Early evolution and transformation of organic matter in the active continental Jordan Rift Valley

A sequence of alternating lacustrine marls, peat and basalts was penetrated in the Notera-3 well in the northern part of the Jordan Rift, Israel. The 2781 m thick sequence, ranges from Upper Miocene to Recent, reflects high sedimentation rate in the active continental rift associated with the Dead Sea Transform. The deep burial and the relatively high geothermal gradient (40°C km⁻¹) compensate for the short time span so that coalification expressed by vitrinite reflectance consistently increases with depth, from about 0.32% Ro at 1040 m to 0.48% Ro at 2495 m.Analysis of the peat reveals that the O/C, S/C and δ¹³C of the humic acids (HA) and the heavy to light normal alkane ratios are the only parameters sensitive enough to express this slight maturation increase with depth. A sharp δ¹³C change from about − 18‰ prevailing in the uppermost meters to an average of − 27.5‰ at 15 m and deeper reflects a change in the higher plant source of the peat (from C4 to C3 plants) rather than an early diagenetic modification.The δ¹³C, O/C, S/C and N/C ratios are usually lower in the kerogens than in the corresponding HA. The decrease in the δ¹³C and the O/C ratios are explained by elimination of oxygen-containing functional groups during transformation and by polymerization effects. The gradual decrease in the ¹²C and the O/C of the HA with depth are attributed to decarboxylation coupled with kinetic effects. The N/C depletion during the transformation from HA to kerogens probably results from the breakdown of amino acids. The S/C ratio which decreases both during this transformation and also with maturation is most readily explained by the breakdown of ester sulfate-containing groups such as sulfated polysaccarids, which formed diagenetically during the humification process.