Recent (2002, 2005) expeditions to the Galapagos Rift, 86°W, have conducted integrated experiments to assess temporal and spatial changes in both the chemistry and associated biology of diffuse-flow hydrothermal vents. The chemical data add significantly to time series observations of hydrothermal processes at the Galapagos Spreading Center (GSC), where seafloor venting was first discovered in 1977. Assuming quartz-fluid equilibrium (0.55 NaCl solution), end-member values for dissolved silica in 2002 and 2005 indicate temperatures ~50°–100°C lower than in 1977 (~400°C). The higher predicted temperature in 1977 is consistent with observed chloride depletion in fluids issuing from Oyster Beds vent, indicating fluid phase separation, whereas all fluid samples from 2002 and 2005 are at or above seawater concentration. Recent data for dissolved Mn further suggest a lower temperature hydrothermal reaction zone relative to three decades ago. The Li/Si ratios in 1977, however, do not differ significantly from analogous data in 2002 and 2005. This is in contrast with ratios for species known to be temperature-dependent, suggesting that other factors (e.g., fluid/rock ratio, protolith composition) may exert a greater influence on dissolved Li in the hydrothermal end-member. Geologic evidence indicates that between 1990 and 2002 a volcanic eruption covered the historical Rose Garden vent field, ending years of biological observations. Although the precise age of the lava flow is uncertain, the lack of chloride depletion and low H2S/heat ratios in the 2002 and 2005 vent fluids suggest that the 2002 expedition arrived well after the eruption based on observations from other diffuse-flow systems perturbed by volcanic activity. Lower dissolved silica, Mn, and H2S in the 2005 vent fluids relative to 2002, however, may indicate continued cooling at depth in the aftermath of the recent eruption. Geochemical modeling results of evolved seawater in equilibrium with a moderately oxidizing mineral assemblage are consistent with the chemistry of recent GSC fluid samples, predicting H2S/Fe > 1, with little Fe in fluids diluted to the extent typical of 86°W vents. In situ chemical sensor measurements indicate that at temperatures below ~10°C, dissolved sulfide species change in a complex way with temperature, suggesting the involvement of processes other than simple dilution, including, perhaps, the effects of microbial metabolism, recognized in other diffuse-flow vent systems.
|Original language||English (US)|
|Title of host publication||Magma to Microbe|
|Subtitle of host publication||Modeling Hydrothermal Processes at Oceanic Spreading Centers|
|Editors||Robert P. Lowell, Michael R. Perit, Anna Metaxas, Jeffrey S. Seewald|
|Publisher||American Geophysical Union|
|Number of pages||22|
|State||Published - Mar 19 2013|
Bibliographical noteFunding Information:
Master, officers, and crew of the R/V Atlantis. Without their help this research would not have been possible. We greatly appreciate the counsel of Rachel Haymon, Ken Macdonald, and Michael Perfit, all of whom provided important insight into the geological and geophysical characteristics of the GSC. We also thank Rick Knurr and Mikaella Rough for their careful and diligent chemical analysis of the 2005 fluid samples, and Justin Revenaugh for creating the algorithm used for the end-member extrapolations. Finally, thoughtful reviews by Wolfgang Bach, Mike Mottl, and Bob Lowell helped to greatly improve the clarity and content of this manuscript. Research funding was largely provided (to T.S.) by NOAA’s Office of Ocean Exploration (NA04OAR4600084), the Cooperative Institute for Climate and Ocean Research (NA17RJ1223), and WHOI through the Vetlesen and Richards Foundation Grants and a Fellowship through the Deep-Ocean Exploration Institute. This publication is also partially funded by the Joint Institute for the Study of the Atmosphere and Ocean (JISAO) under NOAA Cooperative Agreement No. NA17RJ1232, contribution no. 1418, PMEL contribution no. 3088 and through NSF grants OCE-0351069 (W.E.S.), OCE-0525907 (K.D.), and EAR-0649044 (M.H.).
- Hydrothermal circulation (Oceanography)-Mathematical models
- Hydrothermal vents-Microbiology
- Mid-ocean ridges
- Sea-floor spreading
- Seawater-Thermodynamics-Mathematical models