in EAC ranged from +1.76‰ to +0.70‰.  The Open system (steady-state) model showed δ30SibSi with a fractionation factor of (ε) = -1.8 ± 0.5, which is similar to estimates made with previous Southern Ocean studies under Fe-depleted conditions and in the Peruvian Upwelling. These insights confirm that the isotopic composition of biogenic silica is a promising tool to improve our understanding of the Si biogeochemical cycle during a spring bloom.