Cd isotopes under extreme euxinia: Tracing productivity and redox in palaeo-oceans

Cadmium (Cd) exhibits nutrient-type behaviour in the modern ocean and its isotope system has emerged as a promising tracer of primary productivity and carbon burial. Phytoplankton preferentially assimilate lighter Cd isotopes across a wide range of oceanic conditions, leaving surface waters comparatively enriched in heavier isotopes. This biologically-driven fractionation underlies the application of Cd-isotope ratios as a tracer for nutrient availability and the intensity of primary productivity in both modern marine settings and  palaeo-oceans. However, Cd-isotope systematics are also strongly influenced by redox conditions, specifically through the formation and removal of isotopically light Cd sulphides under euxinic conditions. The extent to which sedimentary Cd-isotope signatures faithfully record overlying water-colum processes under such conditions remains poorly constrained.

Here we present new Cd-isotope data from both the water column and sediments of Framvaren Fjord in Norway, the most intensely reducing modern marine basin. Framvaren Fjord serves as a modern analogue for strongly euxinic marine conditions that prevailed during extreme climate events throughout Earth’s history. Notably, the redoxline separating oxic from anoxic waters is uniquely located within the photic zone, in close proximity to the depth of maximum biological productivity. These data allow us to deconvolve Cd-isotope fractionation associated with biological uptake from that linked to Cd sulphide precipitation, and to shed light on how these processes are transferred to and preserved in the underlying sediment.