A 1.5-Million-Year Record of Orbital and Millennial Climate Variability in the North Atlantic
Authors: Hodell David , Crowhurst Simon , Lourens Lucas , Margari Vasiliki , Nicolson John , Rolfe James E. , Skinner Luke C. , Thomas Nicola , Tzedakis Polychronis C. , Mleneck-Vautravers Maryline J. , Wolff Eric W. 
Published: August 2022

Abstract. Climate during the last glacial period was marked by abrupt instability on millennial time scales that included large swings of temperature in and around Greenland (Daansgard-Oeschger events) and smaller, more gradual changes in Antarctica (AIM events). Less is known about the existence and nature of similar variability during older glacial periods, especially during the early Pleistocene when glacial cycles were dominantly occurring at 41-kyr intervals compared to the much longer and deeper glaciations of the more recent period. Here we report a continuous millennially-resolved record of stable isotopes of planktic and benthic foraminifera at IODP Site U1385 (the “Shackleton Site”) from the southwestern Iberian margin for the last 1.5 million years, which includes the Middle Pleistocene Transition (MPT). Our results demonstrate that millennial climate variability (MCV) was a persistent feature of glacial climate, both before and after the MPT. Prior to 1.2 Ma in the early Pleistocene, the amplitude of MCV was modulated by the 41-kyr obliquity cycle and increased when axial tilt dropped below 23.5° and benthic δ18O exceeded ~3.8 ‰ (corrected to Uvigerina), indicating a threshold response to orbital forcing. Afterwards, MCV became focused mainly on the transitions into and out of glacial states (i.e., inceptions and terminations) and during times of intermediate ice volume. During the MPT (1.2–0.65 Ma), obliquity continues to modulate the amplitude of MCV but in a more non-linear fashion as evidenced by the appearance of multiples (82, 123 kyrs) and combination tones (28 kyrs) of the 41-kyr cycle. At the end of the MPT (~0.65 Ma), obliquity modulation of MCV amplitude wanes as quasi-periodic 100-kyr and precession power increase, coinciding with growth of oversized ice sheets on North America and the appearance of Heinrich layers in North Atlantic sediments. Whereas the planktic δ18O of Site U1385 shows a strong resemblance to Greenland temperature and atmospheric methane (i.e., northern hemisphere climate), millennial changes in benthic δ18O closely follow the temperature history of Antarctica for the past 800 ka. The phasing of planktic and benthic δ18O throughout much of the record is similar to that observed for MIS 3, which has been suggested to mimic the signature of the bipolar seesaw – i.e., an interhemispheric asymmetry between the timing of cooling in Antarctica and warming in Greenland. The Iberian margin isotopic record suggests bipolar asymmetry was a robust feature of interhemispheric glacial climate variations for at least the past 1.5 Ma despite changing glacial boundary conditions. A strong correlation exists between millennial increases in planktic δ18O (cooling) and decreases in benthic δ13C, indicating millennial variations in North Atlantic surface temperature are mirrored by changes in deep-water circulation and remineralization of carbon in the abyssal ocean. We find strong evidence that climate variability on millennial and orbital scales are coupled across different time scales and interact, in both directions, which may be important for linking internal climate dynamics and external astronomical forcing.