Seewald, Wheat

White Paper Title: 
Temporal Evolution of Vent Fluid Chemistry at the Lau ISS

One fundamental aspect of the R2K program is to elucidate causes and effects of temporal variability during the convective circulation of seawater-derived hydrothermal fluids at oceanic spreading centers. Because the abundance of individual chemical species in hydrothermal fluids are influenced to different extents by subsurface processes, temporal variations represent a powerful tool to constrain the influence of important variables such as temperature, pressure, water/rock ratio, phase separation, injection of magmatic volatiles, and the rates and types of subsurface microbial respiration. Temporal variability in fluid flux and composition also has direct implications for physiological diversity and community structure within vent ecosystems. Accordingly, monitoring the temporal evolution of hydrothermal fluid chemistry has been a major research focus for ISSs located at 9-11°N on the East Pacific Rise and the Endeavour Segment of the Juan de Fuca Ridge. ISS related field work at Lau Basin, however, is relatively recent and previous studies of hydrothermal activity are limited, resulting in the near absence of time-series datasets for hydrothermal activity in back-arc environments. The few available observations suggest that hydrothermal activity may evolve rapidly, although it is presently unclear whether existing models for the temporal evolution of mid-ocean ridge hot-springs (e.g., Butterfield et al., 1997) apply to back-arc environments.

Initial expeditions characterized six sites of hydrothermal venting on the ELSC, showing a range of crustal, fluid, and biological characteristics. Unlike the basalt-hosted hydrothermal systems to the north, the southernmost felsic systems (Mariner and Vai Lili) have high concentrations of magmatic gases that influence fluid composition, depositional structures, and biological communities. Owing to the recent initiation of field work at the Lau ISS, time series data for hydrothermal activity in this region is extremely limited. Preliminary data indicate substantial changes in fluid composition at Vai Lili over a 16-year span during which vent fluid temperature decreased from 334°C to 120°C in 2005. These results, sparse data from 2006, and shipboard pH and temperature data from 30 samples collected in 2009 suggest that the temporal evolution of hydrothermal systems influenced by degassing of felsic magmas may be relatively rapid and follow a path that is not analogous to mid-ocean-ridge hot-springs.

We are currently working to fill this gap in our knowledge by analyzing vent fluids collected in 2009 from 30 discrete isobaric, gas-tight samplers and 7 OsmoSamplers (continuous fluid samplers). These data allow us to gauge the time scales over which back-arc hydrothermal systems evolve and the role that magmatic volatiles have in the evolution of hydrothermal systems and will guide a new temporal and/or spatial conceptual model for an individual vent site or for the entire ELSC. New data available for the Portland meeting will help inform discussions that address the composition and evolution of vent ecosystems, temporal variability in the flux of heat and mass during hydrothermal activity in back-arc settings, magmatic evolution, and the formation of seafloor metal sulfide deposits.