Rare Earth Element (REE) Scavenging in the Southeast Pacific Hydrothermal Plume: Implications for Interpretation of Paleo-REE Patterns in Metalliferous Sediments

The rare earth elements (REE) are a particularly useful group of elements for probing scavenging processes and their impact on particle-reactive trace elements. Large hydrothermal plumes provide an opportunity to study these effects, as new authigenic Fe and Mn particles introduce fresh adsorptive surfaces to the deep ocean. While hydrothermal fluids are enriched in REEs, initial dilution with ambient seawater makes this enrichment negligible, and subsequent scavenging within neutrally-buoyant plumes means hydrothermal systems are net sinks for REE from the ocean. Because the whole ocean volume may circulate through such plumes every few thousand years, the effect on the REE absolute concentrations, inter-element ratios, and Ce anomalies in the global ocean could be substantial. Here we present REE distributions in suspended particles in the 4000 km-long hydrothermal plume extending west from the Southern East Pacific Rise, sampled during GEOTRACES GP16. From studies in the near-field of other Atlantic and Pacific hydrothermal systems, it is known that particulate REE concentrations form a convex-upward regression against particulate Fe (representing the major scavenging host), such that REE/Fe ratios increase with distance downplume from the ridge source. Competing explanations for this curvature have been offered previously by two authors of this presentation: (1) kinetically-controlled continual adsorption of dissolved REEs (dREEs) onto Fe particles as they are dispersed down-plume; or (2) ubiquitous maintenance of adsorptive equilibrium (constant Kd), with scavenging drawdown of dREEs in the proximal plume and increasing dREEs downplume upon continued mixing with ambient deepwater. The new pREE data, in combination with complementary dREE data, will allow us to distinguish between these alternative mechanisms, as a basis for exploring generally the behavior of scavenging-prone elements in hydrothermal plumes. We will also show REE data for the unconsolidated fluff layer and consolidated core-top sediments underlying the plume, allowing us to examine whether the ambient REE composition of deepwater in the past ocean may be preserved in regional sediments.