Reduced Marine Molybdenum Inventory Related to Enhanced Organic Carbon Burial and an Expansion of Reducing Environments in the Toarcian (Early Jurassic) Oceans

Abstract

AbstractMany metals present in trace concentrations in the oceans are sensitive to reduction and oxidation reactions (termed redox‐sensitive trace metals or RSTMs) and can therefore be affected during intervals of expanded oceanic anoxia and euxinia (anoxic and sulfidic waters). These RSTMs are important micronutrients; and their availability plays a significant role in controlling metabolic pathways and therefore ecosystem structure. Understanding the links to ecosystem restructuring and potential collapse is important since past deoxygenation events are associated with biological turnover. The Early Jurassic Toarcian Oceanic Anoxic Event (T‐OAE) was one such event. We focus on the RSTM molybdenum (Mo), important in the nitrogen cycle, from a basin transect where local redox conditions permit the tracking of the marine reservoir of Mo. Mo enrichments start to decline in the Pliensbachian, continue to decline in the early Toarcian, drop precipitously during the T‐OAE interval, and remain low afterward before a protracted increase toward the initial values seen in the Pliensbachian. From a compilation of available data, we estimate that ∼41 Gt of Mo was buried during the T‐OAE using a range of possible flux estimates. Given the known correlations of Mo with total organic carbon, the estimated burial of organic carbon during the T‐OAE interval was ∼244,000 Gt. This requires a minimum of 3.25% of the ocean floor to be covered by euxinic waters. Finally, we suggest that the intervals traditionally classified as OAEs are associated with the greatest extent of euxinia.