ALBERT

Description: The equatorial Pacific thermocline is a critical component in determining the ocean-atmosphere interactions of the tropics. During the Pliocene warm period, the tropical thermocline was warm and/or deep and shoaled toward present day. Here we use Mg/Ca values of subsurface-dwelling Globorotalia tumida to reconstruct subsurface temperatures at Integrated Ocean Drilling Program, JOIDES Resolution, Expedition 320/321, Pacific Equatorial Age Transect, Site U1338.

Description: High-resolution seawater δ18O records, derived from coupled benthic foraminifera Mg/Ca and δ18O analyses, can be used to evaluate how global ice volume changed during the Mid-Pleistocene Transition (MPT, ~1250-600 ka ago). However, such seawater δ18O records are also influenced by regional hydrographic signals (i.e. salinity) and changes in deep ocean circulation across the MPT, making it difficult to isolate the timing and magnitude of the global ice volume change. Here we generated a new seawater δ18O from coupled Mg/Ca and δ18O analyses of benthic foraminifera Uvigerina spp. Ocean Drilling Program Site 1208 in the North Pacific. We use PSU Solver (Thirumalai et al., 2016) to propagate error on seawater δ18O for ODP Site 1208 and previously published benthic foraminifera Mg/Ca and δ18O records from North Atlantic DSDP Site 607 (Sosdian and Rosenthal, 2009; Ford et al., 2016; Cibicidoides wuellerstorfi, Oridorsalis umbonatus, and Uvigerina spp.) and South Pacific ODP 1123 (Elderfield et al., 2012; Uvigerina spp.). Using these three records we generated a seawater d18O stack over the Mid-Pleistocene Transition.

Description: During the Mid-Pleistocene Transition (MPT), the dominant glacial-interglacial cyclicity as inferred from the marine d18O records of benthic foraminifera (d18Obenthic) changed from 41 kyr to 100 kyr years in the absence of a comparable change in orbital forcing. Currently, only two Mg/Ca-derived, high-resolution bottom water temperature (BWT) records exist that can be used with d18Obenthic records to separate temperature and ice volume signals over the Pleistocene. However, these two BWT records suggest a different pattern of climate change occurred over the MPT-a record from North Atlantic DSDP Site 607 suggests BWT decreased with no long-term trend in ice volume over the MPT, while South Pacific ODP Site 1123 suggests that BWT has been relatively stable over the last 1.5 Myr but that there was an abrupt increase in ice volume at ~900 kyr. In this paper we attempt to reconcile these two views of climate change across the MPT. Specifically, we investigated the suggestion that the secular BWT trend obtained from Mg/Ca measurements on Cibicidoides wuellerstorfi and Oridorsalis umbonatus species from N. Atlantic Site 607 is biased by the possible influence of D[CO3]2- on Mg/Ca values in these species by generating a low-resolution BWT record using Uvigerina spp., a genus whose Mg/Ca values are not thought to be influenced by D[CO3]2-. We find a long-term BWT cooling of ~2-3°C occurred from 1500 to ~500 kyr in the N. Atlantic, consistent with the previously generated C. wuellerstorfi and O. umbonatus BWT record. We also find that changes in ocean circulation likely influenced d18Obenthic, BWT, and d18Oseawater records across the MPT. N. Atlantic BWT cooling starting at ~1.2 Ma, presumably driven by high-latitude cooling, may have been a necessary precursor to a threshold response in climate-ice sheet behavior at ~900 ka. At that point, a modest increase in ice volume and thermohaline reorganization may have caused enhanced sensitivity to the 100 kyr orbital cycle.