Subtropical gyre persistence in the Gulf of Cadiz, southern Iberian margin, interrupted by extremely cold surface water incursions during the Early – Middle Pleistocene Transition

Abstract. Besides the shift in dominant orbital cyclicity, the mid-Pleistocene Transition or Early-Middle Pleistocene Transition (EMPT) was characterized by a change in the deep thermohaline circulation. Those changes contributed to more intense and longer-lasting glacial periods and cooler sea surface temperatures (SSTs). Within the Atlantic Ocean, the Iberian margin is considered a key location to study climatic variations influenced by both high- and low-latitude processes. In this study we focus on IODP Site U1387 on the southern Portuguese margin to reconstruct surface water circulation and related plankton foraminifera ecosystem changes during the interval of Marine Isotope Stage (MIS) 28 to MIS 18 (1006–750 ka). Our planktonic foraminifera assemblages and SST reconstructions (foraminifera assemblages and U^K'₃₇ alkenone index) demonstrate warm, stable SST conditions during much of the interval due to persistent influence of subtropical gyre waters as indicated by the tropical-subtropical and Azores Current related foraminifera species and the periods with dominant sinistral coiling direction of the species Globorotalia truncatulinoides. Maximum interglacial SSTs were up to 2 °C warmer than at present in both summer and winter, with the exception of interglacial MIS 23 with SSTs ~1.5 °C colder than in the other interglacials. Subsequent the respective glacial inception, the relative warm conditions were periodically interrupted by millennial-scale extreme cold events when polar species Neogloboquadrina pachyderma became abundant (>30 %) and the SSTs, reconstructed from the foraminifera assemblage data, dropped below 10 °C in summer and 5 °C in winter. The most pronounced event, considering the amplitude of cooling and duration, occurred between 870 to 864 ka, marking the terminal stadial event of the MIS 22/MIS 21 transition (Termination X). Extreme cold events, always associated with the incursion of subpolar waters into the Gulf of Cadiz, mark all the terminal stadial events from Terminations XII to IX and the millennial-scale variability during the transitions to full glacial conditions, although the duration of the cooling varied greatly. The extreme cooling was only possible through migration of the subarctic front into the lower mid-latitudes as a consequence of an extreme reduction in the Atlantic meridional overturning circulation. The amplitude of cooling, duration, and frequency of subpolar water incursions during MIS 24 to MIS 22 stands out, providing further evidence for the "900 ka event" being a key feature of the EMPT.