Metadata Report for BODC Series Reference Number 886451

• 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

The pressure sensor failed on the first three casts (12791-1, 12793-1, 12794-1) and the pressure was derived using the wire-out. This introduced a significant error in the pressure channel. Therefore all the data related to these casts should be treated with extreme caution.

Data Access Policy

Public domain 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.

The recommended acknowledgment is

"This study uses data from the data source/organisation/programme, provided by the British Oceanographic Data Centre and funded by the funding body."

Narrative Documents

Neil Brown MK3 CTD

The Neil Brown MK3 conductivity-temperature-depth (CTD) profiler consists of an integral unit containing pressure, temperature and conductivity sensors with an optional dissolved oxygen sensor in a pressure-hardened casing. The most widely used variant in the 1980s and 1990s was the MK3B. An upgrade to this, the MK3C, was developed to meet the requirements of the WOCE project.

The MK3C includes a low hysteresis, titanium strain gauge pressure transducer. The transducer temperature is measured separately, allowing correction for the effects of temperature on pressure measurements. The MK3C conductivity cell features a free flow, internal field design that eliminates ducted pumping and is not affected by external metallic objects such as guard cages and external sensors.

Additional optional sensors include pH and a pressure-temperature fluorometer. The instrument is no longer in production, but is supported (repair and calibration) by General Oceanics.

Specifications

These specification apply to the MK3C version.

Further details can be found in the specification sheet.

Aquatracka fluorometer

The Chelsea Instruments Aquatracka is a logarithmic response fluorometer. It uses a pulsed (5.5 Hz) xenon light source discharging between 320 and 800 nm through a blue filter with a peak transmission of 420 nm and a bandwidth at half maximum of 100 nm. A red filter with sharp cut off, 10% transmission at 664 nm and 678 nm, is used to pass chlorophyll-a fluorescence to the sample photodiode.

The instrument may be deployed either in a through-flow tank, on a CTD frame or moored with a data logging package.

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

SeaTech Transmissometer

Introduction

The transmissometer is designed to accurately measure the the amount of light transmitted by a modulated Light Emitting Diode (LED) through a fixed-length in-situ water column to a synchronous detector.

Specifications

• Water path length: 5 cm (for use in turbid waters) to 1 m (for use in clear ocean waters).
• Beam diameter: 15 mm
• Transmitted beam collimation: <3 milliradians
• Receiver acceptance angle (in water): <18 milliradians
• Light source wavelength: usually (but not exclusively) 660 nm (red light)

Notes

The instrument can be interfaced to Aanderaa RCM7 current meters. This is achieved by fitting the transmissometer in a slot cut into a customized RCM4-type vane.

A red LED (660 nm) is used for general applications looking at water column sediment load. However, green or blue LEDs can be fitted for specilised optics applications. The light source used is identified by the BODC parameter code.

Further details can be found in the manufacturer's Manual.

RRS Discovery 217 CTD Data Documentation

Instrumentation

The 39 CTD profiles were taken with a Neil Brown Systems Mk3B CTD incorporating a pressure sensor, conductivity cell and a platinum resistance thermometer. The CTD unit was mounted vertically in the centre of a protective cage approximately 1.5 m square. Attached to the bars of the frame was a Chelsea Instruments Aquatracka fluorometer and an IOS SeaTech red light (661 nm) transmissometer with a 1 m path length.

On some casts, a PML 2-pi PAR irradiance meter was fitted to measure downwelling irradiance. On other casts, a Kiel University nephelometer was attached. Note that this was connected to the same CTD channel as the light meter, but has been extracted and processed as a separate channel at BODC. A UV nitrate sensor was fitted on most casts for testing and development work.

A General Oceanics rosette sampler fitted with 24, 10 litre Niskin bottles was mounted above the frame. The bases of the bottles were 0.75 m above the pressure head with their tops 1.55 m above it. Some of the bottles were fitted with holders for up to three digital reversing thermometers mounted 1.38 m above the CTD temperature sensor.

Lowering rates were generally in the range of 0.5-1.0 m/sec but could be up to 1.5 m/sec. Salinity bottle samples were acquired on the ascent of all casts except 12810-1, and reversing thermometer readings taken on 5 out of 39 casts (12833-1, 12834-3, 12835-2, 12836-1, 12838-1).

