Metadata Report for BODC Series Reference Number 885576

• 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 Discovery 216 CTD Data Documentation

Instrumentation

The 46 CTD profiles were taken with an RVS Neil Brown Systems Mk3B CTD incorporating a pressure sensor, conductivity cell, platinum resistance thermometer and a Beckman dissolved oxygen sensor. 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 all casts except CTD1, 2, and 33, and reversing thermometer readings taken on all except CTD1-4, 17, 25, 28 and 32-33.

Data Acquisition

The CTD sampled at a frequency of 32 Hz. These data were reduced in real time to a 1-second time series by the RVS Level A microcomputer system. These data were logged as raw counts on the Level C workstation 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 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.

The raw transmissometer voltages were corrected for light source decay using a correction ratio computed from light readings in air taken during the cruise and the manufacturer's figure for the new instrument (4.802 V). The correction was applied as follows:

Transmissometer voltages were converted to percentage transmission by multiplying them by 20 and attenuance computed using algorithm:-

Editing

Using a custom in-house graphics editor, the downcasts and upcasts were differentiated and the limits of the downcasts were manually flagged. Spikes on any of 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 editor. 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. Further edits applied to the data were as follows:

• CTD5 had a series of false steps in the salinity channel of up to 0.01 PSU. Small affected depth intervals have been flagged out, but larger depth intervals have been salvaged by applying an offset correction.
• CTD8 was characterised by a very noisy transmissometer signal between the pressures of 120 db and 550 db. All attenuance values in excess of 0.365 within this pressure range have been flagged suspect.
• CTD19's salinity channel was a problem. Below 2012.4 db the downcast salinity was both noisy and unpredictably inaccurate. This affliction lasted until 3480 db on the upcast, but between 3480 db and 3800 db the upcast salinity was predictably inaccurate (0.009 PSU lower with respect to the rest of the cast). In order to correct the cast, the following actions were taken. Between 2012.4 db and 3480 db, upcast salinities and temperatures have been used and the downcast values rejected. Between 3480 db and 3800 db upcast salinities and temperatures have been used, but an offset correction of 0.009 PSU has been added to the salinities. Below 3800 db the upcast temperatures have been used but there are no salinities (all flagged suspect).
• CTD20 had an attenuance signal that was noisy. Between 908 db and 995 db 0.0239 has been subtracted from the signal. Between 1100 db and 1900 db the signal is extremely noisy and therefore heavy flagging has been applied.
• CTD25 required an empirical correction to the salinity channel of 0.014 PSU between 260 db and 1280.9 db.
• CTD44 had a step in the downcast transmissometer signal which was not replicated in the upcast. In order to correct this, 0.004 has been subtracted from the attenuance values at pressures greater than 675 db.

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, against reversing thermometer data. 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). A mean air value (standard deviation of 0.29 db) was determined for all the data from the cruise, giving the correction:

Temperature

The CTD temperatures were in excellent agreement with the digital reversing thermometer readings. Hence no temperature calibration has been applied.

Salinity

Salinity was calibrated against 262 water bottle samples measured on the Guildline 55358 Autolab Salinometer during the cruise. Samples were obtained from 43 of the 46 casts, usually at 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 casts were subdivided into groups for the purpose of the salinity calibration. The correction applied was of the form:

with the following values used for the correction factor (X):

Oxygen

The dissolved oxygen sensor was calibrated against 222 water bottle samples analysed following the Winkler titration procedures outlined in Carpenter (1965). The samples were taken from 39 of the 46 casts, normally at several depths. The probe was extremely stable throughout the cruise and therefore a single calibration has been applied to the data:

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 109 near-surface CTD bottles. Samples were filtered through Whatman GF/F filters and frozen in liquid nitrogen until analysed on board. 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 resulting calibration 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

None.

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: