Southwest Pacific deep-water carbonate chemistry during the Mid-Pleistocene Transition.

After more than 40 years of research, there is still wide disagreement in defining when theMid-Pleistocene Transition (MPT) occurred, with climate reconstructions ranging from anabrupt versus gradual transition that began as early as 1500 ka and ended as late as 600 ka.Our recent work in the Southwest Pacific (Ocean Drilling Program Site 1123) has providedsome evidence for a rapid transition, suggesting that the MPT was initiated by an abruptincrease in global ice volume 900 thousand years ago [1]. This study uses shallow-infaunalbenthic foraminifera Uvigerina spp. to disentangle the contributions of deep-watertemperature (using Mg/Ca ratios) and ice volume to the oxygen isotopic composition offoraminiferal calcite over the last 1.5 Ma. The resulting sea-level reconstructionacross the MPT shows that the critical step in ice-volume variation was associatedwith the suppression of melting in Marine Isotope Stage (MIS) 23, followed byrenewed ice growth in MIS 22 to yield a very large ice sheet with 120 m of sea levellowering. Here, we built on this work with the aim to investigate further the abrupt eventcentered on MIS 24 to 22 (the '900-ka event’) and try to shed some light on theprocesses and mechanisms that caused the MPT. Different hypotheses account for theorigin of the MPT as a response to long-term ocean cooling, perhaps because oflowering CO2. To better quantify the role of the carbon system during the MPT, wereconstruct past changes in bottom water inorganic carbon chemistry from the traceelement (B/Ca) and stable isotopic composition of calcite shells of the infaunalbenthic foraminifera Uvigerina spp. from 1100 ka to 350 ka at ODP Site 1123.This site was retrieved from Chatham Rise, east of New Zealand in the SouthwestPacific Ocean (41º47.2’S, 171º 29.9’ W, 3290 m water depth) and lies under theDeep Western Boundary Current (DWBC) that flows into the Pacific Ocean, and isresponsible for most of the deep water in that ocean; DWBC strength is directlyrelated to processes occurring around Antarctica. The ratio of boron to calcium(B/Ca) in benthic foraminifer shells has proven to be a reliable indicator of thecalcite saturation state of ocean bottom waters. The comparison between benthicforaminifera δ18O and δ13C shows a similar trend at ODP Site 1123, implying a closerelationship between these climate and carbon cycle signals, and we use our B/Ca recordreconstructed from the same samples to explore the potential processes behind this tightcoupling. These results permit preliminary discussion on the deep-water carbonate saturation stateduring glacial/interglacial cycles. Deep-water temperatures estimates using Mg/Ca andoxygen isotopic composition of seawater (δ18Osw) are available from Site 1123 for the last1.5 million years [1] and the phase relationship between the different signals is tentativelyassessed for the early/middle Pleistocene, when different patterns of climate variability havebeen inferred from marine and ice cores records. [1] Elderfield et al. (2012). Evolution of ocean temperature and ice volumethrough the Mid Pleistocene Climate Transition. Science, vol. 337, 6095, 704-709 [ABSTRACT FROM AUTHOR]