• An Alternate Method of Diluting Dissolved Organic Carbon Seawater Samples for 14C Analysis

      Griffin, Sheila; Beaupré, Steven R.; Druffel, Ellen R. M. (Department of Geosciences, The University of Arizona, 2010-01-01)
      We present a time-saving modification to the ultraviolet (UV) oxidation method for analyzing dissolved organic carbon (DOC) concentration, ∆14C, and 13C measurements in seawater and standard materials. A low background (~0.2 +/- 0.2 M) was reported for pre-irradiated Milli-Q (MQ) water that was used to dilute samples for DOC 14C analysis (Beaupr et al. 2007). We use MQ water without pre-irradiation (background ~0.9 0.2 M) to dilute the sample. This method is suitable for small-volume, high-concentration samples (mass of sample DOC overwhelms mass of MQ water DOC). An acceptable precision of ∆14C measurements (5-9) is maintained. This revised method reduces the preparation time for diluted DOC ∆14C samples from 2 days to 1 day.
    • Compound-Specific Radiocarbon Analyses of Phospholipid Fatty Acids and n-Alkanes in Ocean Sediments

      Druffel, Ellen R. M.; Zhang, Dachun; Xu, Xiaomei; Ziolkowski, Lori A.; Southon, John R.; Dos Santos, Guaciara M.; Trumbore, Susan E. (Department of Geosciences, The University of Arizona, 2010-01-01)
      We report compound-specific radiocarbon analyses of organic matter in ocean sediments from the northeast Pacific Ocean. Chemical extractions and a preparative capillary gas chromatograph (PCGC) were used to isolate phospholipid fatty acids (PLFA) and n-alkanes from 3 cores collected off the coast of California, USA. Mass of samples for accelerator mass spectrometry (AMS) 14C analysis ranged from 13-100 g C. PLFA extracted from anaerobic sediments in the Santa Barbara Basin (595 m depth) had modern ∆14C values (-20 to +54), indicating bacterial utilization of surface-produced, post-bomb organic matter. Lower ∆14C values were obtained for n-alkanes and PLFA from coast (92 m depth) and continental slope (1866 m) sediments, which reflect sources of old organic matter and bioturbation. We present a brief analysis of the blank carbon introduced to samples during chemical processing and PCGC isolation.
    • Variability of Dissolved Inorganic Radiocarbon at a Surface Site in the Northeast Pacific Ocean

      Druffel, Ellen R. M.; Beaupré, Steven; Griffin, Sheila; Hwang, Jeomshik (Department of Geosciences, The University of Arizona, 2010-01-01)
      We report radiocarbon measurements of dissolved inorganic carbon (DIC) in surface water samples collected daily during 12 cruises to Station M in the northeast Pacific off central California. Individual surface ∆14C values ranged from 22 to 70 over 10 yr. Variability of average cruise values is highest during winter likely due to increased mixing. A general decrease of ∆14C values was observed at a rate of about 3 per year between 1994 and 2004, about half of that in atmospheric CO2 during this period (Levin and Kromer 2004). The ∆14C results ranged from 2-18 during individual cruises and were often significantly larger than the total uncertainty for individual measurements (3.9). This indicates that a single ∆14C result from a surface site is not sufficient to capture the true variability of ∆14C in the surface ocean.