Lithological and Geochemical Responses to Abrupt Global and Regional Paleoenvironmental Changes During the Aptian in a Hemipelagic Setting of the Eastern Iberian Peninsula: A Multi-Proxy Approach

Abstract

Intense episodes of environmental perturbations and regionally to globally distributed, oxygen-deprived marine facies characterize the Cretaceous sedimentary record. The Organyà Basin in the Spanish Pyrenees chronicles this period in expanded stratigraphic sequences that enabled high-resolution sampling and detailed analysis of disturbances poorly recorded in more lithologically condensed sections. Here, I present an integrated multi-proxy study aimed at understanding the Basin’s response to changing paleoenvironmental conditions during the early Aptian stage of the Cretaceous. Results from the El Pui section indicate that large-scale (> 1‰), negative carbon isotope excursions (CIEs) that show no corresponding shifts in local sources of organic matter (OM) and are simultaneously recorded at coeval localities result from the increased input of isotopically light carbon into the oceanic/atmospheric system, altering the isotopic ratio (δ13C) of the global carbon reservoir. The El Pujal section, in continuation with the series previously recorded at El Pui, registers a δ13Corg curve that is consistent with the pattern reported for carbon isotope segment C5 within Oceanic Anoxic Event 1a (OAE1a). Pulses of fluvial fluxes associated with climatic fluctuations explain the alternating lithology, with limestones representing the least terrestrially influenced end-member and marlstones representing episodes of highest terrigenous influence. Primary production sustained by fluvial inputs of biolimiting elements provided abundant labile OM conducive to oxygen-deprived conditions, as attested by concomitant peaks in total organic carbon (TOC), relatively lower benthic faunal counts, lower bioturbation index, higher concentration of pyrite and peaks in redox-sensitive trace elements. However, OM preservation was not chiefly controlled by redox conditions as physical encapsulation by clay minerals and coincident inputs of terrestrial OM with more recalcitrant properties likely played a significant role. A demise of previous conditions is registered in the uppermost portion with hard limestone low in TOC, and major elements and with lower ratios of autochthonous to allochthonous OM, indicating a reduction in fluvial fluxes and surface-water fertility suggesting dryer climatic conditions. Changes in microfacies, with coarser grain packstones, fewer planktonic foraminifera and an increase in benthic taxa, imply a shallowing of the basin, similar to conditions previously reported in the neighboring Basque-Cantabrian Basin at the end of segment C5.