Climate cyclicity-controlled recurrent bottom-water ventilation events in the aftermath of the Toarcian Oceanic Anoxic Event: the Jenkyns Event

Abstract

Environmental perturbations of the Toarcian Anoxic Event and its associated carbon isotope excursion (CIE) occurred in a cyclic fashion indicating an orbital control mechanism. Sedimentary strata of the E. elegantulum ammonite subzone in the Lorraine Sub-basin, Luxembourg, exhibit eight sedimentary cycles, most of which postdate the CIE, implying that its termination did not coincide with a full recovery from environmental stress. Sea-level and temperature fluctuations of the Toarcian crisis were linked to a cryosphere demise in the Northern Hemisphere, which modulated stadial versus interstadial phases on the orbital 100 ka eccentricity frequency band. Upon stadial phases, enhanced wind strength in combination with lowered sea level disrupted stratified shelf waters and shifted the storm wave base close to the sea floor. Ventilation of bottom waters interrupted accumulation of laminated and organic-rich black shales, but formed organic-lean and non-laminated gray claystones enriched in terrigenous wax lipids and spores provided via aeolian transport from the hinterland due to the enhanced wind strength. Wind systems are assumed to have been driven by high pressure differences between the cryosphere in the North of the Tethyan shelf and the warmer costal lowland. This distinguishes the atmospheric turbulences after the CIE from the postulated intensification of the cyclones that accompanied the rapid warming at the beginning of the CIE. The deep-water ventilation event following the CIE in the Lorraine Sub-basin was accompanied by a parallel evolution in the SW German Basin, indicating a supraregional driving mechanism in paleobathymetric sub-basins susceptible to lowering of storm wave base. In sub-basins with greater water depth on the NW-Tethyan shelf, post-CIE orbitally driven atmospheric turbulences had a minor or no effect on water column stratification and deposition of organic-rich laminated black shales persisted.

Graphical abstract