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A range of dissolved trace metals (e.g. Fe, Zn, Co, Cd) in the ocean are important micronutrients and essential for life. Amongst these trace metal nutrients, the oceanic cycling of dissolved Cd is enigmatic. Low levels of dissolved Cd do not appear to directly limit marine phytoplankton growth. Yet, global distribution of Cd closely matches that of macronutrient phosphate (PO4), forming a remarkable bi-linear correlation between the two. To date, Cd is still the sole tracer for PO4 concentrations in paleoceanography but direct evidence of the marine biological function of Cd exists in only two culturing studies. Recent Cd isotope studies have provided important insights into the enigmatic role dissolved Cd plays in marine biogeochemistry, and the processes that affect its distribution in seawater. In this talk, I will explore the distributions of dissolved Cd from the tropical and South Atlantic, and the Eastern Tropical South Pacific oxygen minimum zone (OMZ). I will demonstrate that biological uptake and remineralization are the main factors that govern Cd isotope fractionation in the upper ocean. Globally, surface seawater displays relatively homogenous Cd isotope ratios that can be modeled by a steady state model or explained by ligandmetal complexation. This upper ocean Cd isotope pattern directly challenges recent paleoceanography studies that apply sedimentary Cd isotopes as a paleo-productivity tracer. I will further demonstrate that Cd isotopes in intermediate and deep waters are powerful tools to fingerprint individual water masses. Lastly, in light of a recent hypothesis that open ocean OMZs are important sink for isotopically light Cd, I will show that dissolved and particulate Cd isotope evidence does not appear to support this hypothesis in the water column of one of the world’s most intense OMZs offshore Peru.
Ruifang Xie is a paleoceanographer and marine isotope geochemist by training. Xie's research focus is twofold: application of geochemical tracers (trace metals, radiogenic and non-traditional stable isotopes) in the modern and past oceans to (1) study the biogeochemical cycles of trace metal nutrients, and (2) understand the climate forcing and feedback mechanisms on the global carbon cycle. Xie is involved in various projects including reconstruction of atmospheric and oceanic circulation in response to abrupt climate variations over the past 25,000 years using radiogenic isotopes (Nd, Sr, Pb) and trace metals (Fe, Si, Ba); investigating mechanisms controlling the marine biogeochemical cycling of dissolved Ba and Cd isotopes; testing sedimentary Ba and Cd isotopes respectively as potential tracers for surface productivity and subsurface remineralization intensity; constraining the strength of biological carbon pump using 234Th; quantification of trace metal fluxes using both 234Th and 230Th isotopes.