EQUABLE OR NOT? INVESTIGATING THE INFLUENCE OF PROXY BIASES IN RECONSTRUCTIONS OF GREENHOUSE PERIOD TEMPERATURE SEASONALITY

Greenhouse periods such as the Late Cretaceous and Paleogene have long been described as “equable”, referring both to their reduced latitudinal temperature gradients and supposedly muted mean annual range in temperature (MART) relative to modern climate conditions. Prior to the 2010s, the majority of quantitative MART estimates for these greenhouse periods were based on leaf physiognomy methods such as leaf margin analysis and CLAMP, which suggested that even at high latitudes (e.g., the north slope of Alaska) and in continental interiors (e.g., central Siberia), the difference between winter and summer temperatures during the Late Cretaceous and early Cenozoic were typically <20 °C, compared to >60 °C in the modern. This view of greenhouse climate equability has been challenged by new MART reconstructions based on paleosol (fossil soil) carbonate clumped isotope paleothermometry. Clumped isotope data from both the Late Cretaceous and Paleogene suggest that greenhouse period MARTs were similar in magnitude to the modern, conflicting with leaf physiognomy data. To resolve this apparent discrepancy, we apply a spatial autoregressive model to a new dataset of modern North American climate (temperature and precipitation), geographic, and land cover data, and show that statistically significant differences in MART exist between the modern land cover types associated with fossil leaf assemblages (e.g., wetlands and broadleaf forests) and carbonate-bearing soils (e.g., grass- and shrub-lands). In the modern, MART is highly variably across landscapes, and we suggest that rather than being in disagreement, leaf physiognomy- and paleosol carbonate-based reconstructions of MARTs are accurately capturing real MART variability across Late Cretaceous and Paleogene landscapes.