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 (OAEla) 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 OAEl a (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 fades analysis from the four sections are correlated and integrat ed. 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 ten tatively 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 ofC27 to C35 components, with the specific isomers varying amongst the sections due to differences in thermal maturity. Steranes ocrur 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 thennal 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 de rived 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 col umn stratification, and high C2g/C30 hopane ratios suggest anoxia at the water/sediment interface, respectively.Sedimentologic analysis also suggests anoxic conditions during sedimentation, but evidence for strong and per sistent water column anoxia is equivocaL The correlation of the sections reveals that sedimentation of organic-rich fades 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 ofOAEla.

Environmental conditions in the South Atlantic (Angola Basin) during the Early Cretaceous

“…120 Ma), organic-rich black shales were deposited in all major ocean basins (e.g. Sliter, 1989;Bralower et al, 1994;Erba et al, 1999;Ando et al, 2002;Takashima et al, 2004;Dumitrescu and Brassell, 2005;Quijano et al, 2012). Although we had rather coarse sample resolution, the compound specific d 13 C values also do not depict the large negative d 13 C shift that is characteristic of OAE 1a (Menegatti et al, 1998;Erba et al, 1999) and recorded as a ca.…”

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

“…However it has been argued that in overmature OM the original n-alkane composition can be altered leading to conflictive results (Quijano et al, 2012). Since Bernaus et al (2003) reported thermally mature OM for samples studied to the east of the El Pui section in the Organyà Basin;…”

Palaeoenvironmental conditions for the preservation of organic matter during the late Hauterivian in the Neuquén Basin (Western Argentina)

“…This means that fluctuations in the percentage of abundances of this taxon probably do not represent a direct response to environmental changes, but may reflect a measure of the success of other species (Aguado et al, 2014) High abundances of Micrantholithus are recorded at the base of the section. This group was considered as a marginal or neritic taxon and its abundance has been related to neritic factors such as reduced salinity (Roth and Bowdler, 1981;Street and Bown, 2000;Bersezio et al, 2002;Bown, 2005;Tremolada et al, 2009;Quijano et al, 2012).…”