Metadata Report for BODC Series Reference Number 2202157

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• Narrative Documents

• Project Information

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• Fixed Station Information

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Metadata Summary

Problem Reports

No Problem Report Found in the Database

Data Access Policy

Open Data

These data have no specific confidentiality restrictions for users. However, users must acknowledge data sources as it is not ethical to publish data without proper attribution. Any publication or other output resulting from usage of the data should include an acknowledgment.

If the Information Provider does not provide a specific attribution statement, or if you are using Information from several Information Providers and multiple attributions are not practical in your product or application, you may consider using the following:

"Contains public sector information licensed under the Open Government Licence v1.0."

Narrative Documents

Sea-Bird Dissolved Oxygen Sensor SBE 43 and SBE 43F

The SBE 43 is a dissolved oxygen sensor designed for marine applications. It incorporates a high-performance Clark polarographic membrane with a pump that continuously plumbs water through it, preventing algal growth and the development of anoxic conditions when the sensor is taking measurements.

Two configurations are available: SBE 43 produces a voltage output and can be incorporated with any Sea-Bird CTD that accepts input from a 0-5 volt auxiliary sensor, while the SBE 43F produces a frequency output and can be integrated with an SBE 52-MP (Moored Profiler CTD) or used for OEM applications. The specifications below are common to both.

Specifications

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

RAPID cruise DY146 CTD Instrument Description

CTD Unit and Auxillary Sensors

A Sea-Bird 911plus CTD system used on cruise DY146. This was mounted on a 24-way stainless steel rosette frame, equipped with 12 10-litre Niskin bottles. The CTD was fitted with the following scientific sensors:

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.

WETLabs Single-angle Backscattering Meter ECO BB

An optical scattering sensor that measures scattering at 117°. This angle was determined as a minimum convergence point for variations in the volume scattering function induced by suspended materials and water. The measured signal is less determined by the type and size of the materials in the water and is more directly correlated to their concentration.

Several versions are available, with minor differences in their specifications:

• ECO BB(RT)provides analog or RS-232 serial output with 4000 count range
• ECO BB(RT)D adds the possibility of being deployed in depths up to 6000 m while keeping the capabilities of ECO BB(RT)
• ECO BB provides the capabilities of ECO BB(RT) with periodic sampling
• ECO BBB is similar to ECO BB but with internal batteries for autonomous operation
• ECO BBS is similar to ECO BB but with an integrated anti-fouling bio-wiper
• ECO BBSB has the capabilities of ECO BBS but with internal batteries for autonomous operation

Specifications

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

Paroscientific Absolute Pressure Transducers Series 3000 and 4000

Paroscientific Series 3000 and 4000 pressure transducers use a Digiquartz pressure sensor to provide high accuracy and precision data. The sensor comprises a quartz crystal resonator that responds to pressure-induced stress, and temperature is measured for thermal compensation of the calculated pressure.

The 3000 series of transducers includes one model, the 31K-101, whereas the 4000 series includes several models, listed in the table below. All transducers exhibit repeatability of better than ±0.01% full pressure scale, hysteresis of better than ±0.02% full scale and acceleration sensitivity of ±0.008% full scale /g (three axis average). Pressure resolution is better than 0.0001% and accuracy is typically 0.01% over a broad range of temperatures.

Differences between the models lie in their pressure and operating temperature ranges, as detailed below:

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

WETLabs C-Star transmissometer

This instrument is designed to measure beam transmittance by submersion or with an optional flow tube for pumped applications. It can be used in profiles, moorings or as part of an underway system.

Two models are available, a 25 cm pathlength, which can be built in aluminum or co-polymer, and a 10 cm pathlength with a plastic housing. Both have an analog output, but a digital model is also available.

This instrument has been updated to provide a high resolution RS232 data output, while maintaining the same design and characteristics.

Specifications

Further details are available in the manufacturer's specification sheet or user guide.

