CTD Data for Cruise Discovery 207
(19th February 1994 - 31st March 1994)

 


1. Introduction

CTD profile and bottle data are presented from the ADOX-2 cruise Discovery 207, as 
reported by Dickson et al. (1995).


2. Instrumentation

The CTD profiles were taken with a Neil Brown Systems MkIIIb CTD, mounted 
beneath a General Oceanics 24 by 10 litre bottle rosette. The CTD was fitted with a 
pressure sensor, conductivity cell, platinum resistance thermometer, a Beckman 
dissolved oxygen sensor, a Sea Tech 100cm path transmissometer and a Simrad 
200kHz altimeter. 9 SIS (Sensoren Instrumente Systeme) digital reversing 
thermometers (T219, T220, T228, T238, T401, T714, T743, T746 and T790) were 
attached to the Niskin bottles. 2 SIS digital pressure meters (6292H and 6293H) 
were also used. Niskin bottle 24 was a Go-Flow type. No fluorometer was attached 
to the CTD package.

For the bottle rosette system the bases and tops of the bottles were, respectively, 
0.75m and 1.55m above the CTD pressure sensor. The digital reversing 
thermometers were mounted 1.38m above the CTD temperature sensor.


3. Data Acquisition

Lowering rates for the CTD package were generally in the range 0.5-1.0ms-1 but 
could be up to 1.5ms-1. CTD data were logged at 16 frames per second. The CTD 
deck unit passes raw data to a dedicated Level A microcomputer where 1 second 
averages are assembled. During this process the Level A calculates the rate of 
change of temperature and a median sorting routine detects and removes pressure 
spikes. This data is sent to the Level B for archival. The data are then passed to a 
Level C workstation for conversion to Pstar format and calibration.

Water samples were acquired on the up cast with the winch stopped. The CTD data 
acquisition system sends out a bottle firing code at the time of bottle firing. The code 
is logged as serial data by the Level A which timestamps its arrival.

A total of 33 stations were occupied (12628 - 12660). Station 12628 was for CFC 
calibration with all but one of the bottles firing at 2046db. After station 12632 the 
transmissometer was removed.


4. Data Processing

4.1. CTD Data

The 1 second data passed to the Level C were converted to Pstar format and initially 
calibrated with coefficients from laboratory calibrations followed by a number of 
calibration corrections. The up cast data (down cast for oxygen) were extracted for 
merging with the bottle firing codes, on time (pressure for oxygen), thus the CTD 
variables were reconciled with the bottle samples. Final calibrations were applied 
using the sample bottle data. Finally, down cast data were extracted, sorted on 
pressure and averaged to 2db values, with any gaps filled by linear interpolation.

Pressure

A quadratic calibration was applied of the form:

	p (dbar) = a x praw + b x praw2 + c

A correction was applied for the effects of temperature on the pressure sensor:

	pcor = p - pTslope x (Tlag - Tzero)

where pTslope is the rate of change of deck offset with temperature. Tzero is the 
temperature at which the quadratic calibration of above gives the correct deck offset. 
Tlag is a lagged temperature, in °C, formed from the CTD temperature using a first 
order equation with a time constant for the rate of change of lagged temperature of 
tconst seconds. Lagged temperature is updated in the following manner. If T is the 
CTD temperature, tdel the time interval in seconds over which Tlag is being updated, 
and tconst the time constant, then:

	W = exp (-tdel/tconst)

	Tlag (t=t0+tdel) = W x Tlag (t=t0) + (1 - W) x T (t=t0+tdel)

During the cruise, the variation of deck pressure value with ambient temperature was 
monitored.

On the up cast, a further correction is made for the hysteresis of the pressure sensor 
based on laboratory measurements. For a cast in which the maximum pressure 
reached is pmax dbar, the correction to the up cast CTD pressure (pin) is:

	pout = pin - ( dp5500(pin) - ( (pin/pmax) x dp5500(pmax) ) )

where dp5500(p) is the hysteresis after a cast to 5500db.

A comparison of the CTD pressures with measurements from the SIS digital 
reversing pressure meters suggested that there was no change required in the CTD 
calibration.

Temperature

Initial calibration to the temperature was applied of the form:

	T (°C) = a x (0.0005 x Traw) + b

Temperatures are given on the ITS90 scale. For the purpose of computing derived 
oceanographic variables, temperatures were converted to the 1968 scale (T68 = 
1.00024 T90).

In order to allow for the mismatch between the time constants of the temperature 
and conductivity sensors, the temperatures were further corrected according to the 
procedure described in the SCOR WG51 report (Crease et al. 1988). The time 
constant used was 0.20s giving a correction of:

	T = T + (0.20 x deltaT)

where deltaT is the change in temperature over one second calculated by the Level A.

A comparison of the CTD temperatures with measurements from the SIS digital 
reversing thermometers suggested that there was no change required in the CTD 
calibration.

Salinity

Initial calibration of the conductivity using the cell ratio (cfac) was applied:

	C (mmho/cm) = cfac x (0.001 x Craw)

The conductivity was then corrected for the effects of pressure and temperature in 
the manner described in the SCOR WG51 report (Crease et al. 1988) with nominal 
values employed for the temperature expansion and pressure contraction 
coefficients of the material of the cell:

	Cnew = Cold x (1 + (Cpslope x p) + CTslope x (T - Torg)

where Cpslope (rate of change of cell ratio with pressure) = 1.5 x 10-8, CTslope (rate of 
change of cell ratio with temperature) = -6.5 x 10-6 and Torg = 15°C (cfac is the cell 
ratio at pressure zero and temperature Torg). Salinity for both the down and up casts 
was then calculated from the corrected temperature, pressure and conductivity.

For each station the differences between the bottle sample salinities (Sbot) and the 
up cast CTD salinity (Sctd) were calculated. A routine was used to derive coefficients 
for the relationship:

	Sbot - Sctd = a + (b x press) + (c x potemp)

The derived a, b, c coefficients were then used to correct all CTD salinities for both 
the up and down cast.

Oxygen

CTD oxygen was calibrated by fitting to sample values using the formula:

O2 = oxsat(T,S) x rho x (oxyc + offset) x exp(alpha x (W x Tctd + (1 - W) x oxyT) + beta x p)

where the coefficients rho, alpha and beta were chosen on a station by station basis to 
minimise the rms residual.


4.2. Sample Data

Sampling from the Niskin bottles was done in the following sequence: CFCs, oxygen, 
salinity, nutrients, followed by oxygen/hydrogen isotope ratios. CFC samples were 
not drawn from the Go-Flow type bottle 24.

Chlorofluorocarbons

Samples were taken for analysis of CFCs 11, 12, 113 and CCl4. The instrument was 
based on a gas chromatograph used by the PML CFC group previously. The main 
separation element was a megabore fused silica pre-column (DB-624), whose 
exhaust was delivered directly to an electron capture detector (ECD). Seawater 
samples were stripped of volatile dissolved gases by bubbling in a sparging tower. 
An unpacked trap was used to concentrate these compounds as they were liberated 
from the seawater prior to injection into the column. The ECD was calibrated by 
using samples of standard gas containing established proportions of the tracers. 
During the cruise the CCl4 concentration of the standard was poorly established and 
marine air was used as the effective standard.

Oxygen

Bottle oxygen samples were taken in calibrated clear glass bottles. The pickling 
reagents were added on deck immediately after drawing and before capping. The 
temperature of the water at the time of chemical fixation was measured to allow 
corrections to be made for the change in density of the sample between the closure 
of the rosette bottle and the fixing of the dissolved oxygen. Analysis followed the 
Winkler whole bottle method with an automatic photometric endpoint detection. 
Standardisation of the thiosulphate titrant was done using a primary standard iodate 
solution supplied by WAKO Chemicals GmbH, Germany, and guaranteed by the 
Sagami Chemical Research Centre. The thiosulphate was checked on each day of 
use using the secondary standard iodate solution prepared from potassium iodate, 
UNIVAR product no. 504 48 00 batch 27986, SAARCHEM (pty) Ltd.

Problems with the analytical system meant that the endpoint had to be detected 
visually. Using the magnetic stirring and illuminating facilities of the instrument and 
manually operating the Metrohm 665 Dosimat, enabled titration to a visual endpoint 
with starch indicator.

Salinity

Salinity samples were analysed using MAFF Guildline 8400A and IOSDL Guildline 
8400A bench salinometers set to run at 24°C in the temperature controlled 
laboratory (21°C). Standardisation was done using IAPSO Standard Seawater 
batches P120 and P123.

Replicate salinity samples (samples from the same bottle) were taken from rosette 
bottles 12 and 24. Bottle 2 was closed at the same depth as bottle 1, enabling 
duplicate samples to be taken. The differences between the bottle 12 replicates was 
0.0000 ±0.0007, and for bottle 24 it was 0.0001 ±0.0005. The differences for the 
duplicate samples (bottle 1 - bottle 2) was 0.0001 ±0.0021.

Nutrients

Samples were drawn into 1 litre polyethylene bottles and analysed without filtration. 
Samples were analysed for nitrate, silicate and phosphate using the MAFF SKALAR 
SA4000 continuous flow auto-analyser. The auto-analyser carousel uses 8ml cups 
which were thoroughly rinsed with the sample.

Mean concentrations (µmol/l) and relative standard deviations (%) for sub-samples 
from the consistent bottles (21 of the 23 fired) of the CFC calibration station 12628 
were:

             Nitrate          Phosphate       Silicate
             28.20 (0.64)     1.92 (2.60)     59.96 (1.80)

A bulk quality control sample was obtained by draining about 2 litres from each of the 
first 12 of these consistent bottles into a 25 litre polyethylene carboy. 2 aliquots of 
this sample were analysed among the samples from the subsequent CTD stations. 
Mean concentrations (µmol/l) and relative standard deviations (%) for the 60 
analyses of this sample were:

             Nitrate            Phosphate         Silicate
             28.197 (0.938)     1.929 (1.981)     61.19 (0.847)

Oxygen and Hydrogen Isotope Ratios

Samples were collected into 250ml salinity bottles with plastic neck inserts, 250ml 
salinity bottles of the older metal screw top type and 150ml sample bottles also with 
metal screw tops. The latter 2 types of bottles were sealed with paraffin wax.

Delta oxygen-18 of water was measured using a mass spectrometer. No analyses 
were conducted for hydrogen isotope ratio.

Reversing Thermometers

9 SIS digital reversing thermometers were attached to the Niskin bottles.


5. BODC Data Processing

No further calibrations were applied to the data received by BODC. BODC were 
mainly concerned with the screening and banking of the data.

5.1. CTD Data

The CTD data were received as 2db averaged pressure sorted down cast data. The 
data were converted into the BODC internal format (PXF) to allow the use of in-
house software tools, notably the graphics editor. Spikes in the data were manually 
flagged 'suspect' by modification of the associated quality control flag. In this way 
none of the original data values were edited or deleted during quality control. These 
data from cruise Di207 required little flagging and just a few points were set suspect.

Once screened, the CTD data were loaded into a database under the Oracle 
relational database management system. The start time stored in the database is the 
CTD deployment time, and the end time is the time the CTD was removed from the 
water. Actually these times are more precisely the start and end of data logging. 
Latitude and longitude are the mean positions between the start and end times 
calculated from the master navigation in the binary merged file.

5.2. Sample Data

BODC conducted extensive quality control to eliminate rosette misfiring and any 
incorrectly assigned flag codes. Before loading to the database the data were 
averaged if bottles fired within ±4.0db of each other. At station 12643 the 2 bottles 
firing at 4002.06db and 4006.12db were also averaged.


6. References

Crease, J. et al. (1988). The acquisition, calibration and analysis of CTD data. 
UNESCO Technical Papers in Marine Science. No. 54, 96pp.

Dickson, R.R. et al. (1995). RRS Discovery Cruise 207. Ministry of Agriculture 
Fisheries and Food Directorate of Fisheries Research, Cruise Report, 79pp.


