The Eastern Lau Basin Integrated Studies Site (ISS): Recent Progress and Future Plans

Rapid progress is being made in understanding the Eastern Lau Spreading Center (ELSC) Integrated Studies Site, which is a new focus area in the Ridge 2000 Program. The ELSC, located in the western Pacific near Tonga, is a 390 km-long first-order ridge segment that displays a broad range of effects of the back-arc environment. Its southern end, at only 40 km from the Tonga arc volcanic front, is propagating southward into a back-arc rift. Its northern end is 100 km from the volcanic front and terminates at a large, nontransform offset. The ELSC undergoes substantial and systematic changes in primary parameters affecting crustal accretion including spreading rate and mantle source composition. As a consequence it displays large changes along its length in lava chemistry, axial depth and morphology, melt lens characteristics, and crustal thickness and structure. A focus of the work at the ELSC is to understand how changes in these forcing functions affect crustal accretion, hydrothermal venting, and faunal composition and abundance. A geophysical/hydrothermal study of the entire ELSC during an initial R2K cruise in April-May 2004 (PI: Martinez) disclosed a surprisingly high level of hydrothermal activity along the ELSC. This survey involved shipboard multibeam, two deep-towed sonars (DSL120A and IMI30), and concurrent MAPR/Chemical Scanner, CTD/rosette tow-yos, and vertical casts. Hydrothermal activity as indicated by water column plumes increases toward the north, even though magmatic robustness decreases. An ancillary study (PI: Thurnherr) also deployed autonomous floats during this cruise to investigate deep circulation patterns that affect hydrothermal plumes and faunal dispersal. A second cruise in September 2004 (PI: Langmuir) focused on locating vent sources using ABE, petrological sampling, and determining water column properties. Three additional cruises are scheduled during 2005. The first (PI: Tivey) will provide an initial characterization of vent fields, fluid chemistry, mineralogy, and biodiversity using Jason II and net tows; a following cruise (PI: Childress) will investigate community ecology using Jason II; and a final cruise (PI: Vrijenhoek) will study genetics and species diversity in the hydrothermal vent communities.