Synchronous Marine and Terrestrial Carbon Cycle Perturbation in the High Arctic During the PETM

The Paleocene-Eocene Thermal Maximum (PETM; 56 Ma) is considered to be one of the best analogs for future climate change. The carbon isotope composition (δ¹³C) of n-alkanes derived from leaf waxes of terrestrial plants and marine algae can provide important insights into the carbon cycle perturbation during the PETM. Here, we present new organic geochemical data and compound-specific δ¹³C data from sediments recovered from an early Cenozoic basin-margin succession from Spitsbergen. These samples represent one of the most expanded PETM sites and provide new insights into the high Arctic response to the PETM. Our results reveal a synchronous ∼−6.5‰ carbon isotope excursion (CIE) in short-chain n-alkanes (nC₁₉; marine algae/bacteria) with a ∼−5‰ CIE in long-chain n-alkanes (nC₂₉ and nC₃₁; plant waxes) during the peak of the PETM. Although δ¹³C_n-alkanes values were potentially affected via a modest thermal effect (1‰–2‰), the relative changes in the δ¹³C_n-alkanes remain robust. A simple carbon cycle modeling suggests peak carbon emission rate could be ∼3 times faster than previously suggested using δ¹³C_TOC records. The CIE magnitude of both δ¹³C _n-C19 and δ¹³C_n-C29 can be explained by the elevated influence of ¹³C-depleted respired CO₂ in the water column and increased water availability on land, elevated pCO₂ in the atmosphere, and changes in vegetation type during the PETM. The synchronous decline in δ¹³C of both leaf waxes and marine algae/bacteria argues against a significant contribution to the sedimentary organic carbon pool from the weathering delivery of fossil n-alkanes in the Arctic region.