Organic carbon accumulation at the northern Gondwana paleomargin (Tunisia) during the Toarcian Oceanic Anoxic Event: Sedimentological and geochemical evidence

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During the early Toarcian (Early Jurassic; ~183 Ma) global environmental changes promoted the expansion of oxygen-deficient conditions across vast shelf areas around the globe. Ocean margin deoxygenation favored the preservation of organic matter in shelf sediments and is considered to reflect a Toarcian Oceanic Anoxic Event (T-OAE). Higher-level global drivers controlled temperature fluctuations, sea level evolution, oceanic circulation patterns, marine primary productivity, and stable carbon isotope excursions. Lower-level factors, controlled by individual basin bathymetry and paleogeography, operated on local to regional scale and here regulated organic matter accumulation. Accordingly, the T-OAE has been described as global event with a strong regional character. During the Toarcian, preferentially organic matter-lean sediments have been deposited at the northern Gondwana paleomargin. Enhanced connectivity to the Tethys Ocean and low marine primary productivity are considered to have been major factors preventing the formation of oxygen-deficient conditions and organic matter preservation. We here report on early Toarcian sediments from the Châabet El Attaris Section (Tunisia), located at the northern Gondwana paleomargin that episodically became enriched in organic matter. Bio- and chemostratigraphy both confirm that organic matter-rich sediments preferentially formed during the T-OAE but also predate the T-OAE. Sedimentological and micropaleontological characterization in concert with bulk, molecular and stable isotope geochemical investigations reveal that accumulation of marine organic matter occurred under anoxic, but not under euxinic, conditions. Limitation in the supply of reactive iron regulated organic matter sulfurization indicating that during most of the depositional history the levels of free H 2 S in bottom waters were rather low but limited to the sediment pore water. We conclude that the chemocline was either situated within the sediment or close to the sediment water interface. Spatiotemporally-restricted occurrence of organic matter enriched sediments and anoxic pore waters was promoted by restriction in water column ventilation in a local graben structure. • Organic matter-enriched strata in Tunisia predates but maximizes in the T-OAE. • Organic matter preservation occurred under anoxic but non-euxinic conditions. • Formation of oxygen deficient conditions was promoted by local paleobathymetry and basin restriction.