Considerable technical problems were encountered with the CTD on this cruise. The pressure sensor was found to drift with time to such an extent as to make it unusable. The backup instrument couldn't be used because the new multiplexed analogue channels could not be used with it. Stations prior to 12796 were therefore worked with no operational pressure sensor. Manually logged wire out values provided the only source of instrument depth information.

The ship put into Falmouth where a new pressure sensor and interface board were delivered. The pressure sensor functioned well until station 12830 when the instrument developed a problem with discrete jumps of up to 30 decibars appearing in the pressure channel during upcasts. This problem continued until the end of the cruise.

After station 12793, the CTD rosette top plate was snagged by the hydrographic winch wire and wrecked. A replacement was obtained when the ship put into Falmouth. The CTD rosette was therefore not operational for cast 12794.

The conductivity sensor was snapped off prior to cast 12837. No salinity data are available for this cast. A replacement sensor was fitted for cast 12838.

Data Acquisition

CTD data were sampled at a frequency of 32 Hz. Data reduction, in real time, to a 1-second time series was done by the RVS Level A microcomputer system. This was logged as raw counts on the Level C workstation via a Level B data buffer.

SOC Data Processing

The raw data were passed to a Sun workstation running the P-EXEC data processing software. Using this system, the following calibrations and data processing procedures were applied.

Pressure

Stations 12791-1, 12793-1 and 12794-1 were deployed with no pressure sensor. For these casts, a data channel containing manually logged wire out values was merged into the data. This is subject to obvious errors such as no allowance for wire angle. As pressure was used as a term in the computation of salinity and sigma-theta, the quality of these channels will also be compromised. Users should be aware that the data from these three casts are therefore of comparatively poor quality.

All the subsequent casts were processed using a linear calibration:

where A is the intercept and B is the pre-cruise determined slope (1.002348; SOC).

Stations 12796-1 to 12829-1 have been treated as one group, with a constant intercept of 3.5 applied. At station 12830-1 the pressure sensor started drifting on the upcast and that erratic behaviour continued through to the last station. These stations have had their downcast intercept adjusted individually as follows:

After the initial calibration was completed, various obvious jumps were removed from the upcast pressure, using a comparison with wire-out at the bottle firing depths. The intercept (A) and slope (B) were then adjusted thus to bring the pressure at the start and end of each cast to zero.

Temperature

The temperature was calibrated based on a pre-cruise tank calibration with slope of 0.998564 and intercept of 0.01655.

Conductivity - Salinity Conversion

The conductivity ratio was corrected by a factor of 0.9966263, to bring the CTD salinity data in line with the water bottle salinities. Salinity (Practical Salinity Units, as defined by the Practical Salinity Scale; Fofonoff and Millard, 1982) was then computed from the adjusted conductivity ratio and a time lagged temperature using SAL78 function described in UNESCO Report 37 (UNESCO, 1981).

Attenuance

Raw counts were converted to volts by multiplying them by 0.00122.

Correction of voltage and conversion to transmission were then computed as follows:

The attenuance value was computed from the % transmission using the equation:

Chlorophyll

A chlorophyll calibration was determined using PML fluorometric extracted chlorophyll data by linear regression of the log of chlorophyll against fluorometer voltage. The resulting equation was obtained:

Downwelling Irradiance

The 2-pi PAR data were converted from Volts to W/m² using the equation:

BODC Data Processing and Quality Control

The data were submitted to BODC in P star format for incorporation into the OMEX database.

Reformatting

The BODC Transfer System was used to convert the data into the BODC internal format. In addition to the reformatting, the program carried out the following additional processing on the data.

For the three casts with no pressure data the wire depths were converted to pressure using the inverse of the UNESCO algorithm. All the values in excess of 10000, caused by wrap-round of the cable meter, were set null.

For the remaining casts, the salinity data in the source file had been computed at SOC using the raw pressure data. To correct this, the conductivity ratio was computed using SAL78 function in inverse mode (UNESCO, 1981) and raw pressure data. Salinity was then recomputed with respect to corrected pressure, using SAL78 in normal mode. The difference was significant. For example a change in pressure of 30 db produced a difference in salinity of 0.012.

Sigma-theta was recomputed for all casts.

Editing

Using a custom in-house graphics editor, the limits of the downcasts were manually flagged. Any spikes on all the downcast channels were manually flagged 'suspect' by modification of the associated quality control flag.

