Incorporation of Al into diatom frustule as a proxy of seawater Al/Si ratios in the Antarctic Zone of the Southern ocean: Implications for surface silicic acid concentrations during the last glacial period

Abstract

Knowledge of silicic acid concentrations at the surface Southern Ocean helps better understand the role of the biological pump in regulating atmospheric CO2 concentrations in the past. While the silicon isotope compositions of diatom provide a measure of Si utilization efficiency, proxies on surface silicic acid concentration are not yet available. Here we determined the diatom frustule Al/Si ratio and dust flux to constrain the surface silicic acid concentrations in the Atlantic sector of the Southern Ocean. Our results based on multiple methods, including inductively coupled plasma mass spectrometry (ICP-MS), scanning electron microscope (SEM) and energy-dispersive spectroscopy (EDS), and nuclear magnetic resonance spectroscopy (NMR), suggest the isomorphic substitution of Al for Si by frustule biosynthesis. This enables us to transfer the diatom Al/Si ratio into seawater Al/Si ratio using the partition coefficient of Al between diatom silica and seawater. By further constraining dust flux using the sediment 230Th-normalized 232Th flux, we found that the markedly higher frustule Al/Si ratios cannot be solely explained by increased Al dissolution from dust and call for much lower surface silicic acid concentrations in the Antarctic Zone during the Last Glacial Period. The lower silicic acid concentration coupled with lower Si utilization efficiency inferred previously from silicon isotopes in the Antarctic Zone is likely due to the combined effects of reduced upwelling and iron-fertilization-regulated uptake stoichiometry, contributing to a lower atmospheric CO2 during the Last Glacial Period.