Continental climate and ocean circulation change during the Pliocene-Pleistocene intensification of Northern Hemisphere Glaciation

Abstract

The overall aim of this thesis, comprising three main chapters, is to investigate the characteristics and mechanisms of climate change across the Pliocene-Pleistocene intensification of Northern Hemisphere Glaciation (iNHG) through the application of geochemical techniques to sediment cores with high rates of accumulation from the North Atlantic Ocean.

Chapter 3 assesses the origin of sediment colour cycles at Integrated Ocean Drilling Program Site U1313 (41°N, 3.4 km water depth) that show a remarkable correlation with global climate variability over the past 5 million years. The work presented shows that these cycles are controlled by variations in %CaCO3 driven by eolian dust deposition from North America not CaCO3 dissolution (the classic interpretation). Observed change at the secular timescale in a proxy record for dust accumulation from this site is consistent with wetter-than-modern conditions on North America during the warm early Pliocene.

Chapter 4 presents a record of the Nd isotope composition of the deep North Atlantic (Site U1313) between 3.3 and 2.4 Ma, measured on fish debris. This represents the first orbitally resolved record of variations in water mass mixing in this region across iNHG derived using a quasi-conservative proxy. In contrast to existing benthic foraminiferal ?13C records, the Site U1313 dataset provides evidence for large glacial incursions of southern sourced water masses to the deep North Atlantic Ocean through iNHG. An important role for Atlantic meridional overturning circulation variability in amplifying glacial-interglacial cycles during this interval is inferred.


Chapter 5 presents new, sub-orbitally resolved, palaeoceanographic records (Nd isotope, benthic ?18O, benthic ?13C and ice rafted debris) spanning the key Early Pleistocene glacial Marine Isotope Stage 100 (2.52 Ma) from sites situated in the deep (Site U1313, 3.4 km water depth) and intermediate (Ocean Drilling Program Site 981, 2.2 km water depth) North Atlantic. In contrast to Late Pleistocene records, Site U1313 Nd isotope measurements reveal no evidence for shoaling of North Atlantic deep water beyond the background glacial state during sub-orbital ice rafting events. At Site 981, Nd isotopes demonstrate the continuous influence of Iceland-Scotland Overflow Water (ISOW). High frequency variability in benthic ?13C at this site therefore records the changing composition of ISOW, suggesting that Dansgaard-Oeschger paced climate variability was a feature of the high northern latitudes during MIS 100 even when such variability is not expressed in deep-ocean overturning at Site U1313.

Together these results provide significant insights into Pliocene-Pleistocene climate and ocean circulation change, and overturn several existing paradigms. In contrast to previous interpretations of benthic ?13C records (from which northern sourced deep water was inferred to have dominated the Pliocene to Early Pleistocene Atlantic), new Nd isotope records reveal incursions of southern sourced water, including during key glacial intervals across iNHG. These previously unobserved changes in North Atlantic overturning were likely an important feedback on atmospheric carbon dioxide decline during iNHG. Further, evidence against a Pliocene “superconveyer” helps to reconcile the paleoclimate record with numerical model expectations of future climate change. Finally the work highlights the advantages to complementing traditional palaeoclimatic/palaeoceanographic proxies with high-resolution radiogenic isotope records.

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ORCID for Paul Wilson: orcid.org/0000-0001-6425-8906

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