Organic and inorganic carbon, biogenic silica, and mass flux measurements were made from neutrally-buoyant sediment trap deployments at three stations occupied during R/V Falkor cruise FK170124 from Honolulu, HI to Portland, OR in January 2017.
The NBST platforms were constructed around Sounding Oceanographic Lagrangian Observer (SOLO) profiling floats and carried four cylindrical sediment trap tubes with collection areas of 0.0113 m2 and an integrated transmissometer (WETLabs C-Rover 6b). Prior to deployment, three trap tubes were filled with filtered seawater from beneath the mixed layer. 500 mL of formalin-poisoned brine (70 ppt) was gravity-fed through tubing to form a layer below the filtered seawater to preserve settling particulate matter for carbon analysis. The fourth trap tube was not used for bulk flux determination.
NBSTs were programmed to descend to a single measurement depth (150 or 170 m), sample for 2–3 days until a burn wire mechanism closed the tube lids, and then ascend to the surface for recovery. NBSTs were programmed to hold depth within ±25 m of the measurement depth.
Upon recovery, NBST tubes were allowed to settle for at least 1 h in the laboratory. The overlying seawater layer was suctioned out of the tops of all tubes. The remaining brine layers from the three tubes were drained through a single, acid-cleaned, 350-μm nylon mesh screen and combined into a 4-L bottle. The screen was picked clean of zooplankton under a dissecting microscope, and the remaining screen contents were rinsed back into the 4-L bottle with filtered seawater. The 4-L bottle was split into eight fractions using a custom-built rotary splitter (Lamborg et al. 2008). One fraction was further split into two fractions to provide a subsample for collaborators. Laboratory space limitations prohibited the use of a shaker table to mix the 4-L bottle during splitting, and instead the bottle was agitated by hand.
Three splits were filtered onto precombusted GF/F filters (Whatman) and dried at 45 ± 5°C using a consumer-grade food dehydrator (blank comparisons vs. a standard laboratory oven showed no difference). Filters were stored dry at room temperature until analysis on shore. On shore, filters were gravimetrically split and half of each filter was analyzed for total carbon (TC) at the UC Davis Stable Isotope Facility. Split-to-split reproducibility was poor, possibly due to the lack of a shaker table during wet splitting.
Six splits (including the two 1/16th splits) were filtered onto pre-weighed, 25-mm diameter, 0.2-μm pore size polycarbonate membrane filters (Nuclepore) and rinsed with pH 8.5 borate-buffered Milli-Q water. All were dried as described above. Four of the six splits were stored dry at room temperature until analysis on shore for CaCO3 and biogenic silica (bSi). Two of the six splits were shared with collaborators.
On shore, all polycarbonate filters were re-dried and weighed to constant mass on a microbalance with ±0.01 mg precision to determine mass flux. To increase the number of TC replicates and partially remedy the poor reproducibility of the TC splits (likely due to splitter variability stemming from hand agitation of the sample bottle), approximately ¼ of each polycarbonate filter was gravimetrically split and used for a second set of TC analyses at the University of Rhode Island Elemental Analysis facility. The specific carbon content (mass C per mass filter) of clean Nuclepore filters was determined empirically. The mass fraction of each ¼ split was used to calculate the carbon content of that split from the original sample filter tare mass. Then the sample carbon was determined by difference between the total carbon and the calculated filter carbon.
Two of the four ¾ polycarbonate splits remaining were analyzed for particulate inorganic carbon (PIC). Filters were extracted overnight in 5% nitric acid with 2% La (as La2O3) releasing agent. Extracts were analyzed using flame atomic absorption spectrometry for Ca (422.7 nm, air-acetylene flame) and Na (589 nm, air-acetylene flame). Sodium levels were used to correct Ca for residual sea salts, then PIC was calculated assuming all Ca was present as CaCO3.
The remaining two ¾ polycarbonate splits were analyzed for bSi. Filters were extracted in 0.2 N NaOH for a total of 2 hours at 95°C, then neutralized with 1 N HCl. Subsamples were taken for immediate analysis for dissolved silicate following standard spectrophotometric methods.
A replicate set of trap tubes was prepared as described above, held in the shipboard laboratory during the deployment, and then analyzed in parallel to provide a process blank determination. The blanks from the three stations were averaged to determine the mean process blank for the cruise (Table 1, See Supplemental Docs, below).
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