Intensified hydrologic cycle and increased marine productivity during Oceanic Anoxic Event 2

The late Cretaceous (~94 Ma) Oceanic Anoxic Event 2 (OAE2) represents one of the most pronounced OAEs and ranks among the largest global carbon cycle perturbations in the Phanerozoic. An extended succession of strata spanning OAE2 and the Cenomanian-Turonian Boundary, was recovered from the Bass River borehole, New Jersey (Ocean Drilling Program Leg 174AX). The OAE2 interval at this site, identified by biostratigraphy and a ~2.5‰ positive shift in the δ13C of organic carbon and foraminifer calcite, comprises a 15 m thick, dark gray laminated, fossiliferous silty clay section, deposited at relatively shallow paleodepth. We employed biomarker analyses and dinoflagellate cyst (dinocyst) micropaleontology to assess temperature, environmental, and ecosystem changes across the event.

TEX86H sea surface temperature (SST) reconstructions show a relatively rapid (~30kyr) 2.5ºC pre-OAE2 warming starting from background values of ~34 ºC. During the onset of OAE2, SSTs remain stable at ~36,5ºC. Two distinct cooling events are recorded within OAE2, of ~3ºC and ~1ºC. These cooling pulses were recorded previously and attributed to decreasing atmospheric CO2-levels as a result of enhanced global carbon burial. Towards the termination of OAE2 SSTs rose to ~37ºC, while they gradually decrease towards pre-OAE2 values after the event. Pre-OAE2 dinocyst assemblages are dominated by typical open marine species (e.g. Spiniferites spp.). Dinocyst assemblages change during the onset of OAE2 and become dominated by Paleohystrichophora and Senegalinium, suggesting intensified continental run off, leading to water column stratification, and increased marine productivity. The runoff indicating species decrease in abundance during the two cooling events. Furthermore, the cooler episodes are marked by the influx of high latitude species (e.g. those belonging to the Cyclonephelium compactum-membraniphorum complex). Additionally, saccate gymnosperm pollen, presumably transported by wind, are abundant during these cooler episodes, suggesting drier conditions with less runoff. After OAE2, abundances of fresh water tolerant dinocyst species diminish, absolute abundances of dinocysts sharply decrease and assemblages point to conditions similar to before OAE2. The combined results, including inorganic geochemical evidence, strongly suggests an intensified hydrological cycle during OAE2.