Metadata Report for BODC Series Reference Number 1373807

• 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 Quality Report

Both altimeters had poor at bottom detection so were not transferred. The altimeter connector had leaked due to being badly fitted onto the CTD breakout box. This connector, along with the BBRTD light scattering sensor connector which had also leaked was replaced at the end of the cruise.

The stainless system fluorometer had a persistent depth related noise problem around 80 to 200 metres, as stated in the cruise report, these have been flagged by BODC. Changes to the sensor, cable and data channel failed to cure the fault during cruise D285. There appeared to be no correlation between this and other instrument operation. The fluorometer problem was investigated again during D286 but no cause was found. The fluorometer along with the attenuance due to backscatter on both CTDs had many negative values which were also flagged.

There were only data avaialble from the PAR sensor for 10 CTD casts (1554, 15560, 15566, 15574, 15580, 15590, 15603, 15604, 15620 and 15623) the other CTD casts had zero values.

Data Access Policy

Public domain 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.

The recommended acknowledgment is

"This study uses data from the data source/organisation/programme, provided by the British Oceanographic Data Centre and funded by the funding body."

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

Two CTD systems were used during the cruise, a standard stainless steel CTD with aluminium, titanium and plastic instrument housings, used for physics and biology sampling, and a titanium CTD with plastic construction, used for iron sampling. The instruments were very similar, consisting of Seabird 911+ CTD with dual C/T and oxygen sensors. Auxiliary sensors included a transmissometer, fluorometer and light backscatter sensor. The primary conductivity sensor 1 was on the outside of the frame whilst the backup sensor from conductivity sensor 2 was inside the frame and so may have been affected by trapped water pulled along with the large CTD frame.

Additional instruments on the stainless steel unit included a lowered ADCP for all casts, and the occasional fitting of a PAR light sensor. The secondary C/T sensors and an experimental oxygen sensor were fitted to the stabilising vane, to remove the effects of water entrainment within the CTD package.

Stainless steel setup

Titanium setup

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 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.

Chelsea Technologies Photosynthetically Active Radiation (PAR) Irradiance Sensor

This sensor was originally designed to assist the study of marine photosynthesis. With the use of logarithmic amplication, the sensor covers a range of 6 orders of magnitude, which avoids setting up the sensor range for the expected signal level for different ambient conditions.

The sensor consists of a hollow PTFE 2-pi collector supported by a clear acetal dome diverting light to a filter and photodiode from which a cosine response is obtained. The sensor can be used in moorings, profiling or deployed in towed vehicles and can measure both upwelling and downwelling light.

Specifications

Further details can be found in the manufacturer's 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.

Paroscientific Absolute Pressure Transducers Series 3000 and 4000

Paroscientific Series 3000 and 4000 pressure transducers use a Digiquartz pressure sensor to provide high accuracy and precision data. The sensor comprises a quartz crystal resonator that responds to pressure-induced stress, and temperature is measured for thermal compensation of the calculated pressure.

The 3000 series of transducers includes one model, the 31K-101, whereas the 4000 series includes several models, listed in the table below. All transducers exhibit repeatability of better than ±0.01% full pressure scale, hysteresis of better than ±0.02% full scale and acceleration sensitivity of ±0.008% full scale /g (three axis average). Pressure resolution is better than 0.0001% and accuracy is typically 0.01% over a broad range of temperatures.

Differences between the models lie in their pressure and operating temperature ranges, as detailed below:

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

BODC data processing

The data were received in 59 Pstar files, one per CTD cast. The originator derived channels were not loaded, with potential temperature and sigma-theta re-derived by BODC. The remaining data channels were converted into the internal BODC format.

The following table shows how the variables within the Pstar files were mapped to appropriate BODC parameter codes:

All reformatted data were visualised using the in-house Edserplo software. Suspect and missing data were marked by adding an appropriate quality control flag.

Originator's processing

Sampling Strategy

A total of 60 CTD casts, 44 casts using the stainless steel CTD and 16 casts with the titanium CTD were carried out during the cruise D286, with one CTD was aborted.

Processing

The processing and calibration methodology were provided by Raymond Pollard. All CTDs went through several stages of processing. The original Sea-Bird .cnv files were converted to Pstar files. The 24 Hz files were reduced to 1 Hz containing both down and up casts (.ctu). It is then thought that the final CTD files (.2db) were extracted from the .ctu files and contained only the good (monotonic in pressure) down cast data averaged to 2 dbar intervals on pressure. The .2db files were then calibrated.

Calibration

Both salinity and oxygen data were calibrated by Raymond Pollard. There were four separate conductivity cells, two from each CTD and two oxygen sensors, one from each CTD which potentially needed calibration.

Salinity

A total of 229 samples for salinity were taken from Niskin bottle firings from the stainless steel CTD only. Excluding six bottle salinity values outside the range of ±0.010, the remaining 223 samples were used to calculate the mean difference between the bottle salinity and the CTD salinity from sensor 1 which was 0.0001 and standard deviation 0.0017. The mean difference between the bottle salinity and the CTD salinity from sensor 2 was 0.0015 and standard deviation 0.0015. Neither calibration was applied.

As there were two conductivity sensors, an offset between the two could also be applied. Salinity from sensor 1 and salinity from sensor 2 from depths of bottle firings were compared. All values from the stainless steel CTD were around 0.0015, consistent with salinity 1 being correct at the 0.001 level, and salinity 2 being low by 0.0015. This correction was not made, because salinity 2 was the secondary sensor, and the cast to cast means varied from about 0.0005 to 0.0020.

There were no salinity samples taken from the titanium CTD, in order to minimise potential iron contamination, therefore it was attempted to calibrate the salinity from the titanium CTD casts by comparison with the approximate stainless steel CTDs. Titanium casts were almost always associated with stainless steel casts close by in space and time at the major iron and productivity stations (see the table below for CTD pairs). Titanium and stainless steel casts were merged on pressure, but reasonable comparisons could only be obtained when both casts extended deeper than 2000 dbar. Only five station pairs could be cross-calibrated, and mean differences were unreliable biased by offsets in shallow values. Instead the peak of the histogram of salinity differences were extracted, which gave consistent results at the 0.001 level for the five stations. Salinity values from sensor 1 were 0.001 too small, and from sensor 2 were 0.0075 too small by comparison with salinity from the stainless steel CTD casts. Therefore, salinity 1 from the titanium CTD were corrected by adding 0.001 and salinity 2 from the titanium CTD by adding 0.0075, after which both salinity 1 and salinity 2 were correct to 0.001.

Oxygen

Dissolved oxygen samples were drawn from bottles on each stainless CTD cast, except those taken for thorium samples, and analysed using the Winkler whole bottle titration method. After chemical analysis, the bottle oxygen values were converted from µMol l^-1 to µMol kg^-1. Two methods for the conversion were compared, one was to calculate density in the bottle files using CTD values of temperature, conductivity and pressure, and the other method was to assume a constant value for density of 1027 kg m^-3 or 1.027 kg l^-1. Differences in converted oxygen values between these two methods were so small (usually < 0.2 µMol kg^-1), that use of constant factor (1.027 kg l^-1) was preferred, as it could easily be reversed and avoided the need for a fixing or other temperature value. Differences between bottle and CTD oxygen values were calculated and it was found that there was no obvious drift with time but a straight line could be fitted against oxygen.

Linear regression of all bottle oxygen values against CTD stainless steel oxygen values gave a best fit line of; corrected oxygen = 1.6 + 1.01485684 * oxygen, but this line was skewed by some large offsets from large oxygen values. After removing 22 values with a difference between bottle oxygen values and CTD oxygen values outside the range -4 to 15, the least squares fit line became; corrected oxygen = -1.4 + 1.02943 * oxygen. To test whether this fit was significantly different from the previous D285 fit the D285 calibration was tried; corrected oxygen = 1.7 + 1.01626 * oxygen. The greatest difference, at high or low oxygen values, was 1.9 µMol kg^-1. Differences are mostly <1 µMol kg^-1 except for high, near surface oxygen values which were >300 µMol kg^-1. It was found that one of the fits was significantly better than the others. For compatibility with the calibration already applied to D285 oxygen values the same calibration was used; corrected oxygen = 1.7 + 1.01626 * oxygen. After calibration, the oxygen values were correct within about 2 µMol kg^-1. However, oxygen errors plotted against pressure, appears to have less scatter than against CTD oxygen values. Thus, if more accurate oxygen values are required, particularly in the top 500 m, correction as a function of pressure could be applied.

As with salinity, no oxygen samples were taken from the titanium CTD, therefore oxygen values from the titanium CTD were calibrated by comparison with the approximate (time and space) stainless steel CTDs (see the table below for CTD pairs). For each CTD pair, a straight line was fitted to the oxygen values, matched on pressure. Comparisons for the deepest pair of CTDs, titanium CTD 15581 and stainless steel CTD 15582 the linear regression line was; corrected oxygen (15581) = 5.79 + 1.1344 * oxygen (15581) (=oxygen (15582)), assuming that CTD 15582 had the correctly calibrated oxygen values. After this calibration was applied, there was a slight offset between CTD 15582 and the bottle values showing that marginal improvements in the calibration could still be achieved. Note that the best fit line is accurate for the deep part of the profiles, deeper than about 1000 dbar. This is consistent with the match obtained for titanium D285 CTDs. While a fit was achieved for all but the shallowest CTDs, the fit was less reliable for the shallower casts. For the shallowest CTDs, the mean fit was used. In the absence of further comparisons, the calibration values A and B were applied to the individual casts, as tabulated below (corrected titanium oxygen=A+B*titanium oxygen). Applying all A and B pairs to a high input of 270 µMol kg^-1, the outputs range from 307 to 327 µMol kg^-1, and applying them to a low input of 160 µMol kg^-1 gives outputs from 177 to 202 µMol kg^-1. Thus oxygen values could be up to ±10 µMol kg^-1 in error.

Stainless steel and titanium CTD pairs and oxygen calibration values

Project Information

CROZet natural iron bloom EXport experiment (CROZEX)

The multidisciplinary CROZet natural iron bloom EXport experiment (CROZEX) was a major component of the Natural Environment Research Council (NERC) funded core strategic project Biophysical Interactions and Controls over Export Production (BICEP). The project is the first planned natural iron fertilisation experiment to have been conducted in the Southern Ocean.

The overall objective of CROZEX was to examine, from surface to sediment, the structure, causes and consequences of a naturally occurring phytoplankton bloom in the Southern Ocean. The Crozet Plateau was chosen as the study area. This area typically exhibits two phytoplankton blooms a year, a primary bloom in that peaks in October and a secondary bloom in December or January. Specific aims with respect to these were to:

• Determine what limits the primary bloom
• Determine the cause of the secondary bloom

The project was run by the George Deacon Division (GDD), now Ocean Biogeochemistry and Ecosystems (OBE) at the National Oceanography Centre Southampton (NOCS). Participants from five other university departments also contributed to the project.

The project ran from November 2004 to January 2008 with marine data collection between 3^rd November 2004 and 21^st January 2005. There were 2 cruises to the Crozet Islands Plateau, which are summarised in Table 1.

Table 1: Details of the RRS Discovery CROZEX cruises.

The two cruises aimed to survey two areas at different phases of the bloom cycle described above. A control area to the south of the Crozet Islands, which is classified as High Nutrient Low Chorophyll (HNLC), where the blooms do not occur and a second area in the region of the blooms to the north of the Crozet Islands.

Sampling was undertaken at ten major stations (see Pollard et al., 2007) numbered M1 to M10. The following observations/sampling were conducted at each station where possible:

• Several CTD casts sampling:
  • Iron (using a titanium rig)
  • ²³⁴Th
  • Physical parameters (temperature, salinity etc)
  • Oxygen
  • CO₂
  • Nutrients using a stainless steel rig including a Lowered Acoustic Doppler Current Profiller (LADCP)
• At each thorium cast there was an associated Stand Alone Pump System (SAPS) deployment
• At some stations, a drifting PELAGRA trap was deployed for the duration of the work
• Megacoring was undertaken at M5 and M6
• Gravity coring was undertaken at M5, M6 and M10
• Longhurst Hardy Plankton Recorder (LPHR) tows were undertaken at a few major stations

For each of the major stations (M1 to M10), the following were determined:

• Primary productivity
• New Production
• Phytoplankton community composition
• Bacterial activity
• Iron
• Nutrient drawdown
• Thorium export

Sampling between major stations included:

• SeaSoar runs instrumented with:
  • CTD
  • Optical Plankton Counter (OPC)
  • Fast Repetition Rate fluorimeter (FRRf)
• Physics CTD casts on several lines
• Argo float deployments
• Zooplankton nets at nearly every CTD and major station
• Underway and on-station CO₂ measurements
• Underway nutrients and radium sampling
• 5 to 6 day ship-board iron-addition incubation experiments
• Checks against near-real-time satellite and model data
• Mooring deployments based on the satellite imagery in support of the CROZET (Benthic CROZEX) project.

The CROZEX cruises included 6 extra days in support of the CROZET (Benthic CROZEX) project, whose main cruise took place one year after the CROZEX cruises. The CROZET work undertaken during the CROZEX cruises was primarily the moored sediment trap deployments, although some of the coring work is applicable to both projects.

CROZEX produced significant findings in several disciplines, including confirmation that iron from Crozet fertilised the bloom and that phytoplankton production rates and most export flux estimates were much larger in the bloom area than the HNLC area (Pollard et al. 2007). Many of the project results are presented in a special CROZEX issue of Deep-Sea Research II (volume 54, 2007).

References

Pollard R., Sanders R., Lucas M. and Statham P., 2007. The Crozet natural iron bloom and export experiment (CROZEX). Deep-Sea Research II, 54, 1905-1914.

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: