Collaborative Research: Elucidating Brine-Dominated, Segment-Scale Hydrothermal Discharge Along The Cleft Segment, Juan de Fuca Ridge

University Of Alaska Fairbanks Campus, Fairbanks AK

Investigators

Abstract

Hydrothermal Brines on the Juan de Fuca Ridge New oceanic crust is formed by magma that is injected beneath the seafloor and then cooled by seawater circulation. This hydrothermal circulation exchanges heat and chemicals between hot rocks and seawater, which produces mineral deposits and unique seafloor ecosystems. The injection of magma is irregular as is subseafloor hydrothermal circulation and the composition of the discharging fluid. During and after a magmatic injection event, the heated seawater separates into a low salinity vapor phase lacking many dissolved metals and a high salinity metal-rich brine phase. Seafloor discharge of the vapor is typically short-lived (years) and is followed by a much longer phase (decades to centuries or longer) of brine discharge. Knowledge of brine composition and its temporal and spatial evolution is currently limited. During this investigation hydrothermal fluids along the Cleft Segment of the Juan de Fuca Ridge off the coast of Oregon will be sampled and analyzed for their chemical composition. This portion of young oceanic crust experienced a magmatic event in 1986 and has been discharging brines for more than three decades. Systematic variations in the composition of these fluids will allow us to document the long-term evolution (decades) of brine-dominated submarine hydrothermal systems, characterize the transport of dissolved minerals and gases in brine phases, and contribute to our understanding of element exchange between Earth’s crust and ocean. The intrusion of magma into the oceanic crust results in the thermal and chemical modification of seawater during convective circulation within the oceanic lithosphere before discharging at the seafloor. This hydrothermal discharge impacts biogeochemical cycles in the ocean, the formation of seafloor metal-rich mineral deposits, and seafloor ecosystems. The net effect is dictated by cyclic temporal and spatial evolution of hydrothermal activity. Many systems begin with discharge of a short-lived (years), vapor phase and evolve to a longer-lived (decades to centuries to longer) discharge of brine phase. Relative to vapor-dominated hydrothermal systems, systematic time-series studies of brine-dominated systems are limited. Understanding brine-dominated hydrothermal systems is particularly important because numerical and conceptual models of magma-driven subseafloor hydrothermal circulation indicate that such circulation of brine-dominated fluids can last for centuries or longer, and potentially impact global fluxes of elements from the lithosphere to the ocean. Gas-tight fluid samplers will collect hydrothermal discharge from the Cleft Segment, which has discharge brine-dominated, hydrothermal fluids from the same seafloor features for at least 32 years. In addition, the investigators will measure discharge temperatures, deploy and recover short-term continuous fluid samplers, and deploy temperature recorders for legacy studies. Results from a shore-based fluid characterization program will address the long-term stability, connectivity, and potential changes in subsurface, segment-scale processes within brine-dominated hydrothermal systems This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.