Large benthic fluxes of dissolved iron in China coastal seas revealed by 224Ra/228Th disequilibria

Abstract

We report benthic flux estimates of Fe from China coastal seas based on a recently developed 224 Ra/ 228 Th disequilibrium approach. There were considerable temporal and spatial variabilities in benthic Fe fluxes, which spanned over 4-5 orders of magnitude, from <10 mmol m À2 d À1 up to $100 mmol m À2 d À1. Nonetheless, we have identified a prominent trend in China coastal seas showing that benthic Fe fluxes tended to decline exponentially with bottom depth. This trend is probably a result of kinetic energy and chemically bound energy gradients that culminate in the coastal seas. In the meantime, large benthic Fe fluxes were more frequently observed within a narrow redox window of 100-150 lM dissolved oxygen (DO) in the bottom water. This observation contradicts an early assumption that iron release from continental margin sediments increases with decreasing DO concentration in the bottom water. It possibly reflects a compromise of oxygen-demanded irrigation that acts to enhance Fe release from bottom sediments, and re-oxidation of dissolved Fe(II) that results in more efficient Fe retention near the sediment-water interface. Notably, benthic fluxes of Fe based on 224 Ra/ 228 Th disequilibria were one order of magnitude larger than those derived from the porewater concentration gradient method. Moreover, the isotopic estimates were >30 times higher than the historical measurements based on the traditional incubation method (i.e., the benthic chamber method) in other coastal seas. However, our estimates of benthic Fe flux were in general agreement with the reported flux value based on a new two dimensional imaging technique.

Dissolved Fe in the East China Sea Under the Influences of Land Sources and the Boundary Current With Implications for Global Marginal Seas

“…Studies on sediment‐water interface flux are more rare than those for water columns, limited by the difficulties in sample accessibility and by scarcity of measurements. The dFe flux from sediments to the overlying water is usually estimated via concentration gradients in pore water near sediment‐water interface and/or in‐situ benthic chamber incubations (Almroth et al., 2009; Berelson et al., 2003; Dale et al., 2015; Elrod et al., 2004; Klar et al., 2017; McManus et al., 1997; Noffke et al., 2012; Pakhomova et al., 2007; Severmann et al., 2010; Slomp et al., 1997; Warnken et al., 2001); Fe isotopes and 224 Ra/ 228 Th disequilibria have also been used in quantifying sediment‐water exchanges in recent studies (Conway & John, 2014; John et al., 2012; X. Shi et al., 2019). These studies showed that bottom sediment works as a very important dFe source contributing a large but variable amount of dFe, which can be even more significant than atmospheric depositions.…”

“…Factors influencing benthic dFe flux mainly include the oxygen concentration in the overlying water column, the organic carbon consumption rate, reactive Fe content in the sediment, as well as bio‐irrigation (Dale et al., 2015; Elrod et al., 2004). Previous studies reported a wide range of dFe concentration (∼0.2–37.5 μM) in the pore water of the surface sediment from the North Sea, Celtic Sea, Gulf of Finland, California coast, and coastal seas of China (Berelson et al., 2003; Elrod et al., 2004; Hong et al., 2018; Klar et al., 2017; McManus et al., 2003; Pakhomova et al., 2007; Severmann et al., 2010; Slomp et al., 1997; X. Shi et al., 2019; X. Zhu et al., 2017), which can be used for a preliminary evaluation combined with the near‐bottom dFe concentration in this study, through application of the Fick's first law of diffusion (Berner, 1980). With the low limit of 0.2 μM as pore water dFe concentration, the diffusive flux of Fe from sediment can be constrained in the range of 0.12–1.73 nmol/m 2 /d with an average of 0.73 ± 0.40 nmol/m 2 /d, corresponding to an annual input of (8.25 ± 4.49) × 10 3 kg Fe over the ECSS.…”

“…In comparison to other input pathways in Table S1 in Supporting Information S1, such an estimate of benthic flux for dFe can be 1-4 orders of magnitude higher than the sum of dFe flux from terrestrial sources, including riverine flux and atmospheric depositions, these values are also orders of magnitude higher than our evaluations on benthic flux mentioned above. However, results of Shi et al (2019) provide evidence that benthic sediment-water exchange should not considered as "closed systems" when dealing with its impact on chemical budgets of the water column. Similarly, the estimate based on the values from Ra/Th disequilibrium can also be higher than the dFe influx along with the exchange with Kuroshio, including contributions from TSWW and subsurface water incursion, shown in Table S1 in Supporting Information S1.…”

Dominance of benthic flux of REEs on continental shelves: implications for oceanic budgets

“…The spatial trend of diffusive Nd flux is probably affected by multiple depth-related processes. At shallow water depths the exchange between porewater and overlying seawater is fast (Shi et al, 2019;Patton et al, 2021), resulting in a small Nd gradient at sediment-water interface. In comparison, in some deep ocean sediments high reactive authigenic [Nd] might contribute to a high benthic flux (Abbott et al, 2016;Haley et al, 2017).…”

Organic Iron-binding ligands in the Arctic, Antarctic and subtropical Regions

“…Since Fe bound to organic ligands stays in solution longer, organic complexation increases the possibility of Fe-ligand complexes to be transported further away from its sources (Gerringa et al, 2015). Thus, the supply of DFe from shelf regions, specifically shelf sediments, could be an important factor in global Fe cycling as suggested previously (Shi et al, 2019). We postulate that supply of Fe from shelf regions to the open ocean is intimately linked to the dynamics of ligand production where notably microbial remineralization might play an important role (see section below).…”