Because the samples are initially processed on deck and radiocarbon is being measured, the engines of the R/V Blue Heron are shut down during this time allowing the vessel to drift (engines not going and thus less possibility for contamination from the stack), until the processing of the samples has been completed.

Related files and references:
Sampling procedures, instrumentation, discussion of blanks, etc, included in Zigah, P.K., E.C. Minor, J.P. Werne, and S. L. McCallister, 2011. Radiocarbon and stable carbon isotopic insights into provenance and cycling of carbon in Lake Superior. Limnology and Oceanography 56(3), 867-886.

Isotopic data were processed as described in Radiocarbon Data & Calculations.

Concentration data for DOC and DIC are from a Shimadzu TOC-Vcsh Analyzer.

For DIC, the analyzer was calibrated using primary standard grade sodium carbonate and ACS reagent grade sodium bicarbonate. The inorganic carbon in the sample was volatilized with 25% H₃PO₄ (by weight) in a CO₂-free closed reaction vessel, and the CO₂ evolved was measured by a non-dispersive infrared (NDIR) gas detector.

For DOC, the samples were acidified to pH 2 with ACS reagent grade HCl at the time of sampling. The TOC analyzer was run in NPOC mode and calibrated with potassium hydrogen phthalate (KHP). Deep seawater from Florida Strait at 700 m (Batch-8 2008) reference waters obtained from the DOC Consensus Reference Program (Hansell laboratory, University of Miami, USA) and additional KHP standards were interspersed and analyzed along with the samples to assess instrumental performance.

Our mean DOC value of the external reference sample was 43.9 ± 3.9 μM (n = 5), and agrees with the consensus value of 41-43 μM. For each sample and standard, three injections were performed. If the standard deviation was not ≤ 2.5%, two more injections were performed and the closest three of the five injections were averaged to yield sample concentration.

Particulate organic samples (on GF/F filters) were fumigated with HCl, dried, then loaded into tin capsules and analyzed for particulate organic carbon (POC) and particulate organic nitrogen (PON) concentrations using a Costech ECS 4010 elemental analyzer (EA) coupled to Finnigan Delta Plus XP isotope ratio mass spectrometer (IRMS). The EA was calibrated with acetanilide (Costech Analytical Technologies Inc., Valencia, CA, USA) and caffeine (SigmalUltra; Wheaton Science Products, NJ, USA) reference standard materials; C and N were quantified using the IRMS. Typical instrumental precisions for POC and PON measurements were respectively, 0.2% and 0.4% of the measured concentrations.

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