Intercalibration studies of short-lived thorium-234 in the water column and marine particles

Abstract

Intercomparision of 234Th measurements in both water and particulate samples was carried out between 15 laboratories worldwide, as a part of GEOTRACES inter-calibration program. Particulate samples from four different stations namely BATS (both shallow and deep) and shelf station (shallow) in Atlantic and SAFE (both shallow and deep) and Santa Barbara station (shallow) in Pacific were used in the effort. Particulate intercalibration results indicate good agreement between all the participating labs with data from all labs falling within the 95% confidence interval around the mean for most instances. Filter type experiments indicate no significant differences in 234Th activities between filter types and pore sizes (0.2-0.8 μm). The only exception are the quartz filters, which are associated with 10% to 20% higher 234Th activities attributed to sorption of dissolved 234Th. Flow rate experiments showed a trend of decreasing 234Th activities with increasing flow rates (2-9 L min-1) for > 51 μm size particles, indicating particle loss during the pumping process. No change in 234Th activities on small particles was observed with increasing flow-rates. 234Th intercalibration results from deep water samples at SAFe station indicate a variability of < 3% amongst labs while dissolved 234Th data from surface water at Santa Barbara Station show a less robust agreement, possibly due to the loss of 234Th from decay and large in-growth corrections as a result of long gap between sample collection and processing.