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

David A. Hodell*, Simon J. Crowhurst, Lucas Lourens, Vasiliki Margari, John Nicolson, James E. Rolfe, Luke C. Skinner, Nicola C. Thomas, Polychronis C. Tzedakis, Maryline J. Mleneck-Vautravers, Eric W. Wolff

*Corresponding author for this work

Research output: Contribution to journalArticleAcademicpeer-review

Abstract

Climate during the last glacial period was marked by abrupt instability on millennial timescales 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. After 1.2 Ma, obliquity continued to play a role in modulating the amplitude of MCV, especially during times of glacial inceptions, which are always associated with declining obliquity. A non-linear role for obliquity is also indicated by the appearance of multiples (82, 123 kyr) and combination tones (28 kyr) of the 41 kyr cycle. Near 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 the 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 kyr. The phasing of millennial planktic and benthic δ18O variation is similar to that observed for MIS 3 throughout much of the record, 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 that 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 that 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 is coupled across different timescales and interacts in both directions, which may be important for linking internal climate dynamics and external astronomical forcing.

Original languageEnglish
Pages (from-to)607-636
Number of pages30
JournalClimate of the Past
Volume19
Issue number3
DOIs
Publication statusPublished - 16 Mar 2023

Bibliographical note

Funding Information:
This research has been supported by the Natural Environmental Research Council (grant nos. NE/J00653X/1, NE/K005804/1, NE/J017922/1, and NE/R000204/1), the Leverhulme Trust (project no. RPG2014-417), and the Netherlands Earth System Science Centre (gravitation grant 024.002.001).

Publisher Copyright:
© 2023 David A. Hodell et al.

Funding

This research has been supported by the Natural Environmental Research Council (grant nos. NE/J00653X/1, NE/K005804/1, NE/J017922/1, and NE/R000204/1), the Leverhulme Trust (project no. RPG2014-417), and the Netherlands Earth System Science Centre (gravitation grant 024.002.001).

Keywords

  • Meridional overturning circulation
  • Deep-water
  • Ocean circulation
  • Southern-ocean
  • Glacial terminations
  • Persistent influence
  • Bering strait
  • Scale changes
  • Time scales
  • Site u1385

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