Reconstructing changes in marine carbonate production during the PETM using stable strontium isotopes

The Palaeocene-Eocene Thermal Maximum (PETM) at ~55Ma represents a global climatic optimum that is characterised by an increase in sea surface temperatures and a co-incident increase in continental weathering rates^1-3. It is also associated with an abrupt negative δ¹³C excursion that is indicative of a large and rapid input of isotopically light carbon into the ocean-atmosphere system⁴. This sudden release of carbon during the PETM had a major impact on the marine carbonate system, and resulted in the dissolution of carbonates in the deep oceans and associated shoaling of the carbonate compensation depth (ccd) due to a depletion in the availability of the carbonate ion^2,5. This study provides new constraints on the changes in the marine carbonate flux during the PETM event using the stable strontium isotope system (δ^88/86Sr). The long residence time of Sr in the oceans (~2.5 Ma) means that seawater has an isotopically homogenous δ^88/86Sr composition that is dependent on the balance between the principle Sr inputs (rivers, hydrothermal fluids and porewaters) and outputs (incorporation into carbonate)⁶. Changes in the flux of Sr to the oceans during the PETM can be accounted for using the radiogenic Sr isotope system (⁸⁷Sr/⁸⁶Sr), enabling variations in δ^88/86Sr to be attributed to shifts in the carbonate output flux. New δ^88/86Sr, ⁸⁷Sr/⁸⁶Sr and Sr/Ca data from the PETM are presented from well-preserved planktonic foraminifera collected from Shatsky Rise in the Pacific Ocean. Foraminiferal calcite has been demonstrated to preserve the δ^88/86Sr composition of seawater⁷, and all samples were handpicked to ensure that only specific species and size fractions were analysed. The new data is compared to previously reported variations in Sr/Ca ratios from the Pacific Ocean, Southern Ocean and Tropical Atlantic³, and the implications for global carbonate dissolution as a consequence of ocean acidification during the PETM are discussed. ⁽¹⁾Zachos et al. (2003). Science, 302, 1551-1554. ⁽²⁾Komar and Zeebe (2011). Paleoceanography, 26, PA3211. ⁽³⁾Stoll et al. (2007). Earth Planet. Sci. Lett., 258, 192-206. ⁽⁴⁾Dickens et al. (1995) Paleoceanography, 10, 965-971. ⁽⁵⁾Zachos et al. (2005). Science, 308, 1611-1615. ⁽⁶⁾Pearce et al. (2011). AGU Fall Meeting, Abstract PP51B-1843. ⁽⁷⁾Böhm et al. (2012) Geochem. Cosmochem. Acta. In Press.