PELAGIC DYNAMICS IN A STABLE, WARM CLIMATE STATE: INSIGHTS FROM THE MIDDLE EOCENE

Extreme climate events of the Cenozoic (e.g. PETM, EECO, E/O-boundary) have been linked to extinctions, radiations, biotic turnovers, and abundance changes in marine and terrestrial settings. However, relatively little effort has gone into characterizing short-and long-term ecosystem variability during the in-between times making it surprisingly difficult to say how unusual these changes are relative to background ecosystem dynamics. As such, resolved baselines of ecosystem dynamics are critically needed, particularly in stable (and warm) climate states of the Early Cenozoic and in deeper time intervals.

Here we present high resolution biogenic opal and carbonate accumulation records documenting pelagic ecosystem dynamics during a relative stable 2 Myr interval of the middle Eocene (~42-44 Ma; North Atlantic IODP Site U1408; 41°26.30’N 49°47.15’W, ~3022 m; 32°N paleolatitude). Two primary factors affect each record: the standing population size and flux of biomineralizing taxa to the deep sea (silica: primarily radiolarians and diatoms; carbonate: primarily nannoplankton and planktic foraminifera) and sedimentary preservation potential. In our records, biogenic carbonate and opal accumulation rates are tracking obliquity-dominated cyclicity as expressed in the complementary high-resolution (~2 kyr) benthic and bulk stable isotope (δ¹³C/δ¹⁸O) records, but are anti-correlated. Biogenic opal variability is dampened as compared to the carbonate record, suggesting a stronger response of marine calcifiers to the regional expression of orbital forcing.

On the long-term, we observe an increase in the calcifier/silicifier ratio in relation to long-term bottom water cooling (~0.6 ‰ δ¹⁸O-equivalent) leading towards the Carbonate Accumulation Event 3 (CAE-3). Similar long-term patterns in biogenic sedimentation are observed in published opal-dominated Equatorial Pacific records suggesting a certain degree of homogeneity in global pelagic dynamics during background conditions, in contrast to the geographic heterogeneity of ecological change often observed during extreme events.