A Trans-Indian Ocean hydrographic section at latitude 32 degrees South : data report of RRS Charles Darwin cruise #29

A trans-Indian Ocean hydrographic section employing CTD j O2 profilers was conducted between Africa and Australia during austral spring 1987.The cruise track ranged between 29°S and 34°S; the average latitude of the crossing was 32°S.The purpose of the cruise was to explore various aspects of the South Indian Ocean including the characteristics of the core water masses of this ocean, the strength of the subtropical gyre, the structure and transport of deep western-boundary currents, and the net meridional heat flux.A total of 109 CTDj02 profiles with associated rosette water sample measurements and 347 XBT profiles were collected, supplemented by underway upper ocean velocity, bathymetric and sea surface temperature and salinity data.This report details the data collection, calibration, and reduction methods, and summarizes the hydrographic observations.VB = i: Vll Equipment Corporation (DEC) MicroVAX II computer systems (Figure 2).Acquisition data were loaded onto the Micro VAX system via Kennedy cartridge tape drives and displayed graphically using Zeta-8 plotters.Two independent MicroVAX systems were employed: the first devoted to basic processing, the second to data archiving, higher level processing and analysis.Nine-track and DEC TK50 cartridge tapes served as media for data archiving.Two Guildline AutoSal Model 8400A salinometers were utilzed to determine water sample salinities.These were installed in a portable laboratory capable of maintaining constant environmental temperature within :l l°C.The nominal laboratory temperature was 22°C.A standardization check was performed once per day, using Standard Seawater Batch P-97.No drift of the Autosal was observed during the cruise, thus no standardization adjustments were made.It should be noted that, based upon a comparison of Batch P-97 and PSS78 DCL Standard, Mantyla (1987) has recommended a correction (which has not been made to these data) of + 0.0008 for rosette samples analyzed with this batch.The uncertainty in the rosette salinity data is believed to be :l 0.003 psu, the manufacturer's stated accuracy of the AutoSaL.Water sample dissolved oxygen analyses were also performed in the constant temperature laboratory using a modified Winkler titration technique.The measurements were conducted on 50 me aliquots of the samples.A Metrohm Titroprocessor controlling a Metrohm Dosimat was used to titrate to an amperometric endpoint as described by Knapp et al. (1989).Standardization checks were performed prior to and following the use of each batch of titrant (typically every third day).No observable drift occurred between standardization checks.These data are reproducible to :l 0.02 mfjf with ¡:,ccuracy of better than 2%.The inorganic nutrient determinations were carried out by Dr. Louis i. Gordon's group from Oregon State University.Samples were analyzed for dissolved, reactive nutrients at sea using an Alpkem Corporation RFA-300 continuous, segmented flow analyzer (RFA).Nutrients analyzed included orthophosphate, silcic acid, nitrate plus nitrite, and nitrite.The phosphate method was basically that of Atlas et al. (1971), modified for the RFA.The remaining methods were those furnished by the Alpkem Corporation for use with the RFA (Alpkem, 1986; Patton, 1983).We have established that all other methods are linear to a few tenths of 1 % and give results comparable to, or better than, the AutoAnalyzer-II -based methods we employed in the past (Atlas et ai., 1971).The dissolved nutrients were measured at all station locations; in most cases, these analyses were performed immediately after each CTD cast and were completed within two to three hours after the cast.The short term precision (1 standard deviation), estimated from replicate analysis of the same sample and on occasions where two rosette bottles were tripped at the same depth, was approximately 0.2%, 0.5%, and 1.0% of regional deep water values for silicic acid, nitrate plus nitrite, and phosphate, respectively.Nitrite precision is typically 0.02 micromolar.Due to problems with the autosampler (mentioned below), long term precision and accuracy were estimated at 1-2% for silcic acid and nitrate plus nitrite, 3-5% for phosphate, and 0.04 micromolar for nitrite.Data which seemed clearly in error were rejected during the post cruise quality control review of the data.Chlorofluorocarbon (CFC) samples (F11 and F12) were drawn from rosette bottles at about 70% of the stations.An analytical system similar to that of Bullister and Weiss (1988) was used.CFC concentrations are reported relative to the SI086 calibration scale (Weiss, personal communication).A combination bottle and handling blank was used to correct for contamination from the Niskin bottles, and from the collection and storage of samples.This blank was estimated by rotating Niskin bottles, double tripping them and measuring what was believed to be CFC-free water.For F11 the blanks varied throughout the cruise, generally decreasing with time.They ranged from 0.04 pmoljkg to zero.For F12 the blanks were zero; however, contamination problems preclude the use of some of the F12 data.We estimate our precision based on analysis of 166 duplicate samples from the same syringe.The standard deviation of the series of replicates for F11 was as follows: for concentrations in the range zero to 0.10 pmoljkg precision :l 0.004 pmoljkg, in the range 0.1-0.5 pmoljkg precision :l 0.007 pmoljkg, in the range 0.5-1.0pmoljkg precision :l 0.012 pmoljkg, and greater than 1.0 pmoljkg precision :l 0.092 pmoljkg.The standard deviation of the series of replicates for F12 was as follows: for concentrations in the range zero to 0.10 pmoljkg precision :l 0.009 pmoljkg, in the range 0.1-0.5 pmoljkg precision :l 0.011 pmoljkg, in the range 0.5-1.0pmoljkg precision :l 0.035 pmoljkg, and greater than 1.0 pmoljkg precision :l 0.04 pmoljkg.Marine airs for F11 were 224 :l 6 ppt.The water sample salinity, oxygen, nutrient, and CFC observations are presented in Appendix B of this report.