Fluid mixing and the deep biosphere of a fossil Lost City-type hydrothermal system at the Iberia Margin.
Subseafloor mixing of reduced hydrothermal fluids with seawater is believed to provide the energy and substrates needed to support deep chemolithoautotrophic life in the hydrated oceanic mantle (i.e., serpentinite). However, geosphere-biosphere interactions in serpentinite-hosted subseafloor mixing zones remain poorly constrained. Here we examine fossil microbial communities and fluid mixing processes in the subseafloor of a Cretaceous Lost City-type hydrothermal system at the magma-poor passive Iberia Margin (Ocean Drilling Program Leg 149, Hole 897D). Brucite-calcite mineral assemblages precipitated from mixed fluids ca. 65 m below the Cretaceous paleo-seafloor at temperatures of 31.7 ± 4.3 °C within steep chemical gradients between weathered, carbonate-rich serpentinite breccia and serpentinite. Mixing of oxidized seawater and strongly reducing hydrothermal fluid at moderate temperatures created conditions capable of supporting microbial activity. Dense microbial colonies are fossilized in brucite-calcite veins that are strongly enriched in organic carbon (up to 0.5 wt.% of the total carbon) but depleted in (13)C (δ(13)C(TOC) = -19.4‰). We detected a combination of bacterial diether lipid biomarkers, archaeol, and archaeal tetraethers analogous to those found in carbonate chimneys at the active Lost City hydrothermal field. The exposure of mantle rocks to seawater during the breakup of Pangaea fueled chemolithoautotrophic microbial communities at the Iberia Margin, possibly before the onset of seafloor spreading. Lost City-type serpentinization systems have been discovered at midocean ridges, in forearc settings of subduction zones, and at continental margins. It appears that, wherever they occur, they can support microbial life, even in deep subseafloor environments.
Department of Marine Chemistry and Geochemistry, Woods Hole Oceanographic Institution, Woods Hole, MA 02543; fklein@whoi.edu.
Frieder
F
Klein
Department of Geology and Geophysics, Woods Hole Oceanographic Institution, Woods Hole, MA 02543;
Susan E
SE
Humphris
Department of Geology and Geophysics, Woods Hole Oceanographic Institution, Woods Hole, MA 02543;
Weifu
W
Guo
Department of Geosciences and Center for Marine Environmental Sciences, University of Bremen, 28334 Bremen, Germany;
Florence
F
Schubotz
Department of Geosciences, Virginia Polytechnic Institute and State University, Blacksburg, VA 24061.
Esther M
EM
Schwarzenbach
Department of Marine Chemistry and Geochemistry, Woods Hole Oceanographic Institution, Woods Hole, MA 02543;
William D
WD
Orsi
D001267:Atlantic Ocean; D018533:Biomass; D002119:Calcium Carbonate / Q000737:chemistry; D002244:Carbon / Q000737:chemistry; D052837:Chemoautotrophic Growth / Q000502:physiology*; D002851:Chromatography, High Pressure Liquid; D005580:Fossils; D060086:Hydrothermal Vents; D008276:Magnesium Hydroxide / Q000737:chemistry; D013058:Mass Spectrometry; D064307:Microbiota; D010163:Paleontology; D012623:Seawater / Q000737:chemistry* / Q000382:microbiology; D013696:Temperature
D002244:Carbon; D002119:Calcium Carbonate; D008276:Magnesium Hydroxide
brucite−calcite; lipid biomarkers; microfossils; passive margin; serpentinization