[B-PT08] Evolution of Chemosynthetic Ecosystem in Earth History

[BPT08-04] Mass occurrence of the enigmatic gastropod Elmira in the Late Cretaceous Sada Limestone seep deposit in southwestern Shikoku, Japa

*Takami Nobuhara¹, Daigaku Onda¹, Takuya Sato¹, Hidemi Aosawa¹, Toyoho Ishimura², Akira Ijiri³, Urumu Tsunogai⁴, Naoki Kikuchi⁵, Yasuo Kondo⁶, Kiel Steffen⁷ (1.Science Education (Geology), Faculty of Education, Shizuoka University, 2.Department of Chemistry and Material Engineering, National Institute of Technology, Ibaraki College, 3.Kochi Institute for Cores Sample Research, JAMSTEC, 4.Graduate School of Environmental Studies, Nagoya University, 5.Museum of Nature and Human Activities, Hyogo, 6.Sciences Unit, Natural Sciences Cluster, Kochi University, 7.Department of Palaeobiology, Naturhistoriska riksmuseet, Stockholm)

Keywords:chemosynthesis, cold seep, Gastropods, Cretaceous, Shikoku

Elmira is a medium- to large-sized gastropod genus, which has so far been recorded only from a presumably Eocene methane-seep deposit in Cuba, and its systematic affinity and paleoecology are unknown. We report a mass occurrence of Elmira sp. from a Late Cretaceous seep deposit in Shikoku, Japan, called Sada Limestone, with its mode of fossil occurrence, carbonate petrology, and stable carbon isotope analyses. Sada Limestone is characterized by the dominant occurrence of a large-sized thyasirid bivalve "Thyasira" hataii and serpulid worm tubes. The mass occurrence of Elmira sp. occurs as a lens-shaped carbonate body, 6.5 m in length and less than 2 m in thickness, intercalated in the thyasirids-rich limestone. The Elmira-rich lens body has a flat top and a concave base, and consists of multiple shell accumulation layers, which were formed by winnowing and filling of a depression in slope mud. The rare occurrence of Elmira sp. elsewhere in the Sada Limestone suggests that it lived in local aggregations in the vicinity of the depression. The matrix of the Elmira mass occurrence is rich in dolomite and ankerite and is less depleted in ¹³C (δ¹³C values of calcite: –5.3 to –2.4‰; of dolomite: –8.3‰) than the calcitic matrix of the surrounding limestones. This suggests that the Elmira mass occurrence was cemented below the sulfate reduction zone and thus with little influence of anaerobic methane oxidation. It is, therefore, difficult to consider that Elmira sp. harbor chemosymbiotic bacteria. As some trochiform gastropods do in seep sites, Elmira sp. was maybe a bacteria grazer gregarious on bacteria mats and/or hard bottoms.