Proxy development and application for reconstructing the surface ocean carbonate system

Over the last two centuries, human activities have led to an unprecedented rate of carbon input into the atmosphere and oceans resulting in an alarmingly rapid decline in surface ocean pH, a process referred to as ocean acidification (OA). This process is leading to an observed decline in the carbonate ion concentrations ([CO32-]) in seawater – an ion that a large number of marine organisms (e.g. corals, foraminifera) utilize to secrete their skeletons and shells. The ability to identify past ocean acidification events using the marine sedimentary record can shed light on future impacts of the modern OA dilemma. Planktonic foraminifera have the ability to record the physical and chemical properties of the seawater in which they calcified, therefore the fossil shells of foraminifera serve as archives for past climatic and oceanographic conditions. Here, we present a new proxy surface ocean [CO32-] – planktonic foraminferal area density – and establish methods for a known proxy for surface ocean pH – the boron isotopic composition of foraminiferal calcite (δ11B). These proxies are used to reconstruct changes in the surface ocean carbonate system of the eastern equatorial Pacific over the last 35, 000 years using marine sediment core TR163-19 collected from the Cocos Ridge (2°16’N, 90°57’W, 2,348 m). The stable carbon and oxygen isotopic compositions of two morphotypes of planktonic foraminifer Orbulina universa collected from the Cariaco Basin, Venezuela are also investigated, with results suggesting that the two morphotypes record different environmental signals in the calcite shells and should not be used together during paleoceanographic reconstructions.

Marshall B. J., 2015. Proxy development and application for reconstructing the surface ocean carbonate system. PhD thesis, University of South Carolina, 187 pp. Thesis (subscription required).


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