The role of scavenging and early diagenesis in controlling hydrothermal fluxes into the ocean along the Endeavour segment of the Juan de Fuca ridge

Abstract

Mixing of high-temperature hydrothermal vent fluids with seawater leads to the precipitation of a large fraction of the non-conservative element load and scavenging of elements from seawater onto the particles formed. This substantially modifies hydrothermal fluxes into the ocean relative to the flux across the seafloor at vents. However, the particles formed in the plume are rarely in equilibrium within the uppermost sediments, and scavenged elements may be loosely bound to particle surfaces, meaning that the net flux from the ocean can be readily modified by processes operating during early diagenesis. Using bulk geochemistry, and progressive leaching experiments, we characterize sediments from around the Endeavour segment of the Juan de Fuca ridge to investigate scavenging and early diagenetic processes. Changes in bulk sediment composition with distance from the ridge (e.g., decreases in Fe/Ti) support a decreased hydrothermal input off-axis. Bulk-sediment and leachate data are interpreted as indicating substantial scavenging of elements such as P, V, Cr and REEs as previously suggested, with different elements scavenged with different efficiencies in different parts of the plume. Correlations of these elements with Fe in the HCl leachates, suggests their uptake is associated with Fe-oxyhydroxides. Water column scavenging is overprinted by fluxes associated with benthic scavenging for V and REE. In contrast, early diagenesis leads to complete loss of scavenged P back to the ocean. Leaching the samples with NaOH and acetic acid provides evidence for recrystallization of reactive (biogenic and hydrothermal) silica and CaCO₃ during early diagenesis in all locations. The fraction of Fe and Mn leached by 1 M HCl in 1 h versus 24 h, and the amount leached in 24 h, vary systematically; these data can be explained by progressive recrystallization of hydrothermally-derived Fe and Mn minerals during early diagenesis. These early diagenetic reactions release almost all hydrothermally-derived Mn back into the ocean, while Fe remains largely fixed in the sediment. Large-scale loss of Co, Zn, Mo, Ag and Cd also occurs during early diagenesis, with some evidence As, Pb and U are also lost. In deeper, more reducing, sediments there is uptake of As, Mo, Ag, Cd and U, presumably via diffusion through the shallow oxic sediments. Such diagenetic processes mean that the net fluxes of elements into and out of the ocean associated with hydrothermal systems differ from those estimated from studies of either vent fluids or hydrothermal plumes.