Geobiology (2007), 5, 127–139 DOI: 10.1111/j.1472-4669.2007.00106.x © 2007 The Authors Journal compilation © 2007 Blackwell Publishing Ltd 127 Blackwell Publishing Ltd ORIGINAL ARTICLE Lateral faunal changes in a Late Cretaceous chemosymbiotic assemblage Methane-flux-dependent lateral faunal changes in a Late Cretaceous chemosymbiotic assemblage from the Nakagawa area of Hokkaido, Japan ROBERT G. JENKINS 1,2 , ANDRZEJ KAIM 1,3 , YOSHINORI HIKIDA 4 AND KAZUSHIGE TANABE 1 1 Department of Earth and Planetary Science, University of Tokyo, Tokyo 113-0033, Japan 2 University Museum, University of Tokyo, Tokyo 113-0033, Japan 3 Institute of Paleobiology, Polish Academy of Sciences, ul. Twarda 51/55, PL-00-818 Warszawa, Poland 4 Nakagawa Museum of Natural History, Hokkaido 068-0835, Japan ABSTRACT A Late Cretaceous carbonate body (2 m in maximum diameter) surrounded by clastic rocks, recently discovered in the Nakagawa area (Hokkaido, Japan), is interpreted as a methane-seep deposit, on the basis of negative carbon isotopic composition (as low as -43.5‰), variable sulphide sulphur isotopic composition, high carbonate content, and in situ fractures. It most likely formed owing to methane-bearing pore-water diffusion. We estimate that the concentration of methane decreased toward the margin of the carbonate body, and that only small carbonate concretions were precipitated at a certain distance from the methane-seep centre. These spatial characteristics coincide well with the observed pattern of faunal distribution. The gastropod-dominated associ- ation (indeterminate abyssochrysids and ataphrids and the acmaeid limpet Serradonta sp. are most common) co-occurs with lucinid and thyasirid bivalves (Thyasira sp., Myrtea sp., and Miltha sp.), and was found within and just above the methane-derived carbonate body. Acharax and Nucinella (solemyoid bivalves) are more typical of the peripheral part of the methane-influenced sediments. We suggest that this pattern of faunal dis- tribution reflects the decreasing concentration of methane and apparently also hydrogen sulphide when moving from the centre of discharge toward the periphery of the methane seep. Received 13 April 2006; accepted 31 January 2007 Corresponding author: R. G. Jenkins; e-mail: robert@um.u-tokyo.ac.jp. INTRODUCTION The unexpected discovery of hydrothermal-vent and cold- seep communities dependent on chemosynthetic bacteria for their major energy sources (Lonsdale, 1977; Corliss et al., 1979; Suess et al ., 1985) stimulated large-scale investigation in all the world’s oceans. Since the 1970s, such highly specialized animals have been found at various sites where bottom water is enriched with sulphide and methane supplied from hydrothermal vents, cold-seeps, ground-water seeps, whale skeletal remains, sunken driftwood, and even from rotting cargo of a sunken ship (Marshall, 1988; Smith et al ., 1989; Dando et al ., 1992; Tunnicliffe, 1992; Hasegawa, 1997; Sibuet & Olu, 1998; Van Dover, 2000). The methane discharging from a methane seep usually originates from the activity of methanogens and through carbonate reduction or acetate fermentation under anoxic conditions. It can also be formed by the thermal decomposition of organic matter (Whiticar, 1999; Kotelnikova, 2002). The methane migrating from its place of origin up to the sea- floor undergoes both aerobic and anaerobic oxidation. The oxidation of organic matter generally takes place owing to several oxidants in the marine water column, which penetrate methane-containing sediment (Aller, 2004). Sulphates are utilized in the last phase of this process (Aller, 2004). For this reason, methane oxidization from sulphates takes place first in the base of the sulphate-reducing zone (Valentine, 2002). Recently, it has been shown that the anaerobic oxidation of methane (AOM) is mediated by consortia of anaerobic methane-oxidizing archaea (ANME) and sulphate-reducing bacteria (SRB; Orphan et al ., 2002). When the methane discharges into a well-oxidized environment, the aerobic oxidation of the methane may also be mediated by methano- trophic bacteria (Aharon, 2000). The AOM reaction leads to an increase in alkalinity, which is favourable for the precipitation of carbonate minerals