Thorium speciation in seawater

Abstract

Since the 1960s, thorium isotopes occupy a special place in the oceanographer's toolbox as tracers for determining rates and mechanisms of oceanic scavenging, particle dynamics, and carbon fluxes. Due to their unique and constant production rates from soluble parent nuclides of uranium and radium, their disequilibrium can be used to calculate rates and time scales of sinking particles. In addition, by ratio-ing particulate 234 Th (as well, in principle, other Th-nuclides) to carbon (and other elements), and linking this ratio to the parent-daughter disequilibrium in the water column, it is possible to calculate fluxes of carbon and other elements. Most of these applications are possible with little knowledge of the dissolved chemical properties of thorium, other than its oxidation state (IV) and tendency to strongly sorb to surfaces, i.e., its bparticle-or surface-activityQ. However, the use of any tracer is hindered by a lack of knowledge of its chemical properties. Recent observations in the variability of carbon to 234 Th ratios in different particle types, as well as of associations of Th(IV) with various marine organic biomolecules has led to the need for a review of current knowledge and what future endeavors should be taken to understand the marine chemistry of thorium. D

A review of present techniques and methodological advances in analyzing 234Th in aquatic systems

“…Consequently, the MnO 2 adsorbers carry a net negative charge at seawater pH. In seawater, Th appears to be complexed by organic compounds, such as humic acids and acidic polysaccharides, and incorporated into submicron colloids, which also carry a net negative charge (see Santschi et al 2005). Hence, repulsion between these Th complexes and the MnO 2 surface may occur, thereby reducing collection efficiency.…”

“…Details of 234 Th speciation, modelling, conversion of 234 Th flux into the flux of carbon and other elements, and fields of expected future applications of 234 Th can be found in companion papers of Santschi et al (2005), Savoye et al (2005), Buesseler et al (2005), and Waples et al (2005), respectively, in this issue.…”

Quantifying lithogenic inputs to the North Pacific Ocean using the long-lived thorium isotopes

“…To determine 230 Th production, 234 U concentrations are estimated using measured salinity and published estimates of the salinity-U relationship in North Pacific seawater (Chen et al, 1986) and the seawater 234 U/ 238 U ratio (Andersen et al, 2010). More than 99.8% of Th in seawater, however, is 232 Th, a primordial isotope added to seawater in the dissolved pool through the partial dissolution of lithogenic materials (Santschi et al, 2006). Thorium (and therefore all Th isotopes) is highly insoluble in seawater and is rapidly removed from solution by scavenging onto particulate matter (Moore and Sackett, 1964).…”

Novel analytical strategies for tracing the organic carbon cycle in marine and riverine particles

“…By contrast, the POC fluxes we report (4.5 ± 3.9 mmol m -2 d -1 ) are on average three times lower than fluxes from other studies (12.6 ± 13.3 mmol m -2 d -1 ) due to lower POC: 234 Th ratios measured in >51 µm particles. In general, POC: 234 Th ratios can vary widely as a function of season, ecosystem composition, size-fraction, depth, and particle sampling methodology (Coppola et al, 2005;Buesseler et al, 2006;Santschi et al, 2006;Jacquet et al, 2011). In GB1 and GB2, an ecosystem effect likely accounts for the 14-fold difference in POC: 234 Th between oligotrophic waters (e.g, 0.8 µmol dpm -1 at GB2-106) and polar waters (e.g., 10.8 µmol dpm -1 at GB1-85) ( Table 3).…”