US JGOFS Arabian Sea Cruise: TN054 HYDROGRAPHIC BOTTLE DATA
L.A. Codispoti (lou@ccpo.odu.edu) Old Dominion University, July 1996 General Comments: This "readme" file pertains to the salinity, dissolved oxygen, and nutrient data taken from sampling bottles with the hydrographic rosette that was equipped with 24 10-liter Niskin type bottles during RV T.G. Thompson cruise TN054. This cruise was the seventh JGOFS Arabian Sea Process Leg and took place during Nov.- Dec. 1995. Dr. Wilford Gardner of the Department of Oceanography at Texas A&M University was the chief scientist (wgardner@astra.tamu.edu). NOTE THAT MULTIPLE CASTS WERE TAKEN AT MOST STATIONS AND THAT, IN SOME CASES, THE GEOGRAPHIC POSITIONS OF CASTS AT THE SAME STATION MAY VARY SIGNIFICANTLY. Some questionable data are not included in this report. These data are still retained in files at Old Dominion University and are available upon request. No units are given for salinity in this report because the most recent definitions of salinity define it as a dimensionless number. To accommodate every preference, Winkler oxygen values are reported in ml/l, micromolar and micromoles per kg. The latter values can only be calculated with a knowledge of the oxygen sample temperatures when the samples were drawn. These "draw temperatures" are not reported here, but can be obtained by contacting lou@ccpo.odu.edu. Nutrient values are reported in micromolar. They can be converted to micromoles per kg, by combining laboratory temperature on the Thompson (approx. 24.5 deg C during this leg) and the salinity of the sample to compute density and then dividing the value in micromolar by this number. Methods: In general, the methods employed for the bottle salinity, Winkler dissolved oxygen, and nutrient analyses did not differ significantly from those described in the JGOFS protocols that were distributed in June, 1994. Minor differences included the following: 1) Sea Bird CTD systems and bottle carousels were employed (SBE-9+ underwater units, SBE-11 deck units, SBE-32 carousels). These units represent a newer generation of equipment than the units described in the JGOFS protocols. 2) The weights of the salts used for primary standards for dissolved oxygen and nutrients were not adjusted to an "in vacuo" basis as suggested in the protocols. It is unlikely that this departure from procedure would cause significant errors. Our calculations suggest that the maximum differences arising from our decision to not correct to an "in vacuo" basis would range from 0.02% (oxygen standards) to 0.06%(ammonium standards). 3) The protocols give one a choice of adjusting nutrient methods so that calibration curves are strictly linear, or opting for more response and taking into account non-linearities. We choose the latter method. 4) No corrections were made for "carryover" between nutrient samples run on the Technicon Autoanalyzer. Carryover effects in our nutrient analyses are generally less than ~2% of the concentration difference between adjacent samples. Examination of cases where more than one sample was taken from a depth at which there was a significant increase in nutrient concentrations will help the user determine the carryover effect for many individual casts. 5) Calibration and re-calibration of volumetric ware were not exactly as described in the JGOFS protocols, but this was largely compensated for by comparing independent standards diluted with independent volumetric ware, and by re-calibration of some of the volumetric ware after cruises TN045 and TN050. WE HAVE NOT YET RECALIBRATED THE VOLUMETRIC WARE USED DURING TN054. WE WILL UPDATE THE DATA IF RECALIBRATION SUGGESTS A NEED TO DO THIS, BUT WE DO NOT EXPECT SIGNIFICANT CHANGES 6) Duplicate oxygen samples were not drawn from every Niskin or Go-Flo bottle, but there were several comparisons of bottles tripped at the same depth. 7) Azide was added to the Winkler oxygen pickling reagents to destroy nitrite that can be present in relatively high concentrations in the Arabian Sea. ON LEGS PRIOR TO THIS ONE, OXYGEN STANDARDIZATIONS WERE RUN USING REAGENTS THAT DID NOT CONTAIN AZIDE, BUT DISCUSSIONS AND TESTS SUGGESTED THAT IT WOULD BE BETTER TO STANDARDIZE WITH AZIDE, DESPITE SOME CONFUSION IN THE LITERATURE ON THIS MATTER. CONSEQUENTLY, WE SWITCHED PROCEDURES BEGINNING WITH LEG TN053 AND USED REAGENTS CONTAINING AZIDE TO STANDARDIZE. OUR TESTS SUGGEST THAT THE MAXIMUM CHANGE IN OXYGEN CONCENTRATIONS ARISING FROM THIS CHANGE WOULD OCCUR AT THE HIGHEST OXYGEN CONCENTRATIONS AND BE < ~0.01 ML/L. Temperature: The temperature data associated with each bottle depth were taken by the CTD system during the bottle tripping process. Consult the companion CTD data report for this cruise to learn more about the CTD system. Sampling: The samples in this report were taken from 10 liter Niskin bottles. Because there is little or no lag time between triggering a bottle and bottle closure with the new SeaBird rosette systems, bottles were generally held at the sampling depth for at least 30 seconds before tripping or until the deck read-outs stabilized if this took more than 30 seconds. NOTE THAT THE MID-POINT OF THE SAMPLING BOTTLES WAS ONE METER ABOVE THE CTD SENSORS. THE DATA HAVE NOT BEEN CORRECTED FOR THIS ONE METER OR 1.01 DECIBAR DIFFERENCE BETWEEN CTD SENSOR AND SAMPLING BOTTLE POSITIONS. Salinity: Salinities were determined with Guildline Autosal salinometers. New vials of standard sea-water were used to standardize before and at the end of every run. Agreement between bottle salinities and the recently calibrated sensors on the Sea Bird CTD systems was usually better than 0.02 (except in regions of strong gradients) before post-cruise data processing which employs the bottle salinities to correct the CTD salinities. More information on the quality of the salinity data are given in