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Using the boron isotope-pH proxy to investigate CO2 driven retreats of the Antarctic Ice Sheet in the geological past

Using the boron isotope-pH proxy to investigate CO2 driven retreats of the Antarctic Ice Sheet in the geological past
Using the boron isotope-pH proxy to investigate CO2 driven retreats of the Antarctic Ice Sheet in the geological past
Understanding the relationship between atmospheric CO2 levels and ice sheet stability is of great importance given the anthropogenic emissions of CO2 over the past 250 yrs. Since the inception of a permanent Antarctic ice sheet at the Eocene-Oligocene transition, fluctuations in global temperature and ice volume are reconstructed to have taken place in the absence of large changes in atmospheric CO2. However, the output from ice sheet modelling experiments suggest that once large ice sheets have grown on East Antarctica they are inherently stable as a result of a powerful hysteresis effect and consequently higher levels of CO2 are needed in order to initiate deglaciation. To further investigate this paradox, boron isotope (?11B) records have here been used to reconstruct CO2 across two intervals of the Miocene- the middle Miocene climatic optimum (MCO) and the Mi-1 glaciation- both of which coincide with major deglaciation events. In order to calculate CO2 from ?11B a number of additional parameters are needed. Here, changes in the boron isotope composition of seawater (?11Bsw) and the life habit of the planktic Foraminifera Globigerina bulloides (G. bulloides) have been investigated through time.

The most useful species of foraminifera for generating ?11B records across the Mi-1 glaciation is G. bulloides, however, its life habit may have been different from the modern. The life habit of G. bulloides is investigated using carbon and oxygen isotope (?13C and ?18O) data of different size fractions to show that, unlike the modern species, Oligocene-Miocene G. bulloides had symbionts. A compilation of published ?13C data for the Neogene suggests that G. bulloides acquired its modern non-symbiotic lifestyle in the late Miocene. Another essential constraint is the boron isotope composition of seawater. Changes in ?11Bsw through time are investigated here using the ?11B and ?13C compositions of planktic and benthic foraminifera. This approach suggests that ?11Bsw was ~ 38‰ at the Oligocene-Miocene boundary before decreasing to ~37 ‰ during the middle Miocene. ?11Bsw then increased by ~ 2.5 ‰ to modern day values (39.61‰). A comparison of this record to the isotopic composition of other major and minor ions in the ocean suggests that variations in riverine input may have been one of the key drivers of the change in ?11Bsw between 14 Ma and the present.

Armed with this improved understanding of the proxy a new ?11B record from across the Mi-1 glaciation suggests that CO2 is low (~ 250 ppm) and invariable prior to, and across the Mi-1 glaciation event and, increases to ~ 400 ppm during the deglaciation. A new ?11B record from between 15.5-17 Ma is also presented that shows CO2 is elevated to maximum levels of ~ 500 ppm between 15.5 and 16.5 Ma but that it is also extremely variable – regularly oscillating between ~300 ppm and ~500 ppm on orbital timescales. When both of these new records are compared to ice volume and climate records it appears that a dynamic Antarctic ice sheet existed even with a relatively modest CO2 forcing across the Miocene. Assuming the Miocene can be used as an analogue for the future, this study suggests that the Antarctic ice sheet can be surprisingly dynamic even at relatively low CO2 (<500 ppm).
Greenop, Rosanna
9a08d945-03bb-41b9-b8f2-f6e84731057e
Greenop, Rosanna
9a08d945-03bb-41b9-b8f2-f6e84731057e
Wilson, Paul
f940a9f0-fa5a-4a64-9061-f0794bfbf7c6

Greenop, Rosanna (2014) Using the boron isotope-pH proxy to investigate CO2 driven retreats of the Antarctic Ice Sheet in the geological past. University of Southampton, Ocean & Earth Science, Doctoral Thesis, 194pp.

Record type: Thesis (Doctoral)

Abstract

Understanding the relationship between atmospheric CO2 levels and ice sheet stability is of great importance given the anthropogenic emissions of CO2 over the past 250 yrs. Since the inception of a permanent Antarctic ice sheet at the Eocene-Oligocene transition, fluctuations in global temperature and ice volume are reconstructed to have taken place in the absence of large changes in atmospheric CO2. However, the output from ice sheet modelling experiments suggest that once large ice sheets have grown on East Antarctica they are inherently stable as a result of a powerful hysteresis effect and consequently higher levels of CO2 are needed in order to initiate deglaciation. To further investigate this paradox, boron isotope (?11B) records have here been used to reconstruct CO2 across two intervals of the Miocene- the middle Miocene climatic optimum (MCO) and the Mi-1 glaciation- both of which coincide with major deglaciation events. In order to calculate CO2 from ?11B a number of additional parameters are needed. Here, changes in the boron isotope composition of seawater (?11Bsw) and the life habit of the planktic Foraminifera Globigerina bulloides (G. bulloides) have been investigated through time.

The most useful species of foraminifera for generating ?11B records across the Mi-1 glaciation is G. bulloides, however, its life habit may have been different from the modern. The life habit of G. bulloides is investigated using carbon and oxygen isotope (?13C and ?18O) data of different size fractions to show that, unlike the modern species, Oligocene-Miocene G. bulloides had symbionts. A compilation of published ?13C data for the Neogene suggests that G. bulloides acquired its modern non-symbiotic lifestyle in the late Miocene. Another essential constraint is the boron isotope composition of seawater. Changes in ?11Bsw through time are investigated here using the ?11B and ?13C compositions of planktic and benthic foraminifera. This approach suggests that ?11Bsw was ~ 38‰ at the Oligocene-Miocene boundary before decreasing to ~37 ‰ during the middle Miocene. ?11Bsw then increased by ~ 2.5 ‰ to modern day values (39.61‰). A comparison of this record to the isotopic composition of other major and minor ions in the ocean suggests that variations in riverine input may have been one of the key drivers of the change in ?11Bsw between 14 Ma and the present.

Armed with this improved understanding of the proxy a new ?11B record from across the Mi-1 glaciation suggests that CO2 is low (~ 250 ppm) and invariable prior to, and across the Mi-1 glaciation event and, increases to ~ 400 ppm during the deglaciation. A new ?11B record from between 15.5-17 Ma is also presented that shows CO2 is elevated to maximum levels of ~ 500 ppm between 15.5 and 16.5 Ma but that it is also extremely variable – regularly oscillating between ~300 ppm and ~500 ppm on orbital timescales. When both of these new records are compared to ice volume and climate records it appears that a dynamic Antarctic ice sheet existed even with a relatively modest CO2 forcing across the Miocene. Assuming the Miocene can be used as an analogue for the future, this study suggests that the Antarctic ice sheet can be surprisingly dynamic even at relatively low CO2 (<500 ppm).

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Published date: 5 December 2014
Organisations: University of Southampton, Paleooceanography & Palaeoclimate

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Local EPrints ID: 377298
URI: http://eprints.soton.ac.uk/id/eprint/377298
PURE UUID: e8948b2c-cb9b-4666-924d-67eda8cb2a09

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Date deposited: 21 May 2015 11:26
Last modified: 31 May 2018 04:01

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