Geochemical characterization and redox signals from the latest Barremian to the earliest Aptian in a restricted marine basin: El Pui section, Organyà Basin, south-central Pyrenees

• Geochemical signals from the late Barremian–earliest Aptian in a restricted basin
• Role of basin physiography and terrigenous fluxes in nutrient-driven productivity
• High productivity and limited vertical circulation induced water column dysoxia
• Restricted basins can act as important carbon sinks.

The Organyà Basin located in the south-central Spanish Pyrenees developed in the northern part of the Iberian Peninsula mainly during a Barremian to early Albian phase, when Iberia rifted and rotated counter-clockwise away from Europe. Extension was followed by strong subsidence in the basin resulting in the development and accumulation of carbonate platforms and deeper-water hemipelagic sediments.Here we present the multiproxy results of a high-resolution lithostratigraphic and geochemical study of the lowest 85 m of the El Pui section in the Organyà Basin, Catalunya, Spain. The sequence comprises series of limestone and marlstone interbeds spanning the latest Barremian and the earliest Aptian. Our results show that intermittent dark limestone and marlstone layers associated with extremely low diversity and scarce benthic fauna, low bioturbation index (0–3) and high TOC (up to 1.7 wt.%), indicate recurrent oxygen-deficient conditions within the lowest 31 m of the section and more uniform oxygenation in the upper 54 m. SEM analyses reveal high abundance of calcareous nannofossils as the source of carbonate throughout the section. EDS analyses confirmed the presence of clastics (mainly aluminum silicates) in the matrix. Sustained high primary productivity was the result of constant supply of nutrients, especially biolimiting elements (P, Fe), from terrestrial fluxes as indicated by excellent linear correlation with Al, Si and Ti. δ13Corg values within the high TOC intervals show excursions of up to 2.0‰ suggesting important isotopic changes in the carbon reservoir. Relative high enrichment of Ni, and minor of U and Cr with respect to the average shale concurrent with high Ni/Co ratios, and punctuated high U/Th values, suggest the existence of intermittent reducing benthic conditions in these intervals. However, V is depleted, indicating that reducing conditions were weak with the absence of an anoxic/euxinic phase.In the present study we document that basin physiography along with the nature of the adjacent geologic terranes surrounding the Organyà Basin induced sustained nutrient supply, thereby enhancing constant productivity, which intensified OM respiration in the water column. Density stratification may have also temporarily exacerbated oxygen deficiency (without an anoxic phase) that enhanced OM preservation from the late Barremian to the earliest Aptian.