Fluid mixing and the deep biosphere of a fossil Lost City-type hydrothermal system at the Iberia Margin
Frieder Kleina,1, Susan E. Humphrisb, Weifu Guob, Florence Schubotzc, Esther M. Schwarzenbachd, and William D. Orsia
aDepartment of Marine Chemistry and Geochemistry, Woods Hole Oceanographic Institution, Woods Hole, MA 02543;
bDepartment of Geology and Geophysics, Woods Hole Oceanographic Institution, Woods Hole, MA 02543;
cDepartment of Geosciences and Center for Marine Environmental Sciences, University of Bremen, 28334 Bremen, Germany;
dDepartment of Geosciences, Virginia Polytechnic Institute and State University, Blacksburg, VA 24061
Edited by David M. Karl, University of Hawaii, Honolulu, HI, and approved August 4, 2015 (received for review March 7, 2015)
Significance
We provide biogeochemical, micropaleontological, and petrological constraints on a subseafloor habitat at the passive Iberia Margin, where mixing of reduced hydrothermal serpentinization fluids with oxic seawater provided the energy and substrates for metabolic reactions. This mixing zone was inhabited by bacteria and archaea and is comparable to the active Lost City hydrothermal field at the Mid-Atlantic Ridge. Our results highlight the potential of magma-poor passive margins to host Lost City-type hydrothermal systems that support microbial activity in subseafloor environments. Because equivalent systems have likely existed throughout most of Earth's history in a wide range of oceanic environments, fluid mixing may have provided the substrates and energy to support a unique subseafloor community of microorganisms over geological timescales.
Abstract
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 13C (δ13CTOC = −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.
serpentinization microfossils lipid biomarkers brucite−calcite passive margin
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