maximum depth of 67 m. The southern side of Majuro Atoll is almost entirely surrounded by a continuous island. The northern side of the atoll is surrounded by very shallow (0-2 m deep) reefs with a chain of sand cays. Calalin Channel, situated at the northern end of the atoll, is the only deep passage, and has a maximum depth of 46 m. This topography effectively isolates the wide interior lagoon from the offshore ocean. Majuro Atoll is subject to a trade-wind climate throughout the year. The present investigation was conducted during the rainy season.
Fig-2.
Majuro Atoll. Surface salinity increased with the distance from the windward reef to the southwest corner of the lagoon. Samples of bottom sediments in the lagoon were taken at stations A, C, and D. A topographic section of Majuro Atoll along the X-X' line is also shown.
2-2. Analytical procedures
(1) Dissolved CO2 system
Water sampling was conducted from small boats at various points in the lagoon of Majuro Atoll from September 9 to 12, 1994. Samples for salinity, pH, total carbon dioxide (CT) and total alkalinity (AT) were drawn from a plastic bucket or a Niskin bottle by means of a siphon. Unfiltered seawater samples for CT determination were immediately poisoned with a saturated solution of HgCl2. Seawater samples for AT were filtered through Whatman GF/F filters. Salinity, pH, CT, and AT measurements were carried out in a laboratory aboard the R/V Hakurei-Maru.
A combined electrode (Model GK2401C, Radiometer) with a Radiometer ION ANALYZER (Model I0N85) was used for pH measurements. The electrode was calibrated in the NBS buffers.
The total carbon deoxide (CT) was measured using a modification of the gas chromatographic method7). The precision of the analysis, based on replicate measurements, was＜0.18%.
The total alkalinity (AT) was obtained using the potentiometric acid titration method. A Radiometer titrator (Model VIT9O) with an automated burette(Model ABU91)was used. The precision of the AT change was 0.15%(3 μmol kg-1).
The salinity was determined with a Guildline salinometer (Autosal Model 8400B). The density of all solutions was calculated from salinity and temperature using the equation reported in Millero and Poisson8).
Smith and Kinsey9) pointed out the necessity of taking extreme care in order to detect small changes in the seawater CO2 system. In this study, measurements of AT and CT in all stored samples were made at the same time, in order to eliminate the effect of day-to-day drift in standardization. The pH measurements were made on the day samples were taken; however, offshore pH values at 25℃ ranged between 8.298 and 8.302 over 3 days and were found to be quite uniform. This fact indicates that the standardization of the pH meter was adequate.
The three analytical parameters of the carbonic acid system (pH, AT, CT) were measured on 40 samples. Values of PCO2 at 25℃ were obtained from the two pairs, PH-AT and PH-CT, by using thermodynamic relations. The dissociation constants of the NBS scale were used10）． The solubility constant of carbon dioxide in seawater was obtained from Weiss11). Correction of PCO2 due to temperature variation was made using the GJ formula12). Details of calculation are as described in the Appendix of Suzuki et al.13). The PCO2 values calculated from pH and AT were compared with those calculated from pH and CT. Internal consistency was found to be good. This indicates that the accuracy of the measured CT and AT is adequate for further discussion.
(2) Suspended materials
Samples for determining suspended particulate matter were taken at 20 stations inside and outside the atoll. Seawater samples were divided into two portions. One was to determine total particulate carbon content and the other was for organic carbon content. Subsamples were filtered through precombusted 25 mm GF/F glass-fiber filters(450℃, 4 hours). Subsample volume for total carbon and organic carbon determination varied from 0.5-5L and 1.5-6L, respectively. The GF/F filters were then rinsed with 50 mL of 0.7M NaCl and stored at -2O℃ in a freezer. Samples were freeze-dried prior to analysis with a Yanaco MT-5 CHN analyzer. Pretreatment for removal of carbonates was necessary for organic carbon determination. The vapor method14) was applied to the filtered samples.
(3) Bottom sediments
Samples of the bottom sediment in the lagoon were collected at Stations A(60 m depth; the deepest point in the lagoon; Fig-2), C(2m)and D(1.5m). Offshore bottom sediment samples were
