Orbital (Hydro)Climate Variability in the Ice-Free Early Eocene Arctic.

Early Eocene (∼56-48 Ma) climates are useful to investigate polar climate dynamics in the absence of ice. We explore early Eocene orbital variability of Arctic climate using sediments recovered by the Arctic Coring Expedition (ACEX). 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 cyclic signals related to ∼20-kyr precession, ∼40-kyr obliquity, and ∼100-kyr eccentricity. Biomarkers indicate obliquity and precession variability representative of sea surface temperature (SST) variations up to ∼1.4 and ∼0.5°C, respectively. Peak SSTs coincide with an elevated supply of pollen and spores and increased marine productivity. 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, 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 a seasonal GDGT bias. The reconstructed complete orbital imprint refutes a bias to one end of the orbital variability. Eccentricity-related SST variability was ∼0.8°C, ∼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 and that this had pronounced polar amplification, despite the absence of ice-albedo feedbacks.