Barium Isotopes Indicate Spatiotemporal Heterogeneity of Marine Primary Productivity During the Toarcian Oceanic Anoxic Event

Abstract The Toarcian Oceanic Anoxic Event (T‐OAE, ∼183 Ma) was characterized by globally enhanced organic‐carbon burial and a negative carbon‐isotope excursion (N‐CIE). However, the role of marine productivity at this time, and its spatiotemporal variability, is unclear. We present the first carbonate barium‐isotope (δ 138 Ba carb ) records across the T‐OAE from a shallow‐water platform (Nianduo, SE Tethys) and a pelagic basin (Dogna, Alpine‐Mediterranean Tethys) to reconstruct productivity dynamics. Both sites show positive δ 138 Ba carb shifts at the N‐CIE onset, indicating supra‐regional productivity enhancement. At Nianduo, δ 138 Ba carb rises through the onset, consistent with increased export production driven by weathering‐derived nutrient inputs. At Dogna, δ 138 Ba carb declines within the N‐CIE onset due to reduction‐driven barite dissolution, followed by a rise during the N‐CIE recovery. The Dogna data suggest protracted elevation of pelagic productivity supported by nutrient upwelling and aeolian fertilization. As such, pelagic basins may have acted as an important carbon sink regulating the T‐OAE carbon cycle.