|Title||The Carbon:(234)Thorium ratios of sinking particles in the California current ecosystem 1: relationships with plankton ecosystem dynamics|
|Publication Type||Journal Article|
|Year of Publication||2019|
|Authors||Stukel M.R, Kelly T.B, Aluwihare LI, Barbeau K.A, Goericke R, Krause J.W, Landry MR, Ohman MD|
|Type of Article||Article|
|Keywords||biological pump; central equatorial pacific; chemistry; export fluxes; frontal zone; oceanography; particulate organic-carbon; sediment traps; th-234 ratios; twilight; upwelling system; water column; zone|
We investigated variability in the C:Th-234 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:Th-234 ratios of sinking particles collected by a surface-tethered sediment trap ((CThST)-Th-:234) varied from 2.3 to 20.5 mu mol C dpm(-1) over a depth range of 47-150 m. C:Th-234(ST) was significantly greater (by a factor of 1.8) than C:Th-234 ratios of suspended > 51-mu m particles collected in the same water parcels with in situ pumps. C:Th-234 ratios of large (> 200-mu m) sinking particles also exceeded those of smaller sinking particles. C:Th-234(ST) decreased with depth from the base of the euphotic zone through the upper twilight zone. C:Th-234(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-mu m. Principal component analysis and multiple linear regression suggested that decaying phytoplankton blooms exhibited higher C:Th-234(ST) than actively growing blooms at similar biomass levels. C:Th-234(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:Th-234(ST) decreased with increasing fecal pellet content. C:Th-234(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:Th-234(ST) and the ratio of vertically-integrated POC to vertically-integrated total water column Th-234 (C-v:Th-234(tot)). A mechanistic one-layer, two-box model of thorium sorption and desorption was invoked to explain this correlation. Two empirical models (one using C-v:Th-234(tot); one using depth and vertically-integrated Chl) were developed to predict C:Th-234 ratios in this coastal upwelling biome. The former regression (log(10)(C:Th-234(ST)) = 0.43 x log(10)(C-v:Th-234(tot)) + 0.53) was found to also be a reasonable predictor for C:Th-234(ST) from diverse regions including the Southern Ocean, Sargasso Sea, Subarctic North Pacific, and Eastern Tropical North Pacific.