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1 School of Ocean and Earth Science and Technology, University of Hawaii, Honolulu, Hawaii 96822
2 Institute of Ocean Sciences, University of Alaska, Fairbanks, Alaska 99775
3 Pacific Marine Environmental Laboratory, National Oceanic and Atmospheric Administration, 7600 Sand Point Way, Seattle, Washington 98115
4 Pacific Geoscience Centre, Geological Survey of Canada, P.O. Box 6000, Sidney, British Columbia V8L 4B2, Canada
5 Marine Resources Institute, University College, Galway, Ireland
6 Rosenstiel School of Marine and Atmospheric Sciences, University of Miami, Miami, Florida 33149
7 School of Oceanography, University of Washington, Seattle, Washington 98195
8 Department of Biology, Western Washington University, Bellingham, Washington 98225
We have located warm springs on an isolated basement outcrop on 3.5 Ma crust on the eastern flank of the Juan de Fuca Ridge in the northeast Pacific Ocean. These are the first ridge-flank hydrothermal springs discovered on crust older than 1 Ma. The springs are venting altered seawater at 25.0 °C along a fault near the summit of Baby Bare outcrop, a high point along a ridge-axis-parallel basement ridge that is otherwise buried by turbidite sediment. Baby Bare is a small volcano that probably erupted off-axis ca. 1.7 Ma; it is thermally extinct, but acts as a high-permeability conduit for venting of basement fluids. The springs have been sampled from the manned submersible Alvin. Compared with the ambient ocean bottom water, they are heavily depleted in Mg, alkalinity, CO2, sulfate, K, Li, U, O2, nitrate, and phosphate, and enriched in Ca, chlorinity, ammonia, Fe, Mn, H2S, H2, CH4, 222Rn, and 226Ra. The springs appear to support a community of thysirid clams. Although we saw no obvious bacterial mats, the surficial sediments contain the highest biomass concentrations ever measured in the deep sea, based on their phospholipid phosphate content. Areal integration of Alvin heat-flow and pore-water velocity data yields flux estimates of 4–13 L/s and 2–3 MW for the total (diffuse and focused) hydrothermal output from Baby Bare, comparable to that from a black smoker vent on the ridge axis. Warm springs such as those on Baby Bare may be important for global geochemical fluxes.
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