Metadata Report for BODC Series Reference Number 962068

• 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 198 Data Documentation

Instrumentation

The CTD profiles were taken with an RVS Neil Brown Systems MkIIIB CTD incorporating a pressure sensor, conductivity cell, platinum resistance thermometer and a new Beckmann dissolved oxygen sensor. This 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.

Above the frame was a General Oceanics rosette sampler with 12, 10-litre Niskin bottles fitted with stainless steel springs. The bases of the bottles were 0.75m above the pressure head with their tops 1.55m above it. One of the bottles had a holder for up to three digital reversing thermometers mounted 1.38m above the CTD temperature sensor.

Two PML 2-pi PAR (photosynthetically available radiation) meters were fitted. One, facing upward, was above the bottle rosette to measure downwelling scalar irradiance. The second, facing downward, was located near the base of the CTD frame to measure upwelling scalar irradiance. Note that there was a physical separation of approximately 2m between the two sensors. No geometrical offset corrections have been applied to the data.

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

The CTD data were acquired using the RVS ABC system.

CTD data were sampled at a frequency of 32 Hertz. Real-time data reduction by the Level A interface converted the 32 Hz data to a 1-second time series, incorporating a temperature time offset to correct for thermal lag of the temperature sensor. This correction wasn't implemented until station 12205 due to a software problem. Consequently there were more problems with salinity noise on the thermocline prior to this station.

The 1-second data were logged on the Level C workstation via the Level B disk buffer.

On-Board Data Processing

The data were transferred from the Level C into P* format on a workstation running the P EXEC oceanographic data processing package as developed for processing the UK WOCE CTD data. The ctdcal program was run to convert the data from raw counts into engineering units. Each CTD cast was split into an upcast and downcast. The downcast data file was then calibrated following the procedures described below and inspected graphically. Any spikes identified in the temperature or salinity channels were replaced by absent data values.

Data were written onto magnetic tape in the P* archive subset of the GF3 format and submitted to BODC. A 'bottle' file was prepared containing averaged CTD parameter values corresponding to the bottle firings. All channels except oxygen were derived from the upcast files. Oxygen was obtained from the downcast file by locating data corresponding to the isopycnal determined for the bottle firing from the upcast data. The individual data channels were then calibrated as follows:

Pressure

The pressure calibration applied was:

The constant offset was adjusted to -7.1 from the laboratory-supplied value of 5.13287 to adjust the mean pressure value logged in air to zero.

Temperature

The laboratory-based calibration:

was used throughout the cruise.

The SIS reversing thermometers (T220 and T238) were fired as a pair on a single bottle and showed excellent agreement with a mean difference of 0.00085 ±0.00082 °C. Comparison between the CTD temperatures and the reversing thermometer values showed a mean offset of -0.00448 ±0.00174 °C. However, no correction was applied to the CTD temperatures because it was considered that the SIS calibrations were no more accurate than the CTD temperature calibration. Temperature accuracy was reported as ±0.005 °C.

Salinity

Salinity was computed from time-offset temperature and conductivity. For the first part of the cruise calibration salinity samples were taken from all twelve bottles and using a Guildline Autosal bench salinometers. Once it was established that the CTD conductivity cell was stable, the number of bottles sampled was reduced to 6 and then 4 per cast.

An intercalibration cast with samples analysed on both Discovery and James Clark Ross showed salinometer measurements between the two vessels to be within 0.002 PSU.

Comparison between the CTD salinities and bottle salinities led to the following correction being applied to the CTD salinity data:

The standard deviation of the calibration was ±0.002 PSU.

Oxygen

Oxygen concentration was computed from the CTD temperature (T_CTD) and pressure (P), sensor current (C_oxy) and temperature (T_oxy) readings using the algorithms:

The coefficients were determined as:

based on a set of 126 bottle water samples analysed using the Winkler technique as described in Williams and Jenkinson (1982). Oxygen saturations were computed using the algorithm of Benson and Krause (1984). The computed oxygen saturations were believed to be correct within 2 per cent.

Transmissometer

An A/D converter that translated 10 Volts to 4096 counts logged the raw counts. The raw counts were therefore converted into a voltage by multiplying by 0.002442. As the transmissometer output 5 Volts for 100% transmission, the raw voltage was converted to percentage transmission by multiplying by 20.

The voltages were corrected for source decay by a ratio correction based on a manufacturer's air voltage of 4.744 Volts and a cruise-determined air voltage of 4.763 Volts.

Light

The upwelling and downwelling irradiance counts were converted using the equations:

Chlorophyll

The CTD fluorometer voltages (determined by scaling raw counts by 0.002441) were calibrated against extracted chlorophyll determinations on samples taken from the CTD rosette. As there was evidence of quenching, samples were only considered for the calibration if they were collected in darkness (PAR >2W/m²).

The following calibration equations were derived for subsets of the CTD stations:

The different calibration equations were attributed to different phytoplankton species assemblages. No quench correction was attempted due to insufficient quantity of suitable data.

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.

Editing

Reformatted CTD data were transferred onto a high-speed graphics workstation. A number of tasks were performed here, using an in-house graphics editor.

• Downcasts and upcasts were differentiated and the limits of the downcast were marked using flags on the pressure channel.
• Spikes identified by manual inspection of the downcast data were flagged as suspect.
• Datacycle groupings on the upcast accompanied by dramatic oxygen spikes were flagged as possible bottle firings, including logging of the pressure range sampled whilst the bottle was being fired. 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 subset of datacycles 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

Most of the CTD calibration work was completed during the cruise. However, the form of the light calibrations was unfamiliar to BODC and there was some concern that the first terms were numerically equal to the exponential calibrations usually applied to these instruments.

Consequently, the data were recomputed as voltages and the following calibrations (determined in June 1992) were applied to convert the data into units of µE/m²/s:

If the data are required in units of W/m² then divide the calibrated data by the empirically-derived scaling factor of 3.75.

Binning

The final CTD data set contains profiles that 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 excluded any data that had been flagged as suspect. If no good data were available within the pressure range of a bin, linear interpolation was used to assign data values. Gaps larger than three bins were left null.

The full resolution data have been archived by BODC and may be obtained on request.

Data Warnings

None

References

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

Williams P.J. Leb., Jenkinson N.W. 1982. A transportable microprocessor-controlled precise Winkler titration suitable for field station and shipboard use. Limnol. Oceanogr. 27, 567-585.

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: