Metadata Report for BODC Series Reference Number 1069006

• 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

Problem Report

The fluorometer voltage output was extremely low, as it normally falls in the range 0-5 V. This casts doubt on the units for this channel.

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

James Clark Ross Cruise AMT10 CTD Instrumentation

Instrumentation

Sea-Bird Electronics 911plus CTD (with SBE9 CTD and SBE11 deck unit)
Rosette fitted with 12 30-litre Niskin water bottles
Chelsea Instruments MkIII Aquatracka fluorometer (SN unknown)

Sea-Bird Electronics SBE 911 and SBE 917 series CTD profilers

The SBE 911 and SBE 917 series of conductivity-temperature-depth (CTD) units are used to collect hydrographic profiles, including temperature, conductivity and pressure as standard. Each profiler consists of an underwater unit and deck unit or SEARAM. Auxiliary sensors, such as fluorometers, dissolved oxygen sensors and transmissometers, and carousel water samplers are commonly added to the underwater unit.

Underwater unit

The CTD underwater unit (SBE 9 or SBE 9 plus) comprises a protective cage (usually with a carousel water sampler), including a main pressure housing containing power supplies, acquisition electronics, telemetry circuitry, and a suite of modular sensors. The original SBE 9 incorporated Sea-Bird's standard modular SBE 3 temperature sensor and SBE 4 conductivity sensor, and a Paroscientific Digiquartz pressure sensor. The conductivity cell was connected to a pump-fed plastic tubing circuit that could include auxiliary sensors. Each SBE 9 unit was custom built to individual specification. The SBE 9 was replaced in 1997 by an off-the-shelf version, termed the SBE 9 plus, that incorporated the SBE 3 plus (or SBE 3P) temperature sensor, SBE 4C conductivity sensor and a Paroscientific Digiquartz pressure sensor. Sensors could be connected to a pump-fed plastic tubing circuit or stand-alone.

Temperature, conductivity and pressure sensors

The conductivity, temperature, and pressure sensors supplied with Sea-Bird CTD systems have outputs in the form of variable frequencies, which are measured using high-speed parallel counters. The resulting count totals are converted to numeric representations of the original frequencies, which bear a direct relationship to temperature, conductivity or pressure. Sampling frequencies for these sensors are typically set at 24 Hz.

The temperature sensing element is a glass-coated thermistor bead, pressure-protected inside a stainless steel tube, while the conductivity sensing element is a cylindrical, flow-through, borosilicate glass cell with three internal platinum electrodes. Thermistor resistance or conductivity cell resistance, respectively, is the controlling element in an optimized Wien Bridge oscillator circuit, which produces a frequency output that can be converted to a temperature or conductivity reading. These sensors are available with depth ratings of 6800 m (aluminium housing) or 10500 m (titanium housing). The Paroscientific Digiquartz pressure sensor comprises a quartz crystal resonator that responds to pressure-induced stress, and temperature is measured for thermal compensation of the calculated pressure.

Additional sensors

Optional sensors for dissolved oxygen, pH, light transmission, fluorescence and others do not require the very high levels of resolution needed in the primary CTD channels, nor do these sensors generally offer variable frequency outputs. Accordingly, signals from the auxiliary sensors are acquired using a conventional voltage-input multiplexed A/D converter (optional). Some Sea-Bird CTDs use a strain gauge pressure sensor (Senso-Metrics) in which case their pressure output data is in the same form as that from the auxiliary sensors as described above.

Deck unit or SEARAM

Each underwater unit is connected to a power supply and data logging system: the SBE 11 (or SBE 11 plus) deck unit allows real-time interfacing between the deck and the underwater unit via a conductive wire, while the submersible SBE 17 (or SBE 17 plus) SEARAM plugs directly into the underwater unit and data are downloaded on recovery of the CTD. The combination of SBE 9 and SBE 17 or SBE 11 are termed SBE 917 or SBE 911, respectively, while the combinations of SBE 9 plus and SBE 17 plus or SBE 11 plus are termed SBE 917 plus or SBE 911 plus.

Specifications

Specifications for the SBE 9 plus underwater unit are listed below:

Further details can be found in the manufacturer's specification sheet.

Chelsea Technologies Group Aquatracka MKIII fluorometer

The Chelsea Technologies Group Aquatracka MKIII is a logarithmic response fluorometer. Filters are available to enable the instrument to measure chlorophyll, rhodamine, fluorescein and turbidity.

It uses a pulsed (5.5 Hz) xenon light source discharging along two signal paths to eliminate variations in the flashlamp intensity. The reference path measures the intensity of the light source whilst the signal path measures the intensity of the light emitted from the specimen under test. The reference signal and the emitted light signals are then applied to a ratiometric circuit. In this circuit, the ratio of returned signal to reference signal is computed and scaled logarithmically to achieve a wide dynamic range. The logarithmic conversion accuracy is maintained at better than one percent of the reading over the full output range of the instrument.

