Metadata Report for BODC Series Reference Number 885029

• 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

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 Charles Darwin 94 CTD Data Documentation

Instrumentation

A total of 51 CTD casts returned data on this cruise. Of these, 11 (CTD3A, CTD3B, CTD8A, CTD8B, CTD15, CTD21, CTD26, CTD31, CTD35, CTD42 and CTD49) were taken with a CTD system belonging to the Institute of Oceanographic Sciences Deacon Laboratory (now part of the Southampton Oceanography Centre). The remaining casts were taken using a CTD supplied with the ship by Research Vessel Services. These are referred to in this document as the IOS CTD and RVS CTD respectively.

The IOS CTD was a Neil Brown Systems Mk3B instrument incorporating a pressure sensor, conductivity cell, platinum resistance thermometer and a Beckman non-pulsed oxygen electrode. The package also included a Chelsea Instruments Aquatracka fluorometer and an experimental UV-absorption nitrate sensor. The primary purpose of the deployments of this package was the development and testing of the nitrate sensor.

The RVS system comprised a Neil Brown Systems Mk3B CTD incorporating a pressure sensor, conductivity cell, platinum resistance thermometer and a Beckman dissolved oxygen sensor. The oxygen sensor was supplied with water by a SeaBird submersible pump. 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 a SeaTech red light (661 nm) transmissometer with a 25 cm path length.

A General Oceanics rosette sampler fitted with 12, 10 litre Niskin or lever-action Niskin (externally sprung for trace metal work) 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. One of the bottles was fitted with a holder 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. Bottle samples were acquired on the ascent of the package.

Data Acquisition

The CTD data were sampled at a frequency of 32 Hz. Real time data reduction to a 1-second time series was done by the RVS Level A microcomputer system. The resulting data stream was logged as digital counts on the Level C via a Level B data buffer.

On-Board Data Processing

RVS software on the Level C (a SUN workstation) was used to convert the raw counts into engineering units (Volts for the PAR sensor, transmissometer and fluorometer, ml/l for oxygen, mmho cm-1 for conductivity and °C for temperature).

Salinity (Practical Salinity Units, as defined by the Practical Salinity Scale (Fofonoff and Millard 1982)) was calculated from the conductivity ratios (conductivity / 42.914) and a time lagged temperature.

Data were written onto Quarter Inch Cartridge tapes in RVS internal format and submitted to BODC for post-cruise processing and data-banking.

Post-Cruise Processing

Reformatting

The data were converted into the BODC internal format 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:

Dissolved oxygen was converted from ml/l to µM by multiplying the values by 44.66 for the IOS CTD casts. For the RVS CTD, dissolved oxygen was computed from the sensor current, sensor temperature and CTD temperature and salinity using algorithms taken from SeaBird CTD manuals appropriate for a pump-fed sensor. Calibration constants were included, derived using the full University of Hamburg Winkler data set, following the procedures established for this instrument during the LOIS Shelf Edge Study.

The raw transmissometer voltages from the RVS CTD were corrected for light source decay using a correction ratio computed from light readings in air taken during the cruise (4.787 V) and the manufacturer's figure for the new instrument (4.802 V). No transmissometer was fitted to the IOS package.

Editing

Using a custom in-house graphics editor, the downcasts and upcasts were defined by the manual insertion of flags into the cycle number channel. Any spikes on all the downcast channels were manually flagged 'suspect' by modification of the associated quality control flag. In this way none of the original data values were edited or deleted during quality control.

The pressure ranges over which the bottle samples were taken were logged by manual interaction with the software. Usually, the marked reaction of the oxygen sensor to the bottle firing sequence was used to determine this. These pressure ranges were subsequently used, in conjunction with a geometrical correction for the position of the water bottles with respect to the CTD pressure transducer, to determine the pressure range of data to be averaged for calibration purposes.

Once screened, the CTD downcasts were loaded into a database under the Oracle relational database management system and later migrated to the National Oceanographic Database.

Calibration

With the exception of pressure, calibrations were done by comparison of CTD data against measurements made on water bottle samples or, in the case of temperature, from the reversing thermometers mounted on the water bottles. In general, values were averaged from the CTD downcasts but where inspection on a graphics workstation showed significant hysteresis, values were manually extracted from the CTD upcasts.

All calibrations described here have been applied to the data.

Pressure

The pressure offset was determined by looking at the pressures recorded when the CTD was clearly logging in air (readily apparent from the conductivity channel). The pressure sensors on both instruments exhibited considerable drift. Consequently, corrections were determined for the individual casts as follows. IOS CTD casts are denoted by an asterisk appended to the cast number.

The pressure corrections have been added to the raw pressures to effect the correction.

Temperature

The CTD temperature readings were in excellent agreement with digital reversing thermometer readings. Hence no temperature calibration was applied to either instrument.

Salinity

Salinity was calibrated against water bottle samples measured on the Guildline Autosal Salinometer during the cruise. Samples were usually obtained from several depths on each cast.

Samples were collected in glass bottles filled to just below the neck and sealed with plastic stoppers. Batches of samples were left for at least 24 hours to reach thermal equilibrium in the constant temperature laboratory containing the salinometer before analysis.

The IOS CTD salinity calibration showed two distinct groupings giving the corrections:

Note that the salinity data from casts CTD3A, CTD3B and CTD35 exhibited severe hysteresis and they have not been included in the database. In general, the IOS salinity data are poorer quality than the data from the RVS CTD and data from the latter should be used in preference wherever possible.

The RVS CTD calibration was determined for batches of casts that were defined by empirical analysis of the differences between the bottle values and CTD values.

The downcast salinity between 53 and 150db on cast CTD45 had a dramatic offset towards low salinity, indicative of biological fouling of the conductivity cell. The data from 100 to 150 db have been salvaged by adding an empirical correction of 0.03 PSU. The remaining affected data could not be corrected and have been deleted.

Oxygen

The dissolved oxygen sensor was calibrated against water bottle samples analysed following the Winkler titration procedures outlined in Carpenter (1965) by personnel from Hamburg University. The following calibration was determined for the IOS CTD.

The RVS CTD oxygen data were initially calibrated as part of the reformatting procedure against the entire bottle data set. However, it was reported that the result was systematically high. As a result, the 'calibrated' values were recalibrated against a quality controlled bottle data set giving the following additional correction:

It is appreciated that this methodology falls short of the ideal and had resources permitted the entire data set would have been reprocessed from scratch using the reduced bottle data set. However, the procedure has brought the deep data into line with other OMEX cruises sampling the same stations and has therefore been deemed successful.

Oxygen saturation present in the data files was computed using the algorithm presented in Benson and Krause (1984).

Chlorophyll

Chlorophyll was measured with a Chelsea Mk2 Aquatracka fluorometer calibrated against discrete samples taken from near-surface CTD bottles. Samples were filtered through Whatman GF/F filters and frozen in liquid nitrogen until analysed. The frozen filters were extracted in 2-5 ml of 90% acetone using sonification and centrifuged to remove cellular debris. Analysis was carried by reverse phase HPLC. The resultant regression equation is:

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).

Data Warnings

The salinity data from the IOS CTD are of poorer quality than the RVS CTD data. Wherever possible the latter data should be used to ascertain the salinity profile for a particular station.

The RVS CTD oxygen calibration appears empirically effective but an unconventional protocol was used. Users should bear this in mind when deciding whether to use the data. The calibrated results may also be slightly high due to a small systematic offset in the bottle data.

References

Benson B.B. and Krause D. jnr. 1984. The concentration and isotopic fractionation of oxygen dissolved in fresh water and sea water in equilibrium with the atmosphere. Limnol. Oceanogr. 29 pp.620-632.

Carpenter J.H. 1965. The Chesapeake Bay Institute techniques for the Winkler dissolved oxygen method. Limnol.Oceanogr. 10 pp.141-143.

Fofonoff N.P. and 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: