Measurements of Dissolved Organic Carbon in the
North Atlantic Ocean During Wintertime in 1994

Introduction

Meteor cruise no. 27 combined the activities of four programs focusing on the interactions of climatic change and the oceanic ecosystem: Leg 2 focused on processes in the Northeast Atlantic at the 47°N, 20°W BIOTRANS station. Objectives of this cruise were to quantify the rates of biological carbon fixation and to measure the fluxes of carbon and other compounds of biological importance under winter conditions. These investigations focused on the upper ocean and mixed-layer (JGOFS) and the benthic boundary layer (BIO-C-Flux) where physical, chemical and biological processes are intensified. Of special importance was the measurement of the exchange rate of CO2 between the atmosphere and the upper mixed layer.

A summary of this cruise (Meteor-Berichte 94-6) is available. Excerpts from this report describe:

Leg 2 was planned to occur between January 20th and February 15th, but due to severe damages to the ship and scientific gear by a hurricane force gale (see narrative) Meteor had to make for a shipyard in Lisbon for a refit hence the cruise ended a week early on February 8, 1994.

DOC Measurement (P. Kahler and E.T. Peltzer)

Dissolved Organic Carbon (DOC) had been measured during the North Atlantic Bloom Experiment (NABE) during 1989 in the North Atlantic employing "2nd generation" high-temperature catalytic oxidation methods introduced by Sugimura and Suzuki (1988). At that time, the method still suffered from problems mainly due to the instrument blanks, which resulted in the widely discrepant DOC concentrations reported by different workers. This notorious situation led to an intercalibration exercise and workshop organized by J. Hedges and held in Seattle during 1991 with 35 laboratories taking part (Hedges and Lee 1993). Shortly thereafter Suzuki (1993) retracted much of his earlier work on the measurement of DOC. Meanwhile, with due attention paid to the blank problem, it was shown that the results of different workers' DOC analyses can compare well (Sharp et al., 1995).

With these improvements in the measurement technique, DOC has recently been shown to vary both spatially and temporally in the open ocean (Carlson et. al., 1994; Carlson and Ducklow, 1995; Peltzer and Hayward, in press). As such it is thought to be important in the oceanic carbon cycle both as a reservoir for the end products of primary production and as a source or substrate for bacterial production. Precise measurements are required to see these changes, and uncertainties remain regarding absolute concentrations and temporal variations. It was hoped that by directly comparing two instruments at sea using freshly collected samples, that some of these uncertainties could be resolved.

There were two major objectives of the DOC measurement program during M 27/2. The first was to obtain a detailed wintertime profile of the distribution and near surface variability of DOC on short time scales at the 47°N, 20°W BIOTRANS-site. This information is important in its own right and is necessary to round off the survey of the annual cycle of DOC at this station established during 1992/93 (PK) with a season not yet covered. The second objective was to compare two independent analytical systems at sea; both are based on the new high-temperature combustion technique. PK's was a modified commercial system (Dimatec) and ETP's was a home-made analyzer based upon improvements in Suzuki's design (Peltzer and Brewer, 1993; Peltzer et al., 1996).

Results

Comparison of the two analyzers produced several results. First, both analyzers yielded the same DOC concentrations for identical samples. However, significant differences were observed between various sample collections and pre-treatment procedures. Contamination during sample collection and filtration was thought to occur and it was concluded that the most consistent results were obtained whenever the samples were handled the least, meaning not filtered.

Mixed-layer DOC concentrations were observed to be quite patchy based upon the underway snorkel samples. Results from these samples are shown in Figure 1 and Figure 2. Generally, the concentrations are low. This is consistent with the wintertime situation of low production and deep convection.

Figure 3 shows three typical profiles obtained during the small-scale survey conducted upon our arrival at the BIOTRANS-station at 47°N, 20°W. The subsurface maximum was typical of all the profiles obtained and surface concentrations were consistent from profile to profile. The subsurface maximum was a bit of a surprise as it suggests rather shallow mixing during this time of year. There were already slightly elevated stocks of phytoplankton in the surface water and nutrient levels were already significantly depleted (cf. W. Koeve's contribution). Also the distribution of DOC showed what we believe to be spring features. The slightly elevated DOC content in the surface water is from freshly produced DOC, otherwise the concentration would be uniform down to the maximum mixing depth near 400 m.