Two variants of the instrument are available, both manufactured in titanium, capable of operating in depths from shallow water down to 2000 m and 6000 m respectively. The optical characteristics of the instrument in its different detection modes are visible below:

* The wavelengths for the turbidity filters are customer selectable but must be in the range 400 to 700 nm. The same wavelength is used in the excitation path and the emission path.

The instrument measures chlorophyll a, rhodamine and fluorescein with a concentration range of 0.01 µg l^-1 to 100 µg l^-1. The concentration range for turbidity is 0.01 to 100 FTU (other wavelengths are available on request).

The instrument accuracy is ± 0.02 µg l^-1 (or ± 3% of the reading, whichever is greater) for chlorophyll a, rhodamine and fluorescein. The accuracy for turbidity, over a 0 - 10 FTU range, is ± 0.02 FTU (or ± 3% of the reading, whichever is greater).

Further details are available from the Aquatracka MKIII specification sheet.

Paroscientific Absolute Pressure Transducers Series 3000 and 4000

Paroscientific Series 3000 and 4000 pressure transducers use a Digiquartz pressure sensor to provide high accuracy and precision data. The sensor comprises a quartz crystal resonator that responds to pressure-induced stress, and temperature is measured for thermal compensation of the calculated pressure.

The 3000 series of transducers includes one model, the 31K-101, whereas the 4000 series includes several models, listed in the table below. All transducers exhibit repeatability of better than ±0.01% full pressure scale, hysteresis of better than ±0.02% full scale and acceleration sensitivity of ±0.008% full scale /g (three axis average). Pressure resolution is better than 0.0001% and accuracy is typically 0.01% over a broad range of temperatures.

Differences between the models lie in their pressure and operating temperature ranges, as detailed below:

Further details can be found in the manufacturer's specification sheet.

James Clark Ross Cruise AMT10 CTD Processing

Originator's Processing

The CTD profiles were processed onboard using Sea-Bird's data processing software.

BODC data processing

• Reformatting

  The data files were sent to BODC in Sea-Bird's data processing software output. All available channels were listed into ASCII format. Non-null channels were then converted to PXF, a BODC internal format. The data were processed from 1Hz averaged down- and upcast data. Sigma-theta was calculated and output from the raw data during the conversion to PXF format.

  Parameters

  Originator's Parameter	Originator's Units	Description	BODC code	BODC Units	Comments
  Pressure	decibars	Pressure	PRESPR01	decibars	-
  Temperature	°C	Temperature	TEMPCU01	°C	-
  Temperature, 2	°C	Temperature	TEMPCU02	°C	-
  Salinity	-	Salinity	PSALCU01	-	-
  Salinity, 2	-	Salinity	PSALCU02	-	-
  Voltage 0	V	Fluorometer voltage	FVLTAQ01	V	-
  -	-	Calibrated fluorescence	CPHLPS01	mg m-3	-
  -	-	Potential temperature	POTMCV01	°C	Computed using UNESCO POTEMP function (using primary T and S)
  -	-	Sigma-theta	SIGTPR01	kg m-3	Computed using UNESCO SVAN function

• Screening

  The PXF data were compared with the original data files to ensure that no errors had been introduced during the conversion process. The data channels were then screened on a graphics workstation using in-house visualisation software. This allows multiple channels to be viewed simultaneously. The start and end-points of the downcast were marked. The pressure ranges over which bottles were fired were also marked. The bottle firing events were identified by disturbances in CTD parameters on the upcast profiles. All spurious and null data were flagged with appropriate BODC quality control flags.The secondary temperature and salinity channels were used to aid screening of the primary channels only. The primary channels should be used in preference to the secondary channel as they have been quality controlled.

  The following notes were made during the screening of AMT10 CTD data.

  There were few spikes in the temperature and salinity channels, but there was a significant difference between the salinity values obtained from the two different sensors.

  The fluorometer voltage output was extremely low, as it normally falls in the range 0-5 V. This casts doubt on the units for this channel.

• Loading into the BODC database

  After the data had been screened and quality controlled, the data were loaded into the BODC database under the Oracle RDBMS.

  40 entries were made in table EVENT, which stores metadata for the CTD casts. The start and end times were taken from the data files. The position was taken from the ship's navigation stored in the underway file at the start time of the CTD cast.

  The data from all 40 casts were loaded into the data tables.

Calibrations

• Pressure

  The pressure sensor had a mean reading of -0.542 decibar while the CTD was logging in air - this was identified during BODC processing. A pressure correction was, therefore, applied to the data when they were listed from the BODC database.

  PRESPR01(corr) = PRESPR01(raw) + 0.542 decibar (BODC calibration 2283)

• Temperature

  No reversing thermometer data were available for AMT10, so the CTD sensor data have not been calibrated against another data set. However, the Sea-Bird sensors were tested and calibrated before and after the cruise, and manufacturer's calibrations were applied during Sea-Bird processing. No further correction has been applied to the data at BODC.

• Salinity

  Unfortunately, no bench salinometer data are available for AMT10, so no further calibration of the salinity channel is possible. Users should be aware that the data are uncalibrated and may not be accurate. There is particular concern with the data from this cruise, as the data from the two sensors were so different. It is impossible to say which sensor logged the more accurate data. However, data from the two sensors up to 7 m were compared with the values obtained from the ship's thermosalinograph. Data from sensor 1 more closely followed the underway salinity (0.007 higher than the underway data). Sensor 2 was 0.011 higher than the underway data.

• Fluorescence

  The fluorometer on AMT10 was a Chelsea Instruments Aquatracka. The manufacturer's laboratory calibrations of the instrument indicated the following relationship

  Chl-a conc (µg/l) = 0.00945 * 10^voltage - 0.0172

  The fluorescence data from the CTD were compared with fluorometric chlorophyll-a measurements made on acetone extracts. When all casts were considered together, the relationship was strongly affected by the data from casts with higher fluorescence signals and chlorophyll-a concentrations. The resulting calibration over-estimated chlorophyll-a concentrations at low fluorescence values. As a result, the casts were split into three groups according to their maximum chlorophyll-a concentrations. A calibration was produced for each group.

  Low concentration casts: CTD01 - CTD14

  CPHLPS01 = 9.62 * 10^FVLTAQ01 - 10.2 (BODC calibration 2663; r² = 0.603; N = 49)

  The r² value was fairly low due to the sensitivity of low values to errors, but the minimum and maximum predicted values were very close to the range of chlorophyll-a measurements available.

  Medium concentration casts: CTD15 - 16, CTD19 - 20, CTD26 - 33

  CPHLPS01 = 7.12 * 10^FVLTAQ01 - 7.47 (BODC calibration 2667; r² = 0.717; N = 39)

  High concentration casts: CTD17-18, CTD21-25, CTD34-40

  CPHLPS01 = 4.84 * 10^FVLTAQ01 - 5.04 (BODC calibration 2668; r² = 0.952; N = 31)

Project Information

The Atlantic Meridional Transect (AMT) - Phase 1 (1995-2000)

Who was involved in the project?

The Atlantic Meridional Transect (AMT) programme was designed by and implemented as a collaboration between Plymouth Marine Laboratory (PML) and Southampton Oceanography Centre (SOC). The programme was hosted by Plymouth Marine Laboratory and involved additional researchers from UK and international universities throughout its duration.

What was the project about?

When AMT began in 1995 the programme provided a platform for international scientific collaboration, including the calibration and validation of SeaWiFs measurements and products. The programme provided an exceptional opportunity for nationally and internationally driven collaborative research and provided a platform for excellent multi-disciplinary oceanographic research. As an in situ observation system, the data collected by the AMT consortium informed on changes in biodiversity and function of the Atlantic ecosystem during this period of rapid change to our climate and biosphere.

The scientific aims were to assess:

• mesoscale to basin scale phytoplankton processes
• the functional interpretation of bio-optical signatures
• the seasonal, regional and latitudinal variations in mesozooplankton dynamics

When was the project active?

The first phase of the AMT programme ran from 1995 to 2000 and consisted of a total of 12 cruises. A second phase of funding allowed the project to continue for the period 2002 to 2006 with a further 6 cruises.

Brief summary of the project fieldwork/data

The AMT programme undertook biological, chemical and physical oceanographic research during the annual return passage of the RRS James Clark Ross between the UK and the Falkland Islands or the RRS Discovery between the UK and Cape Town, a distance of up to 13,500 km. This transect crossed a range of ecosystems from sub-polar to tropical and from euphotic shelf seas and upwelling systems to oligotrophic mid-ocean gyres. The transect route was covered north-south in September/October and south-north in April/May of each year.

The measurements of hydrographic and bio-optical properties, plankton community structure and primary production completed on the first 12 transects (1995-2000) represent the most coherent set of repeated biogeochemical observations over ocean basin scales. This unique dataset has led to several important discoveries concerning the identification of oceanic provinces, validation of ocean colour algorithms, distributions of picoplankton, identifying new regional sinks of pCO2 and variability in rates of primary production and respiration.

Who funded the project?

The programme was funded by the Natural Environment Research Council (NERC) and further support was received from the National Aeronautics and Space Administration (NASA) with equipment and funding from the Sea-viewing Wild Field-of-view Sensor (SeaWiFS) project.

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: