Metadata Report for BODC Series Reference Number 683278

• 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.

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.

Sea Tech Light Back-Scatter sensor

The instrument projects light into the sample volume using two modulated 880 nm Light Emitting Diodes. Light back-scattered from the suspended particles inthe water column is measured with a solar-blind silicon detector. A light stop between the light source and the light detector prevents the measurement of direct transmitted light so that only back-scattered light from suspended particles in water are measured.

The sensor has two ranges permitting the user to measure nearly all suspended particle concentrations found in open ocean or coastal waters. Range for the measurement of suspended particle concentration in water will be approximately 10 mg l^-1 if High_Gain is selected. If Low-Gain is selected full scale will be a factor of 3.3 higher or approximately 33 mg l^-1.

Specifications

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

SeaTech Transmissometer

Introduction

The transmissometer is designed to accurately measure the the amount of light transmitted by a modulated Light Emitting Diode (LED) through a fixed-length in-situ water column to a synchronous detector.

Specifications

• Water path length: 5 cm (for use in turbid waters) to 1 m (for use in clear ocean waters).
• Beam diameter: 15 mm
• Transmitted beam collimation: <3 milliradians
• Receiver acceptance angle (in water): <18 milliradians
• Light source wavelength: usually (but not exclusively) 660 nm (red light)

Notes

The instrument can be interfaced to Aanderaa RCM7 current meters. This is achieved by fitting the transmissometer in a slot cut into a customized RCM4-type vane.

A red LED (660 nm) is used for general applications looking at water column sediment load. However, green or blue LEDs can be fitted for specilised optics applications. The light source used is identified by the BODC parameter code.

Further details can be found in the manufacturer's Manual.

RRS Discovery 285 CTD Data Documentation

Chief Scientist

Prof. Raymond Pollard, National Oceanography Centre, Southampton, UK

Data originator

Prof. Raymond Pollard, National Oceanography Centre, Southampton, UK

Content of data series

Instrumentation and data processing by originator

CTD operations

Two CTDs systems were used during the cruise. One of standard stainless steel construction, with aluminium, titanium and plastic instrument housings, used for physics and biology sampling. The second, of titanium and plastic construction, used for iron sampling. The instrument suits were basically very similar, consisting of Seabird 9+ CTDs with dual C/T sensors and oxygen. Auxiliary sensors were Chelsea Instruments transmissometer and fluorometer and Seatech light backscatter sensor.

Additional instruments on the stainless steel unit included an RDI 300kHz workhorse lowered ADCP, for all casts, and the occasional fitting of a PML par light sensor. Also the secondary T/C sensors and an experimental oxygen sensor were fitted to the stabilising vane, to remove the effects of water entrainment within the CTD package.

The following sensor and configuration changes occured during D285: Titanium No changes. Stainless The transmissometer was changed prior to 15489s. The fluorometer was changed prior to 15523s. The P.A.R. was changed prior to 15523s. The fluorometer channel was changed prior to 15540s.

Data quality:

In general both systems worked well, with two notable exceptions. Both altimeters were poor at bottom detection and the stainless system fluorometer had a persistent depth related noise problem around 80 to 200 metres. Sensor, cable and data channel changes failed to cure the fault. There appeared to be no correlation between this and other instrument operation. This problem was still unresolved at the end of D285.

Originator's data calibrations

Salinity calibration for stainless CTD

Four separate conductivity cells potentially needed calibration, two each on the titanium and stainless CTDs. Salinities were mostly within a few ppm of salinometer derived values so calibrations were applied only to salinity, not conductivity. On most stainless CTD casts four to eight calibration samples were drawn, trying to use depths where vertical salinity gradients were weak. Comparisons of bottle values (botsal) with sensor 1 (sal1 - mounted on the tail) and sensor 2 (sal2 - mounted within the frame) suggested that sal1 should be reduced by 0.002.

The cast to cast offsets in sal1-sal2 are caused by changes in the secondary sensor sal2. As this in mounted within the frame, it is subject to offsets resulting from water trapped by the frame, so is not used except as backup, and so has not been calibrated. However, after the first few casts, sal1-sal2 remains close to zero for nearly all casts and depths, confirming that no calibration beyond the 0.002 offset in sal1 is justified. Overall, we estimate that the 0.002 is an upper limit to the accuracy of sal1, much of this being errors in the bottle values.

Salinity calibration for titanium CTD

To minimize potential iron contamination, no salinity samples were drawn from the titanium CTD. However, TiCTD casts were almost always associated with ssCTD casts close by in space and time at the major iron and productivity stations. We therefore attempted cross-calibration by comparing adjacent casts. The casts were merged on pressure, or potential temperature. Density cannot be used, as any error in salinity will affect density. Potential temperature proved the more useful parameter on which to merge, as internal waves can offset profiles at all depths. Above 2200 dbar, the difference in the primary salinity sensors was not stable at the 0.001 level, varying by typically ±0.002 as the pressure difference varied. Below 2200 dbar however, there was less than 0.001 variation with depth.

By eye, the titanium CTD primary salinity is correct at the 0.001 level. Plots of the primary-secondary salinities indicated that sal1-sal2 had similar standard deviation to the stainless steel CTDs and a mean of 0.006. Thus sal2 is too low by 0.006. This correction was not made.

Oxygen calibration for stainless CTD

Oxygen samples were drawn for calibration at most depths. After chemical analysis, the bottle oxygen values were converted from uMol/l to uMol/kg using the fixing temperature. The mean and standard deviation of bot-CTDoxy for all values in the range 5 to 20 uMol/kg was 5.9 ± 2.9 uMol/kg. Linear regression suggested a correction to the CTD oxygen values oxygen (corrected) = 1.7 + 1.01626 * oxygen uMol/kg and this has been made. This reduced mean, but the standard deviation only marginally, to 0.2 ± 2.6 uMol/kg. On reexamination, the slope has probably been overestimated because of the high outliers at high oxygen values, and postcruise recalibration could slightly improve the calibration. Similarly, plotting the corrected oxygen differences against station number indicates station to station changes of order ± 2 uMol/kg. Nevertheless, overall the CTD oxygens are remarkably good, with errors of order 2 in 200, or 1%.

Oxygen calibration for titanium CTD

Drawing oxygen samples from the titanium rosette would pose a serious contamination risk to the iron sampling, so cross calibration was attempted as for salinity. No variation of oxygen calibration with depth could be determined because of variations of up to 5 uMol/kg with pressure difference and pressure for pressures less than 2200 dbar, though there was some evidence for such drift. Below 2200 dbar, the differences ranged from 23.6 to 30.1 uMol/kg, although the cast to cast differences in the stainless CTD values (corrected as above) may contribute. The offset corrections, ranging from +24 uMol/kg to +30 uMol/kg, have been applied to the titanium CTDs as described in the cruise report.

Fluorimeter

None was done during the cruise.

BODC post-cruise processing and screening

Reformatting

The data were converted into BODC internal format (QXF) and quality-controlled with in-house software, notably the workstation graphics editor EDSERPLO. No significant problems were detected and a few salinity and temperature spikes were flagged suspect.

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: