Metadata Report for BODC Series Reference Number 847974

• 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.

Chelsea Technologies Photosynthetically Active Radiation (PAR) Irradiance Sensor

This sensor was originally designed to assist the study of marine photosynthesis. With the use of logarithmic amplication, the sensor covers a range of 6 orders of magnitude, which avoids setting up the sensor range for the expected signal level for different ambient conditions.

The sensor consists of a hollow PTFE 2-pi collector supported by a clear acetal dome diverting light to a filter and photodiode from which a cosine response is obtained. The sensor can be used in moorings, profiling or deployed in towed vehicles and can measure both upwelling and downwelling light.

Specifications

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 Charles Darwin 91B CTD Data Documentation

Components of the CTD data set

The CTD data set for cruise CD91B consists of 10 vertical profiles. The data parameters are temperature, salinity, upwelling and downwelling irradiance, and optical attenuance.

Data Acquisition and On-Board Processing

Instrumentation

The CTD profiles were taken with an RVS Neil Brown 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 (661 nm) transmissometer with a 25cm path length.

Above the frame was a General Oceanics rosette sampler fitted with twelve 10-litre Niskin water bottles. The bases of the bottles were 0.75 metres above the pressure head and their tops 1.55 metres above it. One bottle was fitted with a holder for twin digital reversing thermometers mounted 1.38 metres above the CTD temperature sensor.

Above the rosette was a PML 2pi PAR (photosynthetically available radiation) sensor pointing upwards to measure downwelling scalar irradiance. A second 2pi PAR sensor, pointing downwards, was fitted to the bottom of the cage to measure upwelling scalar irradiance. It should be noted that these sensors were vertically separated by 2 metres with the upwelling sensor 0.2 metres below the pressure head and the downwelling sensor 1.75 metres above it.

No account has been taken of rig geometry in the compilation of the CTD data set. However, all water bottle sampling depths have been corrected for rig geometry and represent the true position of the midpoint of the water bottle in the water column.

Data Acquisition

On each cast, the CTD was lowered continuously at 0.5 to 1.0 ms^-1 to the closest comfortable proximity to the sea floor. The upcast was done in stages between the bottle firing depths.

Data were logged by the RVS ABC data logging system. Output channels from the deck unit were logged at 32 Hz by a microprocessor interface (the Level A) which passed time-stamped averaged cycles at 1 Hz to a Sun workstation (the Level C) via a buffering system (the Level B).

On-Board Data Processing

The raw data comprised ADC counts. These were converted into engineering units (volts for PAR meters, fluorometer and transmissometer; ml/l for oxygen; mmho/cm for conductivity; °C for temperature; decibars for pressure) by the application of laboratory determined calibrations. Salinity (Practical Salinity Units as defined in Fofonoff and Millard, 1983) was calculated from the conductivity ratios (conductivity/42.914) and a time lagged temperature using the function described in UNESCO Report 37 (1981).

The data set was submitted to BODC in this form on Quarter Inch Cartridge tapes in RVS internal format for post-cruise processing and data banking.

Post-Cruise Processing and Calibration at BODC

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.
• Transmissometer voltages were corrected to the manufacturer's specified voltage by ratio by using transmissometer air readings taken during the cruise.
• Transmissometer voltages were converted to percentage transmission by multiplying them by a factor of 20.
• The transmissometer data were converted to attenuance using the algorithm:-

Editing

Reformatted CTD data were transferred onto a high-speed graphics workstation. Using a custom in-house graphics editor, 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, the marked reaction of the oxygen sensor to the bottle firing was used to determine this. However, on this cruise a 'quiet fire' system was used to close the bottles and data point groupings were used in conjunction with CTD log sheets.

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.

Calibration

With the exception of pressure, calibrations were done by comparison of CTD data against measurements made on water bottle samples or from the reversing thermometers mounted on the water bottles as in the case of temperature. In general, values were averaged from the CTD downcasts but where visual inspection of the data showed significant hysteresis values were manually extracted from the CTD upcasts.

All calibrations described here have been applied to the data except for the calibration to express attenuance in terms of suspended matter concentration.

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). The following correction was applied:

NB This correction is based on data from two casts that were the only ones to log data in air.

Temperature

The CTD temperature was compared with the digital reversing thermometers attached to the instrument frame. These were found to agree within 0.002 °C and consequently no temperature calibration has been applied.

Salinity

Salinity was calibrated against water bottle samples measured on the Guideline 55358 AutoLab Salinometer during the cruise. Samples were generally taken from the bottom bottle plus one or two other depths on deep casts. However, on the later casts the samples were taken from all bottles in an attempt to resolve the problem of identifying bottle misfires. (See data warnings).

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 lab containing the salinometer before analysis.

The correction determined for this cruise was:

Upwelling and Downwelling Irradiance

The PAR voltages were converted to W m^-2 using the following equations determined in February 1990 supplied by RVS.

Note that these sensors have been empirically calibrated to obtain a conversion from W/m² into µE/m²/s which may be effected by multiplying the data given by 3.75.

Optical Attenuance and Suspended Matter

The air correction applied for this cruise was based on an air reading obtained during the cruise (4.757V). The manufacturer's voltage for the instrument used (SN116D) was 4.810V.

Chlorophyll

200ml of seawater collected at several depths on each cast were filtered and the papers frozen for acetone extraction and fluorometric analysis on land. Uncertainties in the bottle firing order reduced the sample data set for calibration purposes to 26 values in the range 0.02 to 0.27 mg/m³. The following relationship was found between extracted chlorophyll levels and corresponding fluorometer voltages:

Oxygen

No Winkler titration data were available from this cruise for the calibration of the dissolved oxygen sensor. It was hoped to use the fact that the North Atlantic is deeply mixed and in equilibrium with the atmosphere at this time of year to effect a calibration. However, there were significant differences in the form of the oxygen profiles between casts indicating instrumental problems. Consequently, no calibration was attempted and the oxygen data have been deleted from the data set.

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.

Data Warnings

The tone fire system used on this cruise reported a large number of misfires, but for a significant proportion of these the bottle had actually fired. This led to a degree of uncertainty in the bottle firing depths (see the report in the Appendix for details). Consequently, salinity samples were taken from several bottles on each cast. However, owing to the well-mixed nature of the water during this stormy cruise, the salinity data did little to resolve the issue and there is still some uncertainty about the sample depths for some samples. The above calibrations have been only used data from samples where BODC has confidence in the bottle firing depths.

Reference

Fofonoff N.P., and Millard Jr., R.C. 1982. Algorithms for Computation of Fundamental Properties of Seawater. UNESCO Technical Papers in Marine Science 44.

Appendix

CD91B Bottle Data Quality Control

The problem

Ten CTDs were carried out on cruise Charles Darwin 91B and several water bottles fired on each cast. Unfortunately, the CTD electronics unit in the lab registered 'misfires' ('short' or 'long') on most attempts to fire bottles, rather than alternate 'odd' and 'even' codes which indicate successful bottle firing. A 'long misfire' is registered when the rosette has received the firing signal but does not return a confirmation signal - the rosette spindle has turned and the bottle has probably closed properly. A 'short misfire' is registered when the rosette has not received a clear firing instruction - it may or may not have attempted to fire the bottle, so the spindle may not move.

There is therefore some uncertainty as to which bottles on the rosette closed at which depth. The resolution of this problem is hindered by the lack of proper CTD operator log sheets - as none were available a rough log was used to scribble down the information.

Reversing thermometer temperatures, bridge salinities, chlorophyll profiles and gravimetric data have been received for this cruise which need some decisive bottle assignment if they are to be useful at all. In addition, the originator of POC/PON data for this cruise is awaiting a 'definitive' bottle assignment for the working up of his data.

Assumptions

Several broad sweeps at this problem have been made by BODC, with basically unsatisfactory results and tentative conclusions. With the benefit of hindsight and more knowledge of the CTD system used, a more successful attempt was made.

Of course, one has to have some starting point to an investigation. This was based on the following set of assumptions:

• The reversing thermometers were attached to the first bottle to fire.
• The CTD and reversing thermometers agree within 0.006°C.
• The CTD salinity calibration is between +0.026 and +0.034 PSU.
• For casts 4 to 10 (excluding 5 which was a transmissometer calibration cast) the number of salinity bottles filled indicates the number of rosette bottles closed.
• All 'ODD' and 'EVEN' return codes indicate successful bottle firing.

With this set of rules, and armed with the rough log, salinity data, reversing thermometer temperatures and SERPLO, a cast by cast analysis was undertaken with the following results:

CTD 1 (25/3/95 11:30 - 12:05 at N140)

The cast when the problem started. The log sheet is a mess and the water is very well mixed. Nothing we can do so all bottles to be flagged 'O'.

CTD 2 (26/3/95 09:20 - 09:52 at N200)

The water column was extremely well mixed, so salinity and temperature data offer no clues.

9 bottles were fired, but on recovery it was seen that only 8 had closed. The 8th request (at 21m) registered return code 'ODD', leading to the conclusion that it was the 7th to close. Therefore, the 11m bottle also obeyed orders. This suggests very strongly that the failure occurred among the 7 deeper firings.

The RVS CTD operator on the cruise concluded that the second bottle (at 225m) had failed. This is consistent with my findings but not very convincing.

CTD 3 (26/3/95 18:35 - 19:15 at N300)

All 11 bottle firings returned 'misfire' codes, but on recovery 11 bottles were seen to have closed successfully. Therefore, there is no reason to suspect that anything untoward really happened on this cast.

CTD4 (27/3/95 19:45 - 20:40 at S700)

13 requests for bottle firing were made (2 attempts at 600m); all returned 'misfire' codes. On recovery, 11 bottles were sampled for salinity. I therefore conclude that 11 bottles closed successfully.

The reversing thermometer temperature was 8.925 °C, compared with 8.885 °C and 8.927 °C recorded by the CTD on the upcast at the time of the 715m and 700m bottle firings respectively. This is a strong indication that no bottle closed at 715m, and that the reversing thermometers (and hence rosette bottle 1) closed at 700m.

Making the further assumption that only one of the two attempts to fire a bottle at 600m was successful, the following comparison of bridge and CTD salinity data results:

This leads to a salinity correction of 0.024±0.003 which is possibly a bit low. (Taking out bottles 8 and 9, the result is 0.026±0.002).

CTD 5 (28/3/95 13:00 - 13:10 at S300)

A transmissometer calibration cast to 20m where all bottles were fired. There is no record of how many closed, but 3 returned 'ODD' or 'EVEN' codes. Hence at least three bottles fired at 20m on this cast.

CTD 6 (28/3/95 16:30 - 17:10 at S300)

Very well mixed water, no tell-tale return codes so nothing we can do. However, it is obvious that the salinity bottle values from Niskins 6, 7, 8 and 9 are about 0.01 PSU higher than the values for the other bottles. This is to be borne in mind when considering the salinity calibration.

CTD 7 (29/3/95 12:00 - 13:00 at S700)

This is a disappointing cast in many ways. The number of bottles sampled for salinity was 7, but the final return code was 'EVEN'. Fortunately, the chlorophyll data solved this paradox by quoting the bottles sampled as numbers 1,2,3,5,6,7 and 8 - in other words, it was a long misfire on bottle 4, moving the spindle on but not opening the bottle. Hence, the last return code was 'EVEN'.

So data from Niskins 7 and 8 were taken from 30m and 15m respectively. All other bottle entries are flagged 'O'.

CTD 8 (29/3/95 15:42 - 17:32 at N1500)

A most satisfying mystery to solve.

The reversing thermometer agrees strikingly well with the CTD temperature at 800m (being more than 4 °C higher than the bottom temperature). This indicates that the reversing thermometer snapped at 800m (on the 7th or 8th press of the bottle-fire button).

16 attempts to fire a bottle were made, and 8 bottles were sampled for salinity. The return code of the 15m bottle (15th press) was 'ODD', indicating that Niskin 7 closed at 15m. Hence Niskin 8 (the last to close) must have been at 5m.

So far, we have deduced that Niskin 1 fired at 800m, 7 at 15m and 8 at 5m. What happened in between is a thorny problem, but there are only 5 combinations allowed by the rough log records. Unfortunately, the strength of the analysis (and hence the credibility of the conclusions) is weakened by two factors. First, all the salinity samples were put through the salinometer, and there is some suspicion about the salinity of water collected in Niskin 4 as the CTD trace shows a consistent increase in salinity between 600m and 30m. It is also a shame that the 15m bottle, which can be confidently assigned, seems a little too saline. A few duff salinities here and there are to be expected however as the bottles were not analysed until Charles Darwin 93, more than a month later.

I think it is quite clear that rosette bottle 3 is more likely to have fired at 200m than at 600m. Scenarios 3, 4 and 5 bind the CTD and bridge salinity values together more tightly than scenarios 1 and 2. Sadly, there is simply not enough evidence to tie up the loose ends of bottles 2, 4 and 5. If only the other three salinity bottles has been put through the Autosal.

In practice, the bottle entries for depths 1531m through to 1000m will be flagged 'M', and the bottle entries for depths 600m and 100m will be flagged 'O'.

CTD 9 (31/3/95 14:29 - 31/3/95 14:56 at S140)

The bottle fire button was pressed 13 times during this cast, and all 12 bottles on the rosette were closed. The water column was very well mixed, salinity varying by less than 0.002 PSU. A quick comparison between the CTD value (~34.342) and the bridge salinities indicates that the bottle data from Niskins 9, 10 and 12 should be flagged suspect.

The tenth press (15m) returned an 'EVEN' code indicating that this bottle fired OK. This was followed by a maximum of 3 presses so must have been the tenth bottle to fire. Hence, the first 9 presses also successfully closed bottles at the prescribed depths. The last two presses, at least one of which was successful, occurred at 5m. It follows from the rough log that bottle 11 could have fired at 15m or at 5m, we can't tell which.

Data will be assigned to depths according to the rough log. No data will be loaded from bottle 11.

CTD 10 (1/4/95 13:48 - 14:15 at S200)

Another cast with very well mixed water. The fire button was pressed 9 times, and on recovery 8 bottles had closed.

The first bottle returned an 'ODD' code, so we can confidently say that Niskin bottle 1 was fired successfully at 230m. Unfortunately, there is nothing we can say about the rest - they will all be flagged 'O'.

Salinity values from Niskin bottles 5, 7, 9 and 10 have been excluded from consideration in the salinity calibration.

Project Information

Land Ocean Interaction Study (LOIS)

Introduction

The Land Ocean Interaction Study (LOIS) was a Community Research Project of the Natural Environment Research Council (NERC). The broad aim of LOIS was to gain an understanding of, and an ability to predict, the nature of environmental change in the coastal zone around the UK through an integrated study from the river catchments through to the shelf break.

LOIS was a collaborative, multidisciplinary study undertaken by scientists from NERC research laboratories and Higher Education institutions. The LOIS project was managed from NERC's Plymouth Marine Laboratory.

The project ran for six years from April 1992 until April 1998 with a further modelling and synthesis phase beginning in April 1998 and ending in April 2000.

Project Structure

LOIS consisted of the following components:

• River-Atmosphere-Coast Study (RACS)
  • RACS(A) - Atmospheric sub-component
  • RACS(C) - Coasts sub-component
  • RACS(R) - Rivers sub-component
  • BIOTA - Terrestrial salt marsh study
• Land Ocean Evolution Perspective Study (LOEPS)
• Shelf-Edge Study (SES)
• North Sea Modelling Study (NORMS)
• Data Management (DATA)

Marine Fieldwork

Marine field data were collected between September 1993 and September 1997 as part of RACS(C) and SES. The RACS data were collected throughout this period from the estuaries and coastal waters of the UK North Sea coast from Great Yarmouth to the Tweed. The SES data were collected between March 1995 and September 1996 from the Hebridean slope. Both the RACS and SES data sets incorporate a broad spectrum of measurements collected using moored instruments and research vessel surveys.

LOIS Shelf Edge Study (LOIS - SES)

Introduction

SES was a component of the NERC Land Ocean Interaction Study (LOIS) Community Research Programme that made intensive measurements from the shelf break in the region known as the Hebridean Slope from March 1995 to September 1996.

Scientific Rationale

SES was devoted to the study of interactions between the shelf seas and the open ocean. The specific objectives of the project were:

• To identify the time and space scales of ocean-shelf momentum transmission and to quantify the contributions to ocean-shelf water exchange by physical processes.

• To estimate fluxes of water, heat and certain dissolved and suspended constituents across a section of the shelf edge with special emphasis on net carbon export from, and nutrient import to, the shelf.

• To incorporate process understanding into models and test these models by comparison with observations and provide a basis for estimation of fluxes integrated over time and the length of the shelf.

Fieldwork

The SES fieldwork was focussed on a box enclosing two sections across the shelf break at 56.4-56.5 °N and 56.6-56.7 °N. Moored instrument arrays were maintained throughout the experiment at stations with water depths ranging from 140 m to 1500 m, although there were heavy losses due to the intensive fishing activity in the area. The moorings included meteorological buoys, current meters, transmissometers, fluorometers, nutrient analysers (but these never returned any usable data), thermistor chains, colour sensors and sediment traps.

The moorings were serviced by research cruises at approximately three-monthly intervals. In addition to the mooring work this cruises undertook intensive CTD, water bottle and benthic surveys with cruise durations of up to 6 weeks (3 legs of approximately 2 weeks each).

Moored instrument activities associated with SES comprised current measurements in the North Channel in 1993 and the Tiree Passage from 1995-1996. These provided boundary conditions for SES modelling activities.

Additional data were provided through cruises undertaken by the Defence Evaluation and Research Agency (DERA) in a co-operative programme known as SESAME.

Data Activity or Cruise Information

Cruise

Complete Cruise Metadata Report is available here

Fixed Station Information

Fixed Station Information

LOIS (SES) Repeat Section R

Section R was one of four repeat sections sampled during the Land-Ocean Interaction Study (LOIS) Shelf Edge Study (SES) project between March 1995 and September 1996.

The CTD measurements collected at repeat section R, on the Hebridean Slope, lie within a box bounded by co-ordinates 56° 29.4' N, 9° 41.4' W at the southwest corner and 56° 32.4' N, 8° 55.8' W at the northeast corner.

Cruises occupying section R

Related Fixed Station activities are detailed in Appendix 1

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:

Appendix 1: LOIS (SES) Repeat Section R

Related series for this Fixed Station are presented in the table below. Further information can be found by following the appropriate links.

If you are interested in these series, please be aware we offer a multiple file download service. Should your credentials be insufficient for automatic download, the service also offers a referral to our Enquiries Officer who may be able to negotiate access.