Metadata Report for BODC Series Reference Number 672938

• 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.

CD171 CTD Instrument Description

Sampling Strategy

144 CTD casts were completed on the cruise utilising a 24-way frame arrangement with the following configuration.

The following sensor changes were made during the cruise

Cast 001 - The RDI Workhorse (upward looking) LADCP flooded and was removed from the frame.

Cast 003 - Altimeter not working, replaced with Benthos PSA-916T, serial # 1040

Cast 005 - Shift in secondary conductivity sensor, replaced with serial # 04C-3054

Cast 037 - Scattering meter not working properly, replaced with serial # BBRTD-182

Cast 039 - Failed connector on BreakOut Box, scattering meter output changed from V6 to V4

Cast 055 - BreakOut Box leaked under pressure, replaced with serial # BO19109T

Cast 055 - Change in 'BOB' connector configuration resulted in fluorometer output change from V3 to V6

Cast 059 - Change altimeter output from V2 to V3

Cast 067 - Noise/spiking in secondary conductivity sensor, changed to serial # 04C-3502

Cast 077 - Continual noise/spiking in secondary conductivity sensor, changed back to serial # 04C-3054

Cast 113 - Jump in secondary conductivity sensor, replaced with serial # 04C-3052

Cast 119 - Replaced BreakOut Box with serial # BO19110 for testing purposes; altimeter output changed from V3 to V2

For further details please see the cruise report

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.

WETLabs Single-angle Backscattering Meter ECO BB

An optical scattering sensor that measures scattering at 117°. This angle was determined as a minimum convergence point for variations in the volume scattering function induced by suspended materials and water. The measured signal is less determined by the type and size of the materials in the water and is more directly correlated to their concentration.

Several versions are available, with minor differences in their specifications:

• ECO BB(RT)provides analog or RS-232 serial output with 4000 count range
• ECO BB(RT)D adds the possibility of being deployed in depths up to 6000 m while keeping the capabilities of ECO BB(RT)
• ECO BB provides the capabilities of ECO BB(RT) with periodic sampling
• ECO BBB is similar to ECO BB but with internal batteries for autonomous operation
• ECO BBS is similar to ECO BB but with an integrated anti-fouling bio-wiper
• ECO BBSB has the capabilities of ECO BBS but with internal batteries for autonomous operation

Specifications

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

Tritech Digital Precision Altimeter PA200

This altimeter is a sonar ranging device that gives the height above the sea bed when mounted vertically. When mounted in any other attitude the sensor provides a subsea distance. It can be configured to operate on its own or under control from an external unit and can be supplied with simultaneous analogue and digital outputs, allowing them to interface to PC devices, data loggers, telemetry systems and multiplexers.

These instruments can be supplied with different housings, stainless steel, plastic and aluminum, which will limit the depth rating. There are three models available: the PA200-20S, PA200-10L and the PA500-6S, whose transducer options differ slightly.

Specifications

Common specifications are presented below

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

CD171 CTD BODC Processing

The data were received in Pstar format 1hz, ctu and 2db files for 144 casts.

The 2db files were selected and transferred (using transfer trn360) to QXF format, a BODC-defined subset of NetCDF and BODC's format for 2-dimensional data storage. Null data were set to the appropriate absent data values for the code in the BODC parameter dictionary and flagged 'N', data outside parameter dictionary range flagged 'M' and already flagged data given an 'L' flag.

Transfer Mapping

The following table shows a summary of the variables.

Other parameters were derived during the transfer of the data

Depth in the water column was calculated by combining pressure and altimetry data. To calculate the maximum and minimum depth of the CTD casts, the pressure sensor maximum and minimum (db) were converted to depth (m) via the Matlab script 'ptodep' using the Saunders and Fofonoff method. Total water column depth was then calculated by adding the maximum depth value calculated to the corresponding altimeter height above the seabed.

Saunders, P.M. / Fofonoff, N.P., Deep Sea Research and Oceanographic Abstracts, 23 (1), p.109-111, Jan 1976.

Quality Control and Screening

The reformatted data were visualised using the in house EDSERPLO software, with no further quality control required. Following the data audit secondary channels were dropped prior the banking.

CD171 CTD Originators Calibration

Temperature Calibration

The SBE35 Deep Ocean Standards Thermometer fitted to the CTD fin provides a reference for evaluating performance of the primary and secondary CTD temperature sensors. At each bottle fire, 8 cycles of SBE35 data are averaged and temperature computed.

Below 2000 dbar the spatial and temporal variability of water properties were anticipated to have only a small effect on the temperature difference between the SBE35 and CTD sensor. The variations were found to be within the nominal accuracy of the CTD sensor and therefore no attempt at calibration of either CTD sensor was made.

Further explanation and detail can be found in the cruise report below.

Conductivity Calibration

CTD conductivities are calibrated by comparing them to bottle conductivities derived from salinity samples obtained during the CTD upcast. The CTD upcast is calibrated and applied to the downcast.

A slope correction (K) was applied to account for sensor drift -

C_corr = K*C_CTD

where K = C_bot/C_CTD, C_bot = bottle conductivity and C_CTD = upcast CTD conductivity for the 10s of the bottle fire.

To compute K, only bottle salinities flagged as good were used. C_diff = C_bot - C_CTD and C_ratio = C_bot/C_CTD were computed and samples not satisfying |C_diff| < 0.1 and 0.9998 < C_ratio < 1.0002 were rejected. The station K value was the mean C_ratio of the remaining samples.

This procedure removed the influence of outlying bottle salinities resulting from poor sample collection, analysis or the effect of spatial and temporal variabilty in water properties at the time of bottle firing. Under the assumption that deep ocean properties are stable, the calibration was not varied on a station to station basis, instead a mean K was taken.

Further explanation and detail of this calibration can be found in the cruise report below.

Oxygen Calibration

The CTD oxygen calibration was reworked at the National Oceanograhy Centre, Southampton (NOCS) after the cruise. CTD oxygen data were compared with bottle sample oxygen values in units of µmol/kg, to attempt to produce a set of CTD downcast oxygen profiles which were in agreement with the bottle samples collected on the upcast.

An initial bulk calibration was estimated. It's form was a scaling of the oxygen concentration, plus a fit to residuals of an offset and a dependence on pressure and potential temperature. Thus-

O_{ctd_bulk} = 1.2 * O_{ctd_raw} - 0.0042 * P - 0.50 * θ - 15.8 (µmol/kg)

The bulk calibration was applied to all stations, and residuals between bottle samples and matched CTD cycles were calculated.

The residuals were examined on a station by station basis. For each bottle -

R₁ = O_samp - O_{ctd_bulk}

For each station, a scaling factor B_n was determined by a least squares fit to the residuals. The further set of residuals (R₂ = R₁ - B_n * O_{ctd_bulk}) was examined to determine whether each bottle was to be included in the determinationof B_n for that station. Iterative visual inspection and exclusion of large values of R₂ was performed for each station until a satisfactory determination of B_n was made.

For further details please see the cruise report

CD171 CTD Originators Processing

Sampling Strategy

144 CTD casts were completed on the cruise utilising a 24-way frame arrangement.

Data Processing

Raw CTD files from the logging PC were transferred to a networked PC on which SEASOFT modules were run manually and the output files transferred to the UNIX system. The processing included the following steps -

Sea-Bird Processing

• Data Conversion - to Pstar format
• Align CTD - alignment of data streams to reduce spiking or hysteresis
• Wild Edit - mean and standard deviation calculated for flagging of outlying data values. Applied to temperature, conductivity pressure temperature, oxygen, transmission and altimeter height data.
• Cell Thermal Mass - to remove conductivity thermal mass effects with a recursive filter (permitting salinity accuracy > 0.01 in regions of steep gradients).
• Filter - low pass pressure filter
• Translate - to create an ASCII version of the binary file.

Pstar Processing

CTD Data Files

The following is a summary of the routines used to process CTD data from resolution of 24hz to 2db and annotate the header accordingly.

• ctd0 - writes 24hz ASCII data into pstar format. Header information extracted and position manually input from deck logs.
• ctd1 - in the deck unit , fin sensor data is input through the secondary channel and frame sensor data through the primary. When possible fin sensors are used as the primary and temp/temp2 and cond/cond2 names are swapped accordingly. Data was averaged into 1s and from this 10s intervals. Additional variables are derived; t2-t1, c2-c1, salin, salin2, potemp, potemp2, sigma0 and sigma2.
• ctd2 - the 1hz data cycles from the downcast start to upcast end output. The 2db file is obtained from the the downcast cycles sorted on pressure and missing levels filled with linear interpolation. Individual cycles from the start of the downcast, at the maximum pressure and at the upcast are output.
• fir0 - writes ASCII file into pstar format. CTD variables at the bottle stops are extracted by merging on time. Winch data is read into pstar.
• sam0 - CTD data are pasted into a copy of the master file and sample data subsequently added.
• add_positions.exec - GPS latitude and longitude are merged with the CTD times at the start of the downcast, maximum pressure and end of upcast. The corrected position (relative to deck log) is written into headers of the 24hz, 1hz, 10s, ctu, 2db and sam files.
• add_ladcpdepth.exec - maximum instrument depth determined from maximum CTD pressure and water depth from Visbeck processing of LADCP data. Both are written headers of the same files.
• add_simdepth.exec - water depth extracted from the simrad record if the cast was not deep enough for the LADCP to resolve water depth.

Data Treatment

Due to significant wake effects in the frame temperature and conductivity sensor pair due to the 20 litre Niskin bottles, the fin sensor pair are preferentially used as the primary data channels. However, numerous casts before station 80 had unsatisfactory noise from the fin sensors. Where spikes in the data could not be edited out due to loss of property resolution, the fin sensor data were substituted (incorporating sensor offset) over either the affected cycles or used as the primary over the full cast. See the cruise report below for full details of these substitutions.

For further details please see the cruise report

Project Information

36 North Project

Project Overview

The 36 North project aims to investigate and understand the influences on global climate regulation, with a focus on the current role of the North Atlantic subtropical gyre.

It will seek to further understand the mechanisms which control the heat transport, nutrient and carbon budgets of the North Atlantic Ocean. This will impact on our understanding of how the ocean is warming, phytoplankton growth and how oceans uptake CO2.

This project brings together scientists from the National Oceanography Centre, Southampton (NOCS), the University of Liverpool, the Plymouth Marine Laboratory (PML) and the University of East Anglia (UEA). It will run from 1 October 2004 to 30 September 2008 and is funded by a Natural Environment Research Council consortium grant.

Aims

The main aims include:

• To examine how nitrogen, phosphorous and carbon bugets are closed.
• To examine how nutrient and carbon budgets are controlled.
• To assess the heat flux across the 36 N section (and poleward using World Climate Circulation Experiment (WOCE) data).

Methods

This will be achieved by:

• A detailed, hydrographic survey along 36N with a complete biogeochemical analysis of data collected during RRS Charles Darwin cruise 1 May - 15 June 2005 (CD171).
• The development and application of inverse and coupled biogeochemical, isopycnic circulation models.
• Comparison with historical data collected by RRS Discovery during 4 April - 10 May 2004 (D279).
• Comparison with three additional North Atlantic cruises undertaken in 2004/2005, funded under the Rapid Climate Change (RAPID) and Atlantic Meridional Transect (AMT) programmes.

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: