Metadata Report for BODC Series Reference Number 1017224

• 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.

Oceans 2025 Cruise D346 CTD instrument description

CTD unit and auxiliary sensors

The CTD system used on cruise D346 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. 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 D346 cruise report). The table below shows which sensors were attached to the CTD frame and on which 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.

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.

Chelsea Technologies Group ALPHAtracka and ALPHAtracka II transmissometers

The Chelsea Technologies Group ALPHAtracka (the Mark I) and its successor, the ALPHAtracka II (the Mark II), are both accurate (< 0.3 % fullscale) transmissometers that measure the beam attenuation coefficient at 660 nm. Green (565 nm), yellow (590 nm) and blue (470 nm) wavelength variants are available on special order.

The instrument consists of a Transmitter/Reference Assembly and a Detector Assembly aligned and spaced apart by an open support frame. The housing and frame are both manufactured in titanium and are pressure rated to 6000 m depth.

The Transmitter/Reference housing is sealed by an end cap. Inside the housing an LED light source emits a collimated beam through a sealed window. The Detector housing is also sealed by an end cap. A signal photodiode is placed behind a sealed window to receive the collimated beam from the Transmitter.

The primary difference between the ALPHAtracka and ALPHAtracka II is that the Alphatracka II is implemented with surface-mount technology; this has enabled a much smaller diameter pressure housing to be used while retaining exactly the same optical train as in the Mark I. Data from the Mark II version are thus fully compatible with that already obtained with the Mark I. The performance of the Mark II is further enhanced by two electronic developments from Chelsea Technologies Group - firstly, all items are locked in a signal nulling loop of near infinite gain and, secondly, the signal output linearity is inherently defined by digital circuitry only.

Among other advantages noted above, these features ensure that the optical intensity of the Mark II, indicated by the output voltage, is accurately represented by a straight line interpolation between a reading near full-scale under known conditions and a zero reading when blanked off.

For optimum measurements in a wide range of environmental conditions, the Mark I and Mark II are available in 5 cm, 10 cm and 25 cm path length versions. Output is default factory set to 2.5 volts but can be adjusted to 5 volts on request.

Further details about the Mark II instrument are available from the Chelsea Technologies Group ALPHAtrackaII specification sheet.

Oceans 2025 Cruise D346 BODC CTD data processing

The data arrived at BODC in 137 MSTAR format files representing the CTD casts conducted during cruise D346. The data contained in the files are the downcast data averaged to a 2db pressure grid. The casts were reformatted to BODC's internal netCDF format. 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.

Oceans 2025 Cruise D346 Originator's CTD data processing

Sampling Strategy

A total of 137 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°N, completing a section across the Atlantic from North America to Africa, beginning and ending at the edge of the continental shelf. Of the 137 CTD stations occupied, 2 (Casts 200 and 202) were bottle blank stations run for the CFC team, so the principle data collected along the 24°N section comprised of 135 stations. In addition a number of instruments were mounted onto the CTD frame including an LADCP, fluorometer, transmissometer and a dissolved oxygen sensor. Instruments not pressure rated below 6000m were removed for the duration of the deeper casts (Casts 64-69 and 200). Some data for certain parameters are, therefore, unavailable for deeper casts. The chronological order of the casts were as follows: 1-64, 200, 65-100, 202, 101-135.

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 (SBE Data Processing v7.19). 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 (-0.028 for the first oxygen sensor and 0.037 for the second), H2 = function constant (5000 for both oxygen sensors), H3 = time constant for hysteresis (2500 for the first and 1450 for the second oxygen sensor).

In addition a number of minor spurious features were identified in the oxygen and conductivity channels, including spikes associated with CTD telemetry failures and spikes at the beginning and end of a cast where bad start and end scan numbers were chosen. Such problems were resolved during processing by adjusting the start and end scan numbers of files to omit bad data or by removing spikes using a median de-spiking routine.

Further details of this processing routine can be found in the D346 cruise report.

Field Calibrations

Upcast conductivity from the first choice sensor was calibrated against conductivity derived from bottle samples. The final calibration ratios applied to the secondary, second primary and third primary conductivity sensors were 0.9999719, 1.0000574 and 1.0000285, respectively. In addition bottle-CTD conductivity residuals showed some structure against pressure believed to be related to sensor performance. Calibrations were, therefore, applied by correcting conductivity values using an additive factor decided by a pressure lookup table. This table was created for each sensor by calculating median offset in pressure bins. Application of the calibration ratios and pressure corrections reduced rms offset of salinity offset from 0.00128, 0.00238 and 0.00202 to 0.00073, 0.00119 and 0.00064 for the secondary, second primary and third primary conductivity sensors, respectively.

Following the hysteresis correction applied to the oxygen data, upcast oxygen concentrations for each sensor were calibrated against oxygen concentrations derived from bottle samples. For the first oxygen sensor, bottle and CTD oxygen showed a clear linear relationship. Subsequently a multiplicative correction factor, calculated as the median ratio between bottle and CTD oxygen, was applied. This was calculated and applied in bulk for stations 1-22. For stations 23-36 the correction was calculated and applied on a station by station basis to account for the gradual degradation in sensor performance over this period. For the second oxygen sensor, bottle oxygen and CTD oxygen concentration offset was typically > 10µmol/kg, however, a linear relationship was less obvious. Bottle-CTD offset was reduced to ±2µmol/kg by applying a combined second order pressure and first order temperature dependent offset to all data with a potential temperature <7.5°C with the pressure-temperature offset function calculated using a least-squares approach. For data with potential temperature >7.5°C, a simple offset of 7µmol/kg was added.

Project Information

Oceans 2025 - The NERC Marine Centres' Strategic Research Programme 2007-2012

Who funds the programme?

The Natural Environment Research Council (NERC) funds the Oceans 2025 programme, which was originally planned in the context of NERC's 2002-2007 strategy and later realigned to NERC's subsequent strategy (Next Generation Science for Planet Earth; NERC 2007).

Who is involved in the programme?

The Oceans 2025 programme was designed by and is to be implemented through seven leading UK marine centres. The marine centres work together in coordination and are also supported by cooperation and input from government bodies, universities and other partners. The seven marine centres are:

• National Oceanography Centre, Southampton (NOCS)
• Plymouth Marine Laboratory (PML)
• Marine Biological Association (MBA)
• Sir Alister Hardy Foundation for Marine Science (SAHFOS)
• Proudman Oceanographic Laboratory (POL)
• Scottish Association for Marine Science (SAMS)
• Sea Mammal Research Unit (SMRU)

Oceans2025 provides funding to three national marine facilities, which provide services to the wider UK marine community, in addition to the Oceans 2025 community. These facilities are:

• British Oceanographic Data Centre (BODC), hosted at POL
• Permanent Service for Mean Sea Level (PSMSL), hosted at POL
• Culture Collection of Algae and Protozoa (CCAP), hosted at SAMS

The NERC-run Strategic Ocean Funding Initiative (SOFI) provides additional support to the programme by funding additional research projects and studentships that closely complement the Oceans 2025 programme, primarily through universities.

What is the programme about?

Oceans 2025 sets out to address some key challenges that face the UK as a result of a changing marine environment. The research funded through the programme sets out to increase understanding of the size, nature and impacts of these changes, with the aim to:

• improve knowledge of how the seas behave, not just now but in the future;
• help assess what that might mean for the Earth system and for society;
• assist in developing sustainable solutions for the management of marine resources for future generations;
• enhance the research capabilities and facilities available for UK marine science.

In order to address these aims there are nine science themes supported by the Oceans 2025 programme:

• Climate, circulation and sea level (Theme 1)
• Marine biogeochemical cycles (Theme 2)
• Shelf and coastal processes (Theme 3)
• Biodiversity and ecosystem functioning (Theme 4)
• Continental margins and deep ocean (Theme 5)
• Sustainable marine resources (Theme 6)
• Technology development (Theme 8)
• Next generation ocean prediction (Theme 9)
• Integration of sustained observations in the marine environment (Theme 10)

In the original programme proposal there was a theme on health and human impacts (Theme 7). The elements of this Theme have subsequently been included in Themes 3 and 9.

When is the programme active?

The programme started in April 2007 with funding for 5 years.

Brief summary of the programme fieldwork/data

Programme fieldwork and data collection are to be achieved through:

• physical, biological and chemical parameters sampling throughout the North and South Atlantic during collaborative research cruises aboard NERC's research vessels RRS Discovery, RRS James Cook and RRS James Clark Ross;
• the Continuous Plankton Recorder being deployed by SAHFOS in the North Atlantic and North Pacific on 'ships of opportunity';
• physical parameters measured and relayed in near real-time by fixed moorings and ARGO floats;
• coastal and shelf sea observatory data (Liverpool Bay Coastal Observatory (LBCO) and Western Channel Observatory (WCO)) using the RV Prince Madog and RV Quest.

The data is to be fed into models for validation and future projections. Greater detail can be found in the Theme documents.

Oceans 2025 Theme 1, Work Package 1.2: Atlantic Circulation and Transports

This Work Package is run by the National Oceanography Centre, Southampton (NOCS) and aims to establish whether the Atlantic Meridional Overturning Circulation (MOC) is slowing down, and to relate this to changes in regional storage of heat, freshwater and carbon.

More detailed information on this Work Package is available at pages 8 - 10 of the official Oceans 2025 Theme 1 document: Oceans 2025 Theme 1

Weblink: http://www.oceans2025.org/

Oceans 2025 Theme 10, Sustained Observation Activity 3: Monitoring the Atlantic Meridional Overturning Circulation

The Meridional Overturning Circulation (MOC) is the key component of ocean circulation in the Atlantic responding to climate change. Under the NERC directed programme Rapid Climate Change (RAPID) two arrays of instruments have been positioned in the ocean to monitor the MOC, one at 26.5 °N to capture the southward flow at depth, and the Western Atlantic Variability Experiment (WAVE) array across the Canadian-US continental slope, closer to the presumed source variations. Sustained Observation Activity (SO) 3 will continue these measurements to observe the strength and structure of the MOC, working closely with Oceans 2025 Theme 1 and the RAPID-WATCH directed programme.

There are two elements to this SO activity. SO 3.1. relates to the E-W mooring array in the mid-North Atlantic at 26.5 °N while SO 3.2. is concerned with the Western Atlantic array between Cape Cod and the Grand Banks.

Aims and Purpose of SO 3.1.

• The purpose of the observing system is to measure and understand the current state and variability of the MOC. The results will help assess the risks of rapid climate change from a slow down in the MOC, by providing the necessary data for coupled climate models to predict future climate. This work will be carried out by the National Oceanography Centre, Southampton (NOCS).

Aims and Purpose of SO 3.2.

• The overall aim is to maintain and service the WAVE monitoring array, to measure changes in the North Atlantic thermohaline circulation along the western boundary of the North Atlantic. This array complements the NOC 26.5 °N meridional section by providing information from north of the Gulf Stream, closer to the presumed source of variations in MOC. This work will be supported by the Proudman Oceanographic Laboratory (POL).

More detailed information on this Work Package is available at pages 11 - 14 of the official Oceans 2025 Theme 10 document: Oceans 2025 Theme 10

Weblink: http://www.oceans2025.org/

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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