The Carbon:234Thorium ratios of sinking particles in the California current ecosystem 1: relationships with plankton ecosystem dynamics

Abstract We investigated variability in the C: 234 Th ratio of sinking particles and its relationship to changing water column characteristics and plankton ecological dynamics during 29 Lagrangian experiments conducted on six cruises of the California Current Ecosystem Long-Term Ecological Research (CCE-LTER) Program. C: 234 Th ratios of sinking particles collected by a surface-tethered sediment trap (C: 234 Th ST ) varied from 2.3 to 20.5 μmol C dpm −1 over a depth range of 47–150 m. C: 234 Th ST was significantly greater (by a factor of 1.8) than C: 234 Th ratios of suspended >51-μm particles collected in the same water parcels with in situ pumps. C: 234 Th ratios of large (>200-μm) sinking particles also exceeded those of smaller sinking particles. C: 234 Th ST decreased with depth from the base of the euphotic zone through the upper twilight zone. C: 234 Th ST was positively correlated with several indices of ecosystem productivity including particulate organic carbon (POC) and chlorophyll (Chl) concentrations, mesozooplankton biomass, and the fraction of Chl >20-μm. Principal component analysis and multiple linear regression suggested that decaying phytoplankton blooms exhibited higher C: 234 Th ST than actively growing blooms at similar biomass levels. C: 234 Th ST was positively correlated with indices of the fractional contribution of fecal pellets in sediment traps when the proportion of fecal pellets was low in the traps, likely because of a correlation between mesozooplankton biomass and other indices of ecosystem productivity. However, when fecal pellets were a more important component of sinking material, C: 234 Th ST decreased with increasing fecal pellet content. C: 234 Th ST was also positively correlated with the Si:C ratio of sinking particles. Across the dataset (and across depths) a strong correlation was found between C: 234 Th ST and the ratio of vertically-integrated POC to vertically-integrated total water column 234 Th ( v C: 234 Th tot ). A mechanistic one-layer, two-box model of thorium sorption and desorption was invoked to explain this correlation. Two empirical models (one using v C: 234 Th tot ; one using depth and vertically-integrated Chl) were developed to predict C: 234 Th ratios in this coastal upwelling biome. The former regression (log 10 (C: 234 Th ST ) = 0.43 × log 10 ( v C: 234 Th tot ) + 0.53) was found to also be a reasonable predictor for C: 234 Th ST from diverse regions including the Southern Ocean, Sargasso Sea, Subarctic North Pacific, and Eastern Tropical North Pacific.