Abstract
We present evidence suggesting that only a portion of the dissolved organic carbon (DOC) in the seawater analyzed previously by Williams and Druffel (1987) was oxidized by the UV‐radiation method. High temperature catalytic (HTC) methods (Sugimura and Suzuki, 1988) used to reoxidize the central North Pacific gyre water samples reveal that the total DOC (DOCHTC) is about twice that of the UV‐oxidizable DOC (DOCuv). Indications from the original study suggest that this additional DOC contains higher concentrations of radiocarbon than in the DOCuv(Williams and Druffel, 1987). This evidence implies that DOC is older and thus more refractory with respect to biological utilization, yet is more chemically reactive with respect to photooxidation, than the fraction resistant to UV (DOCres).We report accelerator mass spectrometry (AMS) Δ14C measurements of humic, fulvic and hydrophilic acid fractions isolated from water collected at 180 m in the North Pacific (19°N, 158°W) using XAD macroreticular resins. Δ14C values of the humic material are less than those of DOCuv from a similar depth 1200 km further north (Williams and Druffel, 1987) indicating that these humic substances are part of the ‘old’ recycled DOCuv in the ocean.
Highlights
For the past twenty years, measurements of DOC in seawater have been commonly achieved by two methods: photooxidation by UV-radiation from a Hg-arc lamp (Armstrong et al, 1966) and by wet oxidation using peroxodisulfuric acid (Menzel and Vacarro, 1964)
We report measurements of DOC using the HTC method on the same seawater samples from the central North Pacific (31N, 159øW) by Williams
Frozen and thawed aliquots of both UV-oxidized (DOC ) res and unoxidized (DOCu) water samples were analyzed from seale-Vampoulesshipped to Japan
Summary
For the past twenty years, measurements of DOC in seawater have been commonly achieved by two methods: photooxidation by UV-radiation from a Hg-arc lamp (Armstrong et al, 1966) and by wet oxidation using peroxodisulfuric acid (Menzel and Vacarro, 1964). Obtained high DOC results using wet oxidation with chromic acid after removal of most of the chloride ion (Krogh, 1934). Data, not yet rigorously confirmed, imply that virtually all of the AOU (apparent oxygen utilization) in the western North Pacific can be accounted for by the oxidation of DOC and dissolved organic nitrogen (DON). We report measurements of DOC using the HTC method on the same seawater samples from the central North Pacific (31N, 159øW) by Williams. This HTC method suggests that there are 2 to 3 times more DOC in our seawater samples than previously measured. Correlations between DOC and AOU indicate that DOC is responsible for the major amount of oxygen utilization in the upper few hundred meters of the water column only
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