22-7 Hydroclimate, ecosystem and wildfire change across the Eocene–Oligocene Transition on the western North Atlantic margin: a multiproxy organic geochemical record from Bass River, New Jersey

Abstract:

The Eocene–Oligocene Transition (EOT, ~34 Ma) marks Earth's transition from greenhouse to icehouse, yet terrestrial records integrating continental hydroclimate, vegetation and fire remain poorly constrained along the western North Atlantic margin. Here we present coupled leaf-wax hydrogen isotope (δ²H_n-C₂₉), vapor pressure deficit (VPD) and polycyclic aromatic hydrocarbon (PAH; fire-markers) records reconstructing hydroclimate, ecosystem and wildfire history across the EOT from the well-dated Bass River borehole, New Jersey (ODP Leg 174AX).

δ²H_n-C₂₉ varies in three phases an ~45–50‰ ²H-enrichment preceding the transition, an ~25‰ ²H-depletion coinciding with the EOT, and a ~50‰ enrichment rebound into the early Oligocene. The initial enrichment precedes the benthic δ¹⁸O shift and may reflect altered isotope hydrology under high CO₂ and temperatures combined with ecosystem reorganization affecting apparent leaf-wax–water fractionation. The subsequent depletion is consistent with cooling-driven changes in precipitation δ²H and clumped-isotope estimates of 4–6 °C cooling on the conjugate North Atlantic margin. However, sea surface temperature records from the Newfoundland margin and southern Labrador Sea indicate little resolvable cooling, so altered meridional vapor transport associated with North Atlantic circulation reorganization cannot be excluded. Molecular-inferred VPD doubles from 0.26 to 0.58 kPa before relaxing in the early Oligocene, indicating drying. Because rising VPD should enrich leaf waxes through evaporative fractionation, the underlying change in precipitation δ²H likely exceeds the measured signal. Retene, a conifer-derived biomarker, is an order of magnitude more abundant in the late Eocene than the early Oligocene and declines through the transition, indicating progressive loss of conifer and/or woody biomass, while hinterland pollen records establishment of Quercus–Carya mesophytic forest by the early Oligocene, suggesting a shift from conifer-rich to angiosperm-dominated ecosystems. The δ²H rebound coincides with this vegetation turnover and may reflect changing meridional vapor transport in the cooler early Oligocene and a shift in apparent fractionation associated with changing ecosystem type affecting the ²H-enrichment. PAHs decline by an order of magnitude across this transition despite cooler and drier Atlantic margin conditions and a persistent closed mesophytic forest, suggesting that sustained burning required a critical dryness threshold.

We suggest that wildfire at temperate continental margins is influenced more by ecosystem changes rather than fire weather, and that δ²H_n-C₂₉ integrates ecological as well as hydrological change across the EOT.

© Copyright 2026 The Geological Society of America (GSA), all rights reserved.