CADMIUM-ISOTOPIC EVIDENCE FOR INCREASING PRIMARY PRODUCTIVITY DURING THE LATE PERMIAN ANOXIA

The Late Permian is host to the largest extinction event in Earth’s history when more than 90 % of all extant marine species were lost. Severe and large-scale ocean anoxia contributed to the biotic catastrophe, though the driving mechanisms that sustained such conditions are still contested. Broadly speaking, two competing hypotheses have been invoked to explain the anoxia: increasing primary productivity and subsequent heterotrophic respiration as a consequence of increased nutrient supply, or a weakening to total stagnation of ocean circulation, allowing significant quantities of free sulfide to accumulate in seawater. Testing these hypotheses requires the ability to separate upwelling and bottom water redox conditions from changes in surface water productivity in sedimentary records. We address this issue by combining a comprehensive set of commonly utilized sedimentary redox proxies (e.g. nitrogen-isotopic data, pyrite framboid analyses, and organic and inorganic shale geochemistry) with cadmium-isotopic analyses of Upper Permian marine shales from east Greenland and the mid-Norwegian shelf. We use modern ‘core-top’ sediments from a range of oceanographic settings as the interpretative framework for our Cd-isotopic data, which are then used to constrain macronutrient utilization efficiency in the latest Permian surface ocean. Our geochemical and redox data indicate sulfidic conditions with vigorous upwelling, and increasingly anoxic conditions with strengthening upwelling in the Greenland and Norwegian sections, respectively. Since sedimentary Cd is primarily associated with organic carbon throughout both sections, our Cd-isotopic record is indicative of incomplete surface water nutrient utilization through time. The constant degree of nutrient utilization combined with increasing upwelling requires increased primary productivity to balance the larger nutrient fluxes to both study sites during the development of the Late Permian anoxia, rather than a stagnation of ocean circulation. More broadly, these data illustrate that Cd-isotopic analyses of organic-rich sediments can be used to provide valuable information on nutrient utilization – and therefore past productivity – so long as there are independent constraints on bottom water upwelling and redox.