Metadata Report for BODC Series Reference Number 974900

• 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

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

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.

RV Belgica 9521 CTD Data Documentation

Instrumentation and Shipboard Procedures

The CTD profiles were taken with the SeaBird SBE 911 plus system. The instrument had enclosed conductivity and temperature sensors supplied with water by a pump with an inlet at the base of the bottle rosette. The CTD had a temperature and salinity (TC) duct with an inertia balanced pump flow to improve the quality of salinity measurements.

When not in use, the sensors were bathed in MilliQ water. SeaBird temperature sensors are high performance, pressure protected thermistors. Other sensors on the rig were a dissolved oxygen (YSI SBE-13-Y polargraphic membrane) and a SeaBird optical backscatter nephelometer.

The CTD temperature and salinity sensors were sent in January 1995 to SeaBird's NWRCC facility in Washington State for calibration.

A SeaBird rosette sampler fitted with 12, 10 litre Niskin or GoFlo bottles was mounted above the frame. The bases of the bottles were level with the pressure sensor with their tops 0.8m above it.

The CTD was lowered at 0.8-1 m/s. On the upcast, the hauling rate was approximately the same, but was reduced on approach to a bottle firing depth to minimise wake interference.

Data Acquisition

Data were logged at 24 Hz but this was automatically reduced to 2 Hz by the deck unit. The resulting data were logged on a PC using the SeaBird SEASAVE program.

Post-Cruise Processing

The SeaBird DATCNV program was used to convert the binary raw data files into ASCII data in engineering units (PSU, °C, mol/kg, etc).

The salinity computation algorithm in the software is based on Fofonoff and Millard (1982). Salinity spiking on thermal gradients was minimised through software realignment of the temperature and conductivity channels.

BODC were supplied with the ASCII output produced by DATCNV.

Reformatting

The data were converted into the BODC internal format (PXF) to allow the use of in-house software tools, notably the workstation graphics editor. In addition to reformatting, the transfer program applied the following modifications to the data:

Temperature was converted from ITS68 to ITS90 by dividing the values by 1.00024.

Dissolved oxygen was converted from µmol/kg to µM by multiplying the values by (1000 + sigma-theta)/1000.

Editing

Using a custom in-house graphics editor, the top and bottom of the downcast were marked to eliminate noisy data logged whilst the instrument was stabilising.

The data were examined point by point and any obvious spikes were flagged 'suspect'. The salinity data from cast 01A showed severe problems between 30 and 50 metres with the salinity value oscillating wildly. The data from this interval have all been flagged suspect.

Once screened, the CTD downcasts (19) were loaded into a database under the Oracle relational database management system. Note that the loader only included data from the downcast marked during screening.

Calibration

The temperature data are believed to be accurate (within 0.01 °C according to the manufacturer's specification) as supplied and no further calibrations have been applied.

An average of 4 salinity samples were taken per cast, stored in crown-corked beer bottles, and determined on a Beckman RB7 laboratory salinometer, calibrated using OSI standard seawater. This procedure has come out well in ICES intercalibration exercises.

In the MUMM cruise report, a comparison between the CTD salinity data and these salinometer determinations was given. This revealed a systematic difference (mean for cruises BG9521 and BG9521 was 0.016 PSU with a standard deviation of 0.004 PSU) between the two data sets. No corrective action was taken by MUMM but a correction of +0.016 has been applied to the CTD data by BODC.

A comparison between the CTD oxygen data and bottle data supplied by the University of Liege showed that the oxygen sensor had significant problems on this cruise. Trouble of some sort was suspected during visual inspection because the signal was unusually noisy but the true extent of the problem did not become apparent until the bottle data were available.

The difference between CTD and bottle values in the upper 250 m varied from virtually zero to up to 20 µM. On some casts, there was good agreement at the surface, which increased systematically with depth. On other casts, this trend was reversed. A small group of casts showed good agreement at all depths.

Deeper than 250 m, a significant linear relationship could be established between the two data sets from all deep casts.

A complex empirical calibration exercise was undertaken to bring the CTD data into line with the bottle data. The criterion for success was taken to be the reduction of the maximum difference between all points in a given bottle and CTD downcast profile to below 10 µM. Where this could not be achieved, the CTD data were flagged suspect. In many cases, the agreement obtained was significantly better than this.

The first stage in the calibration exercise was to look at the deep casts where bottles were fired between 250 and 1200m. This gave the following calibration:

This calibration is obviously only valid for depths >250m and consequently all data shallower than this for these casts have been flagged out. The casts involved are 04B, 05B, 06A, 07B, 09B and 10B.

Secondly, a calibration was obtained by combining the data from the shallow casts 01A and 04A. The equation obtained was:

The following shallow casts were individually calibrated:

Casts 02A, 03A, 05C and 05D required no adjustment.

Casts 05A and 05E had no bottle data available and no sensible relationship could be obtained for the data from cast 08A. All oxygen values from these casts have been flagged suspect.

The calibrations specified above have all been applied to the data. The calibration exercise has significantly improved the fit of the CTD data to the bottle data. Nevertheless, the CTD oxygen data should be used with caution and should not be used for any purpose where an accuracy better than 10 µM is required.

Data Reduction

The final data set was produced by binning the calibrated data to 1 (casts shallower than 100 m) or 2 decibars. 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.

Data Warnings

A complex empirical calibration was required to obtain anything approaching a reasonable fit between the CTD dissolved oxygen data and the corresponding bottle data. The CTD oxygen data, especially those from the upper 250 m, should therefore be used with caution and should not be used for any purpose where an accuracy better than 10 µM is required.

References

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

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: