Intellectual Merit: Hydrothermal activity at seafloor spreading centers largely controls the near-axis thermal structure and thus the important physical characteristics of the mid-ocean ridge. Hydrothermal processes also sustain a diverse biological ecosystem, form mineral deposits, and are important components of chemical and energy flux between the solid Earth and the oceans. Current models view near-axis hydrothermal venting as concentrated within a very narrow region at the ridge crest. In contrast, seismic tomographic data at fast spreading centers image a crustal low velocity volume (LVV) underlying the ridge axis which rapidly transitions to higher velocities a few km (2-4) from the axis. The high seismic velocity gradients are interpreted as zones of rapid cooling of the bulk of the crust from near magmatic (melted or partially-melted) to much cooler flanking temperatures and broadly coincide with the onset of large flanking faults. The close proximity of large faults surrounding a hot partly molten crustal volume in a zone of high thermal gradients suggests that these areas are favorable sites for hydrothermal venting, as is also predicted by some numerical models and geologic studies at ophiolites ("fossil" geological cross-sections of such hydrothermal systems, exposed on land). Yet to-date no survey has systematically examined the lateral extent and possibly expanded off-axis range of hydrothermal venting suggested by these studies. Here we propose to carry out a closely-spaced near-bottom survey encompassing the near axis region and spanning several segments of the Eastern Lau Spreading Center to determine the distribution of hydrothermal venting and the geologic and geophysical characteristics of the underlying seafloor. The survey will utilize the deep-towed DSL120 side-scan sonar and magnetometer, an array of miniature autonomous plume recorders (MAPRs), an in-situ chemical scanner and CTD hydrocasts. Eh sensors on the new MAPRs measure chemical reduction potential and are thus sensitive to hydrothermal plumes independently of standard MAPR optical turbidity and temperature measurements. The survey will achieve complete sonar imagery of the area at a resolution sufficient to identify geologic structures at the vent scale, map hydrothermal plumes within a ~500 m vertical swath within the surrounding water volume, and determine temperature and chemical characteristics of identified plumes. Additional vertical and "tow-yo" hydrocasts will be made at prominent sites to better resolve vent locations and obtain water column and suspended particulate samples to further examine their chemistry. The selected survey area encompasses the ABE to Kilo Moana vent focus sites of the Lau ISS and is an area of known high plume incidence. It spans the transition from axial high to axial valley, the magma lens to no magma lens transition, the change from volcanically dominated to faulted terrain, and the basaltic to andesitic lava transition along axis. Thus any changes in the nature and distribution of hydrothermal activity can be correlated with the fine-scale structure of the seafloor and with these larger changes in ridge physical parameters. The survey will determine the ridge flank distribution and nature of hydrothermal venting, its association with seafloor structures and test the alternative hypotheses across contrasting ridge terrains that a) hydrothermal activity is highly confined to a very narrow region of the ridge crest or b) that significant venting extends several km from the ridge axis. Resolving these alternative hypotheses will fundamentally affect our understanding of how hydrothermal activity controls ridge axis thermal structure.
Broader Impacts: The area selected for the survey encompasses the primary ABE focus and Kilo Moana comparison sites of the Lau ISS and thus the survey data will compliment and facilitate other studies here. This area is also the site of a planned seismic tomography experiment that aims to determine the crustal and upper mantle velocity structure. Not only will our proposed survey help to optimally plan this major experiment by providing detailed geologic context for OBS locations but once the seismic work is completed the mapped plume distribution and correlated geology and geophysics can be directly linked with the underlying seismic structure. The potential discovery of significant off axis venting will open a new class of biological vent environments for study. Off-axis vents would likely be hosted by large fault structures and therefore potentially be more stable and longer-lived and thus more likely to host large and economically important massive sulfide deposits than less tectonically stable ridge-crest sites. The project will also form a major part of a Master's or PhD level thesis for a graduate student and thus advance NSF educational and human resource development goals.
