EARLY EOCENE PERSPECTIVES ON MESOZOIC GREENHOUSE WORLDS

The most recent greenhouse world of the Early Eocene (56-48.6 Ma) provides climate proxy records often lacking in older sections. The Early Eocene was characterized by high atmospheric CO₂ of 1000-1500 ppm, transient CO₂ variations of 500-1000 ppm, barystatic sea-level (ice-driven) variations of 40 to 71 m (71m = ice free), and global mean geocentric sea level positions (commonly referred to as eustatic) of 100-130 m (implying tectonic component of 60 m). Comparisons of two independently derived sea-level records and the record of glacial sediments near Antarctica show that assumptions of an entirely ice-free Early Eocene are unreasonable. Thus, significant continental ice volume changes must have driven Milankovitch scale (<2.4 Myr) sea-level variations of ~25-30 m. Analysis of Pacific benthic foraminiferal δ¹⁸O records show the influence of tilt (1.2, and 41 kyr) and eccentricity (405, ~100 kyr) periods, with ~10 m barystatic sea-level variations attributable to the 1.2 Myr cycle. Analysis of Pacific benthic foraminiferal δ¹³C records shows dominance by the 405 kyr (amplitude modulator of precession, which is above Nyquist), suggesting control by precipitation-evaporation and the carbon cycle variations. North Atlantic igneous province volcanism was associated with a several Myr global δ¹³C increase and a global warming trend. The linkage between volcanism and higher δ¹³C_MDW is not likely through temperature and sea-level rise, but rather through supply of nutrients and/or weathering feedbacks that enhance C_org burial. The Early Eocene is a redux of the peak warmth intervals of the Santonian and Cenomanian/Turonian Ages with similar tectonic configurations and climate driven by high, but variable CO₂, though it likely differs from the ultimate Greenhouse world of the Triassic with a stronger influence of sea-level and ice-volume changes on climate and the carbon cycle. The Early Eocene thus provides insights into the operation of Mesozoic greenhouse worlds, with high, but variable CO₂ and with punctuations of peak warm by glaciations.