Organic geochemistry as a proxy for unravelling water’s conditions in the Messinian succession of the Sorbas basin (Spain)

The Messinian Salinity Crisis (MSC) resulted in the generation of the last “Salt Giant” on Earth and was one of the biggest ecological crises in recent geological history. Given the large volume of data collected, and the high-resolution of the astronomically tuned and dated sedimentary successions, the MSC offers a unique opportunity to study the effect of short-term environmental variability and its impact on biological communities, particularly the resilience of microbial communities. The late Miocene Sorbas basin in south-western Spain hosts one of the most complete records of the MSC and has been used as a reference for astronomical tuning. However, in the absence of sedimentological and micropaleontological data from the shaly pre-evaporitic successions, it is hard to understand the impact of the extreme salinities, anoxia, and desiccation on the aquatic biosphere and on water conditions (i.e. temperatures, salinities and stratification). In this study, new constraints have been derived from the analyses of organic debris in shales from both pre- and inter-evaporitic deposits from a new 176,5-m drill core section in the Sorbas basin.

Our results from the analysis of the pre-evaporitic Abad formation and the black shales of the Yesares member, deposited in the inter-evaporitic cycles, reveal differences in biomarker-composition that can be related to changes in water conditions before and after the onset of the MSC. Squalane, framboidal pyrite, and sulphur reducing bacteria in the Abad formation suggest the presence of oxygen-restricted and saline waters before the precipitation of primary gypsum. After the establishment of hypersaline conditions (i.e. gypsum precipitation), a new phase of anoxia developed in the basin with the precipitation of marls and organic-rich laminites in the first inter-evaporitic eventof the Yesares member. Here the presence of isorenieratene-derivatives in the form of aryl isoprenoids can be interpreted as evidence of green sulphur bacteria. These bacteria use sulfur as an electron donor under anoxic conditions and perform photosynthesis underlow-light conditions, meaning that anoxia at this time extended up to the photic zone (<200m). Our data suggest that if a chemocline was present before gypsum precipitation it was weaker and perturbed by seasonal variation, while the chemocline was more stable during the inter-evaporitic stage.