Particulate organic carbon : natural radionuclide ratios in zooplankton and their freshly produced fecal pellets from the NW Mediterranean (MedFlux 2005)

To discern controls on particulate organic carbon (POC) : natural radionuclide (RN) ratio variability in order to enhance the accuracy of water column radionuclide‐based carbon flux estimates, 234Th, 210Po, and POC were analyzed in seven size classes of mixed micro‐ and mesoplankton (1‐1,500‐µm size range), in larger zooplankton from different taxa (salps, euphausiids, copepods, pteropods), and in freshly produced feces from zooplankton collected during spring in the NW Mediterranean. POC:RN ratios in zooplankton ranged between 120 and 11,600 and between 89 and 9,200 µmol dpm−1 for 234Th and 210Po, respectively. In fecal pellets, POC:RN ratios were one to three orders of magnitude lower for 234Th and 3‐fold to 30‐fold lower for 210Po; the only exception was euthecosome pteropods, which had a higher POC: 210Po ratio in their pellets than in their whole bodies. Significant increases in POC:RN ratios with organism size were best described by a power relationship for POC: 234Th (p 33‐µm size classes, 210Po specific activity correlates negatively with the surface : volume ratio, while 234Th correlates positively with it (p < 0.004 and p < 0.001, respectively). POC:RN ratios vary greatly among species and to a lesser extent among fecal pellet types, most probably due to differences in zooplankton feeding strategies. Partial removal of most zooplankton "wimmers" from trap samples would not likely confound assessment of 234Th flux; however, it could considerably bias similar measurements of 210Po flux as well as those of POC:RN ratios

A review of present techniques and methodological advances in analyzing 234Th in aquatic systems

The short-lived thorium isotope 234Th (half-life 24.1 days) has been used as a tracer for a variety of transport processes in aquatic systems. Its use as a tracer of oceanic export via sinking particles has stimulated a rapidly increasing number of studies that require analyses of 234Th in both marine and freshwater systems. The original 234Th method is labor intensive. Thus, there has been a quest for simpler techniques that require smaller sample volumes. Here, we review current methodologies in the collection and analysis of 234Th from the water column, discuss their individual strengths and weaknesses, and provide an outlook on possible further improvements and future challenges. Also included in this review are recommendations on calibration procedures and the production of standard reference materials as well as a flow chart designed to help researchers find the most appropriate 234Th analytical technique for a specific aquatic regime and known sampling constraints. <br/