Quantitative reconstruction of sea-level dynamics for the Late Pliocene to Early Pleistocene (Marine Isotope Stages MG1–78): An integrated approach using oxygen isotopes, Mg/Ca ratios and "clumped isotopes"
Final Report Abstract
The principal objective of this project was to increase the overall understanding of glacial-interglacial (G-IG) to millennial-scale sea-level/ice-volume variability for the Late Pliocene/Early Pleistocene (~3.35–2.0 Ma). This issue was tackled with a novel geochemical approach integrating δ18O, Mg/Ca ratios and “clumped isotopes” (∆47). The project was designed to generate δ18O and Mg/Ca records on a high temporal resolution (~800 yr) and ∆47 data on a G-IG-scale resolution based on benthic foraminiferal calcite from ODP Site 849 in the E Pacific. The project followed the proposed framework, but some modifications to the overall objectives and working program as outlined in the original proposal were necessary. These were mainly related to the Covid-19 pandemic and the resulting delay of ∆47 analysis at the University of Bergen (Norway). In a first sub-project, the new benthic δ18O record was used to establish an updated age model for the study site via tuning it to the LR04 stack. As benthic δ13C data arise as a side product from the δ18O analysis, δ13C was used (together with sand-accumulation rates of the same site and time interval) to evaluate changes in biological production in the E Pacific from ~3.35–2.0 Ma. In the second sub-project, Site 849 benthic δ18O and Mg/Ca data were combined to yield a highresolution sea-level record for ~3.35–2.0 Ma. This record, together with another sea-level record based on the same methodological approach but from a different ocean basin, the N Atlantic, has been used to critically assess the stability of the East Antarctic Ice Sheet (EAIS), i.e., the largest ice sheet on Earth. The data suggest an increasingly stable EAIS from ~2.5 Ma due to larger Northern Hemisphere land-ice volumes and therefore lower global sea levels and less basal melt of the EAIS. This interpretation is mainly based on sea-level highstand estimates that turned out to be highly robust. Glacial sea-level lowstands in the same record, however, appear to be slightly overestimated. Preliminary ∆47-derived bottom-water temperatures (BWT) provide evidence that this overestimate is likely a result of uncertainties in the Mg/Ca proxy for glacial BWT and hence sea-level reconstruction – a hypothesis that requires further testing. Results on EAIS (in)stability of this sub-project have been addressed by public media, e.g., the “Deutschlandfunk” („Steigender Meeresspiegel lässt Antarktis schneller schmelzen“), the “Frankfurter Allgemeine Zeitung” („Labile Eispanzer am Südpol“), or the “Rhein-Neckar Zeitung” (“Wenn die Eisriesen sprechen”). In the third sub-project, the BWT difference between E Pacific Site 849 and N Atlantic Site U1313 was used to unravel the water-mass mixing history in the deep N Atlantic at a yet unprecedented temporal resolution for ~2.65–2.4 Ma. This record documents an increased influence of southernsourced deep waters during glacials at the expense of northern-sourced waters. As such, this record does not only contribute to a better overall understanding of deep-water circulation and therefore global climate dynamics, but is also highly important for geochemically-based sea-level reconstructions from the deep N Atlantic. The latter was yet limited by quantitative estimates of deep-water mass fluctuations at a sufficient, (sub-)millennial-scale resolution. In a fourth sub-project, sea-surface temperature and salinity variability during the Pliocene warmth (∼3.24–3.33 Ma) and the colder Pleistocene (∼2.13–2.18 Ma) was evaluated based on planktic foraminifers (Mg/Ca, δ18O) from Site 849. The records support the general picture of an expanded Pacific warm pool with only minor G-IG changes in upwelling strength during the Pliocene warmth. In contrast, the Pleistocene E Pacific is associated with stronger glacial upwelling. Absolute seasurface temperature and salinity, however, appear to be similar to the Pliocene warm interval.
Publications
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(2020): A new sea-level record for the Neogene/Quaternary boundary reveals transition to a more stable East Antarctic Ice Sheet. Proceedings of the National Academy of Sciences, 117, 30980–30987
Jakob, K.A., Wilson, P.A., Pross, J., Ezard, T.H.G., Fiebig, J., Repschläger, J., Friedrich, O.
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(2021): Stable Biological Production in the Eastern Equatorial Pacific across the Plio-Pleistocene Transition (~3.35–2.0 Ma). Paleoceanography and Paleoclimatology
Jakob, K.A., Ho, S.L., Meckler, A.N., Pross, J., Fiebig, J., Keppler, F., Friedrich, O.
