MAJOR AND REDOX SENSITIVE TRACE ELEMENTS AS INDICATORS OF PALEOENVIRONMENTAL CONDITIONS IN APTIAN SEDIMENTS DURING OAE1A OF THE CUCHIA SECTION, CANTABRIA, NORTHERN SPAIN

Oceanic anoxic event 1a (OAE1a) is recorded globally in Aptian sediments and is identified by a strong perturbation in the global carbon cycle often associated with organic-rich “black shales” indicative of severe oxygen-depleted conditions to anoxia. The lower Aptian succession at Playa de los Caballos near the village of Cuchia, northern Spain, is part of the Basque-Cantabrian basin where OAE1a has been well documented in previous studies supported by paleontological (planktonic foraminifera, calcareous nannoplankton and ammonite) and carbon isotope data. Here we present new geochemical data, major and redox sensitive trace elemental (RSTEs) on the 67 m segment of the section that comprises OAE1a to better characterize the prevalent mechanisms of organic matter preservation related to this event in the Basque-Cantabrian basin. The succession consists of mostly shale (TIC < 10 wt%) and calcareous shale (TIC 10-30 wt%) intervals with few intercalated marlstones (TIC 30-60 wt%) and scarce limestone (TIC > 65 wt%) layers. Total organic carbon (TOC) fluctuates between 0.25 wt% and 1.15 wt%, with the majority of the higher TOC values occurring in shale and calcareous shale intervals.

Major elements (Al, Ti, and Si) show increasing values during periods of high TOC (r > 0.5). Similarly, RSTEs (V, Cr, Th, and U) values increase at intervals of higher TOC and show strong positive correlations with TOC (r > 0.5). These increased RSTEs and major elements values occur mostly in the shale and calcareous shale layers with values as high as 88006 (Al), 205916 (Si), 3856 (Ti), 101 (V), 80 (Cr), and 3.8 (U) (concentrations in ppm). However, RSTEs such as Cu, Co, Ni, and Mo, have weak or no apparent correlation with TOC. Phosphorous, a biolimiting trace element also shows no apparent correlation with TOC or any of the other trace elements and has values between 172 and 891 ppm. Strong positive correlations between TOC and the major trace elements suggests that the preservation of organic matter is likely due to rapid burial of organic material by terrestrially derived clays, and is less likely due to reducing conditions as attested by the lack of correlation between TOC and most RSTEs.