The middle Eocene climate transition : insights from foraminiferal and sedimentary records

• Main Author: Dawber, C. F.
• Published: University of Cambridge 2009
• Subjects: 551.7
• Online Access: http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.598412

New seawater oxygen isotope reconstructions support the presence of a dynamic cryosphere since the early Middle Eocene with significant ice storage in both hemispheres. The onset of early Cenozoic glaciation is hypothesised to be intimately linked with levels of atmospheric <i>p</i>CO₂. The relationship between bottom water carbonate saturation and the trace metal composition of the benthic foraminifera <i>Oridorsalis umbonatus </i>is investigated using core top and Middle Eocene data sets. B/Ca, Li/Ca and Sr/Ca ratios are correlated with bottom water carbonate saturation, whilst Mg/Ca ratios primarily record bottom water temperature. Discrepancies in Middle Eocene estimates of bottom water carbonate saturation that are based on applying core top calibrations to down-core records of B/Ca, Li/Ca and Sr/Ca indicate that there are some gaps in our understanding of the proxy systematics. The Barton Clay Formation at Alum Bay on the Isle of Wight was studied to assess whether there is evidence for sea level variations during the Middle Eocene. An integrated magneto-bio-chemostratigraphic age model for the Barton Clay Formation is also presented. Third- and fourth-order sequence stratigraphic cycles are identified in records of grain size, sediment properties, faunal assemblage and foraminiferal diversity indices, and are consistent with 40-60 meter variations in water depth. These cycles can be correlated to other successions within the basin on the basis of litho- and biostratigraphic markers. Evidence of reworked clasts and calcareous nannofossils is consistent with episodic uplift within the basin during the Bartonian and at present, precludes the calculation of eustatic sea level variations. The amplitude and duration of water depth variations in the Barton Clay Formation are consistent with glacio-eustacy.