[M-IS19] Biogeosciences of cold seeps, mud volcanoes, and hydrothermal vents

[MIS19-P04] Formation Processes of Oligocene Methane-seep Carbonates in Urahoro, Hokkaido, Japan.

*Daichi Yanagawa¹, Shigenori Ogihara², Robert Jenkins³, Hitoshi Tomaru⁴ (1.Chiba University Graduate School of Science and Engineering for Advanced Studies, Department of Earth and Environmental Sciences, Course of Earth Sciences, 2.Graduate School of Science, The University of Tokyo, 3.School of Geosciences and Cvil Engineering, College of Science and Engineering, Kanazawa University, 4.Department of Earth Sciences,Chiba University)

Keywords:methane seep, carbonate, Hokkaido, oligocene

Methane is often enriched in the seep fluids in the plate subduction zones and continental margins and anaerobically oxidized by sulfate in shallow marine sediments (anaerobic oxidation of methane: AOM), resulting in the formation of carbonates. Therefore, distribution and geological characteristics of such methane-seep carbonates are useful to understand the marine environments including the intensity and source of methane and microbial activities. Along the Atsunai River in Urahoro, southeastern Hokkaido, large carbonate concretions up to 6 m thick is intercalated in the mudstone of Oligocene Nuibetsu Formation. The molluscan fossils (bivalves and gastropods) typical of methane-seep have been reported, indicating the formation of carbonates in methane-rich marine environments. The objective of this study is to to elucidate the formation process of the carbonates based on the petrological and geochemical analyses.
The carbonate body is divided into five lithological units from bottom to top as; (I) massive non-stratified section with basal carbonate nodules, (II) well-defined lamina, (III) carbonate vein, (IV) complex of bivalves, nodules, and lamina, and (V) gravel-bearing carbonate-lamina complex. The stable carbon isotopic ratios ranging between -50 and -40‰VPDB with positive excursion to -31‰VPDB at the top. This indicates that this carbonate was formed mainly with the biogenic methane and somewhat higher contribution of thermogenic methane at the latest stage. The biomarker analysis also showed that the PMI (2,6,10,15,19-pentamethylicosane) derived from methanogenic and methane-oxidizing archaea and crocetane (2,6,11,15- tetrametylhexadecane) indicative of methane seep were found in all the units. microstructural observations revealed fractured clotted fabrics, probably formed by microbial activity, in at least three units (upper part of Unit I , Units IV and V), which were surrounded by carbonate minerals of anaerobic methane oxidation origin. The Urahoro carbonate body was likely formed by at least two or more significant flow of methane-rich fluids and was fractured by the enhanced fluid pressure. It is possible that the methane flux was most intensive in Unit V of early Oligocene, in which deep thermogenic methane was predominantly derived. This abrupt methane emission could be caused by the large dissolution of marine methane hydrate.