Metadata Report for BODC Series Reference Number 1058893

• 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

RAPID Cruise D279 CTD Data Quality Report

General data quality

Frequent looping occurred in the 1 Hz data, probably as a consequence of ship's heave. In cases where looping was particularly severe, flags were assigned to the data values. As flagged data are excluded from the binning process, some profiles required interpolation across the bin. Interpolated values are indicated by the presence of a flag alongside the data value.

A significant number of data cycles were excluded from the beginning of the downcasts where the CTD was logging on deck.

It should also be noted that the data originator specified a preference for the primary salinity and temperature data.

Salinity

All primary salinity data were calibrated against salinity samples. Data from the secondary sensor were not calibrated independently and consequently are not deemed to have the same reliability. Therefore the data from the secondary sensor has been excluded from the series. Secondary salinity was not logged during Casts 3 - 6.

Temperature

Temperature for both channels was checked against the independent Sea-Bird 35 deep ocean standard temperature sensor. Secondary temperature was not logged during Casts 3 - 6. Data from the secondary sensor were excluded from the series as the data originator expressed a preference for data from the primary sensor.

Pressure

Spikes occurred in the pressure channel of Cast 1, which were flagged suspect.

Beam attenuation

Beam attenuation was not measured for Cast 25 to Cast 125, inclusive.For Casts 1 to 24, when the transmissometer was logging, it is not known whether calibration air readings were made, so the data are qualitative at best. On screening the casts, it was noticed that the profiles were not typical for the deep ocean and did not display the expected mid-water minimum or profile shape. Additionally, the values appear too high in most cases. Taking all of this into account, the data are likely to be unreliable and should be used with caution.

Dissolved oxygen

Due to problems encountered calibrating this sensor, including the discovery of a torn sensor membrane post-cruise, the data originator considers this whole data set suspect. Oxygen was not recorded during Casts 2 to 5 and Cast 7. Cast 6 logged negative values throughout.

Chlorophyll-a

Casts 23 and 25 have no chlorophyll channel. Several casts returned bad data in parts of the profile, resulting in null portions of the profile as follows: Casts 31, 32 and 35 in top 200 m; Cast 33 between 700 and 950 m; Casts 37, 46, 48 and 50 after 4000 m.

Several casts contain slightly negative concentrations, which is due to the presence of a small offset or error in the calibration applied and affects those casts where very low concentrations of chlorophyll are present. The casts affected include Cast 18, Cast 20, Cast 24, Casts 31 and 32, Casts 37 to 115, and Casts 117 to 120. All negative values have been flagged, where appropriate.

Data Access Policy

Open 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.

If the Information Provider does not provide a specific attribution statement, or if you are using Information from several Information Providers and multiple attributions are not practical in your product or application, you may consider using the following:

"Contains public sector information licensed under the Open Government Licence v1.0."

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.

RAPID Cruise D279 CTD Instrumentation

The CTD unit was a Sea-Bird Electronics 911plus system with dual temperature and conductivity sensors plus oxygen sensor, fluorometer, transmissometer, optical backscatter and altimeter. Also attached to the frame were 3 RDI ADCPs. Details of these sensors are documented in the table below. The light backscattering sensor was disconnected after Cast 23, and no output from this instrument or the ADCPs was included in the final data set.

CTD package:

Sampling device

The Sea-Bird 24 position Carousel was equipped with 10 litre sampling bottles, manufactured by Ocean Test Equipment Inc.

More than 2600 salinity samples from the CTD were analysed during the cruise using a Guildline 8400B salinometer.

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.

RAPID Cruise D279 CTD Processing

Sampling Strategy

A total of 125 full depth CTD casts were performed during the cruise. Rosette bottles were fired at regular intervals throughout each profile in order to obtain salinity samples for calibration. In addition, samples were taken from the bottles for analysis of dissolved oxygen, nitrate, silicate, phosphate, CFCs, carbon tetrachloride, total alkalinity and partial pressure of carbon dioxide.

On Cast 17 the winch scrolling gear follower failed with the CTD 26 m off the sea bed. The CTD touched the bottom and had to be towed to deeper water. The duration of this cast was significantly longer than expected as a consequence.

Sea-Bird processing by data originator

The raw CTD files were processed manually through Sea-Bird SBE Data Processing software. 24 Hz binary (.DAT) files were converted to engineering units and nominal values using manufacturer's calibration coefficients (DATCNV). ALIGN CTD was run to advance oxygen by 5 seconds relative to pressure. Coefficients for temperature and conductivity were set to 0, as advancement of these parameters was carried out by the deck unit. The WILD EDIT function was subsequently used to reduce the amount of noise in all CTD profiles. To compensate for conductivity cell thermal mass effects, the files were run through CELLTM, using alpha = 0.03, 1/beta = 7, typical values for this CTD model given in the Sea-Bird literature. The final stage of Sea-Bird processing carried out was TRANSLATE, which generates ASCII versions of the binary .CNV data files.

PSTAR processing by data originator

After initial processing with Sea-Bird software, additional routines were applied to the 24 Hz files in PSTAR and subsequently worked through to the 1 Hz and 10 s versions. Bottle salinity data from the CTD upcast were used to calibrate the primary CTD conductivity channel. Bottle conductivities were re-calculated from bottle salinities and compared with CTD conductivities from the time the bottles were fired. A slope correction was applied to the sensor, as a result of this investigation, to account for sensor drift. The calibration data set excluded samples where (Bottle - CTD conductivity) exceeded specified limits.

