GEOCHEMICAL AND ORGANIC MATTER CHARACTERIZATION OF LOWER APTIAN SEDIMENTS WITHIN THE LA FRONTERA SECTION, SUBBETIC BASIN IN RESPONSE TO GLOBAL EVENT: OAE 1A

The hallmark of the Early Aptian sediments is commonly organic-rich black shales related to Oceanic Anoxic Event 1a (OAE 1a). This event reveals a strong perturbation in the global carbon cycle associated with a lack of circulation, increased organic matter preservation, and a high sedimentation rate. Previous biostratigraphic and carbon isotopic studies identified OAE 1a in the La Frontera section of the Subbetic basin in Southern Spain, and the present study revisits ~28 m of the upper Barremian-Early Aptian succession to further assess the geochemical characteristics that show how the Subbetic Basin responded to global forcing factors associated with OAE 1a. Analyses of sixty-five (65) samples from the section include TIC, TOC, major elements (Al, Si, & Ti), biolimiting elements (Fe & P), redox sensitive trace elements (RSTEs; V, Cr, Ni, Cu, Mo, & U), bulk mineralogy and biomarker data.

The lowermost 13.16 m of the studied section consist of pelagic limestones (>65% TIC), with few marly limestones (60-65% TIC) and marlstones (30-60% TIC). This is overlain by 5 m of mostly black shales (0-10% TIC) that coincide with OAE 1a. The uppermost 10 m of the succession comprises generally marlstones and calcareous shales (10-30% TIC). TOC varies between 0.02 % to a 3.4%, with the highest values corresponding with the black shale interval. Major elements inversely correlate with TIC and show the highest values occurring during OAE 1a. The black shale interval also yields increased quartz, feldspar, and clay content, suggesting increased terrigenous fluxes during that time. RSTE’s show increased values within the black shale interval, however these values correlate strongly with the major elements, thus indicating a common provenance, which implies that the basin most likely did not experience strong reducing conditions. Biolimiting elements (P & Fe) do not show any indication of enhanced primary production as they share a poor correlation with TOC (r= -0.10 and 0.15, respectively). Lipid biomarkers extracted from 25 samples showed n-alkane distributions from nC₁₂ to nC₂₅. Long chain n-alkanes (nC₂₁ to nC₂₅), indicative of terrestrial organic matter, occur only in the sediments within OAE 1a, in agreement with increased terrestrial fluxes as suggested by the major elements and the bulk mineralogy.