Cenozoic records of seawater chemistry: chemical proxies as indicators of past climate.
Mawbey, Elaine M.
The ratios of trace metals to calcium in the calcium carbonate tests of benthic foraminifera have been used as palaeoceanographic proxies for several decades and are now routinely used to reconstruct past climate change on a variety of Cenozoic timescales. Recent research, however, reveals gaps in our understanding of the effects of oceanic variables such as temperature and carbonate saturation state on these trace metal ratios, which limits their application as proxies. Additional uncertainties arise because of physiological effects (“vital effects”) and microhabitat of the living foraminifera. Moreover, much of what is known about trace metal uptake into benthic foraminiferal calcite is derived from modern core-top calibrations and laboratory experiments and it is unknown as to what extent our understanding from these models can be applied to the early and mid Cenozoic.\ud This thesis attempts to address some of these questions using a novel depth transect approach and presenting benthic foraminiferal trace metal records across three major Cenozoic climate change events from Ocean Drilling Program (ODP) deep sea core material. The events are: (i) The Eocene-Oligocene transition (EOT), (ii) The Oligocene- Miocene boundary event (Mi-1) and (iii) The Middle Miocene Climatic Transition (MMCT). These records have allowed examination of the validity of the application of current knowledge of benthic foraminiferal trace metal proxies to Cenozoic records, and also the comparison of the records of two different benthic foraminiferal species, which has shed new light on the importance of foraminiferal habitat in the interpretation of the proxy data.\ud The identification of a dissolution effect operating on benthic foraminiferal Mg/Ca in undersaturated waters has allowed a reasonable estimate of cooling (~2-3˚C) to be obtained from deep-sea records across the Eocene-Oligocene Transition. The transient glaciation at the Oligocene-Miocene boundary is estimated to comprise of a deep-sea cooling of ~2°C and a sea level decrease of ~80 metres based on Oridorsalis umbonatus Mg/Ca. Comparison of new Middle Miocene trace metal records across a water depth transect has shed light on the relative interplay of temperature and saturation state on published Mg/Ca records, enabling more accurate estimates of temperature and ice volume change. The new interpretations suggest that deep sea temperatures cooled by ~4.5C, and sea level fell by 70-100 metres between 16.2 and 11.6 Ma.\ud The recently developed paired Mg/Ca-Li/Ca approach to calculating simultaneous variations in bottom water temperature and saturation state has been used to assess inter-basinal differences in water mass composition for the first time. Down-core benthic foraminiferal Li/Ca records from Ceara Rise Sites do not behave as expected, indicating that there may be times when this proxy is affected by an additional environmental parameter. This is tentatively suggested to be a growth-rate related oxygenation signal. U/Ca in benthic foraminifera does not appear to work as a saturation state proxy in these records, again indicating either different behaviour to the observed modern behaviour, or additional factors absent from the modern calibration, such as bottom water oxygenation. Benthic foraminiferal Sr/Ca follows the linear negative water-depth relationship of Lear et al., (2003) suggesting a pressure-related effect.\ud This study demonstrates that benthic foraminiferal trace metal chemistry can be used to provide information about climate events in the Cenozoic, however additional empirical work is required to fully understand the systematics of trace metal incorporation into benthic foraminiferal tests.