Metadata Report for BODC Series Reference Number 875594

• 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.

FS Poseidon 237_1 CTD Data Documentation

Instrumentation and Shipboard Procedures

Instrumentation

The CTD profiles were taken with a Neil Brown Systems Mk V CTD including a pressure sensor, a conductivity cell, a platinum resistance thermometer and a Beckmann dissolved oxygen sensor.

The following instruments were also attached to the bars of the cage and logged as additional CTD channels:

• Chelsea Instruments Aquatracka configured as a fluorometer.
• Chelsea Instruments Aquatracka configured as a nephelometer.
• SeaTech 25-cm path-length red (661 nm) light transmissometer.

The CTD system could only log one auxiliary channel. Consequently, only one of the above instruments could be logged on a given cast. The fluorometer was logged on casts CP02, CP05, CP08, CP25, CP37, CP58, CP71, CP72, CP74, CP78, CP79, C44A and C51D (13 casts). The nephelometer was logged on casts CP77, CP80-CP93, CP95, C51E, C67A, C94A and C94B (20 casts). The transmissometer was logged on the remaining 63 casts.

A General Oceanics 24-bottle tone-fire rosette pylon was fitted to the CTD frame. 10-litre Niskin bottles were used throughout the cruise.

Data Acquisition and Processing

On each cast, the CTD was lowered continuously at 0.5 to 1.5 m s^-1 to the required depth. The upcast was done in stages between the bottle firing depths. A tone fire system was installed to minimise the disruption caused to the data stream by the bottle-firing signal.

The data were logged on a PC running the Neil Brown EG&G logging software. The data were converted to 'calibrated' 16 Hz ASCII files using the CTDPOST program, which were supplied to BODC. It has been assumed that the calibration included the conversion of fluorometer voltage to nominal chlorophyll concentration.

Post-Cruise Processing and Calibration at BODC

Reformatting

The 16 Hz data supplied were reduced to a sampling frequency of 1 Hz using an averaging procedure designed to reduce the influence of spikes and electrical noise. The 14 data values closest to the median of each group of 16 data values were averaged, whilst the other two values were averaged.

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 multiplication by 44.66.
• Transmissometer voltages were converted to percentage transmission by multiplying them by 20 and to attenuance using the algorithm:

Editing

Reformatted CTD data were transferred onto a high-speed graphics workstation. Using custom in-house graphics editors, downcasts and upcasts were differentiated and the limits of the downcasts and upcasts were manually flagged.

Spikes on all the downcast channels were manually flagged. No data values were edited or deleted; flagging was achieved by modification of the associated quality control flag.

The pressure ranges over which the bottle samples had been collected were logged by manual interaction with the software. Usually, clusters of points recorded while the CTD was held stationary were used to determine this. These pressure ranges were subsequently used 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. The data were later migrated to the National Oceanographic Database.

Calibration

Pressure

The pressure calibration was determined by averaging pressure values that were logged in air. The result was an unusually large correction of -8.17 decibars (n = 74, SD = 0.26), which has been applied to the data.

Temperature

No independent check on accuracy of the temperature data was available. Consequently, the data have not been modified. Previous experience gives confidence in the accuracy of Neil Brown temperature data, but users should be aware that there is the possibility that the data are inaccurate.

Salinity

Deep salinity bottle data were used to derive an initial salinity offset of -0.202 PSU (N=4, SD=0.004). After this had been applied to the data, theta-salinity plots were generated for the theta range 3-9 °C using all CTD casts encountering theta values below 4 °C, together with data from a Charles Darwin CD105 deep cast. The data showed considerable scatter, with a total spread of approximately 0.07 PSU.

Further analysis showed that one cast, CP04, was very much 'out on its own', being some 0.03 PSU clear of the rest. There were obviously instrumental problems with this cast: all downcast data in the top 120 m were deleted because there was no resemblance between the forms of the upcast and the downcast. Consequently, this cast was considered to be an aberration and given a unique correction of -0.17 PSU. Examination of overlain salinity profiles at the 500 m salinity minimum indicated that the intermediate depth cast CP07 wasn't affected, but users should bear in mind that the salinity data from the shallow casts CP01, CP02 and CP05 could possibly be too high.

In summary, the CTD salinity data from this cruise, with the exception of a couple of shallow casts at the beginning, are believed to just meet the 0.02 PSU accuracy criterion. The accuracy of some of the casts is significantly better than this, but users are advised against using the salinity data from this cruise for high accuracy applications.

Dissolved Oxygen

The dissolved oxygen data from this cruise showed massive instrumental drift, to the extent that surface downcast and upcast values differed by as much as 50 µM. No oxygen measurements were made on bottle samples and so no additional calibration work was possible. Consequently, the oxygen data have been deleted from the data set.

Chlorophyll

The fluorometer was calibrated against a set of fluorometrically-assayed extracted chlorophyll-a data. The following regression calibration was produced, which has been applied to the data:

HPLC data were also available for this cruise and were considered for the fluorometer calibration. However, the results were poor, with an R² value of 20%, possibly due to the presence of other chlorophylls and phaeopigments in significant and variable amounts.

Attenuance

The attenuance values were obviously too high with clear water values from deep casts in the region of 1.24 to 1.3 per m. No air readings were taken on the cruise and consequently an alternative calibration strategy had to be devised. It was assumed that the minimum attenuance value (excluding any spikes) for CTD casts deeper than 2000 metres was constant. The fact that the actual CTD data varied between 1.244 and 1.292 per m for these deep casts indicated that the transmissometer was subject to instrumental drift during the cruise. By looking at the deep cast values as a time series, three distinct groups of casts (CP01-CP07, CP08-CP23 and CP24 on) were identified as having 'similar' minimum attenuance values. The following calibrations were derived to normalise the data from each group to a clear water value of 0.36.

The corrected clear water attenuance values lie in the range 0.349 to 0.365 per m, comparing very favourably with the 'ideal' values of 0.35 to 0.36. However, users are advised to carefully consider the influence of the calibrations before using them in high accuracy applications such as quantifying seston gradients perpendicular to the shelf break.

Nephelometer

The values supplied (voltages?) have been included in the database without the application of any further calibration.

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.

Oxygen saturation has been computed using the algorithm of Benson and Krause (1984).

Warnings

The salinity data should not be used if high accuracy is required without careful selection. Whilst some casts are accurate within 0.005 PSU, others only just meet the 0.02 PSU accuracy criterion.

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.

Project Information

Ocean Margin EXchange (OMEX) II - II

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 II - II (1997-2000)

The second phase of OMEX concentrated exclusively on the Iberian Margin, although RV Belgica did make some measurements on La Chapelle Bank whilst on passage to Zeebrugge. This is a narrow-shelf environment, which contrasts sharply with the broad shelf adjacent to the Goban Spur. This phase of the project was also strongly multidisciplinary in approach, covering physics, chemistry, biology and geology.

There were a total of 33 OMEX II - II research cruises, plus 23 CPR tows, most of which were instrumented. Some of these cruises took place before the official project start date of June 1997.

Data Availability

Field data collected during OMEX II - II have been published by BODC as a CD-ROM product, entitled:

• OMEX II Project Data Set (three 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: