Recent Projects


POC Export Measured Using 234Th in the Southern California Bight

(in collaboration with Douglas E. Hammond)

Abstract: Since 2008, 234-Thorium profiles from the sea surface to 200m have been collected on monthly cruises to the San Pedro Ocean Time-Series (SPOT) with the goal of quantifying particulate organic carbon (POC) export in the San Pedro Basin through depth-integrated Thorium deficiency (DITD) calculations and a measured POC:Th ratio on sinking particles. The results show seasonal cycles in Thorium flux out of the upper ocean closely correlated to rainfall events, instead of physical or biological parameters such as upwelling indicies or POC flux into sediment traps. This suggests that runoff particulate near the coast dictates Th flux and the resulting deficiency is transported to the SPOT location before it can grow in again from its parent. Transects of Th from the coast to the sampling site are used to account for the loss to terrestrial runoff.

Export Production Measured Using 234Th and Sediment Traps in the Eastern Tropical South Pacific

(in collaboration with William M. Berelson, Maria G. Prokopenko, Doug E. Hammond, Laurence Y. Yeung, Douglas G. Capone)

Abstract: During February 2010 and April 2011, 234Th profiles were measured from the surface to 300m in the Eastern Tropical South Pacific, in a region bounded by 10S - 20S and 80W - 100W.  Horizontal transport of 234Th was determined using geostrophic current vectors and found to be negligible, thus only vertical transport should be significant.  Depth-integrated Thorium deficiencies (DITD) and measured ratios of particulate organic carbon (POC) to 234Th obtained from sediment trap samples were used in a steady state, one-dimensional model to determine POC export from the upper ocean. In 2010, 234Th flux ranged from 800 +/- 180 to 3120 +/- 260 dpm/m2-d, and DITD calculated POC export flux at 200m ranged from 2.27 +/- 0.5 to 12.9 +/- 1.1 mmolC/m2-d.  At each location, drifting sediment traps were deployed for 23-65 hours at 200m to measure 234Th and POC flux directly. The flux of 234Th into the traps ranged from 48 +/- 3 to 747 +/- 12 dpm/m2-d, which was very low compared to the flux calculated using the Thorium deficiency (DITD).  However, at two stations, the integrated deficiency and trap results agreed to 47% and 76%.  The trap-measured flux of POC was also much lower, ranging between 0.19 +/- 0.1 and 2.27 +/- 1.1 mmolC/m2-d. Net community production (in the mixed layer only), calculated from O2 supersaturation and estimates of piston velocity at each station, was found to predict an upper limit for POC fluxes between 0.0 to 9.5 mmolC/m2-d in 2010. All three estimates of POC export may suffer biases and artifacts, and certainly a 24-hour trap deployment may not be representative of export integrated over the time-scale of the 234Th tracer or of oxygen production and gas exchange.  Other possible explanations for the observed differences include: grazing of zooplankton swimmers on trap material, the vertical transport by swimmers past the trap depth, remineralization of POC and 234Th above the trap depth, and non-steady-state conditions.

Upwelling Velocities and Eddy Diffusivity from 7Be Measurements Used to Calculate Vertical Nutrient Fluxes in the Eastern Tropical South Pacific

(in collaboration with David Kadko and Doug E. Hammond)

Abstract: The cosmogenically produced radionuclide, Beryllium-7(half-life = 53.3d), is deposited on the sea surface by rainfall and subsequently homogenized in the mixed layer. The mass balance of its inventory in the water column can be used to estimate upwelling velocities(w) and vertical diffusivity(Kz) if the atmospheric input can be established, thus giving insight into vertical transport of nutrients across the upper thermocline.  Five 7Be profiles were measured in an area bounded by 10-20S and 80-100W, and used in a one-dimensional diffusion-advection model to determine Kz. Observations indicate an inverse relationship between mixed layer nutrient concentrations and mixed layer 7Be activity, indicating the influence of upwelled nutrient-rich, 7Be-poor water into the mixed layer. Thorium-234(half-life = 24.1d) flux and sediment traps were also used to constrain POC export from the euphotic zone at each site, presumably driven by nutrient fluxes. Values for upwelling velocity, nutrient flux and export POC flux are compared. Neglecting subduction, upwelling velocities were estimated to be up to 2.45m/d, Kz values ranged from 0.4*10-4-2.6*10-4 m2/s, and nitrate fluxes across the 1% light horizon ranged from 0.3-19.0 mmol/m2*d.

Use of Triple Oxygen Isotopes, O2/Ar, and 7Be to Constrain the Effects of Upwelling Velocity on Ecosystem Export Efficiency in the Southern California Bight

(in collaboration with Maria G. Prokopenko and Doug E. Hammond)

(PhD Project in progress)

Project Site:

(14C Production from Tiahlo et al. pers. comm.)

ETSP-I cruise track (NOAA, 2010)

Southern California Bight and its major surface currents.  The X marks the SPOT location.