PECKMANN, JORN¹, JOACHIM REITNER¹, VOLKER THIEL², and WALTER MICHAELIS^2
1Inst. Museum Geologie Palaontologie, Univ. Gottingen, Germany
²Inst. Biogeochemie Meereschemie, Univ. Hamburg, Germany

Abstract: Fingerprints of Methanotrophic Bacteria in a Miocene Cold Seep Carbonate

The Miocene carbonate deposits cropping out near the village of Marmorito in the Monferrato hills east of Torino (northern Italy) have been interpreted to be derived from ancient methaneseepage (CLARI et al., 1994, Geo-Marine Letters, 14: 201-209).

We investigated an intensively in-situ brecciated dolomitic rock with calcitic fracture fillings cross-cutting the micro-crystalline matrix. Dolomite d¹³C: -40 o/oo PDB) and calcite (d¹³C: -29 o/oo) are depleted in carbon-13. Dolomitic overgrowths of cloudy appearance and globular fabric occur on the clast surfaces. They exhibit an intense epifluorescence. A relation of the overgrowths to carbonate aggregation postdating brecciation is indicated by fluoresce residues enclosed in the matrix.

Possible causes for the brecciation of the seep deposit are an explosive gas release or the growth of gas-hydrates within incompletely lithified sediments. In both scenarios the sediment clasts would have been exposed to a methane-rich environment.

Organic geochemical analyzes yielded biomarker evidence for methanotrophic bacteria. Sterols and hopanols carrying an extra methyl group at C-4 and C-3 were extracted from the carbonate. Likely biological precursors of these chemo-fossils were identified in methanotrophs (NEUNLIST and ROHMER, 1985, Biochem. J., 231: 635-639).

FE-SEM images show that the overgrowths consist of two types of precipitate habits typical of a bacterial induction: crusts composed of hemispheres and rod-shaped crystal aggregates with rounded, brushlike terminations.

AAPG Search and Discovery Article #90928©1999 AAPG Annual Convention, San Antonio, Texas