Once screened, the CTD downcasts were loaded into a database under the Oracle relational database management system. The Kiel nephelometer data were loaded into Oracle using a 'one off' program written specifically for the purpose.

Calibration

With the exception of pressure, the BODC calibration checks were done by comparison of CTD values, extracted manually using the graphical editor, against measurements made on water bottle samples.

All calibrations described here have been applied to the data.

Pressure

A check was run at BODC to ensure the pressure was consistently zero when the instrument was logging in air (readily apparent from the conductivity channel). This verified the corrections applied by SOC but the following additional calibrations were applied to some of the casts where no correction had been applied:

Temperature

A very limited number of reversing thermometer measurements (11 readings in all) were available. Comparing these to the CTD data was inconclusive to say the least with absolute differences ranging from 0.003 °C to 1.313 °C and no consistency in the sign of the difference. Previous experience with Neil Brown Mk 3B CTD data gives more confidence in the CTD data than the RT data. Consequently, the RT data have been ignored and the CTD temperatures assumed correct. Users should be therefore be aware that there is no independent verification of the temperature data from this cruise.

Salinity

The salinity calibration was carefully checked at BODC. Whilst the general agreement between CTD and bottle data was excellent (within 0.005 PSU), a number of casts were noticed where the overall cruise calibration applied did not give such good results. The following additional, individual cast corrections have been applied:

Data Reduction

Once all screening and calibration procedures were completed, the data set was binned to 2 db (casts deeper than 100 db) or 1 db (casts shallower than 100 db). The binning algorithm excluded any data points flagged suspect and attempted linear interpolation over gaps up to 3 bins wide. If any gaps larger than this were encountered, the data in the gaps were set null.

Downcast values corresponding to the bottle firing depths were incorporated into the database. Oxygen saturations have been computed using the algorithm of Benson and Krause (1984). Once processing was complete the data were migrated to the National Oceanographic Database.

Data Warnings

Users should be aware that for casts 12830-1 through to 12838-1, the pressure channel required significant additional processing to correct sensor problems.

No reliable, independent verification of the temperature data is available. However, there is no reason to suspect these data.

References

Fofonoff N.P., Millard R.C. 1982. Algorithms for computation of fundamental properties of seawater. UNESCO Technical Papers in Marine Science. 44.

UNESCO 1981. Background papers ad supporting data on the Practical Salinity Scale, 1978. UNESCO Technical Papers in Marine Science. 37.

Project Information

Ocean Margin EXchange (OMEX) I

Introduction

OMEX was a European multidisciplinary oceanographic research project that studied and quantified the exchange processes of carbon and associated elements between the continental shelf of western Europe and the open Atlantic Ocean. The project ran in two phases known as OMEX I (1993-1996) and OMEX II - II (1997-2000), with a bridging phase OMEX II - I (1996-1997). The project was supported by the European Union under the second and third phases of its MArine Science and Technology Programme (MAST) through contracts MAS2-CT93-0069 and MAS3-CT97-0076. It was led by Professor Roland Wollast from Université Libre de Bruxelles, Belgium and involved more than 100 scientists from 10 European countries.

Scientific Objectives

The aim of the Ocean Margin EXchange (OMEX) project was to gain a better understanding of the physical, chemical and biological processes occurring at the ocean margins in order to quantify fluxes of energy and matter (carbon, nutrients and other trace elements) across this boundary. The research culminated in the development of quantitative budgets for the areas studied using an approach based on both field measurements and modeling.

OMEX I (1993-1996)

The first phase of OMEX was divided into sub-projects by discipline:

• Physics
• Biogeochemical Cycles
• Biological Processes
• Benthic Processes
• Carbon Cycling and Biogases

This emphasises the multidisciplinary nature of the research.

The project fieldwork focussed on the region of the European Margin adjacent to the Goban Spur (off the coast of Brittany) and the shelf break off Tromsø, Norway. However, there was also data collected off the Iberian Margin and to the west of Ireland. In all a total of 57 research cruises (excluding 295 Continuous Plankton Recorder tows) were involved in the collection of OMEX I data.

Data Availability

Field data collected during OMEX I have been published by BODC as a CD-ROM product, entitled:

• OMEX I Project Data Set (two discs)

Further descriptions of this product and order forms may be found on the BODC web site.

The data are also held in BODC's databases and subsets may be obtained by request from BODC.

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: