Oxymore

The Mesozoic Era (–250 Ma to –66 Ma) records some of the highest atmospheric CO2 levels of Earth’s history. The lack of robust estimates of the oxygen isotope ratios of seawater (d18OSW) for this interval, however, strongly limits Mesozoic climate reconstructions and hence our understanding of past climate sensitivity to changing CO2 concentrations. The aim of this project is to provide the first reliable Mesozoic d18OSW estimates using three complementary approaches, in order to verify or falsify the following hypotheses: 1) the Mesozoic had “low” d18OSW ratios, implying that its climate was not particularly warm and that other factors than CO2 control Earth’s climate on geological timescales; 2) the Mesozoic had “high” d18OSW ratios, implying very warm conditions and higher-than-expected climate sensitivity; 3) d18OSW ratios changed rapidly and globally during this interval in concert with sea-level changes as a result of greenhouse-icehouse cycles. We propose three complementary axes to test these hypotheses: In a first axis, we will compare skeletal d18O values with two different paleotemperature proxies (primarily Mg/Ca ratios; delta47 on selected samples), measured on the same brachiopod specimens, to reconstruct d18OSW values at multiple locations through the Mesozoic. The fossils will be predominantly sampled from academic collections and completed by fieldwork for key, selected time intervals and their preservation will be assessed using well-proven criteria. In a second axis, we will measure d18O values of skeletal remains of marine endotherms, which incorporate 18O and 16O primarily as a function of d18OSW values, to trace temporal and spatial changes in d18OSW values through the Mesozoic. We will first map intra-skeletal d18O variability in both modern (cetaceans) and Mesozoic marine endotherms (i.e., ichthyosaurs and plesiosaurs) to identify the skeletal elements less affected by regional heterothermy (i.e., cooler body extremities used to reduce heat loss to their highly conductive aquatic environment) and therefore able to more reliably track past d18OSW values. A systematic investigation of d18O values of the selected fossil skeletal elements of different age and paleolatitude will then be performed using both museum collections and fieldwork. In a third axis, Mesozoic d18OSW values of surface and deep oceans will be simulated using modern d18OSW databases and General Circulation Models (GCM). These simulations will be used to investigate the impact of paleogeography on salinity and d18OSW values in three selected Mesozoic time slices, and notably to critically examine the hypothesis according to which high freshwater input in Mesozoic shallow seas, which constitute the depositional environment of the bulk of strata of this age now available for skeletal d18O measurements, produced very low d18OSW values, leading to strong paleotemperature overestimates. The results will be integrated to estimate spatial (axes 1+2+3) and temporal changes (axis 1+2) in d18OSW on both long-term and short-term timescales (i.e., during events of major environmental change) during the Mesozoic and hence examine critically the three, above-mentioned key hypotheses. The expected results of this project will shed a long overdue, ancient light on Earth’s climate sensitivity, with obvious but fundamental implications for global environmental and economical policies.