Marine burial of terrestrial organic carbon modulates past warm climates

The Palaeocene-Eocene Thermal Maximum (PETM), ~56 million years ago, was characterised by large-scale carbon release (~3,000-10,000 petagrams of carbon) and transient global warming (4-6°C over 10,000 years). Erosion and subsequent burial of terrestrial organic carbon (OC) in marine sediments could have sequestered OC during the PETM - stabilising the Earth system - yet direct evidence is lacking. Here we present new source-specific biomarker records from five globally distributed shallow marine sites and show that plant and soil OC contribute ~40-95% of total OC in coastal marine sediments during the PETM. This is higher than modern marine sediments (~12-20% of total OC) and implies greater terrestrial OC burial in marine environments during past warm climates. Terrestrial OC burial fluxes can increase ~10-to-50-fold during the PETM due to enhanced physical erosion and higher coastal sedimentation rates. This implies that marine burial of terrestrial OC modulated climate during the PETM and possibly other hyperthermals. Current models do not account for this expanded delivery of terrestrial OC into the marine realm and are thus missing an important carbon sink. Terrestrial OC burial could have acted as a negative feedback in warmer-than-present climates and may aid the long-term (>10,000 year) recovery of the Earth system.