RAPID Cruise DY146 BODC CTD data processing

The CTD data were supplied to BODC as twenty-five MStar files, which were converted to the BODC internal format (a netCDF subset).

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

RAPID Cruise DY146 Originator's CTD data processing

Sampling strategy

A total of 25 CTD casts were performed during the cruise predominately located in the Eastern boundary array of the RAPIDMOC array. The CTD casts provided start-point calibrations for instruments to be deployed and end-point calibrations for recovered instruments.

Data processing

Raw CTD data were transferred from the Sea-Bird deck unit to a LINUX machine via Sea-Bird software. The binary files are converted using Sea-Bird processing software (SBE Data Processing). The ASCII files were converted to MSTAR format and MEXEC programs run to process the data which included reducing the frequency of the data from 24Hz to 1Hz, calibrating the data, and averaging the downcast to a 2db pressure grid. Calibrations were produced for the CTD conductivity sensors by merging the salinity sample data with the CTD data. Details of the MEXEC programs used and further details of the processing performed can be found in Evans (2022).

Calibrations

Temperature

A SBE35 was mounted on the side of the CTD frame for all casts, data from which were used to compare to bottle stop data. For each sensor a small residual offset of order 0.5mK with small pressure dependence was inferred. For full details of the calibration see Evans (2022).

Conductivity

Independent conductivity samples, obtained from the bottles on the CTD frame and measured with the salinometer, were used to calibrate the CTD data. A pressure dependence was found for each sensor and determined with a by-eye fit to the conductivity residuals. There were 83 bottle salinities from samples taken deeper than 1000dbar. After applying the adjustments, the standard deviation of residuals between reported CTD salinity and bottle salinity for those samples was 0.0014 with mean 0.0002. For full details of the calibration see Evans (2022).

Oxygen

A hysteresis correction was not applied in the SBE processing to allow for experimentation with changing of parameters to optimise agreement between up and down cast data.

The equation for applying a correction for hysteresis - a time-dependent pressure-induced effect - is of the form

Oxycorr(i) = { (OxVolt(i) + (Oxycorr(i-1) * C * D)) - (OxVolt(i-1) * C) } /D

where D = 1 + H1 * (exponential(P(i)/H2)-1
C= exponential(-1 * (Time(i) - Time(i-1))/H3)
i = indexing variable
P(i) = pressure (decibars) at index point i
Time(i) = time (seconds) from start of index point i
OxVolt(i) = voltage output from sensor
H1 = amplitude of hysteresis correction function
H2 = function constant or curvature function for hysteresis
H3 = time constant for hysteresis (seconds)

Experimentation found the optimal solution was achieved by setting H1 for each sensor and letting H3 vary with depth i.e.

H1 = -0.038 for primary sensor and -0.033 for secondary sensor

These values provided good upcast/downcast agreement for all casts, with differences generally of order 1 µmol/kg for much of the water column.

A pressure dependence for each sensor was determined after all oxygen bottle data were available. A single empirical pressure was found for each sensor and used for the whole cruise. After application, the residuals (bottle minus CTD) looked stable over most of the cruise, with both sensors reading a little higher on the first few casts, suggesting the sensors lost a bit of sensitivity between the first and fifth cast. Therefore, a further factor was introduced for each sensor interpolated linearly on casts 1 to 5. The calibration equations were:

Primary sensor : OxyCal = Oxygen * interp1([-10 0 1000 3000 5400 6000],[1.027 1.027 1.033 1.038 1.055 1.055],pressure) * interp1([1 5 25],[0.988 1 1],castnum);

Secondary sensor : OxyCal = Oxygen * interp1([-10 0 1000 3000 5400 6000],[1.045 1.045 1.052 1.062 1.075 1.075],pressure) * interp1([1 5 25],[0.992 1 1],castnum);

After applying the calibrations, the mean and standard deviation of the reported CTD oxygen for deeper samples (62 deeper than 1000dbar) were 0.06 µmol/kg and 0.98 µmol/kg, respectively and for the shallower samples (79 shallower than 1000dbar) were 0.53 µmol/kg and 0.94 µmol/kg, respectively.

Fluorometer, Transmissometer and Backscatter

These data had no extra processing on the cruise beyond applying the manufacturer's calibrations in the SBE processing.

References

Evans, D. Gwyn (2022) RRS Discovery Research Expedition DY146, 4 February 2022 - 9 March 2022. RAPID research expedition report for DY146. Southampton, UK: National Oceanography Centre, Southampton, 158 pp. (National Oceanography Centre Research Expedition Report, No 76)

Project Information

Monitoring the Meridional Overturning Circulation at 26.5N (RAPIDMOC)

Scientific Rationale

There is a northward transport of heat throughout the Atlantic, reaching a maximum of 1.3PW (25% of the global heat flux) around 24.5°N. The heat transport is a balance of the northward flux of a warm Gulf Stream, and a southward flux of cooler thermocline and cold North Atlantic Deep Water that is known as the meridional overturning circulation (MOC). As a consequence of the MOC northwest Europe enjoys a mild climate for its latitude: however abrupt rearrangement of the Atlantic Circulation has been shown in climate models and in palaeoclimate records to be responsible for a cooling of European climate of between 5-10°C. A principal objective of the RAPID programme is the development of a pre-operational prototype system that will continuously observe the strength and structure of the MOC. An initiative has been formed to fulfill this objective and consists of three interlinked projects:

• A mooring array spanning the Atlantic at 26.5°N to measure the southward branch of the MOC (Hirschi et al., 2003 and Baehr et al., 2004).
• Additional moorings deployed in the western boundary along 26.5°N (by Prof. Bill Johns, University of Miami) to resolve transport in the Deep Western Boundary Current (Bryden et al., 2005). These moorings allow surface-to-bottom density profiles along the western boundary, Mid-Atlantic Ridge, and eastern boundary to be observed. As a result, the transatlantic pressure gradient can be continuously measured.
• Monitoring of the northward branch of the MOC using submarine telephone cables in the Florida Straits (Baringer et al., 2001) led by Dr Molly Baringer (NOAA/AOML/PHOD).

The entire monitoring array system created by the three projects will be recovered and redeployed annually until 2008 under RAPID funding. From 2008 until 2014 the array will continue to be serviced annually under RAPID-WATCH funding.

The array will be focussed on three regions, the Eastern Boundary (EB), the Mid Atlantic Ridge (MAR) and the Western Boundary (WB). The geographical extent of these regions are as follows:

• Eastern Boundary (EB) array defined as a box with the south-east corner at 23.5°N, 25.5°W and the north-west corner at 29.0°N, 12.0°W
• Mid Atlantic Ridge (MAR) array defined as a box with the south-east corner at 23.0°N, 52.1°W and the north-west corner at 26.5°N, 40.0°W
• Western Boundary (WB) array defined as a box with the south-east corner at 26.0°N, 77.5°W and the north-west corner at 27.5°N, 69.5°W

References

Baehr, J., Hirschi, J., Beismann, J.O. and Marotzke, J. (2004) Monitoring the meridional overturning circulation in the North Atlantic: A model-based array design study. Journal of Marine Research, Volume 62, No 3, pp 283-312.

Baringer, M.O'N. and Larsen, J.C. (2001) Sixteen years of Florida Current transport at 27N Geophysical Research Letters, Volume 28, No 16, pp3179-3182

Bryden, H.L., Johns, W.E. and Saunders, P.M. (2005) Deep Western Boundary Current East of Abaco: Mean structure and transport. Journal of Marine Research, Volume 63, No 1, pp 35-57.

Hirschi, J., Baehr, J., Marotzke J., Stark J., Cunningham S.A. and Beismann J.O. (2003) A monitoring design for the Atlantic meridional overturning circulation. Geophysical Research Letters, Volume 30, No 7, article number 1413 (DOI 10.1029/2002GL016776)

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: