Similar Millennial Climate Variability on the Iberian Margin Before and After the Middle Pleistocene Transition

Abstract:

Although millennial-scale climate variability (<10 ka) has been well studied during the last glacial cycles, little is known about this important aspect of climate in the early Pleistocene, prior to the Middle Pleistocene Transition. Here we present a late early Pleistocene climate record at centennial resolution for two representative glaciations during the ‘41-ka world’ (Marine Isotope Stages MIS 37–41, from 1235 to 1320 ka) from IODP Site U1385 (“Shackleton Site”) on the southwest Iberian margin. Millennial-scale climate variability was suppressed during interglacial periods (MIS 37, MIS 39 and MIS 41) and was activated during glacial inceptions when benthic δ¹⁸O exceeded 3.2‰. Millennial variability during glacials MIS 38 and MIS 40 closely resembled Dansgaard-Oeschger events from the last glaciation (MIS 3) in amplitude, shape and pacing. The phasing of oxygen- and carbon-isotope variability is consistent with an active oceanic thermal bipolar see-saw between the Northern and Southern Hemispheres. Each of the prominent stadials in MIS 38 and MIS 40 was associated with a decrease in benthic carbon isotopes, indicating concomitant changes in meridional overturning circulation. A comparison to other North Atlantic records of ice-rafting in MIS 38 and MIS 40 suggests that freshwater forcing, as proposed for the late Pleistocene, triggered or amplified a slowdown of oceanic circulation and elicited the bipolar see-saw response. Our findings support similarities in the operation of the climate system occurring on millennial timescales both before and after the Middle Pleistocene Transition despite the increase in global ice volume and duration of the glacial cycles.