Carbon 13 Isotopes Reveal Limited Ocean Circulation Changes Between Interglacials of the Last 800 ka

摘要

Ice core data have shown that atmospheric CO_2 concentrations during interglacials were lower before the Mid-Brunhes Event (MBE, ~430 ka), than after the MBE by around 30 ppm. To explain such a difference, it has been hypothesized that increased bottom water formation around Antarctica or reduced Atlantic Meridional Overturning Circulation (AMOC) could have led to greater oceanic carbon storage before the MBE, resulting in less carbon in the atmosphere. However, only few data on possible changes in interglacial ocean circulation across the MBE have been compiled, hampering model-data comparison. Here we present a new global compilation of benthic foraminifera carbon isotopic (δ~(13)C) records from 31 marine sediment cores covering the last 800 ka, with the aim of evaluating possible changes of interglacial ocean circulation across the MBE. We show that a small systematic difference between pre- and post-MBE interglacial δ~(13)C is observed. In pre-MBE interglacials, northern source waters tend to have slightly higher δ~(13)C values and penetrate deeper, which could be linked to an increased northern sourced water formation or a decreased southern sourced water formation. Numerical model simulations tend to support the role of abyssal water formation around Antarctica: Decreased convection there associated with increased sinking of dense water along the continental slopes results in increased δ~(13)C values in the Atlantic in agreement with pre-MBE interglacial data. It also yields reduced atmospheric CO_2 as in pre-MBE records, despite a smaller simulated amplitude change compared to data, highlighting the need for other processes to explain the MBE transition.