Anne H. Kegel

and 8 more

A series of transient global warming events (“hyperthermals”) in the early Eocene is marked by massive environmental and carbon cycle change. Among these events, the impacts of the Paleocene Eocene Thermal Maximum (~56 Ma), Eocene Thermal Maximum 2 (~54 Ma) and Eocene Thermal Maximum 3 (~53 Ma) are relatively well documented, but much less is known on the many later hyperthermals that apparently occurred on orbital eccentricity maxima until at least the end of the Early Eocene Climatic Optimum (EECO; ~53­–49 Ma). Here, at Ocean Drilling Program (ODP) Site 959 (Equatorial Atlantic Ocean), we report a large negative carbon isotope excursion (CIE) in both organic and carbonate substrates that we correlate to the “V” event sensu Lauretano et al. (2016) (or C22nH1 sensu Sexton et al. (2011)) at ~49.7 Ma, following combined bio- and chemostratigraphic constraints. Through TEX86 paleothermometry, we reconstruct a sea surface temperature rise of 1.1–1.9 ºC associated with this CIE, which, combined with evidence for warming from the deep sea, implies that this event indeed represents a transient global warming episode like the earlier hyperthermals. Organic walled dinoflagellate cyst assemblages indicate a productive paleoceanographic background setting, likely through regional upwelling, which alternated with episodes of stratification. Warming reconstructed across V at Site 959 is relatively similar to the higher-latitude-derived deep ocean reconstructions. However, the presence of upwelling and its variable intensity across the event compromises the use of the reconstructed warming as an estimate for the complete tropical band.
We explore the imprint of orbital variability on Arctic temperature and hydrology using sediments recovered during the Arctic Coring Expedition in 2004. High resolution records of lipid biomarkers (GDGTs; 2-kyr) and palynological assemblages (5-kyr) in the ~4 m interval below Eocene Thermal Maximum 2 (~54 Ma) show highly cyclic signals related to ~20-kyr precession, ~40-kyr obliquity and ~100-kyr eccentricity. The GDGTs indicate obliquity and precession variability representative of sea surface temperature (SST) variations up to ~1.4 and ~0.5 ºC, respectively. Peak SSTs coincide with an elevated supply of pollen and spores and increased marine productivity. Together, this implies an orbital control on precipitation and terrestrial nutrient supply to the Arctic Basin. Assuming that SST maxima correspond to Arctic insolation maxima (precession minima/obliquity maxima), precipitation maxima also correspond to insolation maxima, implying regional hydrological processes as a forcing rather than variations in meridional water transport, starkly contrasting Pleistocene Arctic hydrology. The relative amplitudes of precession and obliquity in the SST record match that of local insolation between spring and fall, corroborating previous suggestions of a seasonal GDGT bias. The reconstructed complete orbital imprint refutes that ACEX temperature reconstructions are biased to one end of the orbital variability. Eccentricity-related SST variability was ~0.8 ºC, ~2–3 times higher than synchronous variability in the deep ocean, and 3–4 times higher than similar variations in the tropics. This confirms eccentricity-forced global temperature variability during the Eocene, and that this had pronounced polar amplification, despite the absence of ice and snow albedo feedbacks.