Onset of ‘Pacific-style’ deep-sea sedimentary carbonate cycles at the mid-Pleistocene transition

The calcium carbonate (CaCO₃) content of deep-sea sediments in the Pacific Ocean increases during glacials of the late Pleistocene in comparison to interglacials, whereas records of sedimentary CaCO₃ in the Atlantic Ocean show an anticorrelated pattern across glacial-interglacial cycles. Here we show that this anticorrelation in inter-ocean CaCO₃ cycles arose comparatively recently, at ~1.10million years ago (Ma), during the mid-Pleistocene transition. Before this time, we show that the CaCO₃ content of Pacific and Atlantic Ocean sediments experienced in-phase cyclicity, both having ‘Atlantic-style’ phasing with respect to glacial–interglacial cycles. The onset of anticorrelated cyclicity at 1.10Ma involved a twofold switch in the Pacific's CaCO₃ cycles: glacial CaCO₃ preservation became consistently better while interglacial preservation became consistently worse. We demonstrate that the cause of this glacial–interglacial ‘mirror imaging’ of Pacific CaCO₃ dissolution cyclicity at 1.10Ma was a switching over of the relative ventilation state of abyssal South Pacific waters between glacials and interglacials. Specifically, we suggest that a consistent strengthening of deep water ventilation within the Pacific sector of the Southern Ocean during glacials drove glacial Pacific CaCO₃ dissolution to diminish, while a contemporaneous weakening of very well-ventilated ‘upstream’ North Atlantic Deep Water during interglacials drove interglacial Pacific CaCO₃ dissolution ‘downstream’ to intensify. We propose that the increased mean alkalinity of the global deep ocean (driven by the geographically and bathymetrically vast Pacific Ocean) during glacials after 1.10Ma may explain part of the drawdown in glacial atmospheric CO₂ levels hypothesised to have been linked to the increased severity of these late Pleistocene glacials.