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

Abstract

Abstract. Lower Pliensbachian hemipelagic successions from the northern Iberian palaeomargin are characterized by the occurrence of organic-rich calcareous rhythmites of decimetre-thick limestone and marl beds as well as thicker black shale intervals. Understanding the genetic mechanisms of the cyclic lithologies and processes involved along with the nature of the carbon cycle is of primary interest. This cyclostratigraphic study, carried out in one of the black shale intervals exposed in Santiurde de Reinosa (Basque–Cantabrian Basin), reveals that the calcareous rhythmites responded to periodic environmental variations in the Milankovitch-cycle band and were likely driven by eccentricity-modulated precession. 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 water. The great accumulation of organic matter on the seabed eventually decreased bottom-water oxygenation, which might also be influenced by reduced ocean ventilation. Thus, deposition of organic-rich marls and shales occurred when annual seasonality was maximal. On the contrary, a reduction in terrestrial inputs at precessional configurations with minimal seasonality diminished shallow organic productivity, which, added to an intensification of vertical mixing, contributed to increasing the oxidation of organic matter. These conditions also favoured greater production and basinward export 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 seawater. 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 orbitally modulated aquifer eustasy.