Abstract
The Paleocene-Eocene Thermal Maximum (PETM; ~ 56 million years ago) is the most severe carbon cycle perturbation event of the Cenozoic. Although the PETM is associated with warming in both the surface (up to 8 °C) and deep ocean (up to 5 °C), there are relatively few terrestrial temperature estimates from the onset of this interval. The associated response of the hydrological cycle during the PETM is also poorly constrained. Here, we use biomarker proxies (informed by models) to reconstruct temperature and hydrological change within the Cobham Lignite (UK) during the latest Paleocene and early PETM. Previous work at this site indicates warm terrestrial temperatures during the very latest Paleocene (ca. 22–26 °C). However, biomarker temperature proxies imply cooling during the onset of the PETM (ca. 5–11 °C cooling), inconsistent with other local, regional and global evidence. This coincides with an increase in pH (ca. 2 pH units with pH values >7), enhanced waterlogging, a major reduction in fires and the development of areas of open water within a peatland environment. This profound change in hydrology and environment evidently biases biomarker temperature proxies, including the branched GDGT paleothermometer. This serves as a cautionary tale on the danger of attempting to interpret biomarker proxy records without a wider understanding of their environmental context.
Highlights
The Paleocene-Eocene Thermal Maximum (PETM; ca. 56 million years ago) is a rapid global warming event associated with the release of ocean-atmosphere system
The relative abundance of tetramethylated Branched glycerol dialkyl glycerol tetraethers (brGDGTs) decreases further within the blocky lignite
We argue that, there is vegetation change, it is unlikely to have exerted a primary control upon biomarker paleotemperature proxies in the Cobham Lignite
Summary
The Paleocene-Eocene Thermal Maximum (PETM; ca. 56 million years ago) is a rapid global warming event associated with the release of ocean-atmosphere system. During the PETM, proxies and models indicate that the hydrological cycle exhibits a globally ‘wet-wetter, dry-drier’ style response (Carmichael et al, 2017). High-latitude and coastal settings are generally characterised by stable and/or increasing rainfall, with proxy evidence for both enhanced terrigenous sediment flux to marginal marine sediments (John et al, 2008) and enhanced chemical weathering (Dickson et al, 2015; Ravizza et al, 2001). Mid-to-low latitude and continental interior settings are typically characterised by decreasing rainfall but an increase in extreme precipitation rates (Carmichael et al, 2018; Handley et al, 2012; Schmitz and Pujalte, 2007)
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