Recent Projects


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

(in collaboration with William M. Berelson (USC), Maria G. Prokopenko (Pomona), Doug E. Hammond (USC), Douglas G. Capone (USC))

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 10°S - 20°S and 80°W - 100°W.  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.

Haskell, W.Z.II, W.M. Berelson, D.E. Hammond, D.G Capone. (2013) Particle sinking dynamics and POC        

        fluxes in the Eastern Tropical South Pacific based on 234Th and sediment trap deployments. Deep-

        Sea Res. I, 81, pp.1-13.

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 (Miami/FIU) and Doug E. Hammond (USC))

Abstract: Five 7Be profiles, measured in an area bounded by 10°S-20°S and 80°W-100°W, were used to determine upwelling velocity (wH) and vertical diffusivity (Kz). A positive correlation between wH and 14C primary production rate and a negative correlation between the inventories of 7Be and phosphate were observed. We interpret this as the influence of deeper, nutrient-rich, 7Be-poor water brought up by upwelling. Excluding two stations that appear to be influenced by non-steady state dynamics or horizontal transport, upwelling velocities were estimated to be 0 to 1.0 m d-1 and Kz values ranged from 0.4 to 2.6 cm2 s-1. From these parameters, NO3- fluxes into the euphotic zone were assessed and ranged from 0.15 to 2.9 mmol m-2 d-1. Using these values, we estimate 1.0 to 19 mmolC m-2 d-1 of new production in the ETSP. New production based on 7Be-derived transport parameters agree with carbon export estimates using a 234Th balance, sediment traps and O2/Ar supersaturation for stations along 20°S, but are higher than export estimates at 10°S, 100°W.

Haskell, W.Z.II, D. Kadko, D.E. Hammond, M.G. Prokopenko, W.M. Berelson, A.N. Knapp, D.G.  

        Capone. Upwelling velocities and eddy diffusivity from 7Be measurements used to compare vertical    

        nutrient fluxes to export POC flux calculated from 234Th, sediment traps, and O2/Ar supersaturation

        in the Eastern Tropical South Pacific. Mar. Chem. (in press)


(14C Production from D. Capone and M. Tiahlo - pers. comm.)

ETSP-I cruise track (NOAA, 2010)

Up.R.I.S.E.E. (Upwelling Regime In-situ Ecosystem Efficiency):

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

(in collaboration with Maria G. Prokopenko (Pomona), Doug E. Hammond (USC)

and Rachel H. R. Stanley (WHOI))

Project Site:


This research has been funded by: the National Science Foundation, the International Association of Geochemistry, USC Earth Sciences Teaching Assistantships, and the Sonosky Foundation.

Internal Wave-generated Turbulent Mixing and Vertical Nitrate Flux During Spring and Neap Tides Along the Mid-Atlantic Bight Shelf Break

(in collaboration with Nicholas Nidzieko and Alyson E. Santoro (UMCES))

        Over the continental margins, the proportion of total organic carbon that is exported is much greater than that in the open ocean, so even small fluctuations in biological productivity in these regions can have a large impact on the global carbon budget. In much of the surface ocean, biological productivity is limited by nutrient availability and the vertical transport of nutrients is controlled by turbulent mixing. During interaction with the shelf/slope topography, a portion of tidal energy is lost to turbulence during conversion into internal waves, which lose further energy to turbulence as they move onshore. This mechanism likely increases the rate vertical nutrient flux and biological productivity over the continental shelves. Given the ubiquity and frequency (hours to months) of tides worldwide, mixing caused by internal tide-shelf break interaction may be a globally important mechanism of nutrient supply to surface ocean ecosystems.

        For my postdoctoral work, I have been awarded an NSF-OCE Postdoctoral Fellowship to collaborate with N. Nidzieko and A. Santoro (UMCES) to use a propelled AUV (Hydroid REMUS 600) capable of making simultaneous in-situ measurements of vertical turbulent mixing rate (via a microstructure shear, temperature and conductivity probes) and nitrate concentration profiles (via a SUNA nitrate sensor) to characterize internal wave structure, vertical turbulent mixing rate, and the vertical diffusive nitrate flux across the thermocline at the continental shelf break in the Mid-Atlantic Bight. Our goal is to quantify the effect of internal waves on vertical nutrient flux over the continental shelf edge, testing the hypothesis that tidal energy dissipation caused by the interaction with the shelf edge will increase the rate of vertical nutrient flux near the base of the euphotic zone. Our efforts will be further supported through collaboration with David Nicholson (WHOI) by deploying a Slocum glider equipped with a CTD, oxygen optode and nitrate sensor to further understand the regional hydrographic and nutrient dynamics on larger spatial and temporal scales prior to and during each REMUS deployment.


Euphotic Zone Net (red) and Gross (black) Oxygen Production at a site in Southern California determined using O2/Ar and Triple Oxygen Isotopes. The ratio of the two (blue) represents an export ratio, which is highest just following the initiation of upwelling in spring (bottom; determined using a mass balance of 7Be), and appears to follow an annually repeating cycle, likely indicating that most fixed organic carbon is available for export in the early spring, but most production is recycled in the euphotic zone in late spring.