Metadata Report for BODC Series Reference Number 1046241

• 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

Sea-Bird Dissolved Oxygen Sensor SBE 43 and SBE 43F

The SBE 43 is a dissolved oxygen sensor designed for marine applications. It incorporates a high-performance Clark polarographic membrane with a pump that continuously plumbs water through it, preventing algal growth and the development of anoxic conditions when the sensor is taking measurements.

Two configurations are available: SBE 43 produces a voltage output and can be incorporated with any Sea-Bird CTD that accepts input from a 0-5 volt auxiliary sensor, while the SBE 43F produces a frequency output and can be integrated with an SBE 52-MP (Moored Profiler CTD) or used for OEM applications. The specifications below are common to both.

Specifications

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

Benthos Programmable Sonar Altimeter (PSA) 916 and 916T

The PSA 916 is a submersible altimeter that uses the travel time of an acoustic signal to determine the distance of the instrument from a target surface. It provides the user with high resolution altitude or range data while simultaneously outputting data through a digital serial port. A wide beam angle provides for reliable and accurate range measurements under the most severe operational conditions. The instrument is electronically isolated to eliminate any potential signal interference with host instrument sensors. The PSA 916 is an upgrade of the PSA 900.

The standard model (PSA 916) has an operational depth range of 0 - 6000 m, while the titanium PSA 916T has a depth range of 0 - 10000 m. All other specifications for the two versions are the same.

Specifications

Further details can be found in the manufacturer's specification sheets for the PSA 916 and the PSA 916T.

Instrument Description

CTD Unit and Auxiliary Sensors

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.

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.

BODC Processing

The data were received in one netcdf file per station and contained data from primary and secondary sensors, with various filtered versions. Most of the uncalibrated filtered data (excluding fluor_filt) were not loaded as they had been superseded by calibrated data. The transmittance and optical backscatter channels were not loaded as these were problematic (see cruise report p.17). The originator derived channels were also not loaded, with potential temperature and sigma-theta re-derived by BODC. In addition, press_filt was not required as press_filt_bin_average was a more appropriate pressure variable to use. The remaining data channels were converted into the internal BODC format.

The reformatted data were visualised using the in-house EDSERPLO software. The data were screened and quality control flags were applied to data as necessary. The conductivity primary sensor 1 was found to have malfunctioned for three of the casts, 27, 38 and 39. This lead to pronounced salinity and density inversions of >0.5 in these profiles. The conductivity from the secondary sensor 2 appeared to be unaffected, whilst sensor 1 and sensor 2 data compared well for the other casts. Therefore, for casts 27, 38 and 39 only sensor 2 data (all variables) have been included and all BODC derived channels have been calculated using sensor 2 data only. The remaining 92 casts have data from the primary sensors only and all derived channels have been calculated from these.

The following table shows how the variables within the netCDF files were mapped to appropriate BODC parameter codes:

References

Benson, B.B. and Krause, D., 1984. The concentration and isotopic fractionation of oxygen dissolved in freshwater and seawater in equilibrium with the atmosphere. Limnology and oceanography, No.29(3), 620-632pp.

Fofonoff, N.P. and Millard, R.C., 1983. Algorithms for computations of fundamental properties of seawater. UNESCO Technical Papers in Marine Science, No.44, 53pp.

UNESCO, 1978. Eighth report of the joint panel on oceanographic tables and standards. UNESCO Technical Papers in Marine Science, No.28, 35pp.

Originator's Data Processing

Sampling Strategy

Ninety six CTD profiles were performed during the cruise, 95 were received (cast 5 abandoned as pumped blocked, cast 6 at same location). CTD casts are labelled from 1 to 98. Cast 1 is a test cast, and casts 69, 76 and 81 were cancelled due to time constraints. Data were measured at 24 Hz and averaged to 2 dbar.

Further details of sampling problems and sensor failures can be found in the cruise report.

Field Calibrations

Bottle samples were drawn for salinity and oxygen from each available bottle depth for every CTD cast. Some bottles were leaking badly and were excluded from any sampling. The bottle depths were chosen from the depths: surface, 25, 50, 75, 100, 150, 200, 300, 400, 600, 800, 1000, (1250), 1500, (1750), 2000, (2250), 2500, (2750), 3000, (3250), 3500, (3750), 4000, (4250), 4500, (4750), 5000, bottom-200, bottom-150, bottom-100, bottom-50, bottom with depths in brackets being dropped (shallowest first) where necessary. On some casts some bottles (especially bottle 20) were double fired, at the expense of another depth, as they were deemed likely to leak. On some casts the bottle depths differed from this scheme due to errors; either deeper depths were dropped in favour of shallower or due to transcription errors.

The following processing steps were taken:
1) Temperature 1 and temperature 2 calibrated from SBE35 thermometer (see section 3c).
2) Bottle conductivity 1 and conductivity 2 back calculated
3) Conductivity calibrations found and applied
4) Salinity recalculated
5) Salinity checked
6) Oxygen calibrated

Conductivity

Conductivity was calibrated rather than salinity using bottle samples from the CTD rosette compared to CTD conductivity. Overall sensor performance was assessed to be good. Bottle sample salinity and CTD salinity were also compared to demonstrate behaviour of sensors. It was concluded that CTD salinity 1 did not have an offset, although temperature 1 did, and so conductivity 1 was calibrated. Salinity 2 had a slight pressure-dependent offset and was therefore calibrated.

The following method provided by Brian King is the standard National Oceanography Centre Southampton scientists' approach for calibrating conductivity;
1) Bottle salinity, as measured by the bench salinometer, is converted to 'potential conductivity' at the CTD bottle location using in situ temperature and pressure, and the CSIRO EOS-80 Seawater algorithms.
2) CTD conductivity is calibrated against this 'potential conductivity'.
3) CTD salinity is derived using the corrected conductivity data and the PSS-78 algorithm.

Temperature

Temperature data were calibrated using independent temperature values from SBE35 thermometer compared to CTD temperature. Both primary and secondary sensors were calibrated independently.

Oxygen

Oxygen data were calibrated against dissolved oxygen measurements taken from discrete bottles and determined using the Winkler method on board the ship. The CTD oxygen sensor behaviour was observed to have jumped between casts 23 and 26. The sensor was changed after cast 68. See cruise report for further calibration details.

Project Information

Antarctic Deep Water Rates of Export (ANDREX) project document

ANDREX is a UK field programme aimed at investigating the role of the Weddell Gyre in the Meridional Overturning Circulation (MOC) and its influence on deep ocean properties.

The MOC is a critical regulator of Earth's climate and is crucial for deep water ventilation across the globe. Surface currents transport waters towards the poles, where they become dense and sink, flowing equatorward as deep, cool currents. The MOC ensures that the deep oceans remain ventilated and conducive to life, and is also important for anthropogenic carbon sequestration. The southern closure of the MOC in the Weddell Sea is strongly influenced by the Weddell Gyre, which facilitates the exchange of waters between the Antarctic Circumpolar Current (ACC) and the waters of the continental shelf. Cooling and sea ice formation in the Weddell Sea lead to overturning of the water column and the ventilation of Antarctic Bottom Water (AABW), which flows out of the Weddell Sea and into the deep oceans to the north. Thus, the Weddell Gyre plays an important role in the properties of deep ocean waters on a global scale.

The goals of ANDREX are to investigate the exchange of water masses between the ACC and the Weddell Sea, including AABW formation and ventilation rates, carbon and nutrient cycling, the influence of fresh water input from sea ice, precipitation and glacial melt, and the role of the Weddell Gyre in anthropogenic carbon sequestration. The project includes hydrographic, ventilation tracer, biogeochemical and bathymetric measurements along the outer rim of the Weddell Gyre.

ANDREX is funded by the UK Natural Environment Research Council (NERC) Antarctic Funding Initiative (AFI) and involves scientists from the National Oceanography Centre, Southampton (NOC), the British Antarctic Survey (BAS), the University of East Anglia (UEA), the University of Manchester, the Alfred Wegener Institut (AWI) and the Woods Hole Oceanographic Institution (WHOI).

For more information please see the official project website at ANDREX

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: