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 234Th 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 234Th fluxes, respectively, in both sampling years. The POC flux decreased with depth for all particle settling velocity intervals, with the greatest decrease (factor of ∼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 234Th associated with the slowly settling particles remained constant with depth, while 234Th fluxes on the rapidly settling particles increased. Taking into account decay of 234Th on the settling particles, the patterns of 234Th flux with depth suggest that either both slow and fast settling particles scavenge additional 234Th during their descent or there is significant exchange between the particle classes. The observed changes in POC and 234Th flux produce a general decrease in POC/234Th of the settling particles with depth. There is no consistent trend in POC/234Th with settling velocity, such as might be expected from surface area and volume considerations. Good correlations are observed between 234Th and POC, lithogenic material and CaCO3 for all settling velocity intervals. Pseudo-Kds calculated for 234Th in the shallow traps (2005) are ranked as lithogenic material ≤opal <calcium carbonate <organic carbon. Organic carbon contributes ∼33% to the bulk Kd, and for lithogenic material, opal and CaCO3, the fraction is ∼22% each. Decreases in POC/234Th with depth are accompanied by increases in the ratio of 234Th to lithogenic material and opal. No change in the relationship between 234Th and CaCO3 was evident with depth. These patterns are consistent with loss of POC through decomposition, opal through dissolution and additional scavenging of 234Th onto lithogenic material as the particles sink.