GEOCHEMICAL CHARACTERISTICS ASSOCIATED WITH OAE1A IN THE LOWER APTIAN SUCCESSION OF THE BASQUE-CANTABRIAN BASIN, NORTHERN SPAIN

Oceanic anoxic event 1 a (OAE1a) is a strong perturbation in the global carbon cycle which has also been identified within the Lower Aptian succession at the Playa de los Caballos beach, or the Cuchía section, in the Basque-Cantabrian Basin, northern Spain. Previous works in the section documented OAE1a through biostratigraphy (planktonic foraminifera, calcareous nannoplankton, and ammonite) and carbon isotope data. Here we present the results of a 67 m high-resolution (n = 83) complementary study along the Patrocinio Formation in order to determine the specific geochemical characteristics associated with the local conditions of the basin during the global event.

The Patrocinio Formation includes medium dark gray (N4) OM-rich silty marlstones to grayish black (N2), soft clayey marlstones grading into the upper part of mica-rich claystones, siltstones, and quartz sandstones. Total inorganic carbon (TIC) varies from ~0 to 86.04% with the section containing mostly shale (TIC < 10 wt%) and calcareous shale (TIC 10-30 wt%) intervals with few marlstones (TIC 30-60 wt%) and scarce limestone (TIC > 65 wt%) layers. Total organic carbon (TOC) ranges between 0.25 wt% and 1.15 wt%. and highest content occurs mostly in the shale layers. Major elements (Al, Ti, & Si) have a positive correlation with TOC (r>0.5) and are generally enriched in the high TOC intervals. Redox sensitive trace elements (RSTEs) V, Cr, and U also appear to correlate strongly with TOC (r>0.5). However, other measured RSTEs (Cu, Co, Ni, & Mo) have weak or no apparent correlation with TOC, suggesting that the bottom waters did not experience strong reducing conditions, and likely the preservation of OM is due to rapid burial. The clay content of 9 samples revealed varying amounts of smectite (0.8%-3.8%), chlorite (6.9%-10.9%), kaolinite (11.3%-21.5%), and illite (64.9%-78.6%). Smectite and illite contents are generally higher in intervals where TIC is high, while kaolinite and chlorite percentages decrease in areas with higher TIC. Lipid biomarkers extracted from 20 samples showed n-alkane distributions from nC₁₁ to nC₃₃ indicating a mixed source of OM in the basin. The terrestrial aquatic ratio (TAR) showed an increased ratio of longer chain to shorter chain n-alkanes in the shale layers, indicating higher terrestrial OM input at those levels.