Particulate organic carbon-234Th relationships in particles separated by settling velocity in the northwest Mediterranean Sea

In order to better understand the relationship between the natural radionuclide Th-234 and particulate organic carbon (POC), marine particles were collected in the northwestern Mediterranean Sea (spring/summer, 2003 and 2005) by sediment traps that separated them according to their in situ settling velocities. Particles also were collected in time-series sediment traps. Particles settling at rates of > 100 m d(-1) carried 50% and 60% of the POC and Th-234 fluxes, respectively, in both sampling years. The POC flux decreased with depth for all particle settling velocity intervals, with the greatest decrease (factor of similar to 2.3) in the slowly settling intervals (0.68-49 m d(-1)) over trap depths of 524-1918 m, likely due to dissolution and decomposition of material. In contrast the flux of Th-234 associated with the slowly settling particles remained constant with depth, while Th-234 fluxes on the rapidly settling particles increased. Taking into account decay of Th-234 on the settling particles, the patterns of Th-234 flux with depth suggest that either both slow and fast settling particles scavenge additional Th-234 during their descent or there is significant exchange between the particle classes. The observed changes in POC and (234)Thflux produce a general decrease in pOC/Th-234 of the settling particles with depth. There is no consistent trend in pOC/Th-234 with settling velocity, such as might be expected from surface area and volume considerations. Good correlations are observed between Th-234 and POC, lithogenic material and CaCO3 for all settling velocity intervals. Pseudo-K(d)s calculated for Th-234 in the shallow traps (2005) are ranked as lithogenic material <= opal <calcium carbonate < organic carbon. Organic carbon contributes similar to 33% to the bulk K-d and for lithogenic material, opal and CaCO3, the fraction is similar to 22% each. Decreases in POC/Th-234 with depth are accompanied by increases in the ratio of Th-234 to lithogenic material and opal. No change in the relationship between Th-234 and CaCO3 was evident with depth. These patterns are consistent with loss of POC through decomposition, opal through dissolution and additional scavenging of Th-234 onto lithogenic material as the particles sink. Published by Elsevier Ltd.