PALEOENVIRONMENTAL RESPONSE RECORDED IN LOWER APTIAN SEDIMENTS CORRELATIVE WITH CHANGES RELATED TO OAE1A IN THE SUBBETIC BASIN, SOUTHERN SPAIN

Oceanic anoxic event 1a (OAE1a) has been recorded globally in Aptian sediments and is usually associated with organic-rich black shales and marked by a strong perturbation in the global carbon cycle. Biostratigraphic studies and carbon isotope data have identified OAE 1a in the La Frontera section, which is part of the Subbetic basin, located in Southern Spain. This study reexamines ~28 m of upper Barremian-Early Aptian succession the La Frontera section in order to better understand the response of the basin to OAE 1a. 65 samples were collected for further high-resolution geochemical analyses, which include TIC, TOC, major elements, biolimiting elements, redox sensitive trace elements, and clay mineralogy to complement previous works. Total inorganic carbon (TIC) in the section fluctuates between 0 and 89.6%.The lower 13 m are made up of mostly limestones (>65% TIC), with few marly limestones (60-65% TIC) and marlstones (30-60% TIC). The overlying 5 m consist mostly of black shales (0-10% TIC) and corresponds with OAE 1a. The uppermost 10 m of the succession are comprised generally of marlstones and calcareous shales (10-30% TIC). Total organic carbon (TOC) content varies between 0.02 to 3.4%, with the highest values coinciding with OAE 1a in the black shale interval, and averaging 0.25% above and below the event. Major element (Al, Si, Ti) show increased values at the shale level indicating heightened terrestrial input at that time. The poor correlation between biolimiting elements (P, Fe) and TOC suggests that preservation of organic matter (OM) was not controlled by enhanced productivity. Redox sensitive trace elements (V, Cr, Cu, U) follows a trend correlative with the major elements, which implies that they are also terrestrially derived and that the bottom waters did not experience strong reducing conditions. The clay content of 13 samples were analyzed and show varying amounts of illite (47-72%), smectite (20-51%), chlorite (0-9%), and kaolinite (0-6%). Relative percentages of chlorite are lowest during OAE 1a, indicative of wetter climatic conditions at that time, in agreement with heighted terrestrial fluxes recorded by the major elements.