Untitled DocumentReconstruction of past ocean temperatures and seawater chemistry by means of molecular biomarkers and isotope studies.

Reconstruction of past ocean temperatures and seawater chemistry by means of molecular biomarkers and isotope studies.

C. Pelejero, E. Calvo, S. Eggins and M.T. McCulloch

Since the beginning of the Industrial Revolution, burning of fossil fuels has increased the atmospheric CO2 content from ~280 to almost 370 ppmv, a level unprecedented in the last 420,000 years. This excess atmospheric CO2 is largely absorbed by the oceans, which are one of the main carbon reservoirs of our planet, basically due to the high solubility of CO2 in seawater and biological processes that transfer carbon from surface into deep waters. However, the capacity of the oceans to respond to this large amount of anthropogenic CO2 is still not known.

A multi-proxy approach to evaluate the role of the oceans in absorbing CO2 is being carried out by means of molecular biomarkers and isotope studies of marine sediments south of Australia, namely C37 long chain alkenones and boron isotopes ((11B). Alkenones are compounds specifically synthesized by phytoplanktonic Haptophyta algae and are characteristic biomarkers in sediments from all oceans. Their relative abundances, expressed as

the UK37 index, show a close relationship with the temperature of the waters where these compounds were biosynthesized, forming the basis of a well established paleothermometer for marine waters (Figure 1). So far, the UK37 method has been set up at Geoscience Australia where a recently acquired Dionex Accelerated Solvent Extraction device (ASE 2000) has been optimised for this purpose, allowing a more rapid sample extraction, increased automation and lower solvent consumption and exposure. Apart from estimating paleo-sea surface temperatures (SST), these compounds are also taken as qualitative indicators of paleo-marine productivity as well as paleo-pCO2 (through the analysis of the carbon isotopic composition, (13C, of these alkenones).


On the other hand, the response of seawater pH to past changes in atmospheric CO2 will also be addressed by means of the analysis of boron isotopes on foraminifera and corals, given that seawater B isotopic composition changes with pH. Data on (11B will be obtained by means of multiple collector inductively coupled plasma mass spectrometry (MC-ICP-MS). In this sense, the new Finnigan Neptune MC-ICP-MS set up at RSES, which provides near constant mass fractionation and high precision, is being optimized to obtain a robust method to routinely analyse boron isotopes.

Figure 22: Representative Gas Chromatography chromatograms of two deep sea sediments from the South China Sea and the Mediterranean Sea depicting the different alkenone pattern.