Inference about rates of thorium and particle cycling in the ocean water column

Insoluble thorium (Th) isotopes are particle reactive while their radioactive parents are fairly soluble. This difference of behaviour has allowed chemical oceanographers to use Th isotopes to develop understanding about the scavenging of particle reactive elements and the cycling of particles in the water column. However, many prior models rely on numerous assumptions. Among these assumptions is vertical uniformity of the rate parameters governing sorption reactions and particle processes in the mesopelagic zone and in the deeper regions of the water column (Th adsorption and desorption, particle remineralization and settling speed, etc.).

In this work, Paul Lerner and co-authors (2016, see reference below) use Th and particle data collected at station GT11-22 of the GEOTRACES North Atlantic transect (section GA03) in order to test two models of Th and particle cycling: a conventional one that assumes uniform rate parameters and another one that allows the rate parameters to vary with depth. They show that the second model leads to a significantly better fit to the data, thereby challenging the assumption of parameter uniformity in the conventional model. Moreover, by combining the second model with the data, they diagnose the various terms in the depth-dependent Th isotope budgets at GT11-22, showing in particular that the behaviour of 230Th is dominated by a balance between adsorption and desorption over most of the water column.

16 Lernerl
Figure: Inversion of radiochemical and particle data for station GT11-22 of section GA03 (19º26’ N, 29º 22’ W). The data used include measurements of dissolved and particulate 228,230,234Th, 228Ra, particle concentration, and observational estimates of 234,238U. Panel (a) shows the budget of dissolved 230Th (left box) and particulate 230Th (right box) in terms of vertical averages (dpm m-3 yr-1) along the water column (below 125 m). The numbers are estimated fluxes and their estimated errors. Panels (b) and (c) show the vertical distribution of the 230Th fluxes.

Reference:

Lerner, P., Marchal, O., Lam, P. J., Anderson, R. F., Buesseler, K., Charette, M. A., Edwards, R. L., Hayes, C. T., Huang, K-F., Lu, Ya., Robinson, L F., Solow, A. (2016). Testing models of thorium and particle cycling in the ocean using data from station GT11-22 of the U.S. GEOTRACES North Atlantic section. Deep Sea Research Part I: Oceanographic Research Papers, 113, 57–79. doi:10.1016/j.dsr.2016.03.008

 

 

Latest highlights

Science Highlights

Extremely high radioactive levels in the manganese nodules

Volz and co-authors demonstrate that radioisotopes in the manganese nodules mostly exceed exempt activity levels…

22.05.2023

Science Highlights

The North Pacific Ocean, a key actor for the zinc oceanic cycle

Sieber and his colleagues lift the veil on some of the mechanisms that control the behavior of zinc in the Pacific Ocean, and more globally.

17.05.2023

Science Highlights

Disentangling what controls the cadmium distribution in the Pacific Ocean

Sieber and his colleagues established the distribution of dissolved cadmium concentrations and isotopes in a section extending from Alaska to Tahiti.

16.05.2023

Science Highlights

Solid-solution distribution of the cosmogenic beryllium-7 in the water column

This work questions the validity and limits of the hypothesis that particulate beryllium-7 can be neglected in the oceanographic applications of this tracer.

12.05.2023

Rechercher