Metadata Report for BODC Series Reference Number 961618

• 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

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.

If the Information Provider does not provide a specific attribution statement, or if you are using Information from several Information Providers and multiple attributions are not practical in your product or application, you may consider using the following:

"Contains public sector information licensed under the Open Government Licence v1.0."

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.

RRS Discovery 191 CTD Data Documentation

Instrumentation

The CTD profiles were taken with an RVS Neil Brown Systems Mk3B CTD incorporating a pressure sensor, conductivity cell, and platinum resistance thermometer and a Beckmann dissolved oxygen sensor. During the first cast, the oxygen sensor failed and salinity exhibited an alarming positive drift. This was due to leakage through the oxygen sensor which was removed and replaced by a blanking cap.

As a precaution against further leakage, this instrument (termed the RVS CTD in this document) was restricted to depths of 300m or less. For deeper casts, a backup CTD unit loaned by IOSDL was used (termed the IOS CTD in this document). This was a Neil Brown Mk3 unit with no oxygen sensor and no connections for an external fluorometer or transmissometer.

The CTD unit was mounted vertically in the centre of a protective cage approximately 1.5m square. Where fitted, a Chelsea Instruments Aquatracka fluorometer, and a SeaTech red light (661nm) transmissometer with a 25cm path length were attached to the bars of the frame.

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. These instruments were only attached for casts of 300m or shallower.

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

RVS software on the Level C (a Sun workstation) was used to convert the raw counts into engineering units (Volts for PAR meters, fluorometer and transmissometer: ml/l for oxygen: mmho/cm for conductivity: °C for temperature) and to apply a nominal calibration to the chlorophyll channel.

Salinity (Practical Salinity Units, as defined by the Practical Salinity Scale, Fofonoff and Millard (1982)) was calculated from conductivity ratios (conductivity / 42.914) and a lagged temperature using the function described in Unesco report 37 (1981).

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:

The voltages were converted to percentage transmission by multiplying them by 20. Correction for light source decay was not possible as no air reading data were available for the cruise. Consequently, absolute values of attenuance will be slightly overestimated.

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

Editing

Reformatted CTD data were transferred onto a high-speed graphics workstation. A number of tasks was performed here, using an in-house graphics editor. Initially, downcasts and upcasts were differentiated and the limits of the downcast were manually flagged.

Secondly, 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.

Finally, the pressure ranges over which bottle samples were being collected, 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 values.

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

With the exception of pressure, calibrations were done by comparison of CTD data against measurements made on water bottle samples or from reversing thermometers mounted on the water bottles. In general, values were averaged from the CTD downcasts (most casts were shallow; therefore hysteresis is assumed to be negligible). 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). Neither instrument exhibited drift but the correction required for the different instruments thus:

Temperature

The CTD temperature sensor was calibrated against deep sea digital reversing thermometers.

Salinity

Salinity was calibrated against water bottle samples measured on the Guildline 55358 Autolab Salinometer during the cruise. Samples were generally taken from the first bottle fired on each cast which would normally be at the maximum depth sampled. On some deep casts, one or even two additional samples were taken spaced through the water column.

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.

An overall correction was determined for the RVS CTD, but sufficient bottles were done on each cast with the IOS CTD to allow calibration of individual casts. The results obtained were as follows:

Note that the salinity data from casts prior to instrument maintenance (stations 12053, 12055 and 12056) are useless and have been rejected.

Oxygen

No operational dissolved oxygen sensor was fitted to the CTD for this cruise.

Chlorophyll

The fluorometer was calibrated in terms of chlorophyll using a regression technique against extracted chlorophyll. Samples were taken from each water bottle, filtered and extracted into acetone. Chlorophyll was determined at sea using a JASCO spectrofluorometer calibrated against pure chlorophyll extracts.

The data set contained too few samples at high light levels (many of the CTDs had no light meters fitted) to allow the determination of a quench correction term. The data from the cruise were treated as a single population yielding the calibration equation:

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

Checks were performed on the analytical precision of the CTD conductivity cell, which exploited the canonical relationship between potential temperature and salinity observed by Saunders and Manning (1984) and given in Saunders (1986).

For deep water in the N.E. Atlantic, observations of salinity, S at potential temperature, theta (defined in Bryden, 1973) less than 2.6 °C have been documented as revealing a linear theta-S relationship: S = 34.968 + 0.098*theta.

This algorithm was used to compute expected salinities from CTD temperature measurements at depth, which were compared to measured salinities, to give an indication of the internal consistency of CTD measurements:

It can be seen from these comparisons that, for the IOS CTD, the JGOFS target accuracy for salinity of 0.02 PSU has easily been achieved for this cruise. Indeed, the accuracy for most of the casts is significantly better than 0.01 PSU. However, whilst by BOFS standards the salinity accuracy is very good, it should be noted that these data should not be used for any purpose for which the third decimal place is critical.

No reliable deep salinity data were returned by the RVS CTD. Consequently, it is not possible to make any statement concerning the salinity accuracy of this instrument. However, there is no reason to suspect that the accuracy is any worse than 0.02.

Data Warnings

Salinity accuracy for the IOS CTD is better than 0.01, but caution should be exercised if using the data for applications where the third decimal place in salinity is critical. The accuracy of the RVS instrument is unknown.

There are no dissolved oxygen data for this cruise.

References

BRYDEN H. 1973. New polynomials for thermal expansion, adiabatic temperature gradients and potential temperature of sea water. Deep Sea Research 20: 401-408.

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.

UNESCO 1981. Background papers and supporting data on the Practical Salinity Scale, 1978. Unesco Technical Papers in Marine Science 37 144pp.

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: