Discovery Cruise AMT15 CTD Processing
Sampling strategy
A total of 105 successful CTD casts were made during the cruise, 68 casts used the stainless steel rig and 37 used the titanium rig. Rosette bottles were fired throughout the water column on the upcast of most profiles. Data were measured at 24 Hz by a PC running SEASAVE, Sea-Bird's data acquisition software. The raw data files were supplied to BODC after the cruise.
Originator's processing
Only a subset of files had been partially processed on board during the cruise. The raw data were therefore reprocessed at BODC to produce a homogeneous set of CTD data files for this cruise.
BODC post-processing and screening
BODC used the latest version of the SeaBird Processing software available at the time to process the raw binary data files (DAT files) based on information held in the sensor configuration files (CON files), and bottle firing files (BL).
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Sea-Bird processing
The CON files were first checked for any changes which may have occurred during the cruise, none were made. The information was also cross checked against information held in the sensors' calibration reports.
The following SeaBird routines were then carried out using SBE Data Processing software version 5.30a: DATCNV, CELLTM, FILTER, LOOPEDIT, DERIVE, BINAVG, STRIP. After CELLTM was run, tests were carried out to check whether an alignment of the conductivity sensor was necessary. In some instances a lag of 0.007 s was found but since this was not consistent on all casts it was decided that no lag need to be applied to conductivity. Details of the routines and settings used were as follows:
DATCNV converts the raw data into engineering units. Both down and upcasts were selected. All channels were selected for transfer.
The manufacturer's calibration for the fluorometer was applied during Sea-Bird processing as follows:
Stainless steel Nominal chl-a conc (µg/l) = (0.00948 * 10 voltage ) - 0.0174 Titanium Nominal chl-a conc (µg/l) = (0.01080 * 10 voltage ) - 0.0270 CELLTM was run on the DATCNV output using SeaBird's recommended settings of alpha= 0.03 and Tau=7.
FILTER was run on pressure using a low pass time constant of 0.15 seconds.
LOOPEDIT was run in order to minimise the marked wake effect linked to ship rolling observed on recent cruises.
DERIVE, BINAVG and STRIP were then run to derive the salinity and oxygen concentration, reduce the data to 2Hz and strip redundant channels from the final sets of ASCII files.
Conversion of transmissometer voltages to beam attenuation
There were no air and blocked path readings provided for this cruise. So the transmissometer output was not processed to transmissance or attenuance during SeaBird processing but retained as a voltage. The conversion to attenuance was carried out after transfer, screening and loading to the database.
Conversion of PAR sensor voltages to irradiance
The PAR sensor output was not processed to irradiance units during SeaBird processing but retained as a voltage. The conversion to irradiance was carried out after transfer, screening and loading to the database.
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Reformatting
The data were converted from Sea-Bird ASCII format into BODC internal format (PXF) using BODC transfer function 357. The following table shows how the variables within the Sea-Bird files were mapped to appropriate BODC parameter codes:
Sea-Bird Parameter Name Units Description BODC Parameter Code Units Comments Pressure, Digiquartz dbar CTD pressure PRESPR01 dbar - Temperature [ITS-90] °C Temperature of water column by CTD sensor 1 TEMPCU01 °C - Temperature, 2 [ITS-90] °C Temperature of water column by CTD sensor 2 TEMPCU02 °C - Salinity - Practical salinity of the water body by CTD sensor 1 PSALCU01 - - Salinity, 2 - Practical salinity of the water body by CTD sensor 2 PSALCU02 - - Oxygen µmol kg -1 Dissolved oxygen concentration DOXYSU01 µmol l -1 Converted from µmol kg -1 to µmol l -1 using sigma-T during transfer Fluorescence mg m -3 Nominal chl-a concentration CPHLPM01 mg m -3 Manufacturer's calibration applied during processing Voltage 4 V Downwelling PAR sensor voltage LVLTPD01 V - Voltage 5 V Upwelling PAR sensor voltage LVLTPU01 V - Voltage 6 V Light Scatter Sensor voltage NVLTST01 V No calibration details - only available as raw voltage Voltage 7 V Transmissometer voltage TVLTDR01 V - - - Practical salinity of the water body by CTD sensor 1 - sample calibrated PSALCC01 - PSALCU01 calibrated against bench salinometer data - - Dissolved oxygen concentration - sample calibrated DOXYSC01 µmol l -1 DOXYSU01 calibrated against Winkler titration data - - Fluorometer - sample calibrated CPHLPS01 mg m -3 CPHLPM01 calibrated against fluorometric chlorophyll-a data - - Downwelling sub-surface PAR irradiance IRRDPP01 µE m -2 s -1 Generated using manufacturer's calibration - - Upwelling sub-surface PAR irradiance IRRUPP01 µE m -2 s -1 Generated using manufacturer's calibration - - Beam attenuance ATTNDR01 m -1 Generated using manufacturer's calibration - - Oxygen saturation OXYSSC01 % Generated by BODC using the Benson and Krause (1984) algorithm wioth parameters DOXYSC01, PSALCC01 and TEMPCU01 - - Potential temperature POTMCV01 °C Generated by BODC using UNESCO Report 38 (1981) algorithm with parameters PRESPR01, PSALCC01 and TEMPCU01 - - Sigma-theta SIGTPR01 kg m -3 Generated by BODC using the Fofonoff and Millard (1982) algorithm with parameters PSALCC01 and POTMCV01 -
References
Benson, B.B. and Krause, D., 1984. The concentration and isotopic fractionation of oxygen dissolved in freshwater and seawater in equilibrium with the atmosphere. Limnol. Oceanogr., 29(3), 620-632
Fofonoff, N.P. and Millard, R.C., 1983. Algorithms for computations of fundamental properties of seawater. UNESCO Technical Papers in Marine Science No. 44, 53pp.
UNESCO, 1981. Background papers and supporting data on the International Equation of State of Seawater 1980. UNESCO Technical Papers in Marine Science No. 38, 192pp
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Screening
The PXF data were compared with the original data files to ensure that no errors had been introduced during the conversion process. Reformatted CTD data were transferred onto a graphics work station for visualisation using the in-house editor EDSERPLO. Downcasts and upcasts were differentiated and the limits manually flagged. No data values were edited or deleted. Flagging was achieved by modification of the associated BODC quality control flag for suspect or null values.
The profiles were generally of good quality with some flagging of the oxygen and fluorometer channels. The PAR channels showed some variation in the surface but these were not flagged as they could be from passing cloud or ship's shadow.
The transmissometer appeared to suffer from slight pressure or temperature hysteresis at depth. The pattern showed a minimum attenuance in the range 150-200 db and then a slight increase in value as the profile went deeper.
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Banking
Once quality control screening was complete, the CTD downcasts were banked. Finally, the data were binned against pressure at 1 dbar increments.
Voltage conversions
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PAR sub-surface irradiance
The PAR sensor raw voltages have been converted to PAR irradiance values in units of µE m -2 s -1 using supplied manufacturer's calibration coefficients.
Rig Casts Sensor s/n Calibration BODC cal ref Stainless steel All 01 IRRDPP01 = 0.0423 * exp (LVLTDP01 * 4.987 - 7.7190) 4279 Titanium All 02 IRRDPP01 = 0.0423 * exp (LVLTDP01 * 5.1010 - 8.3209) 4281 Titanium 1 - 18 03 IRRUPP01 = 0.0423 * exp (LVLTUP01 * 5.0970 - 8.7753) 4282 Titanium 19 - 105 04 IRRUPP01 = 0.0423 * exp (LVLTUP01 * 5.1400 - 8.4029) 4280 -
Attenuance
The transmissometer raw voltages have been converted to attenuance values in units of m -1 using manufacturer air/dark/pure water voltages converted to calibration coefficients as per Sea-Bird Application Note No.7 . No air/dark voltages were provided from the cruise so coefficients have been calculated with the most recent dark/air voltages being those provided by the manufacturer.
M = (T w / (W 0 - Y 0 ) * (A 0 - Y 0 ) / (A 1 - Y 1 ) B = -M * Y 1 where
Stainless steel Titanium T w = % transmission for pure water 100% 100% W 0 = voltage output in pure water 4.2009 V 4.1980 V A 0 = manufacturer's air voltage 4.7810 V 4.4320 V Y 0 = manufacturer's blocked path voltage 0.0184 V 0.0214 V A 1 = cruise air voltage 4.496 V 4.530 V Y 1 = cruise blocked path voltage 0.074 V 0.020 V Rig Sensor s/n Calibration BODC cal ref Stainless steel 161048 ATTNDR01 = -1 / 0.25 * ln (TVLTDR01 * 0.257507 - 0.019055) 4278 Titanium 161047 ATTNDR01 = -1 / 0.25 * ln (TVLTDR01 * 0.234152 - 0.004683) 4277
Field Calibrations
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Temperature
No reversing thermometer data were available for AMT15, so the CTD sensor data have not been calibrated against another dataset. Temperature readings from the two temperature sensors were almost identical and no other independent measurements of better quality were available. No further correction was therefore applied to the data.
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Salinity
Bench salinometer data were provided by UKORS.
Casts Calibration N R 2 BODC cal ref Stainless steel PSALCC01 = 0.9978 * PSALCU01 + 0.0722 52 0.4224 4141 Titanium PSALCC01 = 0.9993 * PSALCU01 + 0.0233 30 0.1328 4140 -
Dissolved oxygen
The oxygen sensor calibrations have been carried out using dissolved oxygen data from Winkler titrations (provided by Nikki Gist, Plymouth Marine Laboratory).
Casts Calibration N R 2 BODC cal ref Stainless steel DOXYSC01 = DOXYSU01 + 1.7590 (sd = 1.66) 124 - 6448 Titanium DOXYSC01 = 1.0602 * DOXYSU01 + 2.7248 77 0.9967 6449 -
Fluorescence
The nominal chlorophyll-a values have been calculated from the fluorometer data (with manufacturer's calibration applied) from the up-cast at bottle firing and the fluorometric chlorophyll-a data from sampled bottles. Where samples were not supplied or too few to generate a calibration and could not be grouped with other casts, the fluorometer profiles have not been calibrated. The sampling strategy for the extracted chlorophyll-a dataset used to calibrate the fluorometer focused on the upper water column, therefore the calibration is biased towards these depths. The calibration may not be as reliable below depths ~150m. Casts 1-7, 60 and 61 have not been calibrated. The extracted chlorophyll-a dataset is available for users to derive their own calibrations should they wish.
Stainless steel
Casts Calibration N R 2 BODC cal ref 8 - 24 CPHLPS01 = 2.0816 * CPHLPM01 31 0.8910 6641 26 - 42 CPHLPS01 = 3.7636 * CPHLPM01 29 0.9886 6642 43 - 58 CPHLPS01 = 2.2671 * CPHLPM01 33 0.8805 6643 64 - 105 CPHLPS01 = 1.2958 * CPHLPM01 69 0.3626 6645 Titanium
Casts Calibration N R 2 BODC cal ref 13 - 22 CPHLPS01 = 4.7916 * CPHLPM01 24 0.9074 6646 25 - 40 CPHLPS01 = 5.5096 * CPHLPM01 47 0.9965 6647 45 - 59 CPHLPS01 = 2.4576 * CPHLPM01 27 0.8088 6648 62 - 104 CPHLPS01 = 2.4869 * CPHLPM01 87 0.7469 6649
