Immediate and delayed biotic and environmental responses to the carbon cycle perturbation at the onset of OAE 1a – Insights from a new high-resolution record from the Western Tethys.

The onset of the early Aptian Oceanic Anoxic Event (OAE 1a) was an interval of rapid global environmental change, linked to a major perturbation of the carbon cycle. A complex response of the biotic and environmental system occurred, ultimately triggered by volcanism of the Ontong-Java Plateau. Here we present a new high-resolution study of the very onset of OAE 1a (ca. 80 kyr), obtained through integration of C-isotope stratigraphy, total organic carbon (TOC) contents, elemental geochemistry and biomarkers, in order to examine the nature and timing of changes in the carbon cycle, biota and the environmental system. This work provides new evidence for an early response of the biotic and environmental system, which predates the more generalized development of oceanic anoxia and widespread increase in primary productivity. The early response took place as soon as few kyrs after the initiation of the carbon cycle perturbation, whereas the major change occurred some 29 kyr later. A probable injection of methane postdates the onset of OAE 1a by 6 kyr, likely triggered by early stages of volcanism and/or sill intrusions into organic-rich sediments. These results provide new insights into the analysis of the onset of OAE 1a, and indicates that future research at higher resolution is required in order to better understand the complex interplay of Earth system processes in response to increases in pCO2.