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Stukel, MR, Kelly TB, Aluwihare LI, Barbeau KA, Goericke R, Krause JW, Landry MR, Ohman MD.  2019.  The Carbon:(234)Thorium ratios of sinking particles in the California current ecosystem 1: relationships with plankton ecosystem dynamics. Marine Chemistry. 212:1-15.   10.1016/j.marchem.2019.01.003   AbstractWebsite

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.

Stukel, MR, Aluwihare LI, Barbeau KA, Chekalyuk AM, Goericke R, Miller AJ, Ohman MD, Ruacho A, Song H, Stephens BM, Landry MR.  2017.  Mesoscale ocean fronts enhance carbon export due to gravitational sinking and subduction. Proceedings of the National Academy of Sciences of the United States of America. 114:1252-1257.   10.1073/pnas.1609435114   AbstractWebsite

Enhanced vertical carbon transport (gravitational sinking and subduction) at mesoscale ocean fronts may explain the demonstrated imbalance of new production and sinking particle export in coastal upwelling ecosystems. Based on flux assessments from U-238:Th-234 disequilibrium and sediment traps, we found 2 to 3 times higher rates of gravitational particle export near a deep-water front (305 mg C.m(-2).d(-1)) compared with adjacent water or to mean (nonfrontal) regional conditions. Elevated particle flux at the front wasmechanistically linked to Fe-stressed diatoms and high-mesozooplankton fecal pellet production. Using a data assimilative regional ocean model fit to measured conditions, we estimate that an additional similar to 225 mg C.m(-2).d(-1) was exported as subduction of particle-rich water at the front, highlighting a transport mechanism that is not captured by sediment traps and is poorly quantified by most models and in situ measurements. Mesoscale fronts may be responsible for over a quarter of total organic carbon sequestration in the California Current and other coastal upwelling ecosystems.