A Global View of Deep Ocean Carbonate Ion Concentration Over Glacial-Interglacial Cycles and the Effects on Atmospheric pCO2

Abstract:

Deep ocean carbonate chemistry is inherently linked to atmospheric CO₂ concentration, a fundamental driver of glacial-interglacial climate change. Deep ocean carbonate ion concentration ([CO₃^2-]), carbon isotope ratio (δ¹³C), and sediment CaCO₃% impose crucial constraints on the carbonate system throughout glacial cycles, providing important indicators of why atmospheric pCO₂ shifted between strict upper and lower bounds. We present the first medium to high resolution deep ocean [CO₃^2-] records that span more than one full glacial cycle, extending to MIS 12, in a number of key locations such as the South Atlantic, Central Pacific, and Indian Oceans. In the South Atlantic Ocean, at 3702m, our results show unprecedented glacial-interglacial [CO₃^2-] increases of 40-50 μmol/kg that are preceded by a positive shift in δ¹³C. Coupled with deglacial [CO₃^2-] peaks in the Pacific and Indian Oceans, and a steady decrease in South Atlantic [CO₃^2-] and δ¹³C during glaciations, our results strongly support suggestions that atmospheric CO₂ was stored in the stratified glacial deep ocean and released during reinvigoration of ocean circulation.

The ocean circulation driven response of deep South Atlantic [CO₃^2-] is reproducible over successive glacial cycles, however this is not replicated in the Pacific Ocean. Deep Equatorial Pacific [CO₃^2-] gradually increased by 25 μmol/kg throughout glaciation from MIS 5 - MIS 2. However, during the MIS 7 - MIS 6 transition, [CO₃^2-] increased by 15 μmol/kg in less than 20Kyr and remained unchanged during a further ~80Kyr of atmospheric pCO₂ decline. This highlights the requirement for long term [CO₃^2-] records to fully constrain carbonate system influences on atmospheric pCO₂. Even a small rise in Deep Pacific Ocean [CO₃^2-] could increase glacial CO₂ storage further than changes in ocean circulation alone.