Impact of Microbial Diagenesis from Deep-Water Carbonate Mound Formation in the Gulf of Cadiz
Judith A. McKenzie1, Stefanie P. Templer1, Crisogono Vasconcelos1, and Vitor Magalhães2
1Geological Institute, ETH Zürich, Zürich, Switzerland
2Department of the Geophysical Sciences, The University of Chicago, Chicago, IL
Carbonate mud mounds, found in marine environments from shallow- to deep-water settings, span from Proterozoic to recent time. Mound building seems to be a fundamental but still enigmatic strategy for life. However, the processes initiating and maintaining mound structures remain obscure. What factors control and sustain their development? Numerous geochemical studies of pore waters and associated authigenic carbonates have effectively implied the importance of microbial metabolism in early diagenetic processes in deep-sea hemipelagic sediments. With the discovery of viable microbial populations at variable, but significant depths in marine continental shelf sequences, the recognition of an important in situ microbial factor in early diagenesis has been further strengthened. The Gulf of Cadiz, situated at the tectonically active, Africa-Iberia plate boundary, provides an exceptional environment to explore the importance of microbial diagenesis in the growth of sub-Recent carbonate mounds, which are found there flanked by giant mud volcanoes in water depths of 500-1000 m. In particular, the presence of widespread methane derived authigenic dolomite in the form of crusts, nodules and chimneys, as well dispersed micritic cement, has been interpreted as evidence for a deep methane flux, both focused flow along fluid conduits and diffusive flow. This interpretation is enhanced by stable carbon-isotope values measured in both the pore waters and solid phases and the pore-water geochemistry, pointing to a symbiotic coupling between bacterial sulfate reduction and anaerobic methane oxidation. The documented importance of microbial diagenesis in the Gulf of Cadiz carbonate mound/mud volcano complex supports an internal biogeochemical control on the development of these structures, which may be an important factor for their potential preservation in the rock record.
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