Metadata Report for BODC Series Reference Number 1102251

• 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.

Instrument Description

CTD Unit and Auxiliary Sensors

A Sea-Bird SBE 911plus CTD with dual pumped temperature and conductivity sensors and Sea-Bird SBE32 carousel comprising twenty-four OTE externally sprung 20 litre Niskin bottles were used for all CTD casts.

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.

BODC Processing

Twenty six CTD profiles were provided as CTD01 was a test cast. The data were received in a structured matlab format and converted into BODC internal format (QXF). Potential temperature of the water body was derived by computation using UNESCO 1983 algorithm, sigma-theta was derived by computation from salinity and potential temperature also using UNESCO 1983 algorithm. The following table shows how the variables within the matlab file were mapped to appropriate BODC parameter codes:

TOKGPR01 was generated at BODC to convert units of kg^-1 to l^-1.

TOKGPR01=1000/(sigma-theta+1000)

where sigma-theta = potential density based on salinity, potential temperature and a reference pressure of 0 dbar (Fofonoff and Millard (1983)). 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. Overall no quality issues with very few flags added.

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.

Originator's Data Processing

Sampling Strategy

Twenty seven CTD profiles including a test cast (CTD01) were performed during the cruise. Unfortunately, due to a medical evacuation, the cruise was aborted on 14/01/2009 after station 27 to return to Port Stanley, Falkland Islands. ANDREX cruise JR239 which took place between 19/03/2010 and 24/05/2010 aimed to complete the rest of the stations intended for this cruise.

Three stations at the start of the section occupied WOCE I06S, in addition several cast occupied WOCE SR04.

Data processing

Initial data processing of the raw data files was performed using Sea-Bird processing software ( SBE.DataProcessing-Win32: v.7.18). Firstly, the raw data were converted into physical units using 'DatCnv'. 'AlignCTD' was then used to to shift the oxygen sensor relative to the pressure data by 5 seconds, compensating for lags in the sensor response time. A de-spiking routine 'WildEdit' scanned the data twice, calculating the standard deviation of a set number of scans, and setting values that were outside a set number of standard deviations (sd) of the mean, to bad data values. The effect of thermal 'inertia' on the conductivity cells was then removed using the routine 'CellTM'. Lastly, the data files were converted from binary into ASCII format so that they could be read into Pstar format.

After the Sea-Bird processing, further processing was undertaken using the National Oceanography Centre's Pstar programs. Firstly, the ASCII output was converted to Pstar and the headers were set, with the information for the header extracted from the Sea-Bird ASCII file where possible (script ctd0). Residual spikes from all of the variables were removed ('pmdian'), the data were averaged into a 1Hz file ('pavrge'), absent data values in the pressure data were interpolated ('pintrp'), salinity, potential temperature, sigma0 and sigma2 (referenced to 2000 dbar) were calculated ('peos83') using and a 10 second averaged file was also created (script ctd1). The head and tail of the 1hz file was then cropped to select the relevant data cycles for just the up and down casts of the CTD, the data were averaged into 2 dbar pressure bins and position information was extracted for the start data cycle of the downcast file and written to the header (script ctd2).

The bottle salinity data were then processed, the files were formatted to Pstar and conductivity calculated. These were used in order to calibrate the CTD conductivities.

Field Calibrations

Salinity

The CTD conductivities were calibrated against CTD bottle conductivities. CTD conductivity calibrations were determined by calculating the conductivity differences for both CTD sensors (bottle minus CTD). For CTD conductivity sensor 1 and the 145 bottle values in the range (-0.005, 0.005 mmho cm^-1), the mean difference was -0.0004 (sd 0.0013). For CTD conductivity sensor2 and the 145 bottle values in the range (-0.005, 0.005 mmho cm^-1), the mean difference was -0.0009 (sd 0.0013). CTD conductivities were adjusted with these offsets, and salinities recalculated.

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.

Oxygen

CTD oxygens were calibrated against bottle oxygens using an offset and the pressure-dependent term, O_bottle - O_CTD = a + bP where a and b are offset and slope parameters of the linear fit. The differences between bottle oxygen concentration (O_bottle) and equivalent downcast CTD oxygen concentration (O_CTD) were calculated and, after the removal of outliers, regressed as a linear function of pressure using the equation above. Taking O_bottle to represent the true value of seawater oxygen concentration and assuming this fit adequately explains the remainder of the data points, the equation was employed to calibrate the downcast CTD oxygen profiles to 'true' oxygen concentrations. The values of a and b were found to be 6.3932 and 0.0015837, respectively, and CTD oxygens adjusted accordingly.

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: