Metadata Report for BODC Series Reference Number 874289
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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."
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.
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.
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 for the SBE 9 plus underwater unit are listed below:
|Parameter||Range||Initial accuracy||Resolution at 24 Hz||Response time|
|Temperature||-5 to 35°C||0.001°C||0.0002°C||0.065 sec|
|Conductivity||0 to 7 S m-1||0.0003 S m-1||0.00004 S m-1||0.065 sec (pumped)|
|Pressure||0 to full scale (1400, 2000, 4200, 6800 or 10500 m)||0.015% of full scale||0.001% of full scale||0.015 sec|
Further details can be found in the manufacturer's specification sheet.
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.
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.
- 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)
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.
RV Pelagia 108, 109, 118, 121, 123, 138 CTD Data Documentation
Instrumentation and Shipboard Procedures
The CTD profiles were taken with a SeaBird SBE911 plus system. The instrument was equipped with a SBE-3 temperature sensor, SBE-4 conductivity sensor and SBE-13 oxygen sensor. However, the latter only returned useful data for cruises PLG108 and PLG138.
A SeaTech transmissometer (25 cm path length: 665 nm monochromatic light source) was also included in the CTD package for every cruise. During PLG138 a WET Labs AC3 system that measured optical attenuance at 676 nm was also fitted. This system also estimated the chlorophyll concentration by comparison of attenuance at different wavelengths.
On all cruises other than PLG138 the CTD package included a Chelsea Instruments Aquatracka fluorometer. However, this only returned useful data on cruises PLG108 and PLG109.
The CTD was fitted with a General Oceanics Rosette sampler carrying 24 12-litre NOEX bottles.
The number of CTD casts available for each Pelagia cruise is as follows:
The data were logged on a PC using the standard SeaBird SeaSave data acquisition software.
Hendrik van Aken's group at the Netherlands Institute for Sea Research worked up the CTD data to a very high standard. Temperature, salinity and dissolved oxygen have been calibrated to WOCE standards using the appropriate water bottle data. If there were any doubts about the quality of the oxygen data then the channel was deleted from the data set.
The chlorophyll values given are based upon manufacturer's calibration coefficients, as no extracted chlorophyll data were available from these cruises. Consequently, the absolute concentration values should be treated with a degree of caution. The data from PLG108 and PLG109 appear credible, but the surface mixed layer values seem lower than those returned by other OMEX II cruises in the same area and at the same time of year. The values from PLG138 are high, especially at depth (>1 mg/m3) and there is a worrying increase of chlorophyll with depth at depths >1000 m. It is recommended for this cruise that the data be considered as having no units and that the deep data be totally ignored.
The SeaTech transmissometer data have been calibrated using the SeaBird software, which incorporates an air correction. However, the clear water attenuance values are of the order of 0.38 to 0.40 per m, which is slightly higher than expected for Atlantic waters (usually 0.35 to 0.36 per m). This may possibly be explained by the wavelength (665 nm) reported for the beam. The 'standard' Atlantic values were measured at 660 nm.
The WET Labs AC3 system was calibrated in terms of attenuance due to suspended sediment load. Consequently, the data give values of near zero for clear water.
In addition to calibration, the data were thoroughly 'cleaned' to eliminate spikes and smoothed. The processed data were supplied to BODC as 1db binned profiles.
BODC Data Processing
The data were converted into BODC's standard format. The only modification made to the data during reformatting was the conversion of dissolved oxygen units from micromoles/kg to micromoles/litre using the sigma-theta channel included with the data.
The data were examined using the BODC in-house graphics editor, which confirmed that they were exceptionally clean. No topping and tailing was required and flagging was confined to a small number of zero values in the attenuance channel.
The screened data were loaded into the Oracle database. Calibration records were set up to indicate that the data had been supplied as fully calibrated. The only exception to this was a salinity correction of -0.07 applied by BODC to the data from PLG121 following the report of a processing error by Dr. van Aken. The data were then migrated to the National Oceanographic Database.
BODC has a standard storage convention for storing CTD data. Shallow (<100 db) casts are held at 1 decibar, but deeper casts are stored at the 2 decibar resolution recommended by SCOR. These data have already been binned, which can cause confusion about what happens when the data are passed through a second binning procedure. Users should therefore be aware how the BODC processing has modified the data.
The BODC binning algorithm defines the top bin as the average of all data with a pressure from zero to 0.9999 (1 db binning) or 1.9999 (2 db binning) and labels these bins as 0.5 or 1.0 respectively. The original data were supplied with the top bin labelled as zero. The following show how the original pressure channel maps to the BODC pressure channel for each of the binning intervals used.
|1db binning:||BODC pressure = input pressure + 0.5 |
(effectively a change in labelling convention from the top to the mid-point of the bin)
|2 db binning:||Input pressures 0.0 and 1.0 averaged to give BODC pressure 1.0 |
Input pressures 2.0 and 3.0 averaged to give BODC pressure 3.0
The chlorophyll data from these cruises have been obtained using manufacturer's calibrations. The absolute values should therefore be used with caution, particularly for cruise PLG138.
Ocean Margin EXchange (OMEX) II - II
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.
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 II - II (1997-2000)
The second phase of OMEX concentrated exclusively on the Iberian Margin, although RV Belgica did make some measurements on La Chapelle Bank whilst on passage to Zeebrugge. This is a narrow-shelf environment, which contrasts sharply with the broad shelf adjacent to the Goban Spur. This phase of the project was also strongly multidisciplinary in approach, covering physics, chemistry, biology and geology.
There were a total of 33 OMEX II - II research cruises, plus 23 CPR tows, most of which were instrumented. Some of these cruises took place before the official project start date of June 1997.
Field data collected during OMEX II - II have been published by BODC as a CD-ROM product, entitled:
- OMEX II Project Data Set (three 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.
|Principal Scientist(s)||Tjeerd van Weering (Royal Netherlands Institute for Sea Research)|
Complete Cruise Metadata Report is available here
No Fixed Station Information held for the Series
The following single character qualifying flags may be associated with one or more individual parameters with a data cycle:
|<||Below detection limit|
|>||In excess of quoted value|
|A||Taxonomic flag for affinis (aff.)|
|B||Beginning of CTD Down/Up Cast|
|C||Taxonomic flag for confer (cf.)|
|E||End of CTD Down/Up Cast|
|G||Non-taxonomic biological characteristic uncertainty|
|I||Taxonomic flag for single species (sp.)|
|K||Improbable value - unknown quality control source|
|L||Improbable value - originator's quality control|
|M||Improbable value - BODC quality control|
|O||Improbable value - user quality control|
|0||no quality control|
|2||probably good value|
|3||probably bad value|
|6||value below detection|
|7||value in excess|
|A||value phenomenon uncertain|
|Q||value below limit of quantification|