The Cenozoic represents the transition from the greenhouse world of the Cretaceous to the ice house world of today. Nonetheless, it was not a steady linear trend from an ice free world to an ice house, and there were several reversals along the way. One such reversal was the Mid Miocene Climatic Optimum (MMCO, 14-17 Ma). Material from the Ocean Drilling Program Site 926 on Ceara Rise in the Equatorial Atlantic Ocean is used to examine two intervals within the MMCO from 16.4-15.9 Ma and 15.7-15.3 Ma. Stable isotopes and trace element ratios in two species of benthic foraminifera, Cibicidoides wuellerstorfi, and Oridorsalis umbonatus, and one planktonic foraminifer, Globigerinoides trilobus are used to reconstruct multiple climatological parameters at 2-4 kyr resolution. Paired Mg/Ca and δ18O measurements from the benthic species examined suggest that sea level varied by as much as 40m during the MMCO. Sea level variability was accompanied by changes in sea surface salinity, as measured using δ18O and Mg/Ca from G. trilobus, which suggests that the Intertropical Convergence Zone (ITCZ) was responding to high-latitude forcing centred upon the Northern Hemisphere. The implication is that significant Northern Hemisphere ice sheets were present during the Middle Miocene, some 13 Ma before their currently held date of inception during the Pliocene. These changes in sea level were accompanied by large changes in benthic and planktonic carbonate saturation states (Δ[CO32-]), which are inferred to be representative of changes to global alkalinity and dissolved inorganic carbon. Changes in global export productivity, as evidenced by benthic foraminiferal accumulation rates and the planktonic-benthic gradient of δ13C, are inferred to be controlling the carbon cycle and atmospheric pCO2ATM during the MMCO. The ITCZ is also inferred to be controlling primary productivity at Ceara Rise. Additionally, the relationship between multiple trace element/Ca ratios and Δ[CO32-] are examined using core-tops taken from the Norwegian Sea. The relationship between Mg/Ca and Δ[CO32-] in C. wuellerstorfi is confirmed by expanding the available holothermal data. Mg/Ca in the benthic species Pyrgo murrhina appears to respond exclusively to changes in Δ[CO32-], and the relationship of U/Ca in P. murrhina to Δ[CO32-] is the opposite of that seen in C. wuellerstorfi and O. umbonatus.
| Identifer | oai:union.ndltd.org:bl.uk/oai:ethos.bl.uk:642491 |
| Date | January 2014 |
| Creators | Bradley, Sam |
| Publisher | Cardiff University |
| Source Sets | Ethos UK |
| Detected Language | English |
| Type | Electronic Thesis or Dissertation |
| Source | http://orca.cf.ac.uk/71479/ |
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