Investigating outgassing of CO2 from the Southern Ocean over the last 65 kyr using boron isotopes in the planktic foraminifera Globigerina bulloides

Abstract

Over the last 2.5 million years the Earth has regularly cycled between cold glacial and warm interglacial periods. Ice core records demonstrate that changes in atmosphericCO2 concentrations are a major climate forcing, which results in these changes in global climate. Although the scale and speed of atmospheric CO2 changes point to the vast deep ocean carbon reservoir as playing a crucial role, the mechanisms of CO2 supply and sequestration between the deep ocean and atmospheric carbon reservoirs remain an ongoing debate in the palaeoclimate community. The Southern Ocean is a major region of modern ocean-atmosphere CO2 exchange and has long been considered to play a key role in controlling glacial to interglacial atmospheric CO2 variability. Two mechanisms have been proposed to drive these changes in the global carbon cycle: 1) Enhanced stratification and sea-ice cover in the Antarctic Zone, and 2) Enhanced biological sequestration of carbon in response to alleviated micronutrient limitation in the Sub-Antarctic Zone. Despite the critical role the Southern Ocean is hypothesised to play, there are relatively few palaeo-records from around the region documenting the nature and character of thisCO2 exchange through time. In this thesis, boron isotopes measured in the planktic foraminifera Globigerina bulloides are used to reconstruct surface ocean pH of the Southern Ocean over the last 65 kyr, which in turn is used to infer past seawater dissolved CO2 (expressed as partial pressure;pCO2sw) and ocean-atmosphere CO2 disequilibrium (∆pCO2). The current application of the boron isotope pH proxy in planktic foraminifera is often focussed on lower latitude species, such as Globigerinoides ruber, which are absent from high latitude core sites. Therefore, this thesis initially focuses on overcoming the challenges associated with the application of the boron isotope pH-proxy in G. bulloides which has lower concentrations of boron but is dominant in Sub-Antarctic foraminifera assemblages (Chapters 2 & 3).An updated methodology is presented herein, which overcomes the analytical challenges that occur during the purification of boron from a low B/Ca carbonate matrix, typical of G. bulloides. This PhD project also enhances current understanding of the biogeochemical and biophysical processes that contribute to boron isotope ‘vital effects’ in this species. It also provides insights into the causes of the variability observed within the existing boron isotope calibration for G. bulloides, highlighting a number of areas for future research. Millennial-scale changes in atmospheric CO2 are characteristic of both the deglacial and last glacial period. There are however limited data to assess whether the mechanisms ofCO2 transfer from the ocean to the atmosphere during these rapid events were similar, or varied with background climate state. To address this, down core boron isotope derived ∆pCO2 records are presented, alongside additional proxy records (e.g. alkenone abundance), from the Sub-Antarctic Atlantic and Pacific over the both the last deglacial(Chapter 4), and from the Sub-Antarctic Pacific during the last glacial from 65-30 kyr (Chapter 5). The Sub-Antarctic records from the last 20 kyr demonstrate that both the Atlantic and South West Pacific sectors of the Southern Ocean were regions of significant CO2 outgassing during the early deglacial 35 ppm increase in atmosphericCO2 associated with Heinrich Stadial 1 (17.5-15 kyr). However, a new global compilation presented here suggests that other locations, such as the North Pacific, may have played a larger role in the second part of deglacial CO2 rise (Younger Dryas; 12.9-11.5 kyr) when atmospheric CO2 increased by 30 ppm. The new Sub-Antarctic Pacific record presented here from the last glacial (65-30 kyr) provides the first direct evidence of millennial-scale CO2 outgassing during this time period. This record highlights that, a sin the early deglacial, outgassing of previously sequestered CO2 from the Southern Ocean is associated with, and hence may be a critical driver for, the concurrent millennial scale increases in atmospheric CO2. The data presented here therefore suggests that outgassing of CO2 from the Southern Ocean is a consistent contributor to millennial scale atmospheric CO2 variability throughout glacial cycles. However, given that the Southern Ocean records presented in this thesis show significant spatial variability, better spatial coverage will be needed to fully understand the mechanisms of past and future change in the Southern Ocean.

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

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