Metadata Report for BODC Series Reference Number 681192

• 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

CD141 CTD profile Data Quality Report

Downcast and upcast data for each profile was provided by the originator. BODC flagged the beginning and end of each downcast and recommend that only the data between these flags be used. Unfeasibly high values of chlorophyll concentration were also flagged. Some profiles of transmittance were constant at zero and these have been 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 Charles Darwin CD141 CTD Instrumentation

A Sea-Bird 911 plus CTD system was used on cruise CD141. The CTD was fitted with the following scientific sensors:

Some calibration dates are unavailable.

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.

Chelsea Technologies Group Aquatracka MKIII fluorometer

The Chelsea Technologies Group Aquatracka MKIII is a logarithmic response fluorometer. Filters are available to enable the instrument to measure chlorophyll, rhodamine, fluorescein and turbidity.

It uses a pulsed (5.5 Hz) xenon light source discharging along two signal paths to eliminate variations in the flashlamp intensity. The reference path measures the intensity of the light source whilst the signal path measures the intensity of the light emitted from the specimen under test. The reference signal and the emitted light signals are then applied to a ratiometric circuit. In this circuit, the ratio of returned signal to reference signal is computed and scaled logarithmically to achieve a wide dynamic range. The logarithmic conversion accuracy is maintained at better than one percent of the reading over the full output range of the instrument.

Two variants of the instrument are available, both manufactured in titanium, capable of operating in depths from shallow water down to 2000 m and 6000 m respectively. The optical characteristics of the instrument in its different detection modes are visible below:

* The wavelengths for the turbidity filters are customer selectable but must be in the range 400 to 700 nm. The same wavelength is used in the excitation path and the emission path.

The instrument measures chlorophyll a, rhodamine and fluorescein with a concentration range of 0.01 µg l^-1 to 100 µg l^-1. The concentration range for turbidity is 0.01 to 100 FTU (other wavelengths are available on request).

The instrument accuracy is ± 0.02 µg l^-1 (or ± 3% of the reading, whichever is greater) for chlorophyll a, rhodamine and fluorescein. The accuracy for turbidity, over a 0 - 10 FTU range, is ± 0.02 FTU (or ± 3% of the reading, whichever is greater).

Further details are available from the Aquatracka MKIII specification sheet.

Biospherical Instruments QCP-2350 [underwater] PAR sensor

A cosine-corrected PAR quantum irradiance profiling sensor. For use in underwater applications with 24 bit ADC systems. Measures light available for photosynthesis on a flat surface. Operation is by a single channel compressed analog output voltage that is proportional to the log of incident PAR (400-700 nm) irradiance. The sensor is designed for operation in waters to depths of up to 2,000 m (standard) or 6,800 m (optional).

For more information, please see this document: https://www.bodc.ac.uk/data/documents/nodb/pdf/Biospherical_QCP2300_QCP2350_Apr2014.pdf

Chelsea Technologies Group ALPHAtracka and ALPHAtracka II transmissometers

The Chelsea Technologies Group ALPHAtracka (the Mark I) and its successor, the ALPHAtracka II (the Mark II), are both accurate (< 0.3 % fullscale) transmissometers that measure the beam attenuation coefficient at 660 nm. Green (565 nm), yellow (590 nm) and blue (470 nm) wavelength variants are available on special order.

The instrument consists of a Transmitter/Reference Assembly and a Detector Assembly aligned and spaced apart by an open support frame. The housing and frame are both manufactured in titanium and are pressure rated to 6000 m depth.

The Transmitter/Reference housing is sealed by an end cap. Inside the housing an LED light source emits a collimated beam through a sealed window. The Detector housing is also sealed by an end cap. A signal photodiode is placed behind a sealed window to receive the collimated beam from the Transmitter.

The primary difference between the ALPHAtracka and ALPHAtracka II is that the Alphatracka II is implemented with surface-mount technology; this has enabled a much smaller diameter pressure housing to be used while retaining exactly the same optical train as in the Mark I. Data from the Mark II version are thus fully compatible with that already obtained with the Mark I. The performance of the Mark II is further enhanced by two electronic developments from Chelsea Technologies Group - firstly, all items are locked in a signal nulling loop of near infinite gain and, secondly, the signal output linearity is inherently defined by digital circuitry only.

Among other advantages noted above, these features ensure that the optical intensity of the Mark II, indicated by the output voltage, is accurately represented by a straight line interpolation between a reading near full-scale under known conditions and a zero reading when blanked off.

For optimum measurements in a wide range of environmental conditions, the Mark I and Mark II are available in 5 cm, 10 cm and 25 cm path length versions. Output is default factory set to 2.5 volts but can be adjusted to 5 volts on request.

