New Foraminiferal Proxies for Cenozoic Seawater Chemistry (ForSea)

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The chemistry of the oceans has changed a lot over geological time and, exerts a major control on the long-term composition of the atmosphere and on how the Earth's environment responds to perturbation, and yet is hard to quantify for the past. Here we propose to develop two new proxies for ancient ocean chemistry in the tiny calcium carbonate shells of single celled organisms called foraminifera. The first proxy is the ratio of sulfur to calcium in the shell. This is likely to record the concentration of dissolved carbonate in seawater because the ratio in the shell is controlled by the ratio of sulfate to carbonate in seawater. Over short time scales sulfate concentrations stay the same, but the concentration of the carbonate ion responds to various parameters including the pH of the oceans and the amount carbon dioxide in the atmosphere. The second proxy is the ratio of barium to calcium in the shell. This is expected to respond to the amount of sulfate in the seawater because the mineral barium sulfate is very insoluble, so as the concentration of sulfate gets lower, barium concentrations are able to increase. The two proxies are linked because we need to know how sulfate concentrations change over long time periods to make the calculations for the dissolved carbonate proxy. Published work shows that for barium calcium ratios in the foraminifera the only control is the amount of barium in seawater. However, for sulfur-calcium ratios there may be other factors which affect this ratio. Therefore, we first propose to analyse foraminiferal shell material for its sulfur-calcium ratio from four modern sites with different characteristics such as temperature and for a range of foraminiferal species to understand what controls these chemical ratios. We will then test this dissolved carbonate proxy across the last two glacial-interglacial time periods where we know the concentration of atmospheric carbon dioxide from ice-core records, and can also apply an established proxy for ocean pH, boron isotopes. We can use the boron isotope and atmospheric carbon dioxide measurements to independently calculate carbonate ion concentration to test the against calculations of carbonate ion concentration from the sulfur-calcium ratios. We will test the sulfate concentration proxy by analysing samples from time periods for which we have independent evidence that sulfate concentrations were lower. We will also test how well each of the proxies is preserved in the geological record by comparing samples known to have exceptional 'glassy' preservation with contemporaneous examples with so called 'frosty' preservation indicative of changes occurring in the shell after it was deposited in the sediment. Finally, we will apply these proxies to generate long term records of ocean chemistry across the last 65 million years. We will also study two more rapid events linked to changes in atmospheric carbon dioxide: a rapid warming event ~55 million years ago (the Palaeocene-Eocene Thermal Maximum) thought to be caused by a rapid increase in atmospheric carbon dioxide, and the transition to an Earth with permanent polar ice caps (the Eocene-Oligocene boundary) which may have been caused by declining atmospheric carbon dioxide. If successful, the development of these proxies will represent a big leap forward in our ability to understand how the chemistry of the ocean has changed through time. The principles that govern these proxies are also likely to apply to other sorts of carbonate minerals formed by other organisms, so the work in this grant would prepare the way for them to be tested and applied across a much longer span of Earth's history. This would represent a step change in our understanding of the evolution of seawater chemistry and the biogeochemical cycles that control it, both during rapid events such as mass extinctions and longer term changes such as those imposed by plate tectonics.