Palaeoceanographic and biotic response during early Eocene extreme global warming events. Geologica Ultraiectina (328)

Abstract

Studying past intervals of abrupt global warming and massive carbon release can improve our knowledge in ways relevant to understanding future climate change. Possible paleo-analogues for future climate change are the early Paleogene hyperthermal events, such as the Paleocene-Eocene Thermal Maximum (PETM; ~56 Ma), during which large amounts of carbon were released. More recently, another distinct period of global warming was discovered, similar in nature to the PETM, occurring approximately two million years after the PETM, named Eocene Thermal Maximum 2 (ETM2; ~53.7 Ma). While the PETM has been extensively studied, documentation of ETM2 is insufficient to unravel whether they share similar characteristics and origin. Sediments drilled during Ocean Drilling Program (ODP) Leg 208 on the Walvis Ridge in the subtropical Southeastern Atlantic, were used to reconstruct changes in the lysocline, sea surface- and deep-water temperatures and the oceanic carbon pool associated with ETM2. In addition, comparison of ETM2 with the PETM will give insight in the relationship between massive carbon input and global environmental change at different scales and boundary conditions. The present study shows that within the Elmo horizon the calcium carbonate content dropped till ~40%, indicating that ~96% of the calcium carbonate flux dissolved. This is caused by a massive and rapid injection of CO2 into the ocean and atmosphere near the onset of ETM2, causing a decrease in seawater pH and [CO32-], shoaling of the lysocline and calcite compensation depth (CCD). Comparison of high-resolution carbon and oxygen stable isotope records derived from benthic foraminiferal tests across ETM2 and the successive short-term carbon isotopic perturbation of H2 with published data for the PETM obtained from the same deep-sea cores shows that the magnitudes of the ?13C and ?18O excursions of ETM2 and H2 are significantly smaller than those during the PETM. However, their coherent relation indicates that the ?13C change of the exogenic carbon pool was similarly related to warming during these events, despite the much more gradual and transitioned onset of ETM2 and H2. Furthermore, the planktic stable isotope results are compared with those of multiple specimen benthic foraminiferal records and show largely the same pattern and magnitude, indicating that the isotopically depleted carbon injected into the ocean-atmosphere system was quickly transferred to the deep-sea. The PETM might actually comprises two short-eccentricity cycles (i.e., two CIE’s), comparable to the succession of ETM2 and H2, as shown by the longer option (~255 kyr) of a newly derived duration of the PETM for Walvis Ridge sites. Finally, the impact of a massive injection of carbon during ETM2 on the preservation and biotic response of calcifying organisms (benthic foraminifera and calcareous nannoplankton) and the vertical carbon isotope gradients in terms of the efficiency of the biological pump and seasonal variations are discussed. Studying ETM2 can assist in evaluating future climate change scenarios and specifically into the coupling of global climate and the carbon cycle as possible forcing factors of changes in marine chemistry, marine productivity and the efficiency of the biological pump.