New constraints on the duration of the onset of the PETM carbon isotope excursion

The response of the climate system to rapid carbon-cycle perturbations can be constrained by studying past transient climate events such as the Paleocene–Eocene Thermal Maximum (PETM, ~56 million years ago). The PETM is marked by a massive input of isotopically light carbon, as recorded by a 3–4‰  negative carbon isotope excursion (CIE) in sedimentary records globally. Estimates of the duration of the rapid onset of the CIE range from a few hundred to several thousand years. The exact duration remains poorly constrained due to the scarcity of marine sedimentary records that 1) have sufficiently high sedimentation rates to resolve rapid decadal- to centennial scale transitions, 2) provide robust controls on sedimentation rates and event timing, and 3) preserve proxy data that record perturbations of the dissolved inorganic carbon (DIC) pool. Consequently, the rate of carbon release during the CIE onset, and its relevance for understanding anthropogenic climate change, remains unclear.

International Ocean Discovery Program (IODP) Expedition 396 recovered expanded PETM successions on the Norwegian Margin, including a microlaminated CIE onset interval that preserves decadal-scale variability. We document the first occurrence of haptophyte alkenones from the onset of the PETM CIE and present a high-resolution record of their stable carbon isotopic variability (δ¹³C_alk) across the onset interval. The δ¹³C_alk records variations in the (isotopic) composition and concentration of the dissolved inorganic carbon (DIC) pool. We show that the δ¹³C_alk record is not strongly influenced by local (volcanically induced) input of ¹³C-depleted carbon based on high-resolution sedimentary mercury and polyaromatic hydrocarbon data from this interval. Finally, we provide robust age control from the diatom laminations, enabling a direct and well-constrained estimate of the duration of the global CIE onset interval.