Abstract
Thorium (Th) isotopes were applied to quantify the supply of lithogenic inputs from the Kerguelen Plateau to the Southern Ocean. The dissolved concentrations of 232Th and 230Th were measured following a novel pre-concentration method using the Nobias resin from 10 stations above and on the slopes of the plateau. Elevated 232Th/230Th ratios in the upper 500 m of the water column confirm the input of lithogenic material from islands, glaciers and the resuspension of shelf-deposited sediments. 230Th concentrations were used to calculate a scavenging residence time for Th, which was then applied to calculate the flux of dissolved 232Th required to match the observed concentrations of dissolved 232Th. The 232Th content of the lithogenic material from the Kerguelen Plateau was used with the solubility of Th reported in the literature, to estimate a lithogenic particle flux of 7 to 810 mg m–2 day–1. This flux is comparable to fluxes obtained using sediment traps (8–777 mg m–2 day–1) in other continental-margin zones of the Southern Ocean. The flux of dissolved iron, generated by the dissolution of particles, was also calculated (4,189–6,800 nmol m–2 day–1) based on the iron/thorium ratio in the material from the Kerguelen Plateau. This is higher than previous estimates (1,342 nmol m–2 day–1), suggesting the dissolution of particles as the missing source of iron, thus closing the iron budget of the region. This study confirms the utility of long-lived Th isotopes to quantify lithogenic inputs from continental margin settings.
Highlights
The different isotopes of thorium (Th) provide important information about a variety of processes that occur in the oceans
The temperature-salinity diagram (Figure 4) indicates that the samples from stations located near HIMI, all from depths < 200 m, have salinities below 34.1 g kg−1 and temperature above 2◦C, consistent with properties of the Antarctic Surface Water (AASW)
300 m there is a transition to Upper Circumpolar Deep Water (UCDW) associated with a temperature maximum of ∼2.5◦C and an oxygen minimum (180 μmol L−1)
Summary
The different isotopes of thorium (Th) provide important information about a variety of processes that occur in the oceans. The 232Th-230Th system has gained popularity as a tracer to quantify lithogenic sources of trace elements to the marine environment (Hsieh et al, 2011; Hayes et al, 2013, 2015, 2017; Deng et al, 2014). The long-lived Th isotopes can be used to track and quantify the input of lithogenic material to the ocean because 232Th acts as the tracer of lithogenic input, while 230Th provides information concerning residence time with respect to scavenging, which is assessed through its disequilibrium from its soluble parent nuclide 234U (Broecker et al, 1973). With knowledge about the composition and solubility of the lithogenic material it is possible to estimate the input and flux of lithogenic particles and trace elements to the ocean (Hsieh et al, 2011; Hayes et al, 2018)
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