Alteration microstructures in seafloor basaltic glass: New material for exploring the subseafloor deep biosphere
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Abstract:
Bioalteration microstructures in basaltic glasses are those pits and grooves formed by microbial metabolisms. Microbes produce organic acid while absorbing nutrients within basaltic glasses, dissolving the basaltic glasses and forming bi-oalteration microstructures. Bioalteration microstructures within basalts not only occur in modern oceanic crusts, but have been discovered in ophiolites and greenstone belts representing ancient oceanic crusts. Studying the morpho-logical characteristics, formation mechanisms and spatio-temporal distributions of bioalterated microstructures within basaltic glasses is thus pivotal to exploring the origin and evolution of early lives on Earth, and provide new materials to understand the composition of microbial organisms in the sub-seafloor deep biosphere and their spatio-temporal dis-tribution characteristics. Morphologically, bioalterated microstructures within basaltic glasses can be divided into granular and tubular microstructures. However, abiotic microstructures can also occur in basalt glasses mimicking those bio-alterted microstructures. Thus, multi-disciplinary studies combining morphologies, elemental and isotopic geo-chemistry, organic and DNA detection are all significant in distinguishing bioalterated microstructures from abiotic ones. A summary of occurrences of bioalteration microstructures from geological records show that they occur more common than previously thought, and the vertical distribution of these microstructures within oceanic crusts may be closely related to the porosity and permeability of basaltic rocks. In addition, different seafloor environmental condi-tions, such as redox, pH, water depth, temperature, and porosity, can all greatly affect the composition of microbial community and their subsequently formed bioalteration microstructures. It remains to be answered, however, that which of these factors play a major control.
