The influence of regional factors in the expression of oceanic anoxic event 1a (OAE1a) in the semi-restricted Organyà Basin, south-central Pyrenees, Spain

Abstract

The global occurrence of oxygen-deprived conditions in the ocean characterizes the early Aptian; hence the hallmark of that time interval is the accumulation of organic-rich sediments during an episode of severe oxygen deficiency known as oceanic anoxic event 1a (OAE1a). As seen elsewhere in the Tethyan domain, an overall change in facies marked by increased input of terrigenous material distinguishes the lower Aptian in the study area, but the nature of the lithologies associated with the organic carbon-rich deposits remain dominated by carbonate sediments and are expressed differently with respect to other areas where the occurrence of black shales characterizes OAE1a. This comparison indicates that significant spatial heterogeneity occurred in sub-Tethyan basins coeval with the global oceanic event.The present work provides a high-resolution, multi-proxy study (petrography, total inorganic carbon (TIC), total organic carbon TOC, carbon isotope [δ13Corg], clay mineralogy and biomarkers) of the upper 155m of the El Pui section, Organyà Basin, including the record of OAE1a. The results reveal that oxygenic conditions fluctuated throughout the studied interval, with episodes of oxygen depletion that concurred with TOC values >1%. However, petrographic observations, molecular biomarkers, and redox sensitive trace elements (RSTEs) indicate that full anoxia was not achieved.The results also provide the first high-resolution δ13Corg profile for the El Pui section that replicates in greater detail the carbon isotopic signature reported for the Tethyan domain. The upper 155m of the El Pui section include expanded and well-defined trends of the carbon isotope segments C2 (upper part), C3, C4, C5, and C6 (partial), which correlate with other Tethyan sections and allow for the recognition of the lower Aptian OAE1a in the uppermost 40m of the section. Such improved high-resolution chemostratigraphic curve could contribute to a more precise chronostratigraphic correlation of Aptian events in a broader scale.