A southern Portuguese margin perspective of Marine Isotope Stage 47 – an interglacial in the 41 kyr world

In order to understand interglacial climate variability we also need to study interglacial periods prior to the Mid-Pleistocene Transition, i.e. within the 41 kyr world. Early Pleistocene interglacial periods, in particular from the interval directly preceding the onset of the Mid-Pleistocene Transition, provide ideal study cases since interglacial atmospheric carbon dioxide levels during that period appear to have been similar to or only slightly higher than during the warmest interglacials of the last 800 ka. Here we present the first results from a high-resolution, multi-proxy study of interglacial Marine Isotope Stage (MIS) 47 (1424-1452 ka) at IODP Site U1387 (36°48´N 7°43´W), drilled into the Faro Drift on the southern Portuguese margin at 559 m water depth. Nowadays, surface waters near Site U1387 originate from the subtropical gyre, whereas the intermediate-depth Mediterranean Outflow Water (MOW) is encountered at the seafloor. For our study, we use the stable isotope data of planktonic foraminifera species G. bulloides and G. ruber white and benthic foraminifera species P. ariminensis and C. pachyderma, biomarker-derived sea-surface temperatures (SST), the weight percentage of the sand fraction, and microfossil evidence.

Following a rapid transition, interglacial conditions were quickly established in the surface waters with SST at levels near or above 24°C, sometimes even exceeding 25°C, throughout much of MIS 47. Those are the warmest SST so far observed for the Pleistocene at that location, being more than three degrees warmer than modern SST. The common occurrence of tropical species in the planktonic foraminifera fauna hints to a persistent contribution of tropical waters to the surface waters and thus probably the northward expansion and/or intensification of the North Atlantic's subtropical gyre. The MOW, on the other hand, experienced an extended period of poor ventilation, most likely associated with low oxygen levels, as indicated by the extremely low benthic carbon isotope values and the occurrence of gypsum crystals in the sediments that formed when the pyrite in the sediments was oxidized after the cores were opened. Following evidence from younger interglacials, this MOW signal should be linked to reduced ventilation and overturning in the Mediterranean Sea as consequence of increased freshwater input caused by an intensified North African monsoon. The benthic δ¹⁸O record of MIS 47 indicates a three phased interglacial period with a minimum separating two maxima. On a subtle level, this phasing might also exist in the surface water records. This and potential causes need to be explored further in the future, when all high-resolution data is available. Overall, the Site U1387 records confirm MIS 47 as a "super"-interglacial, much more so than MIS 31, on the southern Portuguese margin. Insights from this warm interglacial and associated oceanographic conditions and changes in the planktonic and benthic microfossil floras and faunas might provide hints on how future warming in those waters could impact the regional ecosystems.