Metadata Report for BODC Series Reference Number 2024648

• 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

CLASS Cruise JC191 CTD Data Quality Document

Chlorophyll: This was calibrated by originators. Chlorophyll values were frequently negative in all series from this cruise. Any negative values are outside of the parameter limits for this channel and have therefore been flagged.

Transmittance and Attenuance: Some values at the start of 5 of the casts on this cruise suggested instrument problems at the surface, and these values were flagged. One cast in particular (cast 120) shows values consistent with an object becoming stuck in a sensor and then returns to the expected curve, the values in this section have also been flagged.

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.

CLASS Cruise JC191 CTD instrument description

CTD unit and auxiliary sensors

The CTD system used on cruise JC191 was a NOC 24-way stainless steel frame system consisting of a Sea-Bird 911 plus, with a Sea-Bird 32 carousel. The CTD frame was equipped with 20L Niskin bottles from cast 1 and a mixture of 10L and 20L Niskin bottles from cast 31. In addition the CTD was fitted with a number of scientific sensors as detailed in the table below. During the cruise various problems with sensors required them to be replaced on the CTD frame (see Section 1 of the JC191 cruise report). The table below shows which sensors were attached to the CTD frame and on which casts.

The cruise report states that the Altimeter was swapped from Cast 91 onwards however the data files contain the information for the Altimeter which was on Casts 1-92.

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.

WETLabs C-Star transmissometer

This instrument is designed to measure beam transmittance by submersion or with an optional flow tube for pumped applications. It can be used in profiles, moorings or as part of an underway system.

Two models are available, a 25 cm pathlength, which can be built in aluminum or co-polymer, and a 10 cm pathlength with a plastic housing. Both have an analog output, but a digital model is also available.

This instrument has been updated to provide a high resolution RS232 data output, while maintaining the same design and characteristics.

Specifications

Further details are available in the manufacturer's specification sheet or user guide.

CLASS Cruise JC191 BODC CTD data processing

The data arrived at BODC in 134 MSTAR format files representing the CTD casts conducted during cruise JC191. The files contained the downcast data averaged to a 2db pressure grid. The casts were reformatted to BODC's internal netCDF format. The originator indicated that the first choice data was from the secondary sensor so the primary sensor data were removed from the final file. The following table shows the mapping of variables within the MSTAR files to appropriate BODC parameter codes:

The reformatted data were visualised using the in-house EDSERPLO software. Suspect data were marked by adding an appropriate quality control flag, and missing data marked by setting both the data to an appropriate value and setting the quality control flag. In addition any data outside of the allocated range for each parameter have been flagged as null and values set to absent data value for that parameter.

Detailed metadata and documentation were compiled and linked to the data.

CLASS Cruise JC191 Originator's CTD data processing

The following summary information has been taken from the JC191 cruise report (Sanchez-Franks, 2020). For more detailed information refer to the cruise report.

Sampling Strategy

A total of 134 CTD casts were performed during the cruise which aimed to attempt to calculate the transport of water and its constituents along a latitude of 24 degrees N, completing a section across the Atlantic from North America to Africa, beginning and ending at the edge of the continental shelf. In addition to the standard temperature and conductivity sensors, a number of instruments were mounted onto the CTD frame including a fluorometer, transmissometer, dissolved oxygen sensor and an upward and downward looking LADCP. Instruments not pressure rated below 6000m were removed for the duration of the deeper casts (Casts 67 to 70). Some data for certain parameters are, therefore, unavailable for deeper casts.

The CTD computer crashed at the bottom of the downcast on Cast 25. The computer was restarted and logging was made under Cast 26. Before processing these two files were stitched together to form a complete cast, Cast 26.

Data Processing

Raw CTD data were transferred from the Sea-Bird deck unit to a LINUX machine via Sea-Bird software. The binary files are converted using Sea-Bird processing software. The conductivity sensors were advanced by 0.073 seconds which was applied in the deck unit. ALIGNCTD was run to advance the oxygen measurements by 5 seconds, ensuring the calculations of dissolved oxygen concentration are made using measurements from the same parcel of water. CELLTM was run to minimise salinity spiking in steep vertical gradients applying two constants; the thermal anomaly amplitude alpha=0.03 and thermal anomaly time constant 1/ss=7. The ASCII files were converted to MSTAR format, averaging 24Hz data to 1Hz data, and calculating salinity and potential temperature in addition to recording the first choice conductivity-temperature sensor pair for each station.

A correction for downcast-upcast sensor hysteresis was made during the MSTAR processing, applying an algorithm provided by Sea-Bird for oxygen concentration values measured by the SBE 43 sensor.

The algorithm used was as follows:

Oxnew_conc = {(Oxygen_conc (i) + (Oxnew_conc (i-1) x C x D))-(Oxygen_conc (i-1) x C)}/D

where D = 1 + H1 x (exponential(P(i)/H2)-1) and C = exponential(-1 x (Time(i)-Time(i-1))/H3)

and i = indexing variable, P = pressure (db), Time = time (seconds), H1 = amplitude of hysteresis correction function, H2 = function constant, H3 = time constant for hysteresis.

