A cold-seep assemblage is present in slope to outer-shelf strata of the Upper Pliocene to Lower Pleistocene Ofuna Formation and the Lower Pleistocene Koshiba Formation of the Kazusa Group in Yokohama City, on the Pacific side of central Japan. Four cores were taken in order to learn the three-dimensional extent of the assemblage: one core was parallel to bedding and three were normal to bedding. Study of the cores and outcrops shows that: (1) the assemblage is at least 37 m thick stratigraphically and extends across the boundary between the Koshiba Formation and the underlying Ofuna Formation; (2) parallel to bedding, the assemblage is at least 16 m wide in a north-south direction and 30 m long in an east-west direction; (3) the assemblage consists mostly of large (up to 10 cm in maximum diameter), articulated bivalves of the genera Lucinoma, Conchocele, and Acharax, in aggregations or as sporadic shells; (4) authigenic carbonate cement in the matrix enclosing the assemblage ranges from dense to sparse and is greatly depleted in ¹³C (δ¹³C = −47.99‰ to −55.06‰), indicating that the carbonate resulted from the microbial oxidation of methane; (5) there is a subsurface horizon about 10 m thick that is characterized by: (a) scoria beds in which the matrix consists entirely of pure, white carbonate, (b) abundant brecciated clasts of the host sediment occurring along with extremely abundant bivalve fragments, many of which are very small, and (c) lithologic boundaries greatly discordant with the overall dip within the study area. Observations 5a–c together suggest very active seepage and/or an explosive effusion of subsurface material, and the presence of “pockmarks” in the subsurface of the study area.

Twenty tuff beds are observable and accurately correlatable among outcrops where no cold-seep assemblage occurs. However, these tuff beds cannot be correlated with those in the cores. This is probably due to highly active bioturbation and by severe seepage and/or the explosive effusion of seep materials. We infer that these activities and events dispersed, disturbed, brecciated and/or reconcentrated the originally deposited tuff beds.

A cold-seep invertebrate assemblage consisting mainly of the large bivalve Lucinoma sp. is exposed at the Kuge Shrine and surrounding area in Shintomi Town, Koyu County, Miyazaki Prefecture, Kyushu Island, southwestern Japan. The outcrops with this assemblage are in the upper part of the Upper Pliocene Takanabe Formation of the Miyazaki Group. The Takanabe Formation exposed in Shintomi Town is a cyclical sequence of upward-coarsening and upward-fining sediments, which reflects sea-level change: the paleobathymetry inferred from molluscs implies a shallowing upward from 160 m–200 m to 50 m in water depth, followed by a deepening back to 160 m–200 m. The cold-seep assemblage occurs in the coarsest (muddy sandstone) and bathymetrically shallowest (50 m in water depth) horizon, an outer shelf environment near the average storm-wave base, in this shallowing and deepening cycle. The cold-seep assemblage around the Kuge Shrine occurs sporadically in area of about 100 m in a north-south direction and about 60 m in an east-west direction, and is about 12 m in stratigraphic thickness. Many articulated and disarticulated bivalves are preserved with their commissure planes parallel to bedding, but some articulated bivalves are oriented normal to it, in the characteristic life position of lucinids. The muddy sandstone containing this assemblage is cemented strongly to weakly by authigenic carbonate depleted greatly in ¹³C (δ¹³C = −30.69‰ to −53.21‰ vs. PDB), which suggests the influence of methane seepage. The preserved life positions of the lucinid bivalves and the carbon isotope ratios of the associated carbonates suggest that this assemblage is autochthonous. The bedding-parallel orientation of many shells is inferred to have resulted from in-situ reworking by storm-induced waves and currents that are prevalent in such a shallow environment.

Cold-seep carbonate mounds containing Vesicomya (Calyptogena) kawamurai paleocommunity occur in massive siltstones of the Pliocene Tamari and Hijikata Formations, both deposited on the upper to middle slope of a forearc basin in the Sagara-Kakegawa area, central Japan. The shell-rich carbonate mounds vary from lenticular (2 m in diameter and 0.5 m in maximum thickness) to barrel-shaped (0.8 m in diameter and 1.5 m in length). One lenticular shelly concretion consisted of densely crowded vesicomyid shells lying parallel to bedding, and overlies a brecciated siltstone with dolomicritic cement. A barrel-shaped concretion contained abundant articulated valves of V. (C.) kawamurai in life orientation, and preserves a three-dimensional view of a chemosymbiotic habitat maintained over several generations of these large clams. In addition, the carbonate mounds contain various void spaces (open burrows and dissolved-shell molds) which are fringed by authigenic carbonates (splayed fibrous aragonites and dolosparites). The aragonite bur-row-linings in the carbonate mounds and calcite/Mg-calcite micronodules in the siltstone matrix have very ¹³C-depleted isotopic signatures (δ¹³C = −43.59 ‰ to −54.54‰ PDB).

