Metadata Report for BODC Series Reference Number 961201

• 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

Open Data supplied by Natural Environment Research Council (NERC)

You must always use the following attribution statement to acknowledge the source of the information: "Contains data supplied by Natural Environment Research Council."

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 190 CTD Data Documentation

Instrumentation

The CTD profiles were taken with an RVS Neil Brown Systems Mk3B CTD incorporating a pressure sensor, conductivity cell, platinum resistance thermometer and a Beckmann dissolved oxygen sensor. The CTD unit was mounted vertically in the centre of a protective cage approximately 1.5m square.

Attached to the bars of the frame were a Chelsea Instruments Aquatracka fluorometer, and a SeaTech red light (661nm) transmissometer with a 25cm path length. No transmissometer data are available for many of the casts from this cruise due to instrument failure.

Above the frame was a General Oceanics rosette sampler fitted with 12, 10 litre Niskin bottles. The bases of the bottles were 0.75m above the pressure head with their tops 1.55m above it. One of the bottles was fitted with a holder for up to three digital reversing thermometers mounted 1.38m above the CTD temperature sensor. On deep casts, a second bottle was sometimes similarly equipped.

Above the rosette was a PML 2-pi PAR (photosynthetically available radiation) sensor pointing upwards to measure downwelling irradiance. A second such sensor was fitted to the bottom of the cage facing downwards to measure upwelling irradiance. It should be noted that the PAR meters were vertically separated by approximately 2m.

Lowering rates of up to 1.5 m/sec were used, although rates were generally in the range 0.5 - 1 m/sec. Bottle samples and reversing thermometer measurements were acquired on the ascent of each cast.

Data Acquisition

CTD data were sampled at a frequency of 32 Hz. Data reduction was in real time, converting the 32 Hz data to a 1-second time-series (done by the RVS Level A system) which was then passed through an Analogue-Digital Converter and logged as digital counts on the Level C.

On-Board Data Processing

The raw data were passed from the Level C onto a second Sun workstation running the P-EXEC suite of data processing software. Standard procedures were then used to convert the raw counts into engineering units (pressure (db), salinity (PSU), temperature (°C), oxygen current (mA), oxygen probe temperature (°C), irradiance (-1.0*ln(V)), nominal chlorophyll (mg/m³), transmittance (%)). Salinity was computed from conductivity using the algorithms given in Fofonoff and Millard (1982).

In the P-EXEC system, downcasts and upcasts are processed separately. Sophisticated techniques are used to obtain the instrumental readings corresponding to bottle firing points. These are unlike the BODC procedures used on other cruises in that they are fully automated. Salinity, temperature and fluorometer values are extracted from the upcast. Dissolved oxygen values are obtained from the downcast by matching downcast and upcast densities.

Temperature values were checked against the reversing thermometer data, reaching the conclusion that no correction was required.

Salinity values were calibrated against water bottle measurements made on board and the correction equation below was applied to the data:

Dissolved oxygen was iteratively computed from the raw channels with systematic variation of the coefficients RO3, Alpha and Beta until the best fit against the bottle data set (55 samples) was obtained. Final values were R03=1.022242, Alpha=-0.045 and Beta=0.0002.

Nominal chlorophyll was regressed against extracted chlorophyll measurements made on board for each of the two fluorometers used to give the calibration equations:

Transmissometer voltages were converted to percentage transmission by the P-EXEC package. The algorithms applied to the data are not known and there is some concern that at some stage these data may have been inappropriately processed. The attenuance profiles are atypical with unusually high values at depth. The attenuance data from this cruise should therefore be used with caution.

Data were written onto magnetic tape in GF3 format and submitted to BODC.

Post-Cruise Processing

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:

Dissolved oxygen was converted from ml/l to µM by multiplying the values by 44.66.

The 2-pi PAR data were converted back to Volts and then to µE/m²/s using the equations:

Editing

Reformatted CTD data were transferred onto a high-speed graphics workstation. Spikes on the downcast data were manually flagged. No data values were edited or deleted; flagging was achieved by modification of the associated quality control character flags.

The data downcasts were already extracted. However, examination of the data showed that further topping and tailing were required. This was done.

Once screened on the workstation, the CTD downcasts were loaded into a database under the Oracle relational database management system. During the loading process, the transmissometer data were converted to attenuance using the algorithm:

Calibration

The data supplied from this cruise were fully calibrated, with the exception of pressure. Values logged in air were identified and the pressure correction below was applied to the data:

After the cruise, it was discovered that the Turner Designs bench fluorometer used to determine the extracted chlorophyll data during the cruise was malfunctioning. Duplicate samples were frozen and subsequently analysed. The replacement chlorophyll data set was used to correct the values in the database.

The procedure involved restoring the data to 'nominal chlorophylls' and regressing these against the replacement extracted chlorophyll data set. As the initial calibration for the two fluorometers used was so close, the two fluorometers were not differentiated for this exercise. The resulting equation cannot be quoted as the file containing the information (a one-off correction program) was lost during the BODC migration from IBM to Unix computers.

Binning

The CTD data present on the CD-ROM have been binned by averaging over 1 db intervals for casts shallower than 100m and 2 db intervals for casts deeper than 100m. The binning algorithm only included data values associated with good flags. If no good data were available for a bin, linear interpolation was used to fill gaps of up to 3 bins. Gaps larger than this were left null.

The result of this algorithm is that data points are either considered good, in which case there is a value, or null, in which case the field is left blank. This removes the need for quality control flags which are often ignored and consequently make the data much easier to handle. The disadvantage is that some information is lost. The full resolution data have been archived by BODC and may be obtained on request.

