The temporal variability of hydrothermal activity of Wocan hydrothermal field, Carlsberg Ridge, northwest Indian Ocean

Abstract

The Wocan is a basalt-hosted hydrothermal field on the slow-spreading Carlsberg Ridge (CR), northwest Indian Ocean. It consists of two hydrothermal sites: Wocan-1 which is active, and Wocan-2 which is inactive. Three push-cores collected by the manned submersible Jiaolong from these two sites were investigated on their mineralogy, geochemistry, Pb-Sr isotopes, and chronology to understand the characteristics of hydrothermal sediments and their hydrothermal history. Core JL125-S17 (26-cm long) was collected from the Wocan-1 site with its lithology characterized by metalliferous sediments, mainly composed of sulfides, sulfates, Fe-oxides, and Fe-oxyhydroxides. Cores JL123-S03 (18-cm long) and JL124-S04 (20 cm long) were retrieved from the Wocan-2 site, mainly composed of calcareous oozes. Geochemically, Wocan-1 sediments exhibit strong metal and S enrichment. Ag, As, Cd, Co, Cu, Fe, Mg, Pb, Zn, and S are mainly incorporated into sulfides, while As, Ga, Mo, P, U, and V are mainly scavenged from seawater by Fe-Mn oxyhydroxides. In contrast, Wocan-2 sediments are rich in Ca and have much lower heavy metal content. The temporal variations of hydrothermal metal fluxes, which corrected for lithogenic input with Ti show that the intensity of hydrothermal activity at the Wocan-1 site has been extremely strong over the past ~1069 years, with hydrothermal Fe (FeH) fluxes varying from 9483 to 10834 mg·cm−2·kyr−1. During this period, 699–1069 yr BP and 0–411 yr BP represent two stages of increased hydrothermal activity. The Wocan-2 site has been inactive since 4864 ± 114 years before present (yr BP) but the cores still contained weak hydrothermal signatures evidenced by a slightly elevated flux of FeH at the base of the cores, which could be explained by the mineralization of low-temperature diffuse flow and distal plume fall-out from Wocan-1.