Stable Cd isotope fractionation in the oceans – A measure of bio-productivity?

Many trace metals are bio-essential elements that can drive, and often limit, ocean primary production, and therefore influence the functioning of ocean ecosystems and the global carbon cycle. Some trace elements are of concern as contaminants, while others are used to assess modern-ocean processes and the role of the ocean in past climate change. While seawater concentration measurements were successfully performed during the groundbreaking GEOSECS chemical oceanographic survey in the early 70’s, isotope measurements have been limited until recently to studies pioneered by Patterson and colleagues for Pb, and Piepgras and colleagues for Nd, due to the extremely low concentrations and contamination issues. Recent breakthroughs in analytical instrumentation and chemical separation of trace metals, along with ultra-clean seawater sampling systems, have led to improvements in both sensitivity and accuracy of concentration measurements of some key micronutrients (Zn, Fe, Cd) in the oceans. The availability of micronutrients severely limits marine phytoplankton growth and this is particularly so in High-Nutrient-Low-Chlorophyll oceanic regions where iron – an essential nutrient and “fertilizer”, is deficient, resulting, paradoxically, in low biomass and primary production in these otherwise nutrient-rich regions. Because the strength of the “biological pump” regulates the sequestration of atmospheric CO2, and its efficiency depends on the nutrient inventories of the oceans, understanding the biogeochemical cycles of key nutrient elements is thus crucial if we are to assess the impact of the Anthropocene era on the global carbon cycle. The application of non-traditional stable isotope systems, such as natural, mass-dependent fractionation of Fe, Zn and Cd isotopes, has been fostered by the GEOTRACES program which is an international study of the marine biogeochemical cycles of trace elements and their isotopes. One important application is tracing the isotope effects of biological nutrient utilization in which the light isotopes are preferentially incorporated by marine organisms leaving behind the seawater pool isotopically heavier. I will review our present knowledge on the biogeochemical cycling of Cd forty years after the classical Boyle et al. 1976 paper “On the marine geochemistry of cadmium” and show that stable Cd isotope fractionation in the oceans trace both biological nutrient utilization and deep ocean circulation. I will highlight the complex interaction of biological, chemical and oceanic processes shaping the overall modern oceanic Cd distribution and the potential of stable Cd isotopes as a proxy for past productivity in response to changes in the ocean-biosphere-atmosphere dynamics throughout Earth’s history.