Orbitally forced environmental changes during the accumulation of a Pliensbachian (Lower Jurassic) black shale in northern Iberia

Abstract. Lower Pliensbachian hemipelagic successions from the north Iberian palaeomargin are characterized by the occurrence of organic-rich calcareous rhythmites of decimetre-thick limestone and marl beds and thicker black shale intervals. Understanding the genetic mechanisms of the cyclic lithologies and involved processes along with the nature of the carbon cycle is of primary interest. The cyclostratigraphic study carried out in one of the black shales exposed in Santiurde de Reinosa (Basque-Cantabrian Basin) revealed that the calcareous rhythmites responded to periodic environmental variations in the Milankovitch-cycle band, with the prevalence of precession, short eccentricity and long eccentricity cycles.

The main environmental processes that determined the formation of the rhythmite were deduced on the basis of the integrated sedimentological, mineralogical and geochemical study of an eccentricity bundle. The formation of precession couplets was controlled by variations in carbonate production and dilution by terrigenous supplies, along with periodic changes in bottom water oxygenation. Precessional configurations with marked annual seasonality, increased terrigenous input (by rivers or wind) to marine areas and boosted organic productivity in surface waters. The great accumulation of organic matter on the seabed eventually decreased bottom waters oxygenation, which might also be influenced by reduced ocean ventilation. Thus, deposition of organic-rich marls and shales occurrred when annual seasonality was maximum. On the contrary, a reduction in terrestrial inputs at precessional configurations with minimal seasonality disminshed shallow organic productivity, added to an intensification of vertical seawater mixing, contributed to increasing the oxidation of organic matter. These conditions also favoured greater production and basinward exportation of carbonate mud in shallow marine areas, causing the formation of limy hemipelagic beds. Short eccentricity cycles modulated the amplitude of precession driven variations in terrigenous input and oxygenation of bottom seawaters. Thus, the amplitude of the contrast between successive precessional beds increased when the Earth’s orbit was elliptical and diminished when it was circular. The data also suggest that short eccentricity cycles affected short-term sea level changes, probably through orbital modulated aquifer-eustasy.