Refining the roles of productivity, redox, and remineralization on the cadmium isotope composition of marine sediments

• Published in: Geochimica et Cosmochimica Acta
• Main Authors: Tegler, Logan A., Nielsen, Sune G., Wang, Yi, Scholz, Florian, Owens, Jeremy D., Peterson, Larry, Auro, Maureen, Kinsley, Christopher W., Lam, Phoebe, Horner, Tristan J.
• Format: Article in Journal/Newspaper
• Language: English
• Published: Elsevier 2024
• Subjects: Southern Ocean
• Online Access: https://oceanrep.geomar.de/id/eprint/60151/; https://oceanrep.geomar.de/id/eprint/60151/2/1-s2.0-S0016703724001236-mmc1.xlsx; https://oceanrep.geomar.de/id/eprint/60151/8/1-s2.0-S0016703724001236-main.pdf; https://doi.org/10.1016/j.gca.2024.03.010

Cadmium (Cd) has a nutrient-like distribution in the ocean, similar to the macronutrient phosphate. Significant isotope fractionation induced by the biological cycling of Cd makes it a potential tracer for nutrients and productivity. However, the Cd flux and Cd isotope composition of marine sediments can also be influenced by local redox conditions and partial remineralization of organically hosted Cd. These confounding factors are under-constrained and render it challenging to use Cd as a reliable paleoproxy. To understand the relative importance of each of these processes, we examined the Cd isotope systematics of 69 modern sediments deposited across a wide range of environments. We complement these data with four profiles of particulate Cd isotope compositions from the Southern Ocean. We report three main results. First, we show that the sedimentary flux of Cd is tightly coupled to that of organic matter. Second, most Cd burial occurs in regions with some bottom-water oxygen, and the flux of CdS to anoxic regions is, globally, minor. Finally, we find that remineralization can substantially modify sedimentary Cd isotope compositions, though it is challenging to relate pelagic and sedimentary processes. For example, we find that the relationship between sedimentary Cd isotope compositions and surface seawater [Cd] is the reverse of that predicted by isotope reactor models. Likewise, sedimentary Cd isotope compositions are anti-correlated with bottom-water oxygen. While this pattern is consistent with preferential remineralization of isotopically heavy Cd, profiles of marine particulate matter reveal the reverse, whereby the Cd isotope composition of large particles, which are most likely to reach the seafloor, becomes increasingly ‘heavy’ with depth. These results highlight how productivity, redox, and remineralization all influence the flux and isotope composition of Cd to marine sediments. While our study suggests that there is no simple way to relate sedimentary Cd isotopes to surface nutrient utilization, ...