Role of microorganisms in oceanic anoxic events (OAEs)

The accumulation of organic matter (OM) during oceanic anoxic events (OAEs) that took place in the Mesozoic may reflect different biogeochemical pathways and feedbacks related to palaeoenvironmental changes. It originates from a combination of nutrient cycling, metal concentrations and position of the chemocline and/or pycnocline controlling biological production. Because ocean stagnation cannot induce euxinia, an expanded oxygen minimum zone (OMZ) associated with upwelling and improved hydrological cycles is more commonly accepted. In some depositional settings, it permitted the evolution of anoxia/dysoxia to euxinia (sulphidic waters). The latter is recorded by the presence of anoxygenic phototrophs and involved denitrification, anaerobic oxidation of methane and sulphate reduction. In order to better constrain different OAE palaeoenvironments, models are suggested here that show the evolution from main microbial communities thriving through the water column to bacterial and more complex microbial mats. Therefore, microbial processes have a key role in OM production and metabolization. They directly influence the position of the chemocline. Detailed study of the OM preserved in black shales will be key to understanding OAEs.