GEOCHEMICAL AND LITHOLOGICAL CHARACTERISTICS OF CHANGING PALEOENVIRONMENTAL CONDITIONS ASSOCIATED WITH THE END OF CARBON ISOTOPE SEGMENT C5 OF OAE 1A IN THE SOUTH-CENTRAL SPANISH PYRENEES

Here we present a high-resolution geochemical and microfacies analysis of the uppermost 85.82 m-portion of the El Pujal section that includes a continuous Aptian hemipelagic sequence corresponding to Oceanic Anoxic Event 1a (OAE 1a). The basal 24.72 m of alternating limestones, argillaceous limestones and marlstones, reveals periodic enrichments in RSTEs (V, Cr, Co, Ni, Cu, Mo, U), bio-limiting (P, Fe), major elements (Al, Si, Ti), and higher proportion of terrestrially derived OM in marlstone beds, concurrent with higher total organic carbon (TOC). Concomitance of these various parameters implies short-lived, intermittent pulses of fluvial discharge leading to water column stratification and episodic dysoxic bottom waters. The succeeding 57.17 m of marlstones show relatively higher values for TOC, RSTEs, P, Fe, Al, Si, and Ti, indicative of an increase in runoff. The uppermost 3.93 m of hard limestones documents a sharp change in lithology with impoverished TOC, Al, Si, Ti, P and Fe and lower relative proportion of short to long chain n-alkanes, indicating a reduction in fluvial fluxes and surface water fertility likely linked to dryer climatic conditions.

Thermal maturity indices from steranes (C₂₉ 20S/(20S+20R x̄ = 0.52, C₂₉ ββ/(ββ+αα) x̄ = 0.62 and hopanes (C₃₁ 22S/(22S+22R) x̄ = 0.55) suggest that the sediments are thermally immature and thus the preserved organic geochemical data characterizes the original signal. Carbon isotope (δ¹³C_org) results show relatively small variability (-25.31‰ to -23.91‰, x̄ = -24.27‰), but a pronounced negative shift to -25.31‰ (1.04 ‰) in the uppermost 1.4 m is consistent with the pattern reported elsewhere for the closure of segment C5 within OAE1a, which is further corroborated by the sparse presence of Leupoldina cabri, whose Zone parallels that global event. Simultaneous changes in microfacies showing coarser grain packstones, lower planktonic foraminifera % and increase in benthic taxa, in the uppermost hard limestones imply a shallowing of the basin similar to that reported for the Basque-Cantabrian Basin synchronous to the negative δ¹³C_org excursion heralding the end of segment C5.