Impact of organic carbon reworking upon GDGT temperature proxies during the Paleocene-Eocene Thermal Maximum

Glycerol dialkyl glycerol tetraethers (GDGTs) have been widely applied to coastal marine sediments to reconstruct past temperature variability. However, coastal environments are characterised by variability in the source, age and/or thermal maturity of different organic carbon (OC) pools and may bias various GDGT-based proxies. Here we analyse TEX86 and MBT5MEvalues within a shallow marine sediment core (South Dover Bridge, Maryland) from the Paleocene-Eocene Thermal Maximum (PETM; 56 million years ago (Ma)) to explore how changes in OC reworking influence GDGT-derived sea surface and terrestrial temperature estimates, respectively. We demonstrate that TEX86 values are unaffected by an increase in soil- and fossil organic carbon during the PETM. In contrast, we find large and unexpected variations in MBT5ME-derived temperature estimates (-6 to 25 degrees C) during the onset and core of the PETM at some sites. This coincides with input of reworked terrestrial OC from the Cenomanian-aged Raritan Formation. However, there is also an increase in the degree of cyclisation of tetramethylated branched GDGTs, suggesting that branched GDGTs are also derived from marine in-situ production. These factors preclude terrestrial temperature reconstructions at this site. We explored whether OC reworking is problematic in other PETM-aged coastal environments. Using GDGT metrics and the Branched and Isoprenoid GDGT Machine learning Classification algorithm (BIGMaC), we demonstrate that TEX86 values are mostly unaffected by changes in OC sources. However, MBT5ME values are affected by marine and/or freshwater overprints, especially in environments with low terrestrial OC input. Taken together, this study highlights the importance of constraining the provenance of different GDGTs in marine and lacustrine environments.