Astronomical timescale for the early Toarcian (Early Jurassic) Posidonia Shale and global environmental changes

Abstract

Lower Toarcian sediments (Early Jurassic, c. 183 Ma) record the Toarcian Oceanic Anoxic Event (T-OAE), which represents a period of profound environmental changes. The event involved the widespread deposition of organic carbon-rich strata in oxygen-depleted paleo-shelf settings, global warming, and is characterized by a negative Toarcian carbon-isotope excursion (T-CIE). The T-CIE provides a global time-correlative chemostratigraphic marker; however, its age, duration and underlying causes remain uncertain in the absence of an accurate early Toarcian timescale. This article presents a cyclostratigraphic framework for the lower Toarcian Posidonia Shale Formation from the Dotternhausen Quarry (South German Basin), one of the best-studied sediment archives that record early Toarcian environmental and ecosystem changes. The Posidonia Shale in the study site was deposited in the deeper part of the epicontinental shelf sea, where sediment accumulation rates were relatively uniform and major hiatuses unlikely. These factors are rarely found in most sections, and favor obtaining accurate cyclostratigraphic results at the study site. Our astronomical timescale indicates that black shale deposition at the site lasted about ∼3.2 Myr, a remarkably long period for sufficient primary productivity in stagnant bottom-water conditions. The estimated durations of the tenuicostatum and falciferum ammonite zones, and the T-CIE are ∼0.9, ∼1.8 and ∼ 1.2 Myr, respectively. Early Toarcian global warming, accompanying the T-CIE, occurred over a period of ∼400 kyr. Early Toarcian 3rd-order sea-level cycles correlate to 1.2 to 1.6 Myr orbital cycles, reflecting long-term obliquity and eccentricity periods. Orbitally-forced 3rd-order sea-level cycles, in combination with 405 kyr long-eccentricity, and potentially 100 kyr short-eccentricity cycles, controlled sea-floor oxygenation and deposition of black shales. Short-term environmental changes (e.g., redox cycles), responding to long- and short-eccentricity cycles were most pronounced during 3rd-order sea-level lowstands, when the depositional environment was most susceptible to short-term environmental instabilities. Our results highlight orbital-forcing as a key driving mechanism for early Toarcian environmental conditions.