Late Cretaceous-Neogene trends in deep ocean temperature and continental ice volume: Reconciling records of benthic foraminiferal geochemistry (δ18O and Mg/Ca) with sea level history

We reconstruct trends in ice volume and deep ocean temperature for the past 108 Myr, resolving variations on timescales of similar to 2 Myr and longer. We use a sea level record as a proxy for ice volume, a benthic foraminiferal Mg/Ca-bf record as a proxy for temperature, and a benthic foraminiferal d(18)O(bf) record as a proxy for both. This allows us to construct dual estimates of temperature and ice volume variations for the interval 10-60 Ma: extracting temperature from d(18)O(bf) by using sea level as a proxy for ice volume to constrain the d(18)O(sw) component, and extracting seawater d(18)O(sw) (which reflects ice volume) from d(18)O(bf) by using Mg/Ca-bf to constrain the temperature component. Each of these approaches requires numerous assumptions, but the range of plausible solutions are concordant on timescales >2 Myr and within an uncertainty of +/- 2 degrees C temperature and +/- 0.4 parts per thousand delta O-18(sw). The agreement between the two approaches for the last 50 Myr provides empirical justification for the use of d(18)O(bf), Mg/Ca-bf, and sea level records as robust climate proxies. Our reconstructions indicate differences between deep ocean cooling and continental ice growth in the late Cenozoic: cooling occurred gradually in the middle-late Eocene and late Miocene-Pliocene while ice growth occurred rapidly in the earliest Oligocene, middle Miocene, and Plio-Pleistocene. These differences are consistent with climate models that imply that temperatures, set by the long-term CO2 equilibrium, should change only gradually on timescales >2 Myr, but growth of continental ice sheets may be rapid in response to climate thresholds due to feedbacks that are not yet fully understood.