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Title: New insights into Cenozoic Silicon cycling in the Southern Ocean : refined application of silicon isotope ratios in biogenic opal
Author: Egan, Katherine Elizabeth
ISNI:       0000 0004 5357 0680
Awarding Body: University of Oxford
Current Institution: University of Oxford
Date of Award: 2014
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The marine silicon and carbon cycles are intrinsically linked by a unique group of primary producers; the diatoms. These siliceous-walled phytoplankton play a significant role in carbon export, making them a critical component of the global biological carbon pump with the power to affect climatic change. In this thesis, the silicon isotope composition (δ30Si) preserved in diatom opal is used together with the δ30Si of sponge opal, a powerful new proxy for deepwater silicic acid concentration, to document the Cenozoic Silicon cycle, shedding light on its role in carbon cycling and global climatic change. This study has developed a novel size-separation methodology to produce the first core top calibration of diatom δ30Si. The calibration demonstrates that diatom δ30Si exhibits a strong negative correlation with surface water silicic acid concentration, supportive of its application as a proxy for silicic acid utilisation. The refined method is used to produce a diatom δ30Si record, for the first time combined with sponge δ30Si, to gain insight into the Southern Ocean silicon cycle over one of the largest Cenozoic climatic shifts; the onset of Antarctic glaciation (~33.7 Ma). The two δ30Si records yield the first geochemical footprint to demonstrate that diatom proliferation, coincident with the onset of Antarctic circumpolar current flow, was a precursor event to the Eocene-Oligocene Transition. Diatoms are shown to have played a role in climate cooling through enhanced export and burial of organic carbon on the seafloor. The first long term reconstruction of silicic acid concentration in subsurface waters of the Southern Ocean, which spans the Late Eocene to the earliest Pliocene, provides new evidence that oceanic vertical mixing rates, coupled with the efficient removal of silicon from the surface by the diatoms, have been the most important factor in controlling the silicon chemistry of the ocean over the Cenozoic.
Supervisor: Rickaby, R. E. M.; Halliday, A. N. Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available
Keywords: Earth sciences ; Geochemistry ; Diatom ; Silicon Isotope ; Eocene ; Oligocene