Calyptogena-cemented rocks and concretions from the eastern part of Nankai accretionary prism : Age and geochemistry of uranium
Résumé
Calyptogena valves included in a carbonate-rich cement, and fragments of a carbonate-rich chimney, have been examined for their stable isotopic (C and O) composition, 14C activity and uranium series disequilibrium. The fossil shells were formed essentially with seawater carbon and a negligible contribution of cold seepage organic carbon, as shown by their ~13C values. This allows the 14C method to be used to determine their age. A fairly good concordance between the 14C and 23°Th/234U ages of the youngest shells gives confidence in the dating of the older samples
using the latter technique. Thus, the Calyptogena are dated at ca. 150,000 and 20,000 yrs B.P. They have been preserved from dissolution by rapid cementation by a supersaturated carbonate solution. The cement is especially rich in uranium (as high as 75 ppm), whose source is seawater; the enrichment is due to local reducing conditions brought about by the bacterial decomposition of the soft tissues of the bivalves shortly after death. The Calyptogena that probably developed between these two events (the events of ca. 20,000 and 150,000 yrs) have not been preserved from dissolution because, as is presently the case, the cold seepages were undersaturated with calcium carbonate. The two events probably represent periods of intense fluid venting connected with tectonic activity.