Dataset: Export fluxes

See complete methodology in Haskell et al. (2016). In summary:
This study is part of an effort aimed at characterizing the biological response to upwelling at SPOT on 21 cruises between January 2013 and June 2014; the Upwelling Regime In-Situ Ecosystem Efficiency (Up.R.I.S.E.E.) study.

Sediment traps: During 13 of the 22 cruises, one string of surface-tethered drifting sediment traps containing two Particle-Interceptor-Traps (PITs) was deployed at 100 m and 200 m. A 50 m section of 1-inch thick bungee cord was inserted to dampen the movement caused by waves. A series of five surface buoys were attached to the line and to a mast buoy, which held a strobe light, radio transmitter, radar reflector and Pacific Gyre GPS satellite transmitter. Each trap had a total surface area of 0.0851 m2. All traps had 12 collection tubes with an aspect ratio of 6.4 and had 1 cm x 1 cm baffles fitted into the top opening of the tubes. Funnels with centrifuge tubes were attached into the base of each trap tube. The centrifuge tube (50 ML) contained a brine solution of NaCl (in excess of sea water by 5 ppt) prepared from filtered seawater that was poisoned with 3% formaldehyde and buffered with disodium tetraborate. Deployments averaged 25 h, and were typically deployed near noon. After recovery, each trap was covered and allowed to settle for at least 1 h before the water overlying the centrifuge tubes was siphoned off and the tubes removed from the bottom of each trap tube and capped. For all trap deployments, material from each trap were then filtered onto Whatman Nuclepore polycarbonate membrane (0.4 um) filters and left to dry at room temperature for 2 days. Each filter was folded and put in a counting tube, and placed in a gamma detector (same as Th analysis) to measure thorium and beryllium activities. After counting, the filters were removed from the tubes, the trap material scraped off and homogenized into powder with a mortar and pestle, then divided into three splits. One split was placed in silver foil, acidified with HCl fumes to drive off the inorganic C, and then pelletized. One split was placed in tin foil without acidification. Both splits were sent to the UC Davis Stable Isotope Facility (SIF) for C and N elemental analysis via an isotope ratio mass spectrometer. A third split was transferred to a centrifuge tube for digestion in Na2CO3 to measure biogenic silica (SiO2) using colorimetric analysis at USC. The results of each analysis were then scaled up by weight percent to the initial mass weight of the entire trap catch. Uncertainty in surface-tethered sediment trap fluxes are reported here as +/- 50%.

Sediment trap-based export production: Rates of carbon and nitrogen export in sinking particles from the
surface ocean were calculated during 13 surface-tethered sediment trap deployments. Biogenic Si (bSi) was measured in trap material during 9 of the 13 trap deployments.

Thorium-234: Vertical profiles from the surface to 200 m were collected for thorium via Niskin/CTD on every cruise. Ten liters were collected at eight depths, chosen based on the fluorescence profile observed during the CTD's descent. A 229Th spike of known activity was added to the samples as they were being transferred from Niskins into 10 L or 20 L polycarbonate carboys (to an activity ~0.9 dpm/L) and allowed to equilibrate for at least 24 h. The recovery yield of 229Th in each sample was used in all calculations of 234Th to correct for methodological efficiency. The samples were coprecipitated with MnO2 using the technique originally developed by Rutgers van der Loeff and Moore (1999) and detailed in Haskell et al. (2013). Samples were filtered onto a 0.45 um Pall Supor Membrane filter (142 mm). The filters were dried at room temperature, placed in a plastic test tube, and placed in an Ortec low background gamma detector (intrinsic germanium, well-type, 150cc active volume).

234Th/238U disequilibrium: In order to constrain particle export, we calculated 234Th export from the upper 100 m, by integrating the 234Th/238U disequilibrium. The disequilibrium at each station was calculated using trapezoidal integration of the difference between 238U activity and 234Th activity from the surface to the depth at which the sediment traps were deployed. We use the approach outlined in Haskell et al. (2015b) when calculating the particle export of 234Th (PTh), assuming steady-state. The export flux of particulate organic carbon (POC) out of the zone of 234Th deficiency can be estimated from PTh if the ratio of POC:234Th in sinking particles is established. In this study, POC and 234Th were measured on material caught in sediment traps set at 100 m and 200 m. This ratio is then used in the following equation to estimate POC export:

POCflux = PTh * (POC: 234Th)

For 7 of the 20 sampling periods, there were not any accompanying sediment traps to measure the ratio. For these, we used the most recent ratio measured in the calculation of POC flux, except for the first cruise (SP42) where we used the seasonal average since it occurred a month prior to the most recent trap deployment.

Thorium-based export: The 234Th: 238U disequilibrium approach for estimating POC export was used for every cruise except Up-14 and Up-16, providing insight into particle export fluxes between sediment trap deployments. Generally, export estimates calculated using this approach agreed well with those calculated using the sediment trap approach, although in most cases, they were larger in magnitude.