14:15 〜 14:30
[AHW18-13] Geochemistry and stable isotope fractionation of chromium along salinity gradients in two rivers, Kochi Prefecture, Japan
キーワード：クロム同位体、化学種、元素循環、汽水域
Chromium (Cr), a redox-sensitive element, has two stable oxidation states, Cr(VI) and Cr(III), with Cr(VI) being more soluble than Cr(III) under oxidation conditions. The Cr isotope system (53Cr/52Cr relative to NIST979, hereafter δ53Cr) is a promising redox proxy as the Cr stable isotope fractionation observed in the surface environment is highly variable, mainly due to the conversion of Cr(III) and Cr(VI). Oxidative rock weathering releases isotopically heavy Cr(VI) into the rivers, where it is partially reduced by Fe2+(aq) and organic matter, and isotopically lighter Cr(III) is removed by suspended particles and sediment. Recently, δ53Cr values in marine sediments have been used to reconstruct ancient atmosphere-ocean redox conditions. However, only a few studies have investigated the Cr isotope systematics in estuarine environments in order to assess the faithful transport of oxidative weathering signals from the rivers to the oceans (Mallick et al., 2022). To better understand the geochemical and isotopic behavior of Cr in estuaries, we observed Cr concentrations, speciation, and δ53Cr along salinity gradients in the Kuma and Sakura Rivers in Kochi Prefecture, Japan.
The Kuma and the Sakura Rivers are located in Susaki City and Kochi City, respectively. River water samples and riverbed sediment samples were collected from each river in March and June 2023. River sediment samples were centrifuged, and the supernatant was collected as sediment pore water. The major anion and cation concentrations of river water samples were analyzed by IC. Trace element concentrations and Cr speciation in river water and porewater were measured by ICP-MS and HPLC-ICP-MS, respectively. The Cr isotope ratios of total Cr in river water were determined by double-spike TIMS after pre-concentration of Cr by Fe(II) coprecipitation using 0.5 to 2 L river water samples (modified from Bonnand et al., 2011), followed by 3-step column separation (Yamakawa et al., 2009).
The water samples from the Kuma River show higher Cr concentrations than those from the Sakura River, which may reflect the underlying lithology (serpentinite and limestone, respectively). In both rivers, Cr(VI) is predominant upstream, while the concentration of Cr(III) increases with salinity in the Kuma River. The δ53Cr values decreased from ~+2.5‰ to ~+1.0‰ downstream of the two rivers in the estuarine mixing zone. A simple conservative mixing model of river and seawater end members was valid for the Sakura River samples. This suggests that simple mixing with seawater may control Cr concentration and isotopic variation in the Sakura River. In contrast, it was insufficient to explain the trend in Cr concentration in the Kuma River. In the Kuma River, the logarithm of total Cr concentration and δ53Cr were negatively correlated at low salinity. This suggests that Cr(VI) may have been partially reduced by organic matter and removed from river water. Similar trends have been observed in previous studies in the Connecticut estuary, however, the fractionation factor is much smaller than reported, indicating that different isotopic fractionation factors should be considered. On the other hand, a slight increase in total Cr concentration and a large decrease in δ53Cr were observed at high salinity, suggesting the presence of a source of Cr with lower isotopic ratios. Although the source is unknown, Cr speciation analysis indicates that Cr(III) concentrations are relatively higher in pore water than in river water, suggesting that Cr(III)-organic complexes with low isotopic ratios may be percolated into river water via a concentration gradient. These results indicate that the estuarine environment can significantly modify the riverine Cr isotope signatures and that benthic flux may be one source of Cr in the estuarine mixing zone.
The Kuma and the Sakura Rivers are located in Susaki City and Kochi City, respectively. River water samples and riverbed sediment samples were collected from each river in March and June 2023. River sediment samples were centrifuged, and the supernatant was collected as sediment pore water. The major anion and cation concentrations of river water samples were analyzed by IC. Trace element concentrations and Cr speciation in river water and porewater were measured by ICP-MS and HPLC-ICP-MS, respectively. The Cr isotope ratios of total Cr in river water were determined by double-spike TIMS after pre-concentration of Cr by Fe(II) coprecipitation using 0.5 to 2 L river water samples (modified from Bonnand et al., 2011), followed by 3-step column separation (Yamakawa et al., 2009).
The water samples from the Kuma River show higher Cr concentrations than those from the Sakura River, which may reflect the underlying lithology (serpentinite and limestone, respectively). In both rivers, Cr(VI) is predominant upstream, while the concentration of Cr(III) increases with salinity in the Kuma River. The δ53Cr values decreased from ~+2.5‰ to ~+1.0‰ downstream of the two rivers in the estuarine mixing zone. A simple conservative mixing model of river and seawater end members was valid for the Sakura River samples. This suggests that simple mixing with seawater may control Cr concentration and isotopic variation in the Sakura River. In contrast, it was insufficient to explain the trend in Cr concentration in the Kuma River. In the Kuma River, the logarithm of total Cr concentration and δ53Cr were negatively correlated at low salinity. This suggests that Cr(VI) may have been partially reduced by organic matter and removed from river water. Similar trends have been observed in previous studies in the Connecticut estuary, however, the fractionation factor is much smaller than reported, indicating that different isotopic fractionation factors should be considered. On the other hand, a slight increase in total Cr concentration and a large decrease in δ53Cr were observed at high salinity, suggesting the presence of a source of Cr with lower isotopic ratios. Although the source is unknown, Cr speciation analysis indicates that Cr(III) concentrations are relatively higher in pore water than in river water, suggesting that Cr(III)-organic complexes with low isotopic ratios may be percolated into river water via a concentration gradient. These results indicate that the estuarine environment can significantly modify the riverine Cr isotope signatures and that benthic flux may be one source of Cr in the estuarine mixing zone.