Geochemical and Redox Sensitive Trace Elements of the Lower Aptian Sediments in the La Frontera Section, Subbetic Basin, Southern Spain, Implications Concerning OM Preservation Associated with Oceanic Anoxic Event 1a.

Abstract:

The Subbetic Basin part of the Southern Iberian paleomargin in the center of the Betic Cordillera area (Southern Spain) consists of hemipelagic/pelagic facies. The Barremian – lower Aptian sediments of the Subbetic Basin have been well documented to include Oceanic Anoxic Event 1a (OAE 1a) which is marked by a strong perturbation in the global carbon cycle associated with a lack of circulation, increased organic preservation, and a high sedimentation rate.

Here we present new geochemical data on ~28 m of the La Frontera section located ~ 30 km south of Jaén, Spain. The succession contains upper Barremian-Early Aptian pelagic marls, limestones, marlstones, black shales, and radiolarian marls. Biostratigraphic studies have determined that the section is Aptian in age and coincides with OAE 1a. The lower 13 m are mostly composed of 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 coincides with OAE 1a. The uppermost 10 m of the succession are comprised generally of marlstones and calcareous shales (10-30% TIC). Total inorganic carbon (TIC) varies from 0% in the shales and reaches up to 89.6% in the limestones. Total organic carbon (TOC) fluctuates between 0.02 % to a high of 3.4%. TOC values are highest in the shale layers within the OAE 1a interval and remain relatively low before and after the event, averaging 0.26% and 0.24%, respectively. The carbon isotope on the organic fraction (δ¹³C_org) was measured for correlation with other well studied sections. Measured δ¹³C_org values range from -27.77 to -21.49‰ and confirm the presence of segments C1 to C7 (Menegatti et al.,1998). Biolimiting elements (P, Fe) share no apparent correlation with TOC (r= -0.10 and 0.15, respectively) implying that enhanced productivity was not a major factor in the preservation of organic matter (OM). Major elements (Al, Si, Ti) show increased values in the shales and reach up to 192007 (Al), 850382 (Si), and 9358 (Ti) (ppm). Redox sensitive trace elements (V, Cr, Cu, U) correlate strongly with Al (r>0.7) and show enhanced values in the black shales. These results demonstrate that this part of the basin did not experience strong reducing conditions during OAE1a and that the preservation of OM is likely linked to terrestrial fluxes as shown elsewhere.