Insights From the²³⁸U‐²³⁴Th 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.

Ironing Out Fe Residence Time in the Dynamic Upper Ocean

“…Total 238 U was derived from salinity (Owens et al, 2011). Steady‐state assumptions were made, and lateral advection of 234 Th was determined to be negligible when these calculations were possible (e.g., along transects where a 234 Th gradient and current speeds could be determined) or where prior studies had already indicated that these assumptions were valid (Black et al, 2019; Lemaitre et al, 2018; Owens et al, 2015). For the new locations in this study, upwelling was a significant factor for the coastal region of the Pacific campaign only, and the appropriate adjustments were made (Black et al, 2018).…”

Effects of <sup>238</sup>U variability and physical transport on water column <sup>234</sup>Th downward fluxes in the coastal upwelling system off Peru

“…2) are similar to those seen in the open ocean (see compilations in Owens et al, 2011, andVan Der Loeff et al, 2006, andreferences therein). It thus appears that water column suboxic/anoxic conditions alone are not sufficient to remove U, in contrast to sedimentary U studies underlying lowoxygen waters where soluble U(VI) diffuses downward into subsurface sediments and is reduced to insoluble U(IV) (Anderson et al, 1989;Böning et al, 2004;Scholz et al, 2011). Our inference is in accord with water column 238 U studies in intense OMZs in the eastern tropical North Pacific (Nameroff et al, 2002) and the Arabian Sea (Rengarajan et al, 2003), where 238 U concentrations remain constant over the entire upper water column studied.…”

Particulate Trace Element Export in the North Atlantic (GEOTRACES GA01 Transect, GEOVIDE Cruise)

“…Black et al (2019) reported no significant differences in SSF and LSF pP: 234 Th, pCo: 234 Th and pCd: 234 Th ratios contrasting with the higher pMn: 234 Th ratios in the LSF compared to the SSF, leading to a greater contribution of the LSF to the total pMn flux 36 .…”

“…Large particles (>53µm) are generally assumed to represent the sinking particulate matter and are thus responsible for the particulate export flux [77][78][79] . Using the component: 234 Th ratios from the LSF to calculate export fluxes is the most common approach 36,[80][81][82][83] . However, recent studies have shown that the SSF could also contribute to the export flux of particulate carbon or nitrogen in oligotrophic regions 20,21,84 .…”

“…Despite an increasing number of studies addressing TE export, this process remains still poorly understood 23,26,35,36,[27][28][29][30][31][32][33][34] . Besides the strong temporal and spatial variability, another reason likely resides in the difficulty in measuring particulate trace element (pTE) fluxes, as TEs are present at very low concentrations in the ocean and are easily prone to contamination.…”