Further details about the Mark II instrument are available from the Chelsea Technologies Group ALPHAtrackaII specification sheet.

BODC Processing

The CTD data were supplied to BODC as Matlab files and converted to the BODC internal format.

During transfer the originator's variables were mapped to unique BODC parameter codes. The following table shows the parameter mapping.

Following transfer the data were screened using BODC in-house visualisation software. Suspect data values were assigned the appropriate BODC data quality flag. Missing data values, where present, were changed to the missing data value and assigned a BODC data quality flag.

SCIPIO cruise CD141 Originator's CTD data processing

The following information contains extracts from the CD141 cruise report.

Sampling strategy

The RRS Charles Darwin 141 cruise aimed to examine the structure of the mean flow around and over the Mascarene Ridge, Indian Ocean.

A total of 147 CTD casts were deployed during the cruise. There were three main types of CTD profile collected during the cruise:

• single full depth CTD casts at a particular location (about 40 casts).
• repeated full depth CTD casts at a particular station as part of a tidal period (14 hour) time series(about 90 casts).
• shallow (typically 200 m) CTD casts for surface water samples (about 20 casts).

Data Processing

Initial data processing was performed using the SeaBird processing software SBE SEASAVE_Win32 Version 5.22. The following steps were performed:

• Data conversion - converts raw data to physical units.
• 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.

The entire Mstar software suite is written in Matlab and uses a NetCDF file format to store all the data. The 5 CTD files store all the data from the CTD sensors. These arr: raw, 24Hz, 1Hz, psal and 2db. The Mstar software program averages and interpolates the raw data until it has 2db resolution.

BODC will transfer the 2db resolution CTD data.

Field Calibrations

The oxygen sensor was calibrated against discrete samples taken from bottles on the CTD. CTD oxygens were advanced by 6 seconds relative to the pressure record, and oxygen values in units of µmol/kg and % saturation were computed. Including all samples, but disregarding outliers, the mean offset (sample-CTD oxygen) was 6.84 µmol/kg with a standard deviation of 10.63 µmol/kg. The sample oxygens were higher than the CTD oxygens, and there was no significant long-term drift in the differences throughout the cruise, although there was a discontinuity (and slow recovery) near CTD 34 on June 27. (This may have been related to a change in the temperature of the constant temperature (CT) laboratory, in close proximity to the oxygen equipment. This varied between 23-24°C during the earlypart of the cruise, but jumped to 27-29°C on and after June 27, due to a change in the air-conditioning circulation. This also correlated with a change in the conductivity ratios of the Standard Seawatersamples measured on the salinometer in the CT laboratory, which fell from about 2.0005 to 2.0002 at the same time, on June 27.) The sample minus CTD oxygen residuals show a curve with depth, higher residuals occurring in the top 250 m of the water column. Below this the difference between the bottle and CTD oxygens appears to be smaller and more constant, although there are fewer samples for comparison.

Salinity derived from the primary conductivity-temperature sensors were calibrated against salinity derived from bottle samples at the same depths. Samples were used from depths of 2000 metres or more. Ignoring outliers, considering only data below 2000 m and applying a linear fit of the CTD data to the bottle data, the primary sensor showed a start offset of 0.0018 psu a final offset of 0.0003 psu and a drift of -0.0018 psu. The secondary sensor showed a start offset of 0.0065 psu a final offset of 0.0030 psu and a drift of -0.0035 psu, almost twice that of the primary sensor. It should be noted, however, that there appears to be a non-linear component to the drift over the cruise. Because of the large offset and drift of the secondary conductivity sensor, data from the primary sensor are to be preferred.

References

New, A. (2003). 'Satellite Calibration and Interior Physics of the Indian Ocean: SCIPIO'. Cruise Report No. 41 National Oceanography Centre, Southampton.

Project Information

Satellite Calibration and Interior Physics of the Indian Ocean (SCIPIO)

The SCIPIO project took place on the RRS Charles Darwin. The first leg of the cruise left the Seychelles on 1st June 2002 and docked in Mauritius on 21st June 2002. The second leg of the cruise left Mauritius on 22nd June 2002 and returned there on 11th July 2002. A total of 147 CTD casts were completed at 64 stations.

Objectives

The objective was to undertake a multidisciplinary survey of the Mascarene Ridge system in the Western Indian Ocean.The aims of the survey were as follows:

• To study the flow of water masses through the Mascarene Ridge system, together with their decadal-timescale variability.

• To assess the energy fluxes and mixing arising from internal waves.

• To collect in situ data for the calibration of seasurface temperature and ocean colour sensors on the ENVISAT satellite.

• To investigate the biogeochemical properties of the water masses.

• To measure the heat fluxes and winds, and the airflow disturbance around the ship.

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: