Metadata Report for BODC Series Reference Number 1764937

• Metadata Summary

• Problem Reports

• Data Access Policy

• Narrative Documents

• Project Information

• Data Activity or Cruise Information

• Fixed Station Information

• BODC Quality Flags

• SeaDataNet Quality Flags

Metadata Summary

Problem Reports

No Problem Report Found in the Database

RRS Discovery Cruise DY052 CTD Data Quality Report

The data originator reported problems with the CTD altimeter and all of this data has been flagged.

BODC note that the oxygen saturation was high, ranging from 105% to 117% at the surface. However, this supersaturation was not considered to require flagging.

Occasional spikes of chlorophyll significantly deeper than the chlorophyll maximum were flagged.

Data Access Policy

Open Data supplied by Natural Environment Research Council (NERC)

You must always use the following attribution statement to acknowledge the source of the information: "Contains data supplied by Natural Environment Research Council."

Narrative Documents

RRS Discovery DY052 CTD instrument description

CTD unit and auxiliary sensors

The CTD configuration comprised a Sea-Bird 9plus underwater unit, #09P-24680-0637 (changed after cast 64), with accompanying Sea-Bird 11plus deck unit, #11P-24680-0589. The CTD frame was fitted with two Sea-Bird 3 Premium temperature sensors, two Sea-Bird 4 conductivity sensors and a digiquartz temperature compensated pressure sensor.

Additional sensors fitted to the CTD frame include a Sea-Bird 43 dissolved oxygen sensor, WETLabs C-Star transmissometer, WETLabs turbidity sensor, Chelsea Aquatracka MKIII fluorometer, biospherical QCD cosine PAR collectors (upward and downward looking) and a Benthos PSA-916T altimeter.

All instruments were attached to an NMF stainless steel 24 position carousel (#CTD8). SBE35 Deep Oceans Standards Thermometer (#35-0037), a TRDI WHM 300 kHz LADCP (#4275) and a NOCS LADCP battery pack (#WH005).

The table below lists more detailed information about the various sensors.

The discrete salinity samples collected from water bottles on the CTD were analysed in the ship's temperature-controlled room using a Guildline Autosal model 8400B salinometer (#71185) according to WOCE cruise guidelines. Dissolved oxygen concentrations from further water samples were determined using a Winkler titration technique.

Sea-Bird Electronics SBE 911 and SBE 917 series CTD profilers

The SBE 911 and SBE 917 series of conductivity-temperature-depth (CTD) units are used to collect hydrographic profiles, including temperature, conductivity and pressure as standard. Each profiler consists of an underwater unit and deck unit or SEARAM. Auxiliary sensors, such as fluorometers, dissolved oxygen sensors and transmissometers, and carousel water samplers are commonly added to the underwater unit.

Underwater unit

The CTD underwater unit (SBE 9 or SBE 9 plus) comprises a protective cage (usually with a carousel water sampler), including a main pressure housing containing power supplies, acquisition electronics, telemetry circuitry, and a suite of modular sensors. The original SBE 9 incorporated Sea-Bird's standard modular SBE 3 temperature sensor and SBE 4 conductivity sensor, and a Paroscientific Digiquartz pressure sensor. The conductivity cell was connected to a pump-fed plastic tubing circuit that could include auxiliary sensors. Each SBE 9 unit was custom built to individual specification. The SBE 9 was replaced in 1997 by an off-the-shelf version, termed the SBE 9 plus, that incorporated the SBE 3 plus (or SBE 3P) temperature sensor, SBE 4C conductivity sensor and a Paroscientific Digiquartz pressure sensor. Sensors could be connected to a pump-fed plastic tubing circuit or stand-alone.

Temperature, conductivity and pressure sensors

The conductivity, temperature, and pressure sensors supplied with Sea-Bird CTD systems have outputs in the form of variable frequencies, which are measured using high-speed parallel counters. The resulting count totals are converted to numeric representations of the original frequencies, which bear a direct relationship to temperature, conductivity or pressure. Sampling frequencies for these sensors are typically set at 24 Hz.

The temperature sensing element is a glass-coated thermistor bead, pressure-protected inside a stainless steel tube, while the conductivity sensing element is a cylindrical, flow-through, borosilicate glass cell with three internal platinum electrodes. Thermistor resistance or conductivity cell resistance, respectively, is the controlling element in an optimized Wien Bridge oscillator circuit, which produces a frequency output that can be converted to a temperature or conductivity reading. These sensors are available with depth ratings of 6800 m (aluminium housing) or 10500 m (titanium housing). The Paroscientific Digiquartz pressure sensor comprises a quartz crystal resonator that responds to pressure-induced stress, and temperature is measured for thermal compensation of the calculated pressure.

Additional sensors

Optional sensors for dissolved oxygen, pH, light transmission, fluorescence and others do not require the very high levels of resolution needed in the primary CTD channels, nor do these sensors generally offer variable frequency outputs. Accordingly, signals from the auxiliary sensors are acquired using a conventional voltage-input multiplexed A/D converter (optional). Some Sea-Bird CTDs use a strain gauge pressure sensor (Senso-Metrics) in which case their pressure output data is in the same form as that from the auxiliary sensors as described above.

Deck unit or SEARAM

Each underwater unit is connected to a power supply and data logging system: the SBE 11 (or SBE 11 plus) deck unit allows real-time interfacing between the deck and the underwater unit via a conductive wire, while the submersible SBE 17 (or SBE 17 plus) SEARAM plugs directly into the underwater unit and data are downloaded on recovery of the CTD. The combination of SBE 9 and SBE 17 or SBE 11 are termed SBE 917 or SBE 911, respectively, while the combinations of SBE 9 plus and SBE 17 plus or SBE 11 plus are termed SBE 917 plus or SBE 911 plus.

