A 1.5-Million-Year Record of Orbital and Millennial Climate Variability in the North Atlantic

Abstract

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.

A 1.5-million-year record of orbital and millennial climate variability in the North Atlantic

“…All datasets and age models have been deposited with PANGAEA and are available at https://doi.pangaea. de/10.1594/PANGAEA.951401 (Hodell et al, 2022).…”

A Systematic Role for Extreme Ocean‐Atmosphere Oscillations in the Development of Glacial Conditions Since the Mid Pleistocene Transition

“…Abrupt climate variability, as first observed within ice core records from Greenland (Figures 1 and 2), is now known to be a pervasive feature of the Pleistocene, with most archives pointing to enhanced millennial‐scale activity (or Millennial Power) associated with intermediate climate states and the transitions between states (Barker et al., 2019; Hodell et al., 2023; Kawamura et al., 2017; Lambert et al., 2012; McManus et al., 1999). Many studies have also highlighted a direct link between so‐called DO oscillations (abrupt transitions between stadial and interstadial conditions) and changes in circulation within the Atlantic basin, specifically the AMOC (Henry et al., 2016; Kissel et al., 2008; McManus et al., 2004).…”

“…In this respect it is apparent from our results that insolation must play some role (given the observed link between e.g., obliquity and millennial power; Figure 6). Previous studies have shown that millennial variability reaches its maximum power somewhere between full glacial and full interglacial conditions (Barker et al., 2019; Hodell et al., 2023; Kawamura et al., 2017; Lambert et al., 2012; McManus et al., 1999), possibly reflecting the dampening effects of large ice sheets during glacial maxima and high levels of CO 2 during interglacial periods (Galbraith & de Lavergne, 2019; Zhang et al., 2017), but also perhaps the degree of deep ocean stratification as alluded to earlier. The MIS 5/4 transition occurred when CO 2 had already decreased below interglacial values, and well before ice sheets had grown to their maximum size.…”

“…By extension, and because orbital forcing remained unchanged across the MPT, this implies that millennial variability associated with transitions to a glacial state prior to the MPT did not influence the relative evolution of cooling versus atmospheric CO 2 and ice volume in the same manner as in more recent times. And yet the evidence suggests that millennial variability occurred frequently before the MPT (Billups & Scheinwald, 2014; Birner et al., 2016; Hodell et al., 2023; Raymo et al., 1998).…”

Meltwater flux from northern ice-sheets to the mediterranean during MIS 12

“…Those events were linked to huge meltwater discharges from the Fennoscandian icesheet, supporting the instability of these continental ice masses during MIS 12. The most prominent discharge, which occurred between 461 and 456 ka, has been associated with the opening of the Dover Strait and the catastrophic drainage of previous proglacial lakes (Hodell et al, 2019;Toucanne et al, 2009).…”