CONTINUOUS EPISODIC DYSOXIA RELATED TO OAE1A, ORGANYÀ BASIN, EL PUJAL SECTION, CATALUNYA, NE SPAIN

The El Pujal section is a continuation of the Lower Cretaceous series recorded in the El Pui section [1]. In this study we present a high resolution, multiproxy study integrating total inorganic carbon (TIC), total organic carbon (TOC), petrographic, carbon isotope (δ¹³C_org), redox sensitive trace elements (RSTEs), major elements, and biomarkers (n-alkanes), covering a total of 13.77m. The interval between 4.38m and 6.82m shows a stark enrichment in RSTEs (V, Ni, Co, U, Cr, Cu, Mo), increasing by >90% from background average levels, correlative with enhanced preservation of organic matter (OM) with TOC values >1.68 wt%, an increase of 49% from average values of 1.17%, which suggest that the basin continued to experience fluctuating oxygenic conditions with this episode of dysoxia that reaches a peak at ~5.52m. Fe, P, Al, Si, follow an almost identical trend to the RSTEs, with a sharp increase of 57% from background values along the same stratigraphic interval. Calcareous nannos continue to be the major source of CaCO₃ to the sediments with high average TIC (72.28%), decreasing to a low of 62.03% during the dysoxic phase. N-alkanes results reveal a bimodal distribution with predominance of short-chain length homologues (≤ nC₁₉) derived mainly from protists and bacteria, and by longer chain (>nC₂₅) land derived vegetation, with some contribution from macrophytes (nC₂₀ - nC₂₅), thus suggesting a mixed source of OM. The mean values of 0.21 for the terrestrial/aquatic ratios TARs (nC₂₇+nC₂₉+nC₃₁)/(nC₁₅+nC₁₇+nC₁₉) clearly show the high input of terrestrial OM. However, TAR values in the dysoxic interval show an inverse trend relative to RSTEs, TOC and major elements, with decreasing values relative to background levels. This trend might be attributed to a dilution effect due to the higher input of land derived, biolimiting nutrients (P, Fe) into the basin, fueling primary production and increasing the input of OM from shorter chain n-alkanes (≤ nC₁₉), hence decreasing the TAR values during the dysoxic phase. Lower TIC values (<65%) give rise to more marly limestones in which benthic foraminifera and echinoid fragment abundances follow an inverse pattern to that of RSTEs, major elements and TOC, thus further corroborating the presence of a dysoxic phase during this interval.

[1] Sanchez-Hernandez & Maurrasse (2016) Palaeo3 441.