Specifications

Specifications for the SBE 9 plus underwater unit are listed below:

Further details can be found in the manufacturer's specification sheet.

RRS Discovery DY052 CTD data: Processing undertaken by BODC.

The data arrived at BODC in 89 MSTAR format files representing the CTD casts conducted during cruise DY052, one file per cast. The data contained in the files are the downcast data averaged to a 2db pressure grid. The casts were reformatted to BODC's internal NetCDF format. The following table shows the mapping of variables within the MSTAR files to appropriate BODC parameter codes:

The reformatted data were visualised using the in-house EDSERPLO software. Suspect data were marked by adding an appropriate quality control flag, missing data by both setting the data to an appropriate value and setting the quality control flag.

RRS Discovery DY052 Originator's CTD data processing

The following information contains extracts from the DY052 cruise report.

Sampling strategy

The main objective of the cruise was to occupy the annually repeated hydrographic section of the Extended Ellett Line (EEL).

The cruise departed on the 7th June 2016 from the port of Glasgow and returned on the 24th June 2016 to Glasgow.

A total of 89 CTD casts were deployed during the cruise with the deepest cast reaching 2710 m.

Additional sensors on the CTD were dissolved oxygen, PAR, transmissometer, fluorometer, turbidity and an altimeter. However, PAR data were not collected on this cruise.

Data Processing

Initial data processing was performed using the SeaBird processing software SBE Data Processing version 7.21b. The following steps were performed:

• Data conversion - converts raw data to physical units.
• Alignment of the oxygen sensor in time relative to the temperature and salinity sensors to account for the additional length of hose between the T and S sensors and the oxygen sensor.
• Cell thermal mass - takes the .cnv files output from the data conversion and makes corrections for the thermal mass of the cell, in an attempt to minimize salinity spiking in steep vertical gradients.
• Bottle summary - generates an ASCII summary .bl file of the bottle firing data from the cast .ros file.

Further onboard processing was undertaken onboard using the Mstar software suite. The following processing steps were taken:

• Correction for oxygen hysteresis. Correction values of -0.02, 5000, and 2000 were chosen to get the up and downcast to within about 2 µmol kg^-1 of each other below about 500 m and within about 0.3 µmol kg^-1 in the depth range of Labrador Sea Water.
• The original 24Hz data were averaged to 1Hz.
• Salinity was computed from temperature, pressure and conductivity.
• Station position and time information were collected from the TechSAS data stream for the exact start time and end time of each cast.
• The data were averaged to 2 decibars
• Visual inspection and removal of spikes (values changed to NaN) observed via a graphical user interface. Throughout the cruise conductivity spikes due to the ingestion of particles into the conductivity cells of the primary and secondary sensors were removed. Spikes were defined by an anomalously low conductivity value over just a few scans (usually 1-10 scans at 24 Hz), that was not reflected by a similar dip in temperature. With the spikes removed for a particular station, the data were bin averaged and overwritten in the 24 Hz, 1 Hz, 2dbar, and sample files. Casts 65-73 exhibited noisy oxygen data where the signals were amplified in both directions. These points were removed from the raw files. It was not known what the cause for this was and the cables were checked for loose connectivity. From cast 74 onwards this noise was not observed as prolifically. During cast 18 the CTD data acquisition was restarted at the bottom of the down cast. Large spikes in the oxygen signal were removed from the bottom of this downcast.
• Oxygen calibration was undertaken in 2 sections; before and after cast 65. During cast 65, the pumps did not start and the SBE 9plus unit was replaced . The slope and and intercept for CTD casts 001 to 064 were determined to be 1.01335 and 10.99686 µmol kg^-1, respectively. For CTD casts 065 to 089, the slope and intercept were 0.92960 and 21.07409 µmol kg^-1, respectively. After applying these slopes and intercepts to the CTD oxygen data, the residuals were plotted again and it was found that a small pressure adjustment was needed. A piece-wise linear adjustment was determined by computing the average residuals in 3 zones: the upper 100 m (-0.97109 µmol kg^-1); from 900 m to 1100 m (0.19834 µmol kg^-1); and below 2500 m (5.72306 µmol kg^-1).
• Comparison of the primary and secondary temperature sensor against the SBE36 reference showed the difference to be within the 0.002 °C accuracy limit for WOCE quality data.
• The primary and secondary conductivity sensors were calibrated against Niskin bottle salinity samples measured in the laboratory using a linear adjustment to take account of temporal drift of the sensors. The slope for the primary sensor was 1.0000832788 (mean residual stations 1-6: -0.0022095; mean residual stations 10-20: -0.0004107; mean residual stations 85 - 89: 0.0013027) and that for the secondary sensor 0.9999343064 (mean residual stations 1-6: -0.0012403; other sectors mean residual 0 (no adjustment applied). The calibrations were applied to the 24 Hz data and the 1Hz data, sample file and 2decibar files were re-created.

References

Gary, S.F. et al. (2016). Cruise DY052 Extended Ellett Line. Cruise Report Scottish Association for Marine Science

Project Information

No Project Information held for the Series

Data Activity or Cruise Information

Cruise

Complete Cruise Metadata Report is available here

Fixed Station Information

No Fixed Station Information held for the Series

BODC Quality Control Flags

The following single character qualifying flags may be associated with one or more individual parameters with a data cycle:

SeaDataNet Quality Control Flags

The following single character qualifying flags may be associated with one or more individual parameters with a data cycle: