A view of the Mid-Pleistocene Transition from the deep North Pacific

Abstract:

During the Mid-Pleistocene Transition (MPT, ~900 kyrs ago), the climate system’s glacial/interglacial pacing, as reflected in benthic δ¹⁸O, changed from a dominant periodicity of 41,000 to one of 100,000 kyr, without a comparable change in orbital forcing. Proposed changes in the climate system that accompanied or could help explain this transition include an increase in the northern hemisphere and/or Antarctic ice volume, thermohaline circulation reorganization, and atmospheric pCO₂ drawdown. Here we reconstruct bottom water temperature and δ¹⁸O of seawater (δ¹⁸O_seawater) from Mg/Ca and d¹⁸O signatures of Uvigerina spp., and carbonate ion concentration ([CO₃^2-]) from B/Ca values of Cibicidoides wuellerstorfi to evaluate changes in ice volume and deep ocean carbon storage. The study is based on North Pacific ODP Site 1208 (36.13°N, 158.20°W, 3350 m water depth) and covers Marine Isotope Stages (MIS) 26 to 19 (~780-970 kyr). Our high-resolution (~2 kyrs) bottom water temperature record is largely in agreement with the high-resolution record from South Pacific ODP Site 1123 (Elderfield et al., 2012). At MIS 22, Site 1123 shows an abrupt 0.5 per mil δ¹⁸O_seawater increase that persists during subsequent glacial periods, interpreted by Elderfield et al. as a step-wise increase in glacial ice volume in Antarctica. By contrast, our preliminary Site 1208 δ¹⁸O_seawater record exhibits a transient increase in glacial δ¹⁸O_seawater at MIS 22, but suggests no significant, permanent glacial ice volume growth occurred. Preliminary low-resolution B/Ca results indicate a gradual and large decrease in [CO₃^2-] from MIS 25 to MIS 21 at Site 1208, which may have been caused by a marked increase in deep ocean carbon storage over the MPT. Based on our preliminary analyses, we propose that thermohaline reorganization during the MPT may have changed deep ocean carbon storage and contributed to pCO₂ drawdown.