Mid-Cenomanian Anoxic Events controlled by organic accumulation from Jordan
Abstract
 
Mid-Cenomanian oceanic anoxic event is documented in the sedimentary achieve in Jordan. A section of Cenomanian was investigated in the Ajloun area for calcareous nannofossil biostratigraphy, microfacies, carbon and oxygen isotopes composition, organic and elemental compositions. Calcareous nannofossils confirm the age of Cenomanian revealing two biozones; UC-2 and 3. The Mid-Cenomanian Event initiated through a negative organic carbon isotope (δ13C) signatures, highlighting the abundance of light carbon within the Cenomanian seas. The latter abrupt positive shift of δ13C values, suggests that marine organic carbon accumulation rates were high as a result of the increasing ocean organic productivity that marked the onset of the anoxic event during the Mid-Cenomanian. The relatively high TiO2, P2O5, SiO2, and Al2O3 contents confirm this observation since productivity increased significantly followed by the extreme burial of organic carbon. The chemical index of alteration (CIA) values of more than 0.8 also indicate that the bituminous carbonate facies took place during the warm and humid climatic conditions, thereby resulting in an intensity of subaerial weathering. The oxygen isotope (δ18O) also trend toward negative values in samples deposited during the high productivity interval reflecting an increase in temperatures associated with enhancement of the hydrologic cycle in such a restricted and occasionally sub-aerially exposed setting, leading to a sea level rise which could have been the main factor controlling the terrigenous input. The high abundant of alginite (i.e., telalginite and lamalginite) in the bituminous material is a further evidence for a redox condition. The finding of mainly marine-derived organic matter is also demonstrated by the biomarker distributions. The biomarkers are represented by narrow Pr/Ph ratios of up to 0.97, abundant of tricyclic terpanes and high C27 regular sterane, indicating that the OM was primarily derived from mainly phytoplankton algae, along with small amounts of land plant-derived material and were accumulated under reducing conditions.

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