SHALLOW WATER SEEPS FROM JURASSIC–CRETACEOUS BOREAL SEA (SVALBARD, NORWEGIAN BARENTS SEA)

Fossil marine hydrocarbon seeps are known since the Devonian, but most of the better recognized sites are Cretaceous or younger in age. The Jurassic–Cretaceous seep sites from Svalbard (Norwegian Barents Sea) are in addition to 6 known Jurassic sites and comprise one of few fossil seep areas discovered from higher latitudes. They are therefore significant data sources for the study of Mesozoic seep faunas. The Svalbard seeps are 15 carbonate bodies, heavily weathered and up to a few meters in diameter. They are enclosed within organic-rich shales of the Slottsmøya Member, representing sediments of the epicontinental palaeo-Barents Sea. Ammonite dating of 10 out of 15 seeps shows they formed during a relatively continous interval of ca. 9 million years around the Jurassic-Cretaceous boundary. The negative d¹³C carbonate isotope signatures, as light as –46‰, indicate a contribution of methanogenic carbon and confirm their seep origin. The seeps contain a very diverse fauna, of at least 53 species of molluscs, brachiopods, polychaetes and other taxa. The fauna mostly comprises background species which are predominantly bivalves and brachiopods common in coeval shallow-water sediments of the Boreal seas. Two background species, belong to chemosymbiotic bivalve genera Solemya and Nucinella, often found in normal marine reduced sediments. Seep-restricted species comprise less than 8% of the fauna, both in terms of diversity and abundance. These are a species of the wide-spread lucinid genus Tehamatea, the thyasirid genus Cretaxinus, which is currently unique to the Svalbard seeps, and possible provannoid gastropods and siboglinid tubeworms. The Svalbard seep fauna, with background species largely replacing seep specialists, is similar to recent seeps situated on continental shelves and probably results from the relatively shallow water setting, with possible weak methane flux influencing the composition of the chemosymbiotic niche. Ecologically the most similar fossil seeps are probably Tepee Buttes from the Late Cretaceous Mid-Continent Seaway, which formed in less than 100 meters of water. The reasons why the wide-spread latest Jurassic-Cretaceous seep-restricted modiomorphid Caspiconcha is absent are unknown, and might be related either to water depth, palaeobiogeography or local ecological reasons.