Hydrothermal signatures in the southern Okinawa Trough detected by the sequential extraction of settling particles

Abstract Sediment trap samples collected over 6 months at two depths within the southern Okinawa Trough (SOT) have been analyzed for phase associations of Al, Fe, Mn and Pb by six-stage sequential extraction (SSSE). The deeper samples are characterized by the presence of hydrothermal constituents (such as Mn and likely Fe and Pb), while shallower samples are representative of background-settling particles originating from laterally transported sediments. These results suggest that the extraction scheme is appropriate for detecting particles affected by hydrothermal emanations. Analytical results show that shallower and deeper samples have similar total Al contents and total Fe contents; by contrast, Mn and Pb are 1.7- and 1.6-fold higher in the deeper samples than in the shallower samples, respectively. Additionally, the two depths have almost the same phase associations for Al and Fe, but largely different phase associations for Mn and Pb. Compositional differences in Mn and Pb between the two depths depend essentially on the relative abundance of the fresh oxide fraction. The observed phase association suggests that Mn anomalies in the deeper samples are derived preferentially from adsorptive oxidation of dissolved hydrothermal Mn onto settling particles and/or incorporation of very fine Mn-enriched hydrothermal precipitates within settling particles. Pb anomalies in the deeper samples imply either a similar hydrothermal origin and/or scavenging of dissolved Pb from ambient water by newly formed oxides/hydroxides. The constant concentration of Mn within the deeper samples may reflect phase equilibrium between dissolved Mn and settling particles. Hydrothermally derived fluxes, defined as the fraction from hydrothermal contributions and/or indirectly by processes related to hydrothermal activities, have been estimated at 7.0±3.1 mg/m 2 /day for Fe, 2.3±1.0 mg/m 2 /day for Mn and 0.07±0.03 mg/m 2 /day for Pb. These fluxes are higher than those found at other hydrothermal systems owing to high loads of settling and suspended particles.