Recognition of Early Eocene global carbon isotope excursions using lipids of marine Thaumarchaeota

The Paleocene-Eocene Thermal Maximum (PETM; similar to 56 Ma) and Eocene Thermal Maximum 2 (ETM2; similar to 53 Ma) are geological short (<200,000 years) episodes of extreme global warming and environmental change. Both the PETM and ETM2 are associated with the injection of C-13-depleted carbon into the ocean-atmosphere system as revealed through a globally recognized carbon isotope excursion (CIE) and massive dissolution of deep sea carbonate. However, the magnitude of these CIEs vary with the type of fossil matter, i.e. multiple carbonate phases, bulk organic matter, and terrestrial and marine biomarker lipids, making it difficult to constrain the actual CIE in atmospheric and oceanic carbon pools. Here we analyzed the stable carbon isotopic composition (delta C-13) of glycerol dibiphytanyl glycerol tetraether lipids (GDGTs) derived from marine Thaumarchaeota in sediments deposited during the PETM in the North Sea Basin and ETM2 in the Arctic Ocean. The delta C-13 values of these lipids are potentially directly recording variations in delta C-13 dissolved inorganic carbon (DIC) and can thus provide a record of marine delta C-13 DIC across both these Eocene carbon cycle perturbations. Reconstructed pre-CIE delta C-13(DIC) values are slightly lower (0.5-1 parts per thousand) than modern day values, possibly because Thaumarchaeotal lipids are not only derived from surface waters but also from C-13-depleted subsurface waters. Their values decrease by similar to 3.6 (+/- 0.3) parts per thousand. and similar to 2.5 (+/- 0.7)parts per thousand during the PETM and ETM2, respectively. The CIE in crenarchaeol for ETM2 is higher than that in marine calcite from other locations, possibly because of the admixture of deep water C-13-depleted CO2 generated by the euxinic conditions that developed occasionally during ETM2. However, the reconstructed PETM CIE lies close to the CIE inferred from marine calcite, suggesting that the delta C-13 record of crenarchaeol may document changes in marine DIC during the PETM in the North Sea Basin. The delta C-13 of thaumarchaeotal lipids may thus be a novel tool to reconstruct the delta C-13 of DIC in sediments that are devoid of carbonates, but relatively rich in organic matter, such as shallow marine coastal settings. (C) 2013 Elsevier B.V. All rights reserved.