The marine geochemistry of thorium and protactinium

Abstract

Suspended particulate matter was collected by sediment traps deployed in the Sargasso Sea (Site S2), the north equatorial Atlantic (Site E), the north equatorial Pacific (Site P), and the Panama Basin (STIE Site). Additional samples of suspended particles were obtained by in situ filtration at Site F., at the STIE Site, and in the Guatemala Basin. Concentrations of dissolved Th and Pa were determined by extraction onto manganese dioxide adsorbers at Site P, at a second site in the Sargasso Sea (Site D), at the STIE Site and in the Guatemala Basin. Sediment samples were obtained from cores taken near Sites E and P. Results have shown unequivocally that suspended particulate matter in the open ocean preferentially scavenges Th relative to Pa. This behavior could not have been predicted from the known physical chemistry of Th and Pa. Dissolved 230Th/231Pa activity ratios were 3-5 at Sites P and D and 3-8 at the STIE Site. In contrast, unsupported 230Th/231Pa ratios were 22-35 (average 29.7 for 7 samples) in sediment-trap samples from greater than 2000 m at Sites S2, E and P. Ratios were lower in particulate matter sampled at shallower depths. Particles filtered at 3600 m and 5000 m at Site E had ratios of 50 and 40. In contrast to the open ocean samples described above, samples collected by six sediment traps at depths of 667-3791 m in the Panama Basin had unsupported 230Th/231Pa ratios of 4-8, and the deepest samples had the lowest ratios. Fractionation of Th and Pa that was observed at the three open ocean sites either does not occur or occurs to a very limited extent in the Panama Basin. Particulate 230Th/231Pa ratios were negatively correlated with the concentration of suspended particles. However, variable scavenging rates, as indicated by variable particle concentration, do not completely control the ratio at which Th and Pa are scavenged from solution. Major biogenic and inorganic components of trapped material were found in approximately the same proportions in the STIE samples and in samples from Sites E and S2. Lower 230Th/231Pa ratios found in the STIE samples must therefore result from subtle changes in the chemical properties of the particles. Consideration of 230Th/23lPa ratios in several depositional environments indicates that no single factor controls the ratio at which Th and Pa are adsorbed from seawater. Fluxes of 210Th and 231Pa were less than their rates of production in the overlying water column in every trap at Sites S2, E, and P. In the Panama Basin, fluxes measured with the same traps were greater than or equal to their rates of production. These results are a strong indication that even extremely reactive elements such as Th and Pa are redistributed within the oceans. Redistribution occurs because variable scavenging rates in different environments set up horizontal concentration gradients. Horizontal mixing processes produce a net horizontal transport of Th and Pa from areas of 1ow scavenging rates to areas of high scavenging rates. Protactinium is redistributed to a greater extent than Th. Fluxes of 230Th can be used to set lower limits for horizonttal transport of Pa even when absolute trapping efficiencies of the sediment traps are not known. Less than 50% of the Pa produced at the open ocean sites is removed from the water column by scavenging to settling particles. The remainder is removed by horizontal transport to other environments. At Sites E and P, 230Th/231Pa ratios were identical in the deepest sediment trap sample and in surface sediments. However, 230Th/232Th and 231Pa/232Th ratios were 2.5 times higher in trapped particles than in surface sediments. The 230Th/232Th ratios were 5.5 times higher in particles filtered at 3600 m and 5000 m at Site E than in surface sediments. This observation is best explained by dissolution of most of the 230Th and 231Pa scavenged by settling particles during remineralization of labile biogenic phases. The behaviors of certain other radioisotopes were also studied. 232Th is present only in detrital mineral components of trapped material. Concentrations of 232Th in trapped particles correlate closely with Al and K, at ratios approaching that of average shale or crustal abundances at Site E and P and basalts at the STIE Site. High specifìc activities of 228Th and 239+240Pu were found in sediment trap samples throughout the water column at Sites E and P and in the Panama Basin. The dominant source of these isotopes is near the sea surface and also near the sea floor in the case of 228Th. Thus it appears that the bulk of the trapped material is recently derived from the sea surface where it incorporates these isotopes,with little loss during rapid transit through the water column. A bioauthigenic form of particulate uranium is produced at the sea surface and remineralized in the deep ocean along with its labile carrier phase(s). This flux of uranium to the deep ocean is 0.25-1.0 dpm/cm2103 years, which is insufficient to cause a measurable concentration gradient in the uranium distribution within the mixing time of the oceans. Increased concentrations and fluxes of particulate uranium were not found in the eastern equatorial North Pacific under areas of an intense oxygen minimum. Therefore, reduction of uranium to the tetravalent state with subsequent scavenging to settling particles in oxygen minima is not a mechanism removing uranium from the oceans.