Geochemical variation in hydrothermal sediments at the Endeavour Hydrothermal Field, Juan de Fuca Ridge

Abstract:

Sediments near hydrothermal vents are enriched in metals derived directly from hydrothermal fluids (e.g., Fe, Cu, Zn, Mn) and those associated with scavenging by hydrothermally derived Fe-oxyhydroxides (e.g., V, Co, As, Y, REEs). Hydrothermal sediment accumulation rate and chemical composition serve as records of vent activity over time. A high resolution analysis of the chemical composition of a 51 cm sediment core from 3 km off-axis of the Endeavour hydrothermal field (Juan de Fuca Ridge) is used here to investigate variation in hydrothermal accumulation over the past ~6,000 yrs (based on ¹⁴C analysis of foraminifera).

In the core there are two intervals with elevated concentrations of hydrothermally derived elements as well as those elements scavenged from seawater onto hydrothermal particles. A concentration peak in the upper core shows enrichment relative to expected detrital contributions in Zn, Mn, and Fe by 20-40% and Cu by 80%. Similar enrichments are seen in scavenged elements (e.g., V, Co, As, Y, REEs). The lower zone shows similar enrichments in Fe and Mn (20-30%) and are less enriched in Cu and Zn (10-30%). In this zone, scavenged elements also show similar enrichments to hydrothermal elements. The variation in the accumulation of the hydrothermal component on a thousand year timescale over the last 6 kyr can be compared to that seen over glacial-interglacial timescales on the Juan de Fuca Ridge (75% and 160% increase in Fe and Cu, respectively, over glacial to interglacial transitions [1]. Our variations on a thousand year timescale could result from more or less constant hydrothermal input where the composition and accumulation rate varies due to changes in bottom water chemistry. Or these variations could be linked to physical processes that change plume output, plume direction and focussing of sediments on the ridge flank.

[1] Costa et al, 2017, EPSL, 479, 120-132.