Award: OCE-1232578

In this project, we have applied inverse methods to analyze measurements of metal and particle concentrations in oceanic samples collected during the GEOTRACES program. Inverse methods are techniques that can be used to make inferences about natural phenomena from the quantitative combination of observations and theories. Whilst such phenomena may not be directly observable, their very occurrence, and the rates at which they occur, can be inferred from other observations in the presence of a model (a classical example is the inference of the structure of the Earth interior from surface observations in the presence of a geodynamical model). In our project, inverse methods were used to extract information about the interactions of dissolved metals with solid particles in ocean waters. Phenonema of particular interest included (i) the attachment of metals onto the surfaces of particles ("adsorption") and (ii) the subsequent removal of particles by gravitational settling. Both phenomena are responsible for the removal of metals from the ocean to the sediment and are collectively called "particle scavenging". Whereas they cannot easily be observed in situ, the rates at which they occur can be determined by fitting particle scavenging models to measurements of metal and particle concentrations in seawater samples. Our project relied on three ingredients: a dataset, a model, and an inverse method. The dataset consisted in radiochemical and particle measurements at several stations occupied during the GEOTRACES North Atlantic transect GA03 (Fig. 1). We used measurements of the concentrations of three radio-isotopes of thorium (Th-228, Th-230, and Th-234) and particles, in three different particulate size fractions. Thorium (Th) is a chemical element that is known to be very particle-reactive in seawater, hence the use of Th data for this project. We also used measurements of the concentration of radium-228 (the radioactive parent of Th-228) as well as observational estimates of the concentrations of uranium-234 (parent of Th-230) and uranium-238 (parent of Th-234). The model considered for our project constituted a relatively simple description of particle scavenging in the oceanic water column (Fig. 2). It included two components: a Th cycling model and a particle cycling model. The Th cycling model represented the effects of radioactive production and decay, Th adsorption onto particles, Th desorption (i.e., detachment) from particles, particle degradation, and particle settling. The particle model represented the effects of particle degradation and particle settling. Different inverse methods were applied to combine the Th and particle data with the Th and particle cycling model. Each of these methods is a least-squares technique, aimed at a model solution that minimizes the sum of the squared deviations from the measurements, given the measurement errors. Intellectual merit. Our major results are the following. (1) A model that allows variations of rate constants of Th and particle cycling along the water column provides a significantly better fit to the data than a model that does not. (2) The specific rate at which Th attaches to particles relative to that at which it is released from particles is higher in the upper ocean than in the deep ocean (Fig. 3). (3) Noticeable variations in the rate constants of Th cycling and particle cycling were found along GA03 (Fig. 4). (4) The rate constant of Th adsorption (k1) generally decreases with depth (Fig. 5) and increases with particle concentration (Fig. 6), consistent with the notion that Th attachment onto particles increases with the number of surface sites available for adsorption. (5) Among the various components of the marine particulate matter, particulate organic matter is the major agent responsible for the spatial variability of k1 along GA03, consistent with an important role of organic molecules in Th adsorption experiments conducted in the laboratory. Broader impacts. (1) This project provided full support to a graduate student. This student has been involved in virtually all facets of this project (ranging from the development of computer codes to the writing of manuscripts for peer review), has prepared and presented posters at multiple conferences, has attended a summer school on marine particles, and has interacted with a number of established scientists. (2) This project has brought together two researchers with distinct experience: an ocean modeler (O. Marchal) and a chemical oceanographer (P. Lam). (3) By enhancing our understanding of the spatial variability of particle scavenging in the ocean, this project should contribute to refined representations of this process in global ocean models and to the interpretation of deep-sea sediment records of particle-reactive elements, in particular records of the ratio of protactinium-231 to thorium-230. (4) The computational software developed over the course of this project together with a manual will be submited in Fall 2016 to the Biological and Chemical Oceanography Data Management Office. This way, our approach of data analysis could be applied by other researchers to study the cycle of other particle-reactive substances in the ocean. Last Modified: 08/29/2016 Submitted by: Olivier Marchal