Further analysis, following slope correction, revealed a pressure dependence in (Bottle - CTD conductivity) during Casts 38 - 93 and 94 - 120. The pressure dependence in the former set of casts was addressed by favouring data from the secondary sensors, and through application of linear and quadratic pressure fits to the upper and lower water column respectively. The latter set of casts were adjusted by application of pressure fits alone, without the swapping of sensor data.

The final stage of calibration was the addition of a station by station salinity offset to the primary CTD salinity channel, following removal of sample data outliers. No salinity correction was applied to Cast 119 due to significant scatter in the (Bottle - CTD conductivity) values.

BODC post-processing and screening

Reformatting

The 1 Hz version of the data were converted from PSTAR into BODC internal format (QXF) to allow use of in-house visualisation tools. In addition to reformatting, the transfer program (tr360) converted dissolved oxygen from µmol kg^-1 to µmol l^-1. The following table shows how the variables within the original PSTAR file were mapped to appropriate parameter codes;

Screening

Reformatted CTD data were transferred onto a graphics workstation for visualisation using the in-house editor EDSERPLO. Downcasts and upcasts were differentiated and the limits flagged. No data vales were edited or deleted.

Banking

Once BODC quality control screening was complete, the CTD downcasts were loaded into BODC's ocean database under the ORACLE Relational Database Management System.At this stage, the data were binned to 2 dbar. Temperature and salinity data from the secondary sensors were excluded from the final data series as the data originator expressed a preference for data from the primary sensors.

Project Information

No Project Information held for the Series

Data Activity or Cruise Information

Cruise

Complete Cruise Metadata Report is available here

Fixed Station Information

Fixed Station Information

24.5 degrees N Hydrographic Transatlantic section - A05

The hydrographic transatlantic section at 24.5 degrees North has been occupied 8 times, namely in 1957, 1981, 1992, 1998, 2004, 2010, 2015 and 2020. The section is an attempt to calculate the transports of water and its constituents (heat, salinity, nutrients, biological productivity) along this latitude, and to compare the data so that the degree of change in the North Atlantic transports can be revealed. The Hydrographic Programme of the international World Ocean Circulation Experiment (WOCE) designated this section as A05. Since WOCE, the section occupations contribute to the Global Ocean Ship-Based Hydrographic Investigations Program (GO-SHIP).

Map of stations

The station positions for most of the occupied sections are shown in the above figures. Sections occupied in 1957, 1981, 1992, 1998 and 2004 are shown in the upper figure. The section occupied in 2010 is shown in the lower figure.

The 1957 and 1992 sections each went zonally along 24.5 degrees N from the African coast to the Bahama Islands. Because of diplomatic clearance issues, the 1981, 1998, 2004, 2010 and 2015 sections angled southwestward from the African coast at about 28 degrees N to join the 24.5 degrees N section at about 23 degrees W. The 1998, 2004 and 2010 sections angled northwestward at about 73 degrees W to finish the section along 26.5 degrees N. The 2010 and 2020 sections deviated from the previous sections across the Mid-Atlantic Ridge (MAR) in order to sample the deepest channel and capture the passage of the deep water across the MAR.

Table 1: Occupations of the section

References

Baringer, M. O. N. and R. L. Molinari, 1999. Atlantic Ocean baroclinic heat flux at 24 to 26°N. Geophysical Research Letters, 26, 353-356.

Bryden, H. L., H. R. Longworth, and S. Cunningham, 2005b. Slowing of the Atlantic meridional overturning circulation at 25°N. Nature, 438, 655-657.

Fuglister, F. C., 1960. Atlantic Ocean Atlas of Temperature and Salinity Profiles and Data from the International Geophysical Year of 1957-1958. Vol. 1, Woods Hole Oceanographic Institution Atlas Series, WHOI, Woods Hole, Massachusetts.

King, B. A. et al., 2012. RRS Discovery Cruise 346, 05 Jan-19 Feb 2010. The 2010 transatlantic hydrography section at 24.5N. Southampton, UK, National Oceanography Centre Southampton, 177pp. (National Oceanography Centre Cruise Report, No. 16).

King, B. A. et al., 2016. RRS Discovery Cruise DY040 (RAGNARoCC), 06 Dec 2015 - 22 Jan 2016. The 2015 transatlantic hydrography section at 24.5N. Southampton, UK, National Oceanography Centre Southampton, 159pp. (National Oceanography Centre Cruise Report).

Parilla, G., A.Lavin, H. Bryden, M. Garcia, and R. Millard, 1994. Rising temperatures in the subtropical North Atlantic Ocean over the past 35 years. Nature, 369, 48-51.

Roemmich, D. and C. Wunsch, 1985. Two transatlantic sections: meridional circulation and heat flux in the subtropical North Atlantic Ocean. Deep Sea Research, 32, 619-664.

Sanchez-Franks, A., 2020. RRS James Cook Cruise JC191, 19 January - 1 March 2020. Hydrographic sections from the Florida Straits to the Canaries Current across 24N in the Atlantic Ocean. Southampton, UK, National Oceanography Centre Southampton, 262pp. (National Oceanography Centre Cruise Report, No. 70).

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: 24.5 degrees N Hydrographic Transatlantic section - A05

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

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