A new method for the U-Th dating of a carbonate chimney deposited during the last glaciation in the northern Okinawa Trough, East China Sea

Authigenic carbonates from cold seep systems, such as chimney and crust carbonate deposits, are important archives of biogeochemical processes associated with the migration and seepage of methane-rich fluids and seepages on the seafloor. Here we use a new U-Th dating method to precisely determine the ages of the authigenic carbonates of a chimney in the Okinawa Trough (OT), East China Sea. The method uses a series of heavy liquids (HL, a mixture of tribromomethane and alcohol with a density of 2.71-2.80 g/cm(3)) to separate subsamples of carbonate contaminated by detrital material. The contents of relevant elements (Al, Rb, Zr, Nb, and rare earth elements) of the subsamples reveal that the separation of the mineral phase was successful. Five samples from along the radius of a well-structured chimney (PES-1) from station D2-5 in the OT were analyzed and yielded U-Th ages of 28.8 +/- 0.4 ka, 26.1 +/- 0.5 ka, 24.4 +/- 0.7 ka, 22.5 +/- 0.6 ka and 20.1 +/- 0.6 ka, from the exterior to the interior of the chimney section, and AMS C-14 dating of the same samples produced the similar results. The agreement between the Th-230/U and AMS C-14 ages suggests that the application of heavy liquid methods in U-Th dating can be very successful, and it reduces the influence of detrital contamination. Therefore, the approach has a great potential space for studying the evolutionary history of authigenic carbonates in cold seep systems. Moreover, the new Th-230/U ages show that the carbonate chimney was formed during the Last Glacial Maximum (LGM), which corresponding to the large sea-level fall in the East China Sea. Together with information on biogeochemical processes, the results will provide valuable geochronological information for the future study of active fluid venting on continental margins.

Automated summary

Authigenic carbonates from cold seep systems, such as chimney and crust carbonate deposits, are important archives of biogeochemical processes associated with methane-rich fluids on the seafloor. A new U-Th dating method was used to precisely determine the ages of carbonates from a chimney in the Okinawa Trough, showing potential for studying the evolutionary history of these carbonates.