Constraints from new geochemical proxies on temperature and sea level during critical climate transitions.

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Currently the polar regions are exhibit the greatest amount of warming of any region on the planet, a feature which is not well produced in any of the current generation of climate models. The potential response of ice sheets and sea level to projected greenhouse warming is also not known. Currently sea level is globally rising at a rate far greater than predicted, with rates predicted to accelerate as atmospheric CO2 levels increase. Paleoclimate records have the potential to inform us about the amplitude and regional pattern of climate change, and about the processes responsible for such change. Despite several decades of research using oxygen isotopes (d18O; a proxy of both temperature and seawater d18O, the latter of which reflects the growth and melting of ice sheets) and more recently, magnesium thermometry (sensitive to temperature, seawater Mg/Ca, and carbonate ion concentration), the evolution of sea level and temperature throughout Earth history is poorly known and highly controversial. The two approaches we plan to take have not been used before. First, we will use a new technique that measured the abundance of bonds between the isotopes carbon-13 and oxygen-18 in foraminifera, as it is controlled solely by temperature. Biological fractionation and seawater chemistry appear to have a negligible influence on this proxy. However, measurements are very time-consuming to make, so we will only make a few measurements at each location. We will combine these temperatures with foraminiferal d18O measurements to estimate seawater d18O. In addition, we will calculate seawater Mg/Ca ratios using these temperatures and published Mg/Ca records. Seawater Mg concentrations change over timescales of about 10 million years, and Ca concentrations over about 1 million years, so although during the period we are studying, seawater Mg/Ca ratios were probably different from today, it is unlikely that they changed very much over the 15 million years represented by our study. We can apply these seawater Mg/Ca ratios to high-resolution Mg/Ca records for the 3 study locations, in order to more accurately estimate past temperature variations. In addition, we can estimate the influence of changes in carbonate ion concentration on Mg/Ca by bringing in another set of geochemical measurements, the lithium to calcium (Li/Ca) and boron to calcium (B/Ca) ratio of foraminifera. By bringing in multiple geochemical proxies, we can isolate the temperature component of the foraminiferal Mg/Ca record and develop more precise and accurate reconstructions of past temperature. These records can in turn be compared with foraminiferal d18O estimates to study the evolution of seawater d18O. We plan to then compare both sets of reconstructed temperature and seawater d18O measurements with estimates of atmospheric carbon dioxide levels, in order to look at past relationships between climate and greenhouse gases.