As particles sink down the water column in the world ocean, they carry with them biogeochemically important elements such as phosphorus, cadmium, cobalt, and iron and exert strong control on their oceanic distributions. Such elements limit biological production in the surface ocean, help regulate the global carbon cycle, and influence global climate. Despite its importance, accurate and precise particle flux is challenging to estimate from observations. It has been determined traditionally by sediment traps but also more indirectly by radioactive disequilibria, both with their own limitations.
As part of a major US GEOTRACES campaign, Hayes et al.  present an impressive set of water column measurements of four pairs of parent-daughter radionuclides on the same water samples: 238U-234Th, 210Pb-210Po, 228Ra-228Th, and 234U-230Th. A secular equilibrium would be expected for each of the pairs in a closed system, because the daughters all decay much faster than their parents. However, this equilibrium is not reached because the daughters are sufficiently particle reactive that they are preferentially removed from the water column than their parents.
Hayes et al. estimate this removal rate by measuring radioactive disequilibria for the four parent-daughter pairs. The authors then extend this flux calculations to carbon and other biogeochemically important elements. Their flux estimates from different radionuclines agree within a factor of two. Their new depth profiles show varying degrees of importance of biotic remineralization, lateral transport, and particle scavenging for the different elements. The new depth profiles of trace element fluxes are novel and potential targets of future ocean biogeochemical modeling.
Citation: Hayes, C. T., Black, E. E., Anderson, R. F., Baskaran, M., Buesseler, K. O., Charette, M. A., et al. . Flux of particulate elements in the North Atlantic Ocean constrained by multiple radionuclides. Global Biogeochemical Cycles, 32. https://doi.org/10.1029/2018GB005994
—Katsumi Matsumoto, Editor, Global Biogeochemical Cycles