the companion CTD report. Both the CTD salinity data at the time of bottle tripping and the salinities run on the Niskin bottle samples with an Autosal salinometer are reported here. Dissolved oxygen: The Winkler dissolved oxygen set-up was built and supplied by the SIO/ODF group. This system is computer controlled and detects the end-point photometrically. Temperature of the thiosulfate and standard solutions is automatically monitored by this system. Checks on cruises TN039 and TN043 between independent standards prepared with independent volumetric ware gave agreement of +-0.02 per cent. A similar check made during TN054 suggested agreement of better than +-0.15 per cent. The linearity of the "Dosimat" automatic buret was also checked during cruises TN043 and TN054 with good results. Nutrients: Note that the terminology used to describe nutrients has become somewhat loose over the years and that silicate=silicic acid, and phospate=reactive phosphorus. Nutrient analyses were performed on a 5-channel Technicon II AA system that was modified and provided by the SIO/ODF group. In assessing the nutrient standard comparisons outlined below, note that the full-scale ranges for nutrients were as follows: Ammonium =0 to 5 micromolar Nitrate =0 to 45 " Nitrite =0 to 5 " Phosphate =0 to 3.6" Silicate =0 to 180 " These ranges were arrived at after an Internet poll of PI's and were intended to cover the full depth concentration range for the Arabian Sea. Starting with TN050 the nitrite range was expanded to 0-7micromolar because we found maximum nitrite concentrations to be ~6.5 micromolar. On the set-up and calibration cruise (TN039), the SIO/ODF nitrate and nitrite standards and standards from the National Institute of Oceanography in India (provided by S.W.A. Naqvi) were compared with the following results: NIO Nitrate Std.= 22.6 micromolar; SIO/ODF=22.5 micromolar NIO Nitrite Std.= 2.42 micromolar; SIO/ODF = 2.50 micromolar As can be inferred from the above, the nitrate plus nitrite values were almost identical in the mixed standards; 25.02 (NIO) vs 25.00 (SIO)micromolar. On TN039, Lou Codispoti prepared independent primary nitrate, nitrite, silicate and phosphate standards for comparison with SIO/ODF primary standards and made dilutions using glassware entirely independent of the SIO/ODF glassware. The results were as follows: Codispoti SIO/ODF Nitrate 26.96 micromolar 26.85 micromolar Nitrite 2.90 " 2.86 " Silicate 86.4 " 85.8 " Phosphate 2.36 " 2.36 " On TN043, the volumetric equipment used for making routine nitrate and phosphate standards was checked against volumetric ware calibrated by LAC. The average difference between these comparisons of mid-range standards was + or - 0.2% for phosphate and + or -0.4% for nitrate. Because nitrite values in the suboxic waters of the Arabian Sea can attain values of approximately 5 micromolar and because our routine standards contained 22.5 micromoles of nitrate and 2.5 micromoles of nitrite, we kept track of the efficiency of the Cd column that reduces nitrate to nitrite in the nitrate analysis. The lowest column efficiency determined on this cruise was 97.5%. No corrections have been made for any errors in nitrate arising from deviations in cadmium column efficiency. NOTE THAT THE FULL-SCALE NITRITE RANGE FOR THIS CRUISE WAS 7 MICROMOLAR. The ammonium results are the least precise of all the nutrient results. On TN039, three primary standards were compared with agreement of about plus or minus three per cent of the full-scale (5.0 micromolar) value. These standards may have agreed within the precision of the method, but we found a significant salinity effect on the ammonium results that might explain some of these differences since the salinities of the comparison standards varied a bit. Experiments on the first JGOFS Arabian Sea process study cruise (TN043) suggested that the ammonium signal decreases by approximately 3.5% for a salinity increase of 1.00. Comparisons of an independent standard prepared by LAC with an SIO standard on this cruise (TN043), when corrected for salinity differences between the standards agreed to ~ + -0.1% of the full-scale value. The largest absolute difference was 0.025 micromolar and the average difference was 0.013 micromolar for six comparisons between 1-3 micromolar. Thus, the average difference between these two independent standards was + -0.006 micromolar. Comparisons of independent high concentration ammonium standards (~2.5 and 5.0 micromolar) prepared by LAC with SIO standards during TN054 agreed to better than + - 1% for four out of the five standards when corrected for a salinity effect of 4.5%/1.00S on that cruise. One standard agreed to only + - 2.5%, but we assume that this was due to a dilution error. We believe that the suite of ammonium comparisons suggests no systematic differences arising from standards and dilutions, as all of the differences are within the precison of the ammonium analysis. Our results tend to confirm the need to take salinity differences between samples and standards into account when calculating the final ammonium concentrations. THE AMMONIUM VALUES IN THIS REPORT HAVE BEEN CORRECTED FOR THIS EFFECT. ON THIS CRUISE (TN054) THE SALINITY EFFECT CORRECTION IS A 4.5% DECREASE IN SIGNAL FOR A SALINITY INCREASE OF 1.00. The average salinity of the working standards used to calibrate the ammonium method was ~34.96 for casts TN05400101-TN05401302 (inclusive), ~35.27 for casts TN05401303-TN05401902 (inclusive), 35.14 for casts TN05401903-TN05402601 (inclusive), and 34.59 for the remainder of the casts. The ammonium method has additional problems, such as contamination of "baseline" water etc. These problems can introduce inaccuracies on the order 0.1 micromolar, so differences in ammonium concentrations of less than ~0.1 micromolar should not be over-interpreted.