Quantifying Lithogenic Inputs to the Southern Ocean Using Long-Lived Thorium Isotopes

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 232 Th and 230 Th were measured following a novel pre-concentration method using the Nobias resin from 10 stations above and on the slopes of the plateau. Elevated 232 Th/ 230 Th 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. 230 Th concentrations were used to calculate a scavenging residence time for Th, which was then applied to calculate the flux of dissolved 232 Th required to match the observed concentrations of dissolved 232 Th. The 232 Th 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.

Glacial heterogeneity in Southern Ocean carbon storage abated by fast South Indian deglacial carbon release

“…1950) levels (Δ 14 C atm , ShCal13, error bars show 1σ-standard deviations (SD)) 26 compared to predicted (i.e. modelled) 14 C changes in the atmosphere due to variations in cosmogenic production 3 (with error bars showing 1σ-SD), b production-corrected 3 variations in Δ 14 C atm , with error bars showing 1σ-SD, c atmospheric CO 2 (CO 2,atm ) variations 32 , and d Antarctic temperature variations represented by water isotope changes, δD, in the Antarctic EPICA Dome C (EDC) ice core 25 , and e water isotope changes, δ 18 O, in Greenland ice core NGRIP 74 . Vertical bars indicate intervals of rising CO 2,atm levels.…”

“…In addition, sedimentation in these areas can be highly dynamic with common occurrences of highaccumulation drift deposits 14,15 , often preventing robust age models to be developed. In particular, stratigraphic alignments of sedimentary iron concentrations to Antarctic ice-core dust as recently employed for the Southwest Indian Ocean 16 may be problematic as the lithogenic fraction may likely not be solely of aeolian origin 17,18 . Many of these limitations were circumvented through paired uranium series-and 14 C-dated corals in the Drake Passage upwelling hotspot region 8 complemented by deep-ocean 14 C ventilation reconstructions from the South Atlantic 5 .…”

Canadian Arctic Archipelago Shelf‐Ocean Interactions: A Major Iron Source to Pacific Derived Waters Transiting to the Atlantic

“…Therefore, the lack of relationship of Fe distributions and N * (Figure S4), the modest increase of dFe and dMn (up to ∼7 and 11 nmol kg −1 , respectively; Figures 3a and S4), the overwhelming dominance of lithogenic pFe (Figures 3 and S2) and the strong sediment resuspension events (Figure 5), all point to non‐reductive sedimentary sources (desorption/dissolution) of Fe in the CAA. Non‐reductive dissolution of lithogenic material is increasingly being recognized as an important, but often underestimated, source of Fe to ocean waters in the Pacific, Atlantic and Southern oceans (Abadie et al., 2017; Conway & John, 2014; Homoky et al., 2016; Pérez‐Tribouillier et al., 2020). Although no trace metal samples were collected in Jones Sound and Nares Strait during the Canadian GEOTRACES GN02 and GN03 cruises, similar processes are expected to control Fe distributions in these regions, where transmissivity drops and tidal stresses (and thus the prevalence of sediment resuspension) are comparable to those in the central sills area of the CAA (Figures 5 and S3).…”

“…Reductive dissolution of Fe in oxygen‐deficient continental sediments has long been documented as the dominant input mechanism of dissolved species of Fe at the sediment‐water interface (Burdige, 1993; Elrod et al., 2004; Severmann et al., 2010). However, more recently, non‐reductive sedimentary dissolution has been proposed as a major source of dissolved Fe (Abadie et al., 2017; Conway & John, 2014; Homoky et al., 2016; Jeandel & Oelkers, 2015; Pérez‐Tribouillier et al., 2020; Radic et al., 2011).…”

Slowly Sinking Particles Underlie Dissolved Iron Transport Across the Pacific Ocean

“…Observational data suggest that Fe is sourced from continental shelf sediments and supplied via lithogenic particles (Pérez‐Tribouillier et al., 2020). Model studies have demonstrated that lithogenic particles can also have a significant impact on both the source and sink of DFe (Beghoura et al., 2019; Pagnone et al., 2019; Ye & Völker, 2017).…”