of three different methods for measuring non-volatile organic carbon in seawater during the JGOFS North Atlantic pilot study

SUMMARY The two HTCO methods with various modifi- cations look promising. Improvements in sample handling and assessment of response and blanks may lead to better reproducibility, provided that ship-induced interferences with the final CO2 infrared analyzer will be circumvented. New types of infrared CO2 analyzers operating at higher frequencies (approximately 500 Hz) are now available and would appear useful. Absolute TOC values found with both the two HTCO methods as well as the wet chemical oxi- dation method are 100-150pmol in surface waters and 80-100#mol in deeper waters of the northeast Atlantic Ocean. The reasonable agree- ment of the wet oxidation method is mostly ascribed to the higher amount of persulfate used. Earlier investigations of the Atlantic Ocean were based on methods other than HTCO, with reported TOC values either similar to or lower than our results (Table 1 of Sugimura and Suzuki, 1988). Values in the range of 70-140#mol, found in the open northwest Atlantic Ocean (Kepkay and Wells, 1993) are similar to ours. Recently reported values for three Pacific stations are also in the same range as ours and fairly uniform with depth, except for a distinct surface maximum of 170/~mol at one station (Tanoue, 1993). During a recent intercalibration in the Mediterranean Sea the TOC values obtained with a Shimadzu TOC 500 instrument rarely exceeded 125/~mol