Insights From the 238U‐234Th Method Into the Coupling of Biological Export and the Cycling of Cadmium, Cobalt, and Manganese in the Southeast Pacific Ocean

• Published in: Global biogeochemical cycles 2019-01, Vol.33 (1), p.15-36
• Main Authors: Black, E. E., Lam, P. J., Lee, J.‐M., Buesseler, K. O.
• Format: Article
• Language: English
• Subjects: Abundance; Anthropogenic factors; Atmospheric particulates; Biology; Budgets; Cadmium; Carbon; Climate models; Coastal zone; Cobalt; Computer simulation; Data; Dust storms; Dynamics; export; Exports; Fluctuations; Flux; Food chains; Food webs; Global climate; Global climate models; Heavy metals; Human influences; Interactions; Low concentrations; Manganese; Marine environment; Marine organisms; Measuring instruments; Metal concentrations; Metals; Methods; Ocean models; Oceans; Organic carbon; Organic phosphorus; Oxidation; Oxidoreductions; Oxygen; Particulate flux; Phosphorus; Precipitation; Remineralization; residence time; Rivers; Scavenging; Temperature (air-sea); Thorium; Trace elements; Trace metals; Tracers; Upper ocean
• ISSN: 0886-6236; 1944-9224
• DOI: 10.1029/2018GB005985

Better constraints on the magnitude of particulate export and the residence times of trace elements are required to understand marine food web dynamics, track the transport of anthropogenic trace metals in the ocean, and improve global climate models. While prior studies have been successful in constructing basin‐scale budgets of elements like carbon in the upper ocean, the cycling of particulate trace metals is poorly understood. The 238U‐234Th method is used here with data from the GP‐16 GEOTRACES transect to investigate the upper ocean processes controlling the particulate export of cadmium, cobalt, and manganese in the southeastern Pacific. Patterns in the flux data indicated that particulate cadmium and cobalt behave similarly to particulate phosphorus and organic carbon, with the highest export in the productive coastal region and decreasing flux with depth due to remineralization. The export of manganese was influenced by redox conditions at the low oxygen coastal stations and by precipitation and/or scavenging elsewhere. Residence times with respect to export (total inventory divided by particulate flux) for phosphorus, cadmium, cobalt, and manganese in the upper 100 and 200 m were determined to be on the order of months to years. These GEOTRACES‐based synthesis efforts, combining a host of concentration and tracer data with unprecedented resolution, will help to close the oceanic budgets of trace metals. Plain Language Summary Despite trace metals being present at low concentrations in the ocean, they can be essential to many cellular functions for marine organisms. The abundance of these metals can impact marine food webs and, therefore, how we construct global models for simulating past, current, and future ocean conditions. To improve our understanding of the interactions between ocean biology and metals, we need a better understanding of the comparative sources of these metals, such as dust and rivers, and the interior biological, oxidation‐reduction, and scavenging sinks. In particular, we need to know how much of a given metal is being transported out of the upper ocean via sinking particles. Once a particle reaches a depth of a few hundred meters, the attached metals will remain in the deep ocean for hundreds of years to millennia, where they will be unavailable for use by surface organisms. In this study we focus on measuring the quantity of particle‐associated cadmium, cobalt, and manganese leaving the surface ocean daily in the southeast