The Madotz Urgonian platform (Aralar, northern Spain): Paleoecological changes in response to Early Aptian global environmental events

Abstract

Sudden addition of carbon dioxide to the atmosphere can reduce the CaCO3 saturation and weaken the biocalcification potential of marine organisms in shallow water and in open marine settings. In this study, the response of an Aptian neritic carbonate environment to sudden addition of carbon dioxide at the beginning of Oceanic Anoxic Event 1a is investigated. The beginning of the OAE1a was coupled with a major perturbation on the carbon cycle as indicated by a negative carbon isotope excursion in the sedimentary record. This isotope anomaly is regarded as a proxy for massive addition of volcanic or methane-derived CO2 to the atmosphere within only a few 104years. The impact of a rapid change in atmospheric pCO2 on biocalcifiers in low latitude shallow-water settings can be studied in a well preserved Aptian carbonate shelf succession cropping out today in the Aralar mountains (NE Spain). The Madotz section (N Spain) preserves a continuous shallow water record that was deposited on a mid-latitude, Atlantic-oriented mixed siliciclastic–carbonate ramp. Lower Aptian sediments consist of two neritic limestone successions separated by orbitolinid-rich marlstone enriched in organic matter. The lower neritic limestone succession ends with a submarine hardground and the transition from the lower neritic limestone to the orbitolinid marlstone coincides with a negative spike in the organic carbon isotope record. This negative spike can be correlated with the negative carbon isotope anomaly marking the base of OAE1a. The paleoecological change coinciding with the base of OAE1a occurred at a time of sea level rise and it coincided with a demise of heavily calcified nannoconids in the Tethys and Pacific Oceans. The paleoecological change observed in the Madotz section corresponds to a comparable change seen in the more distal and more expanded carbonate ramp section (Igaratza) at the Aralar Mountains. Ocean acidification caused by sudden increase in pCO2 may explain reduced calcification potential of some shallow water calcifiers. Calcification crisis was amplified by rising sea level, increasing temperatures and increased flux of detrital material and nutrients from continents into coastal seas.