Ephemeral signature of anoxia in the southwest Mediterranean coastal lagoon (southeast Tunisia) similar to the sapropel S1

On the southeast Tunisian coast, organic-rich facies were formed at the middle Holocene in a confined Boujmel lagoon bottom (currently Sebkha Boujmel) during the last deglaciation. Sedimentological and organic geochemistry are used to analyze samples from respectively eight and three cores. The total organic carbon (TOC) and humic compounds analysis highlighted the setting of anoxia around 6800 years before present (BP) and its breakdown at about 4000 years BP. Biotic and abiotic anoxic proxies constitute waterproofing of a high degree of confinement occurring under water column stratification. The well-preserved organic-rich material of mixed origin (continental and marine) constitutes a biotic proxy and the abiotic ones are made of polyhalite and magnesian minerals that layer just above the euxinic facies close-fitting water column stratification. The TOC peak registered in the euxinic facies of the Sebkha Boujmel correlates with the highest Holocene sea level change during the transgression of the Mediterranean Sea. Such correlation and the time of the anoxic phase launch the relationship between the Mediterranean sapropel S1 and the euxinic facies of Sabkha Boujmel. It seems that anoxia began early around 9000 years BP in the deepest areas of the Mediterranean Sea and progressively reached its edges around 6000–7000 years BP, recording a diachronism in the installation of the organic-rich facies, which agrees with the marine intrusion highlighted in the Sebkha Boujmel.