Clumped isotope thermometry in deep time palaeoceanography

Abstract

This thesis explores clumped isotope thermometry in deep time palaeoceanography. We have improved the clumped isotope method, and applied it to gain new absolute temperature estimates for records from the surface oceans across critical climate transitions, as well as insights into deep ocean temperature evolution throughout the late Cenozoic. These new results will provide tuning targets for climate models. If the climate models can accurately simulate palaeoclimate, this increases our confidence in predictions for future climate change. With respect to the average Earth System Sensitivity (ESS) of recent clumped isotope studies of ~6.3°C per doubling of CO2, our results from Walvis Ridge Site 1264 and previous results from the Indian Ocean reveal even higher ESS for the time interval encompassing the onset of the MCO until the Pliocene warm period. This may imply that the water masses in which this site bathed were extremely sensitive to greenhouse warming. Below atmospheric CO2 concentrations of 700 ppm, it appears that the sensitivity may have been higher, potentially suggesting a climate state dependence (as explained in von der Heydt et al., 2016). If we look at Pliocene modelling results, an ESS of 6.2°C was suggested previously (Haywood et al., 2020). Though the uncertainty of our Pliocene data is large, the average suggests a larger sensitivity. A previous latest Miocene estimate of ESS of 9.3±3.8°C seems compatible with our estimates (Brown et al., 2022). High-CO2 world clumped estimates corroborate findings from other studies. If we fit a line separately through the data with atmospheric CO2 concentrations >700 ppm, the slope is similar to the younger interval, albeit with a different intercept. In the most recent IPCC report, palaeoclimate estimates of ECS were used to weigh the model ensemble mean in AR-6, where models with high ECS were given less weight. Our new insights from clumped isotopes may indicate that this weighing should be reconsidered. Furthermore, scaling sparse estimates of sea surface temperature from particular sites to a global average is hard, and analysing deep ocean temperatures that scale to changes in GMST may be a way forward. As our palaeoclimate studies are used more and more to anchor climate models, we must become ever more diligent in understanding all the factors that drive our proxy systems. Proper treatment of both measurement and systematic uncertainties will be paramount in the next decades of palaeoceanography research.