A switch from Si(OH)₄ to NO₃⁻ depletion in the glacial Southern Ocean

Abstract

Phytoplankton in the Antarctic deplete silicic acid (Si(OH) 4 ) to a far greater extent than they do nitrate (NO 3 − ). This pattern can be reversed by the addition of iron which dramatically lowers diatom Si(OH) 4 :NO 3 − uptake ratios. Higher iron supply during glacial times would thus drive the Antarctic towards NO 3 − depletion with excess Si(OH) 4 remaining in surface waters. New δ 30 Si and δ 15 N records from Antarctic sediments confirm diminished Si(OH) 4 use and enhanced NO 3 − depletion during the last three glaciations. The present low‐Si(OH) 4 water is transported northward to at least the subtropics. We postulate that the glacial high‐Si(OH) 4 water similarly may have been transported to the subtropics and beyond. This input of Si(OH) 4 may have caused diatoms to displace coccolithophores at low latitudes, weakening the carbonate pump and increasing the depth of organic matter remineralization. These effects may have lowered glacial atmospheric pCO 2 by as much as 60 ppm.