A summary of this cruise (Meteor-Berichte 94-6) is available. Excerpts from this report describe:
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).
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.
The METEOR expeditions are funded by the Deutsche Forschungsgemeinschaft and the Bundesminister für Forschung und Technologie.
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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)
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