A major sea-level drop briefly precedes the Toarcian oceanic anoxic event: implication for Early Jurassic climate and carbon cycle.

Sea-level change is an important parameter controlling the expansion of oxygen-depleted conditions in neritic settings during oceanic anoxic events (OAEs). Despite this fundamental role, it remains on a short timescale (<1 Myr) one of the least constrained parameters for numerous OAEs. Here we present sedimentological and geochemical evidence from Morocco and East Greenland showing that a forced regression shortly precedes (ca.102 kyr) the major transgression associated with the Toarcian OAE. The forced regression can be correlated over distances greater than 3000 km in numerous Tethyan and Boreal basins, indicating that the relative sea-level change was driven by eustastic fluctuations. The major amplitude (>50 m) and short duration of the forced regression suggests that it was most likely related to the transient waxing and waning of polar ice sheet. We suggest that this short-lived glaciation might have a genetic link with the inception of the Toarcian OAE. Indeed, during the deglaciation and the accompanying sea-level rise, the thawing permafrost may have released important quantities of methane into the atmosphere that would have contributed to the Toarcian OAE rapid warming and its characteristic negative carbon isotope excursion. This study offers a hypothesis on how some hyperthermal events might be rooted in short-lived "cold-snap" episodes.