Palynological evidence of vegetation dynamics in response to palaeoenvironmental change across the onset of the Paleocene-Eocene Thermal Maximum at Cobham, Southern England

A high-resolution palynological study is undertaken through the Cobham Lignite Bed (Cobham, Kent, UK) to investigate vegetation response to the rapid climate warming at the onset of the Paleocene-Eocene thermal maximum (PETM). The lower laminated lignite records negative carbon isotope ( 13C) excursions (CIE) (marking the PETM onset) in bulk organic material, n-alkanes and, uniquely, also in hopanes. The upper blocky lignite represents an estimated 4-12 kya after PETM onset. Raw and rarefied palynomorph species richness measures are higher in the PETM but the difference is not statistically significant. Only five (of 24) common taxa have last appearance or major shifts in percentage occurrence close to the PETM onset. One of these, a triporate eudicot, occurs only in the maximum negative CIE sample and the immediately underlying sample, the former at very high percentages, an interesting feature of PETM onset. The palynomorph composition of Late Paleocene samples is significantly different from PETM samples. In the late Paleocene there is a close association of high abundances of Cicatricosisporites (Schizaeaceae) fern spores with microscopic and mesoscopic charcoal representing a low diversity fire prone fern and woody angiosperm community. By contrast, the PETM vegetation is characterised by the loss of ferns and cessation of fires, an increase in wetland plants (including cupressaceous conifers) and a more varied flowering plant community with palms and eudicots. These palynofloras thus indicate little response in plant species across the PETM onset but a major change in vegetation composition, linked to a switch in fire regime.