The taphonomic, petrographic, and stable isotopic evidence confirms that the vesicomyid paleocommunities formed due to biogenic methane seepage. Brecciation of the siltstone shows an explosive collapse of the sediment fabric, possibly triggered by decomposition of gas hydrates in near-surface sediments. The subsequent biological architecture of burrows and large, dead-shell cavities acted as efficient conduits that facilitated continued seepage. The chemosymbiotic bivalves were able to colonize slope mudstone during the methane-seepage associated with gas hydrate decomposition, and their biological activities altered the muddy substrate to promote the methane seepage that sustained several generations of clams.

A well-preserved fossil seep community has been found in a carbonate lens in the Santonian to Campanian Omagari Formation, Upper Yezo Group in the Nakagawa region, Hokkaido, north Japan. The carbonate lens (roughly ellipsoidal in plan view with a diameter of 10 m × 6 m, and a thickness of about 5 m) is composed mainly of various types of high-Mg calcite containing several to 10 mol% magnesium and little iron or manganese. The carbonate lens is divided into an upper tube worm-dominated boundstone and a lower carbonate breccia facies. In the boundstone facies, concentric cements occur in the vestimentiferan tubes, indicating that the worm tubes were conduits for seepage. Layered to veinlike precipitates of high-Mg calcite occur in the boundstone facies. The carbonate breccia facies contains clast-supported carbonate breccia with sideritic, silty and tuffaceous matrices. Chemosynthetic bivalves occur in the upper zone of the carbonate breccia. The most common of these is the lucinid Miltha sp. Others include the lucinid Thyasira sp., and vesicomyid Calyptogena. Many small molluscs occur in the matrices of the carbonate breccia. The most common of these are trochid archaeogastropods; the others are two acmaeid limpets, mesogastropods and nuculacean bivalves. Small terebratulid brachiopods are also common. The carbonate lens, with its chemosynthetic bivalves and vestimentiferan worm tubes, may have been formed by bacterial sulfate reduction and anaerobic methane oxidation, as it shows extreme ¹³C-depletion (δ¹³C = −41 to −45‰). The Omagari community resembles the modern cold-seep communities along the landward slope of the subduction-zone complex off the Pacific coast of Japan.

To better understand Eocene climate and the isotopic record of paleotemperature preserved in shells of the gastropod Conus, we serially sampled and analyzed four modern and two Eocene shells from the U.S. Gulf Coast and the Gulf of Mexico. The modern shells are from nearshore Mexico off Veracruz, off-shore Texas (Stetson Bank), and nearshore Florida (Alligator Point). The fossil shells are of late middle Eocene (ca. 38 Ma) age from the Moodys Branch Formation in Mississippi (U.S.A.). The four modern shells yield three different oxygen-isotopic patterns of seasonality (asymmetrical saw-tooth, cuspate, and irregular) representing different seasonal growth patterns and environments. The asymmetrical sawtooth pattern occurs in the middle shelf specimen (Stetson Bank) and indicates rapid spring and declining autumn growth, presumably in response to increased nutrient supply and productivity associated with spring upwelling. The cuspate pattern indicates winter shutdown and occurs in the most northern specimen. The irregular pattern reflects seasonal freshwater input in a nearshore environment. The Eocene shells yield an asymmetrical sawtooth pattern suggestive of enhanced spring growth during upwelling.

Assuming a constant seawater δ¹⁸O of 0.24‰ (Lear et al., 2000), including correction for latitude (Zachos et al., 1994), oxygen isotope data yield a mean annual range of temperature (MART) for the late middle Eocene of 4–5°C, and a mean annual temperature (MAT) of 23°C. Taking the depth estimation (20–100 m) into consideration, sea surface temperatures are estimated to be >25°C for summer, ∼21°C for winter, and MAT of >23°C. Compared with modern temperatures and isotopic paleotemperatures of modern shells, the late middle Eocene Gulf Coast experienced warmer winter temperatures. The difference between modern and late middle Eocene climate can partly be attributed to the development of a continental cold front during the modern winter, and to the increased marine influence during the middle Eocene caused by the warmer water mass of the ocean.

Coleia uzume sp. nov., a lobster of the polychelidan family Coleiidae, is described from the upper Triassic (Carnian) Nakatsuka Formation of the Mine Group in Yamaguchi Prefecture, southwest Japan. The discovery of C. uzume greatly extends the geologic range for the genus back to the late Triassic. This species represents the first and oldest records of a Triassic decapod in the western Panthalassic realm.