Abstract
Early diagenesis processes and the geochemistry of redox-sensitive elements (RSEs) in four sediment cores in an active hydrothermal field in the southern Okinawa Trough (OT) was investigated. Dissolved Fe, Mn, ΣHS–, and several other RSEs (Mo, U, and V) in pore water were measured. We also studied metal distribution in sediments using sequential extraction methods. Very high dissolved Fe concentrations (∼140 μmol L–1) but insignificant dissolved Mn were observed in surficial pore waters in the station adjacent to the hydrothermal vent, where highly reactive Fe, Mo, U in the sediments were also measured. Such an atypical diagenetic sequence found in those cores could be driven by the overwhelmingly high reactive Fe fraction (mostly Fe oxides) delivered from the vents. Consequently, significant upward benthic fluxes of Fe and Mo were estimated for the studied stations. In addition, we performed a principal component analysis (PCA), together with relative ratios of carbonate-related elements (Sr, Ca, Mg), to identify particles’ origins in the hydrothermal field; two endmembers being the hydrothermal source and hydrogenous processes. This comprehensive study on a unique set of samples collected by advanced technology provided valuable data to demonstrate distinctive geochemical features that occur in hydrothermal sediments.
Highlights
Submarine hydrothermal vents are a widespread feature of the ocean seafloor, in the midocean ridges, and in the island arcs (Hawkes et al, 2014; Beaulieu et al, 2015)
At stations remotely operated vehicle (ROV) 10-2 and 11-1, we observed dissolved Mn peaks in pore water and Mn enrichments, with enrichment factors of 3.4 and 2.9, respectively, in surface sediments (Figures 2C–E, 3G,H), in agreement with the conventional literature reporting on Mn geochemistry in oxic hemipelagic sediments, as well as the sediment pore water profiles we observed in northern Okinawa Trough (OT) (Wang et al, 2019)
Due to the observed higher fractions of FeHR in near-field hydrothermal sediments (ROV 101, Figure 6), we suggest that the unusual diagenetic sequence observed in core ROV10-1 and ROV11-2 was mainly driven by the presence of highly reactive Fe, which was used as the primary electron acceptor, instead of Mn oxides
Summary
Submarine hydrothermal vents are a widespread feature of the ocean seafloor, in the midocean ridges, and in the island arcs (Hawkes et al, 2014; Beaulieu et al, 2015) Since their first discovery in late 1970s, hydrothermal vents are recognized as a potentially important source and sink of elements in the deep ocean (Edmond et al, 1979). Our understanding of how early diagenesis processes affecting those metals’ geochemistry in hydrothermal sediments remains largely fragmented. This is likely due to the difficulties in finding steadily deposited sediment layers close to the hydrothermal vents and collecting such sedimentary materials near the vents
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