North Atlantic Bottom Water Temperature and Ice Volume Records during the Mid-Pleistocene Transition

Abstract:

During the Mid-Pleistocene Transition (MPT), climate cyclicity changed from dominantly 41 thousand year cycles to 100 thousand year cycles in the absence of any obvious orbital forcing. Currently, only two high-resolution bottom water temperature records based on the Mg/Ca values of benthic foraminifera exist and these data record fundamentally different trends across the MPT. These Mg/Ca records have been coupled with oxygen stable isotope records to separate out the effects of temperature and ice volume signals. The DSDP Site 607 record from the North Atlantic suggests bottom water temperature cooled prior to and during the MPT and that global ice volume gradually increased across this interval (Sosdian and Rosenthal, 2009). In contrast, the Elderfield et al. (2012) IODP Site 1123 record from the South Pacific suggests near freezing bottom water temperatures occurred during glacial intervals over the entire length of the 1.5 Myr record and that an abrupt increase in ice volume occurred at ~0.9 Ma (MIS22).

Here we present Mg/Ca records from the North Atlantic DSDP Site 607 and ODP Site 664 based on Uvigerina spp. and use this data to construct a comprehensive view of bottom water temperature and deep ocean ventilation from the North Atlantic across the MPT. Our Uvigerina spp. record from site 607 is similar to the Sosdian and Rosenthal (2009) record and also suggests a long-term cooling trend in glacial and interglacial bottom water temperature prior to the MPT. Before 900 ka and after 600 ka, sites 607 and 664 vary from δ¹³C-enriched, warm northern-component-type water to δ¹³C-depleted, cold southern-component water which suggests strong glacial-interglacial water mass variability. However preliminary analyses indicate from 600-900 ka, δ¹³C-depleted, cold southern-component water flooded the N. Atlantic suggesting weaker North Atlantic Deep Water formation during the MPT. This suggests changes in the character and strength of bottom water circulation may have contributed and/or responded to the MPT.