Abstract
The oceanic crust provides one of the largest habitats for subsurface microbial life on earth, where lithoautotrophs utilize redox gradients between reduced elements in volcanic rocks and oxygenated seawater to form the basis of a deep microbial biosphere. Progressive alteration of the oceanic crust is argued to be “in part” microbially mediated, but identifying robust textural and geochemical biosignatures with good fossilization potential is challenging.
This study investigates pillow basalts from the Antarctic Australian Discordance (AAD) at the South East Indian Ridge (SEIR) containing candidate textural biosignatures in alteration products of the glassy margins (Thorseth et al., 2003). Samples include 2.5 Ma dredged seafloor basalts, and 18–28 Ma drill core samples from the Ocean Drilling Program (ODP) Leg 187. The focused ion beam (FIB) technique was used to prepare electron transparent foils across spherical microtextures in zeolite filled fractures and altered glass (palagonite), and across microtunnels at the interface of fresh and altered glass. Transmission electron microscopy (TEM) was used to map chemical and ultrastructural variations and to evaluate both biotic and abiotic origins of the candidate textural biosignatures in the FIB prepared foils.
Three foils were cut from zeolite hosted, hollow microspheres, which comprise purely Fe-oxyhydroxides, or mixed Mn–Mg, and Fe–Mn oxyhydroxides. The microspheres are 1 to 4 μm across, with a radiating ultrastructure, and have a denser inner surface and a more porous outer surface, suggesting outwards growth from a spherical initial surface. Amorphous organic carbon is associated with some of the microtextures both on the inner and outer walls. These microtextures are interpreted as mineral encrusted microbial cells. A FIB-foil was also cut from palagonite-hosted microspheres, which are more irregular in shape and partially infilled by palagonite. Amorphous organic carbon is abundant in the vicinity of the microtextures but is spatially unrelated, and may be derived from several sources. The results indicate that maturation of the palagonite, involving dehydration and recrystallization, overprints and destroys potential biosignatures in this alteration phase. In contrast, the zeolite-hosted microtextures appear to have a higher preservation potential. Tubular microtextures in the glass at the alteration front comparable to argued “bioalteration” textures are also abundant in the AAD basalts. However, their angular cross-sectional shape and lack of “bio-elements” in the palagonite infill, mean that an abiotic origin cannot be excluded.
In summary, FIB-TEM provides multiple high-resolution lines of information to characterize alteration textures in ocean floor basalts. Comparing the evidence obtained from glass hosted microtunnels, zeolite and palagonite hosted microspheres we conclude that the zeolite hosted microtextures are the strongest candidate biosignature. The combination of the size, rim ultrastructure, and elemental composition is consistent with an origin as cell encrustations, resulting from the biologically induced mineralization of microbial cells that inhabited fractures in pillow lavas both at the seafloor and the subseafloor stage.
This study investigates pillow basalts from the Antarctic Australian Discordance (AAD) at the South East Indian Ridge (SEIR) containing candidate textural biosignatures in alteration products of the glassy margins (Thorseth et al., 2003). Samples include 2.5 Ma dredged seafloor basalts, and 18–28 Ma drill core samples from the Ocean Drilling Program (ODP) Leg 187. The focused ion beam (FIB) technique was used to prepare electron transparent foils across spherical microtextures in zeolite filled fractures and altered glass (palagonite), and across microtunnels at the interface of fresh and altered glass. Transmission electron microscopy (TEM) was used to map chemical and ultrastructural variations and to evaluate both biotic and abiotic origins of the candidate textural biosignatures in the FIB prepared foils.
Three foils were cut from zeolite hosted, hollow microspheres, which comprise purely Fe-oxyhydroxides, or mixed Mn–Mg, and Fe–Mn oxyhydroxides. The microspheres are 1 to 4 μm across, with a radiating ultrastructure, and have a denser inner surface and a more porous outer surface, suggesting outwards growth from a spherical initial surface. Amorphous organic carbon is associated with some of the microtextures both on the inner and outer walls. These microtextures are interpreted as mineral encrusted microbial cells. A FIB-foil was also cut from palagonite-hosted microspheres, which are more irregular in shape and partially infilled by palagonite. Amorphous organic carbon is abundant in the vicinity of the microtextures but is spatially unrelated, and may be derived from several sources. The results indicate that maturation of the palagonite, involving dehydration and recrystallization, overprints and destroys potential biosignatures in this alteration phase. In contrast, the zeolite-hosted microtextures appear to have a higher preservation potential. Tubular microtextures in the glass at the alteration front comparable to argued “bioalteration” textures are also abundant in the AAD basalts. However, their angular cross-sectional shape and lack of “bio-elements” in the palagonite infill, mean that an abiotic origin cannot be excluded.
In summary, FIB-TEM provides multiple high-resolution lines of information to characterize alteration textures in ocean floor basalts. Comparing the evidence obtained from glass hosted microtunnels, zeolite and palagonite hosted microspheres we conclude that the zeolite hosted microtextures are the strongest candidate biosignature. The combination of the size, rim ultrastructure, and elemental composition is consistent with an origin as cell encrustations, resulting from the biologically induced mineralization of microbial cells that inhabited fractures in pillow lavas both at the seafloor and the subseafloor stage.
