Hydrographic Measurements

Two WHOI-modified EG&G Mk-III CTDs were provided for the cruise, although only one was used throughout the entire cruise (CTD #10). It was provided with an optional oxygen current and temperature channel and modified at WHOI to include a thermally isolated titanium pressure transducer, with a separately digitized pressure and temperature channel. The temperature and pressure calibrations were made at WHOI prior to and following the cruise. The CTD pressure, temperature, and conductivity data were processed and corrected according to laboratory calibrations. Pressure values are expected to be accurate to ±3 dbar; temperature values to ±0.002°C.

The water samples for the analysis of salinity, dissolved oxygen, and nutrients were collected from each of the 36 10-L bottles tripped on the upcast of each CTD station, in accordance with the recommendations of the WOCE Hydrographic Office. The vertical distribution of these samples was a compromise between the need to obtain deep samples for the calibration of the CTD conductivity and oxygen sensors and the requirement to define the characteristics of the water masses by the distributions of the various measured parameters.

Salinity samples were drawn into 120-mL Boston Round flint glass bottles with screw caps equipped with Poly-Seal® cones to prevent leakage and evaporation. After determining some problems with the quality of salinity measurements, it was decided to change the bottles. Beginning with station 59, samples for salinity measurements were collected in 200-mL square Kimax bottles, with polyethylene caps and inserts, owned by SIO, and a dramatic improvement was seen. Salinity was determined on the basis of electrical conductivity with a Guildline Autosal® salinometer.

Samples for dissolved oxygen were collected soon after the rosette sampler was brought on board and after CFC and helium samples were collected. Samples were titrated in the volume-calibrated iodine flasks with a 1-mL microburette, using whole-bottle automated Winkler titration. Estimated accuracy was 0.02 mL/L.

Nutrient samples were drawn from all CTD/rosette casts at stations 1 through 94 and at several test stations that preceded station 1. High-density polyethylene bottles of ~30-mL volume were used as sample containers, and these same bottles were positioned directly in the autosampler tray. These bottles were routinely rinsed at least three times with one-third to one-half of their volume of sample before filling and were thoroughly cleaned with 10% HCl every two or three days. The measurements were performed with an Alpkem Rapid Flow Analyzer, model 300. A Keithley data acquisition system was used in parallel with analog strip-chart recorders to acquire the absorbance data. The software used to process the nutrient data was developed at OSU. All of the reagent and standard materials were provided by OSU.


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