The marine geochemistry of thorium and protactinium

Abstract

Suspended particulate matter was collected by sediment traps deployed in the Sargasso Sea (Site 82), 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 GuatemalaBasin. Concentrations of dissolved Tn 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.Resul ts have shown uneauiv~cally that suspended particulate matter in the open ocean preferentially scavenges rh relative to Pa. This behavior could not Mave been predicted from the kno\m physical chemistry of Th and Pa. Dissolved 230Thl231pa activity ratios \.¡ere 3-5 at Sites P and D Particulate 230Th/23lpa ratios _,,!ere negatively correlated \i7ith the concentra tion 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 STIF. samples and in samples from Sites E and S2' Lower 230rh/23lpa ratios found in the SrIE samples must therefore result from subtle changes in the chemical properties of the particles. Consideration of 230Thl23lpa ratios in several depositional environments indicates that no single factor controls the ratio at which Th and Pa are adsorbed from seawater.Fluxes of 210'th and 2jlpa were less than their rates of production' in the overlying w'ater column in every trap at Sites S2, E,-3-and P. In the Panama .Basin, fluxes measured with the same traps ..¡ere greater than or tqual to their rates of production. These resul ts are a strong indication that even exti:emely 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 mixiug processes produce a net horizonUll transport of Tl: and Pa from areas of 10",1 scavenging -i-ates to areas of high scavengi,ng rates. Protactinium is redistrihuted to a gn:ater extent than Th. Fluxes of 230'1h can be used to set 1.ot"re1. 1 imits for liorizoiital transport of Pa even when absolute trapping efficiencies of the sediment traps are not known. Less than 50% of the Pa produced d t the open ocean sites is removed from the ""rater col uwn by scavenging to settling particles The remainder is removed by horizontal transport to other environments.At Sites E and P, 230'1h/23lpa ratios were identical in the deepest sediment tn1p sample and in surface sediments. Ho¡,rever, 230Th/232Tli and 23Lp~/232ih ratios were 2.5tirnes higher in trapped particlE!S than in surface sediments. The 2301'0/232'10 ratios "..ere 5.5 times higli.sr in particles filtered...

Hydrothermal vents: A previously unrecognized source of actinium-227 to the deep ocean

“…Because of its particle affinity, virtually all of the 231 Pa produced in the open ocean water column is scavenged onto particles and accumulates in sediments (Anderson et al, 1983a). Actinium-227 is more soluble than its parent (Nozaki, 1993;Anderson, 1980), thus 227 Ac produced through the decay of sediment-bound 231 Pa is released to the overlying water column.…”

“…Work by Sakamoto et al (2002) suggests that Ac also has an affinity for Fe oxyhydroxides. Although the few existing measurements of particulate 227 Ac in the open ocean suggest that it is not particle reactive (Anderson, 1980;Geibert et al, 2002), Moore (1992) suggests that 227 Ac may be more particle reactive than Ra in particle-rich environments. In this study we examine the activities of both dissolved and particulate 227 Ac in a hydrothermal plume emanating from the high temperature TAG hydrothermal vent.…”

Particle flux and trace metal residence time in natural waters1,1

“…The sources of data for particle fluxes (s), residence times of Th (TP), and mean depth (h in m) are given in Table 2. Li et al 1980Li et al 1980Knauss et al 1978Brewer et al 1980Brewer et al 1980Li et al 1980Anderson 198 1 Moore et al 198 1 Minagawa and Tsunogai 1980Minagawa and Tsunogai 1980Minagawa and Tsunogai 19 80 Santschi et al 1980Santschi et al 1980Santschi et al 1980Li et al 1979, 1981Anderson et al 1983Anderson et al 1983 (Krishnaswami et al 1976;Brewer et al 1980;Li et al 1980 comes from observations (Neihof and Loeb 1972;Hunter and Liss 1979;Lion et al 1982) that natural particles are coated with organic or Fe-Mn-hydroxide films which give them relatively uniform outer surfaces for adsorbing trace metals.…”

Distribution of thorium isotopes between dissolved and particulate forms in the deep sea

“…During the preparation of this paper we became •aware of recen t measurements by Moore [1981] Because of the large volumes of seawater that must be processed for analysis of 23øTh and 228Th, we adopted a scheme for sampling in situ. Submersible, battery-powered, pumping systems, initially described by Sp•;ncer and Sachs [1970•;and later modified during the GEOSECS program [Krishnaswami' et al, 1976b], were used to force the water first through a coarse prefilter (62-/am mesh Nitex), then through fi fine membrane filter' (1.0-/am NuclePore, preweighed), then th,rough a cartridge packed with, Nitex netting that was Coal•ed with manganese dioxide to scavenge the dissolved Th isotopes [Anderson, !981], and fina!ly through a flow meter to record the volume of water that was Complete details of our analytical methods are given by Anderson [ 1981] Table 2. The particulate Th results are based on the 1-62-tam fraction that was caught by the Nuclepore filters.…”

“…It will be argued in the following section that the tendency of the Th isotopes to be partitioned differently between dissolved and particulate forms is related to the differences in radioactive half-life and to the kinetics of the scavenging process. Measurements of U (not listed in Table 2 but given by Anderson [1981]) showed that negligible amounts of the 234Th and 23øTh in the particulate matter are supported by decay of the parent radionuclides. We assume this to be the case also for 228Th.…”