Boron based insights into Plio-Pleistocene carbon cycle changes and global climate evolution

Abstract

From the Pliocene to the modern, the Earth’s climate has undergone a vast and significant change from a world dominated by continental ice restricted only to Antarctica with a rhythmic 41 kyr beat, through a period of declining atmospheric CO2 and cooling culminating with the bihemispheric glaciation known today, dominated by 100 kyr cyclicity. Ocean circulation is often given a central role in the dynamics of the late Neogene although many questions, such as the role of the North Atlantic in glacial-interglacial CO2 change remain. It is a well-studied region however and as such provides an ideal location for further study with novel proxies that may potentially provide new insights. Similarly, atmospheric CO2 is often thought to be the most crucial single variable driving Plio-Pleistocene climate change. Atmospheric CO2 reconstructions so far published beyond the end of the 800 ka Dome C ice core record are however few and of relatively low resolution and/or precision. This is at present hampering our understanding of CO2-climate interaction for climates warmer than the present and must be addressed as a priority given humanity’s ever-increasing CO2 emissions and anthropogenic global warming. This thesis aims to address these issues using boron-based proxies in foraminiferal carbonate. The potential power of these boron based proxies to directly quantify the marine carbonate system in the past has an enormous draw, both as a pH-CO2 proxy, but also for identifying the role of the deep ocean circulation changes in ocean carbon storage and release on orbital timescales. The first half of this thesis aims to better address the role of ocean circulation in rapid climate change and carbon storage over glacial-interglacial cycles. ?11B and B/Ca records from benthic foraminifera (Cibicidoides wuellerstorfi) from three cores in the North Atlantic spanning the last full glacial cycle and making up a depth, latitude and longitude transect are presented. These show that over this period, North Atlantic circulation is both dynamic and complex, presenting new and demonstrable links between climate change and the deep ocean carbonate system. Within this record a high-resolution section was taken focusing on the last 40 thousand years to search for any rapid changes in circulation associated with Heinrich events. It is demonstrated here that the boron based proxies can remove ambiguity from the existing records of deep ocean circulation change and challenge the established theory of deep water formation (DWF) shutdown in the Northern hemisphere during H-events.

In the second half of this thesis atmospheric CO2 records, beyond the reach of the ice cores, derived from the ?11B of planktonic foraminifera (Globigerinoides ruber) from the tropical Atlantic basin and Caribbean Sea are presented. Here the relationship between the climate system (both in terms of ice-volume/sea level and temperature) is examined in climate states warmer than today. These include a suborbitally resolved record from 1.0-1.2 Ma to observe the nature of CO2 cycles before the ‘over thickening’ of the Laurentide ice sheet and the associated switch from 41 kyr to 100 kyr climate cycles at the Mid Pleistocene Transition (MPT). This study reveals the existence, around 1 million years ago, of high amplitude CO2 cycles with a 41 kyr cyclicity, and a mean CO2 level around 25 ppm above the Late Pleistocene. The relationship between CO2 and ice volume/SL prior to the MPT is significantly different to that post MPT, implying that CO2 decline and some other boundary condition change, probably related to the sub-glacial regolith, were both responsible for this most recent major climatic transition. Also reconstructed is a multisite reconstruction of atmospheric CO2, extending through the last 3.5 million years, including the onset of Northern Hemisphere Glaciation (iNHG). In order to gain a quantitative understanding of the role of CO2 decline in Plio-Pleistocene cooling a comprehensive compilation of sea surface temperature data is also presented. A combination of this record of “global” sea surface temperature data with the longterm CO2 data confirms that Plio-Pleistocene cooling was driven by CO2 decline amplified by the ice-sheet albedo feedback.

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Identifiers

ORCID for Gavin L. Foster: orcid.org/0000-0003-3688-9668

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