Using the boron isotope-pH proxy to investigate CO 2 driven retreats of the Antarctic Ice Sheet in the geological past
Understanding the relationship between atmospheric CO 2 levels and ice sheet stability is of great importance given the anthropogenic emissions of CO 2 over the past 250 yrs. Since the inception of a permanent Antarctic ice sheet at the Eocene-Oligocene transition, fluctuations in global temperature...
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| Format: | Thesis |
| Language: | English |
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2014
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| Online Access: | https://eprints.soton.ac.uk/377298/ https://eprints.soton.ac.uk/377298/1/Greenop%252C%2520Rosanna_PhD_May_15.pdf |
| Summary: | Understanding the relationship between atmospheric CO 2 levels and ice sheet stability is of great importance given the anthropogenic emissions of CO 2 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 CO 2 . 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 CO 2 are needed in order to initiate deglaciation. To further investigate this paradox, boron isotope (δ 11 B) records have here been used to reconstruct CO 2 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 CO 2 from δ 11 B a number of additional parameters are needed. Here, changes in the boron isotope composition of seawater (δ 11 B sw ) 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 δ 11 B 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 (δ 13 C and δ 18 O) data of different size fractions to show that, unlike the modern species, Oligocene-Miocene G. bulloides had symbionts. A compilation of published δ 13 C 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 δ 11 B sw through time are investigated here using the δ 11 B and δ 13 C compositions of planktic and benthic foraminifera. This approach suggests that δ 11 B sw was ~ 38‰ at the ... |
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