Organic geochemistry, stable isotopes, and facies analysis of the Early Aptian OAE—New records from Spain (Western Tethys)

Abstract

The Early Aptian Oceanic Anoxic Event (OAE1a) is a time interval characterized by increased organic carbon accumulation in marine sediments, notable sedimentary and biotic changes, and abrupt carbon-isotope excursions indicative of significant major palaeoenvironmental changes linked to a perturbation in the global carbon cycle. Here we present the study of four sections recording the OAE1a (Early Aptian) in Spain, which are located in two broad basins respectively located in the South and the North of Iberia: the Southern Iberian Palaeomargin (Carbonero, La Frontera and Cau sections) and the Basque–Cantabrian Basin (Puentenansa section), which represent depositional settings ranging from shallow marine (distal ramp -Cau- and drowned platform -Puentenansa-) to pelagic environments (Carbonero, La Frontera). Biomarker compositions, C-isotope profiles, biostratigraphic data and facies analysis from the four sections are correlated and integrated. The C-isotope curves all present a clear negative excursion followed by a positive shift. The integration of the C-isotope curves with the biostratigraphic data has been used to correlate the studied sections and to tentatively identify the eight segments formerly proposed from the Alpine domain, and subsequently identified in sections worldwide. Four main groups of compounds are present in all sections: n-alkanes, isoprenoids, hopanes and steranes. n-Alkanes and acyclic isoprenoids (pristane and phytane) are dominant in most samples. The hopanes are represented by a range of C27 to C35 components,with the specific isomers varying amongst the sections due to differences in thermal maturity. Steranes occur as a range of C27, C28 and C29 isomers, whereas diasteranes only occur in the most thermally mature section (Carbonero). Other compounds of interest include gammacerane and dinosterane. Differences in thermal maturity appear to be the first order control on different biomarker assemblages amongst the studied sections. The Carbonero section is thermally mature, whereas the nearby La Frontera and Cau sections are immature. Puentenansa has intermediate values. Organic matter is derived from a range of terrestrial, marine and bacterial sources. The dominance of the C29 sterane isomers in all sections suggests a strong contribution from higher plants. The presence of gammacerane indicates water column stratification, and high C29/C30 hopane ratios suggest anoxia at the water/sediment interface, respectively. Sedimentologic analysis also suggests anoxic conditions during sedimentation, but evidence for strong and persistent water column anoxia is equivocal. The correlation of the sections reveals that sedimentation of organic-rich facies started earlier in pelagic and later in the shallow marine settings, which can be related to an expansion of the favorable conditions for organic matter accumulation and preservation from deep marine waters to shallower platform environments during the development of OAE1a.