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

• δ13C of crenarchaeol from Thaumarchaeota records δ13C of dissolved inorganic carbon.
• A shift of 3.6‰ in the δ13C of crenarchaeol is recorded at a Danish PETM site.
• δ13C of crenarchaeol at Arctic ETM2 site shifts by 2.5‰.

The Paleocene–Eocene Thermal Maximum (PETM; ∼56 Ma) and Eocene Thermal Maximum 2 (ETM2; ∼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 13C-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 (δ13C) 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 δ13C values of these lipids are potentially directly recording variations in δ13C dissolved inorganic carbon (DIC) and can thus provide a record of marine δ13C DIC across both these Eocene carbon cycle perturbations. Reconstructed pre-CIE δ13CDIC values are slightly lower (0.5–1‰) than modern day values, possibly because Thaumarchaeotal lipids are not only derived from surface waters but also from 13C-depleted subsurface waters. Their values decrease by ∼3.6 (±0.3) ‰ and ∼2.5 (±0.7)‰ 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 13C-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 δ13C record of crenarchaeol may document changes in marine DIC during the PETM in the North Sea Basin. The δ13C of thaumarchaeotal lipids may thus be a novel tool to reconstruct the δ13C of DIC in sediments that are devoid of carbonates, but relatively rich in organic matter, such as shallow marine coastal settings.