The default hysteresis parameters were [H1 H2 H3] = [-0.033 5000 1450]. Experimentation of the early deep casts found that better agreement could be found when letting H3 vary with depth. Consequently, the following look up table was devised:

Oxygen1: p < 2000: H3 = 1000; 2000 > p: H3 = 3000

Oxygen2: p < 2000: H3 = 1000; 2000 > p: H3 = 3500

Primary oxygen data from cast 1 read very low and the sensor was subsequently changed and this data was excluded from consideration.

Field Calibrations

Temperature

From the initial cast there was a temperature difference between the two sensors of 2mdeg at the surface and a dependence on pressure. One sensor was swapped before Cast 35 and an SBE25 was added to the CTD frame from Cast 41 onwards to help with the calibration of the temperature sensors. The differences were explored by comparing sensors for the entire downcast of groups of stations. This led to the conclusion that the Secondary temperature on later casts was the good sensor and the other sensors were adjusted to agree with it. the following adjustments were made:

Primary sensor on later casts: +0.60 mdeg

Primary sensor on early casts: +0.4 - 0.5*p/1000 mdeg

Secondary sensor on early casts: +2.42 - 0.10*p/1000 mdeg

Conductivity

The two conductivity sensors showed very good agreement throughout and two offsets were applied after comparison with bottle salts:

Primary conductivity sensor: (1 - 0.0003/35)

Secondary conductivity sensor: (1 - 0.0022/35)

The conductivity sensors showed a pressure dependence residual between the CTD sensor salinity and the bottle salinity drawn from the Niskin and was investigated however a definitive identification of the residual was not found. Consequently, adjustments were made to the conductivity sensors according to the following interpolation lookup table:

P = [0 1000 4000 6500]

c = [0 +0.0015 -0.0005 -0.0005]

This factor was then turned into a conductivity scaling factor: fac = 1 + c/35

Oxygen

Following the hysteresis correction applied to the oxygen data, upcast oxygen concentrations for each sensor were calibrated against oxygen concentrations derived from bottle samples. Residuals are calculated and stored as a scaling factor since a change in sensitivity is considered a more likely sensor error than an offset.

As a first step, the median residual is calculated for each cast and then a series of breakpoints is chosen to enable a piecewise linear fit of a scaling factor. The form of the scaling factor is an initial offset and then a series of slopes, continuous at each breakpoint. The coefficients are determined simultaneously from a single least squares fit to the cast median residuals.

The second step is to apply the cast-varying factor and examine remaining residuals in the vertical. Each sensor showed a highly reproducible shape of pressure dependence. This was deemed to be either a genuine response or an artefact of the way the hysteresis coefficients were chosen. As it was highly reproducible, a single shape was chosen for the entire cruise.

For JC191 sensor coefficients and equations applied were as follows:

Sensor 1:

o1rs_s = [0 20 70 122 135];

o1rs_f = [1.0410 1.0410 1.0586 1.0663 1.0726];

deps = [-10 2000 3000 4000 5000 6000 6600];

o1dfac = [0.9835 0.9959 1.0086 1.0170 1.0223 1.0268 1.0268];

Sensor 2:

o2rs_s = [0 20 70 122 135];

o2rs_f = [1.0370 1.0370 1.0671 1.0693 1.0756];

deps = [-10 2000 3000 4000 5000 6000 6600];

o2dfac = [0.9822 0.9996 1.0124 1.0186 1.0219 1.0212 1.0212];

o1rs_i = interp1(o1rs_s,o1rs_f,stn);

o1dfac_p = interp1(deps,o1dfac,press);

alpha = o1rs_i.*o1dfac_p;

beta = 0;

oxyout = alpha.*oxyin + beta;

Note: Exchange sensor 1 terms for sensor 2 terms when applying to sensor 2 data.

After applying both correction factors, the residuals between the CTD and bottle oxygens were all nearly less than 2 umol/kg at depths below 200dbar.

Fluorometer

Bottle samples taken were analysed for pigments Chlorophyll-a and Pheophytin-a. After Station 118 a calibration was determined as follows:

ctd_chl = 1.85 * (ctd_fluro_raw - 0.02)

Transmissometer

The maximum transmittance on each station was generally over 100% so was scaled to place the maximum just below 100%. The scale factors are given in the following table:

References

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

Project Information

Marine LTSS: CLASS (Climate Linked Atlantic Sector Science)

Introduction

CLASS is a five year (2018 to 2023) programme, funded by the Natural Environment Research Council (NERC) and extended until March 2024.

Scientific Rationale

The ocean plays a vital role in sustaining life on planet Earth, providing us with both living resources and climate regulation. The trajectory of anthropogenically driven climate change will be substantially controlled by the ocean due to its absorption of excess heat and carbon from the atmosphere, with consequent impacts on ocean resources that remain poorly understood. In an era of rapid planetary change, expanding global population and intense resource exploitation, it is vital that there are internationally coordinated ocean observing and prediction systems so policy makers can make sound evidence-based decisions about how to manage our interaction with the ocean. CLASS will underpin the UK contribution to these systems, documenting and understanding change in the marine environment, evaluating the impact of climate change and effectiveness of conservation measures and predicting the future evolution of marine environments. Over the five-year period CLASS will enhance the cost-effectiveness of observing systems by migrating them towards cutting edge autonomous technologies and developing new sensors. Finally, CLASS will create effective engagement activities ensuring academic partners have transparent access to NERC marine science capability through graduate training partnerships and access to shipborne, lab based and autonomous facilities, and modelling capabilities.

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.

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.