Long-term orbital forcing on Lower Jurassic black shale sedimentation: Basque-Cantabrian evidence

Some periods of Earth's history were characterized by worldwide recurrent deep-sea anoxia and black shale accumulation. The Jurassic was one such period, but the reasons behind the recurrent environmental changes remain poorly known. In order to fill this gap, a cyclostratigraphic study of Pliensbachian (Lower Jurassic) hemipelagic successions of the Basque-Cantabrian Basin was undertaken, as these successions contain several black shale intervals coeval with those found in other areas. This study found that Jurassic hemipelagic sedimentation was astronomically forced. The formation of Pliensbachian black shale intervals started around every minimum of very long (~2.4 My) eccentricity cycles, as seabed anoxic conditions developed when variations in seasonality driven by precession cycles decreased and, consequently, relatively stable oceanic conditions prevailed. These conditions lasted for approximately 0.8–1.0 My, until the increasing eccentricity of Earth's orbit produced significant seasonality variations, which in turn intensified seawater circulation and enhanced seabed oxygenation. The comparison of the Basque-Cantabrian Pliensbachian cyclostratigraphy with records from other areas showed that long-term eccentricity cycles had global impact. Thus, temporary but recurrent seabed anoxia occurred at very long eccentricity minima wherever paleogeographic constraints allowed bottom water stagnation. Furthermore, the tuning of the Basque-Cantabrian cyclostratigraphic record with published orbital solutions of Earth's long-term motion is a good fit with solutions La10a and ZB20d, which are regarded as the most reliable.