Several days later a much different profile was obtained in the same area. During the night before the cast the wind had picked up to 40 knots and the air temperature was 1-2 degrees cooler than the sea. This wind event was sufficient to cause the complete overturn of the upper water column down to a depth of almost 400 m. This mixing can be clearly seen in DOC profile 35 (red line in Figure 4) when compared to a profile from before the event (profile 26, in green). The result of this "deep mixing event" is the net downward transport of organic carbon below the level of the summer mixed layer.

DOC in sediment contact water was measured in three cores: there was no gradient with distance from the sediment surface (four 5-8 cm increments), and the concentration was the same as in the deep water. This suggests that the sediments of the BIOTRANS-site are not a major source of DOC to the overlying water during winter time conditions.

Acknowledgments

This research was supported by National Science Foundation grant OCE-9203953. I thank program managers Neil Anderson and Rodger Baier for their guidance and encouragement. We gratefully acknowledge the captain, crew and technicians of the Meteor for their help and support. We are especially indebted to the valiant efforts of the crew that got us safely through the storm. We also thank chief scientist Olaf Pfannkuche for cruise planning and leadership.

The METEOR expeditions are funded by the Deutsche Forschungsgemeinschaft and the Bundesminister für Forschung und Technologie.

References

Carlson, C.A. and H.W. Ducklow (1995). Dissolved organic carbon in the upper ocean of the central equatorial Pacific Ocean, 1992; Daily and finescale vertical variations. Deep-Sea Research II, 42, No. 2-3, 639-656.

Carlson, C.A., H.W. Ducklow and A.F. Michaels (1994). Annual flux of dissolved organic carbon from the euphotic zone in the Northwestern Sargasso Sea. Nature, 371, 405-408.

Hedges, J.I. and C. Lee (1993). Measurement of dissolved organic carbon and nitrogen in natural waters. Proceedings of NSF/NOAA/DOE Workshop, Seattle, WA, USA, 15-16 July 1991. Special issue of Marine Chemistry, 41, 290 pp.

Peltzer, E.T. and P.G. Brewer (1993). Some practical aspects of measuring DOC - sampling artifacts and analytical problems with marine samples. Special issue of Marine Chemistry, 41, 243-252. (Abstract)

Peltzer, E. T., B. Fry, P. H. Doering, J. H. McKenna, B. Norrman and U. L. Zweifel (1996). A comparison of methods for the measurement of dissolved organic carbon in natural waters. Marine Chemistry 54, 85-96. (Abstract)

Peltzer, E.T. and N.A. Hayward, (1996). Spatial and temporal variability of total organic carbon along 140°W in the Equatorial Pacific Ocean in 1992. Deep-Sea Research II, 43, 1155-1180. (Abstract)

Pfannkuche, Olaf, W. Balzer and F. Schott (1994). Carbon cycle and transport of water masses in the North Atlantic-the winter situation, Cruise No. 27, 29 December 1993 - 26 March 1994. METEOR-Berichte, Universität Hamburg, 94-6, 134 pp.

Sharp, J.H., R. Benner, L. Bennet, C.A. Carlson, S.E. Fitzwater, E.T. Peltzer and L. Tupas (1995). Analyses of dissolved organic carbon in seawater: the JGOFS EQPAC methods comparison. Marine Chemistry, 48, 91-108. (Abstract)

Sugimura, Y. And Y. Suzuki (1988). A high temperature catalytic oxidation method for the determination of non-volatile dissolved organic carbon in seawater by direct injection of a liquid sample. Marine Chemistry, 24, 105-131.

Suzuki, Y. (1993). On the measurement of DOC and DON in seawater. Special issue of Marine Chemistry, 41, 287-288.

This page was last updated on 20 April 1999 by Edward Peltzer, MBARI.