[B-PT08] 化学合成生態系の進化をめぐって

[BPT08-04] 高知県の後期白亜紀の冷湧水性石灰岩（佐田石灰岩）より巻貝類 Elmira の大量産出

*延原 尊美¹、恩田 大学¹、佐藤 拓弥¹、青澤 秀実¹、石村 豊穂²、井尻 暁³、角皆 潤⁴、菊池 直樹⁵、近藤 康生⁶、Kiel Steffen⁷ (1.静岡大学教育学部理科教育講座地学教室、2.茨城高等専門学校 物質工学科、3.海洋研究開発機構 高知コア研究所 地下生命圏研究グループ、4.名古屋大学大学院環境学研究科、5.兵庫県立人と自然の博物館、6.高知大学自然科学系、7.ストックホルム自然史博物館)

キーワード：化学合成、冷湧水、巻貝類、白亜紀、四国

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.