Quality Control

There were no sufficiently deep casts on this cruise to test the salinity data against the canonical relationship between potential temperature and salinity for deep Atlantic waters described by Saunders and Manning (1984) and given in Saunders (1986).

However, unusually for BOFS cruises, there were marine physicists on board responsible for the processing and calibration of the CTD data. The P-EXEC system also has a reputation for calibrating CTD data to a high standard. Consequently, there is every reason to believe that the salinity data from this cruise are of good quality.

Data Warnings

Salinity accuracy is not known but the data are believed to be of good quality. Users who rely upon the significance of the third decimal place should make their own critical assessment of the data before using them.

The attenuance profiles are atypical with concern at BODC that the channel may have been inappropriately processed. This parameter was not considered to have sufficient priority to warrant the resources required to investigate and rectify the problem.

References

FOFONOFF N.P., MILLARD R.C. 1982. Algorithms for computation of fundamental properties of seawater. UNESCO Technical papers in Marine Science 44.

SAUNDERS P.M., MANNING A. 1984. CTD Data from the Northeast Atlantic Ocean, 22N-33N, 19-24W, July 1983 during RRS Discovery cruises 138,139. IOS Deacon Laboratory technical report 188.

SAUNDERS P.M. 1986. CTD data from the Madeira and Iberian abyssal plains, Charles Darwin cruises 3/85 and 9A/85. IOS Deacon Laboratory technical report 227.

Project Information

Biogeochemical Ocean Flux Study (BOFS)

The Biogeochemical Ocean Flux Study (BOFS) was a Community Research Project within the Marine and Atmospheric Sciences Directorate (MASD) of the Natural Environment Research Council. The project provided a major United Kingdom contribution to the international Joint Global Ocean Flux Study (JGOFS). The project ran from April 1987 until March 1992 but was extended through bridging funds until March 1993. The BOFS North Atlantic Data Set was collected during the initial five year period. Fieldwork in the bridging year focused on the Antarctic in late 1992. These data will form part of a subsequent electronic publication of Antarctic data and are not included on this CD-ROM.

The primary aims of the BOFS programme were:

• To improve the understanding of the biogeochemical processes influencing the dynamics of the cycling of the elements in the ocean and related atmospheric exchanges with particular reference to carbon.
  
  
• To develop, in collaboration with, other national and international programmes. models capable of rationalising and eventually predicting the chemical and biological consequences of natural and man-induced changes to the atmosphere ocean system.

A Community Research Project brings together scientists from NERC institutes and UK universities to work on a common problem. In this way resources far beyond the scope of individual research groups may be brought to bear on a common problem. The project is coordinated through a host laboratory which has responsibility for financial management, organisation and logistics. The host laboratory for BOFS was the Plymouth Marine Laboratory (PML).

Fieldwork

The BOFS North Atlantic data set was the result of fieldwork carried out on 11 research cruises. Four studies were carried out during three field seasons in 1989, 1990 and 1991; the 1989 North Atlantic Bloom Experiment, the 1990 Lagrangian Experiment, the 1990 BOFS Benthic Study and the 1991 Coccolithphore Study. Measurements taken include:

Physical (e.g. temperature, salinity and optics)
Meteorology and positioning
Chemical (e.g. dissolved oxygen, organic carbon and nitrogen)
Biological (e.g. biomass, pigments and bacteria production)
Geological (sediment traps)

The Sterna 1992 project (the Southern Ocean component of BOFS) aimed to measure the size and variability of carbon and nitrogen fluxes during early summer in the Southern Ocean, with particular emphasis on rates and processes in the marginal ice zone. Fieldwork was carried out between October and December 1992 in the Southern Ocean area, approximately 55°S to 70°S, 60°W to 85°W. A wide range of physical, chemical and biological parameters were measured.

Data Management

Data management services to BOFS were provided by the British Oceanographic Data Centre, funded by the UK Natural Environment Research Council.

Joint Global Ocean Flux Study (JGOFS)

JGOFS was an international and multi-disciplinary programme, which ran from February 1987 to December 2003, with participants from more than 20 nations. JGOFS was launched at a planning meeting in Paris under the auspices of the Scientific Committee of Oceanic Research (SCOR), a committee of the International Council for Science (ICSU) and later became one of the first core projects of the International Geosphere-Biosphere Programme (IGBP) in 1989.

The primary aims of the JGOFS programme were:

• To determine and understand on a global scale the processes controlling the time-varying fluxes of carbon and associated biogenic elements in the ocean, and to evaluate the related exchanges with the atmosphere, sea floor and continental boundaries.
• To develop a capacity to predict on a global scale the response to anthropogenic perturbations, in particular those related to climate change.

JGOFS consisted of fieldwork, synthesis and modelling phases. Further information about JGOFS may be found at the international Joint Global Ocean Flux Study web site.

JGOFS fieldwork

Synthesis and modelling phase

From 1998, as the fieldwork for most process studies were being completed, JGOFS focused on:

1. Integrating regional synthesis and modelling activities
2. Maintaining links to other ocean observing and global change programmes
3. Developing a global synthesis and modelling phase

Data availability

The field data collected during JGOFS has been published on two DVDs. These are available via the World Data Center for Oceanography, Silver Spring and are entitled:

• JGOFS International Collection, Volume 1: Discrete Datasets (1989-2000) DVD
• JGOFS Arabian Sea Process Study, CTD, XBT and SeaSoar Data from 1990-1997

Data sets making up the UK contribution to JGOFS, for which BODC provided data management support, are also available directly 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: