Insights from the U-238-th-234 method into the coupling of biological export and the cycling of cadmium, cobalt, and manganese in the southeast Pacific Ocean

Abstract

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.