Benthic microbial mats: a possible major component of organic matter accumulation in the Lower Aptian oceanic anoxic event

Oceanic anoxic events (OAEs) throughout the Cretaceous were periods of high organic carbon burial leading to drawdown of atmospheric carbon dioxide and lowering of bottom-water oxygen concentration, thereby enhancing the preservation of organic matter (OM). Two dynamic depositional models have been proposed for these events in the Tethyan domain: one is based on strong thermohaline stratification and low surface productivity, the other on high surface productivity with intensified deep-water circulation. Here, we propose another explanation for the concentration of OM, derived essentially from microscopical observations (scanning electron microscope and transmission electron microscope) in the organic-richest interval of an Early Aptian OAE in central Italy (OAE1a or Selli level, 116 Ma). This high-resolution microscopical study of OM highlights benthic microbial mats as the possible source of organic-rich samples where amorphous OM is the major organic constituent. These mats could be more common in OAE black shales than previously thought.