Terrestrial ecosystem response to changing temperature and seasonality in the Paleocene-Eocene Thermal Maximum: Shallow marine records from the Salisbury Embayment, USA

The Paleocene-Eocene thermal maximum (PETM, ∼56 Ma) is marked by a massive and rapid rise in atmospheric CO₂ and ∼5°C of global warming. It is globally characterized by a negative carbon isotope excursion (CIE), and, at least locally, is preceded by a pre-onset excursion (POE). We present palynological and bioclimatic analyses from stratigraphically expanded marginal marine sediment sections from the eastern United States. Late Paleocene forests were dominated by needle-leaved gymnosperms and broad-leaved angiosperms characteristic of warm climates. The POE is marked by a minor expansion of angiosperms and pteridophytes, warmer winters, and altered seasonal precipitation, followed by a return to pre-POE conditions. Increased terrestrial palynomorph concentrations before the CIE are suggestive of increased fluvial discharge before the PETM. Early PETM assemblages are characterized by dominance of ferns, loss of conifers, and expansion of broad-leaved angiosperm forests. Bioclimatic analyses indicate warmer mean atmospheric temperatures in early PETM time, driven primarily by winter warming of ∼3°C. A shift in seasonality, associated with increased severity of storms and floods that scoured the late Paleocene floodplain, facilitated establishment of riparian fern communities at the CIE onset. These flooding events persisted through the early part of the PETM and were severe enough to transport Westphalian-age (Middle Pennsylvanian) reworked material from the central Appalachian Basin and flush large amounts of terrestrial material and carbon onto the continental shelf, resulting in decreased salinity, increased productivity, and water-column stratification.