Combined benthic clumped isotope and osmium isotope data over the Mid-Pleistocene transition: towards better constraints on the weathering and seawater temperatures and d18O

The Mid-Pleistocene transition (MPT) is marked by a progressive increase of glacial-interglacial cycle amplitude, a shift of the climatic response from a 41-ka cycle dominated to a 100 ka-cycle, a prominent asymmetry in large glacial inceptions and an extension of glaciation. This transition is associated with a cooling of the sea surface temperatures and an increase of the atmospheric CO₂ concertation, that could be associated with a change in the ice-sheet volume. One of the hypotheses to explain the MPT transition is the regolith hypothesis, based on the basal erosion of glaciers, resulting in changes in weathering and in ice-sheet volume. Here, we apply clumped isotope thermometer (Δ₄₇) to benthic foraminifera. The Δ₄₇ has also the advantage to be independent of the isotopic composition of the seawater (δ¹⁸O_sw). As consequence, by combining Δ₄₇ and δ¹⁸O from benthic foraminifera, the δ¹⁸O_sw can be reconstructed. Our data are compared to osmium (Os) isotope measurements to observe potential change in weathering intensity. Thanks to this unique combination of Os isotope and Δ₄₇, we can test the regolith hypothesis. We therefor present a new deep temperature dataset, combined with osmium, over the MPT, from the “Shackleton” site (IODP U1385 from exp. 397) in the North Atlantic Ocean.

The deep-temperatures show an unexpected increase between MIS 30 and MIS 22, associated to an increase of δ¹⁸O_sw, while the osmium isotope decrease, indicated a decrease of weathering. These results point toward the regolith hypothesis with changes in ice sheet volume and weathering.