Metadata Report for BODC Series Reference Number 1261344

• 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

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

Niskin Bottle

The Niskin bottle is a device used by oceanographers to collect subsurface seawater samples. It is a plastic bottle with caps and rubber seals at each end and is deployed with the caps held open, allowing free-flushing of the bottle as it moves through the water column.

Standard Niskin

The standard version of the bottle includes a plastic-coated metal spring or elastic cord running through the interior of the bottle that joins the two caps, and the caps are held open against the spring by plastic lanyards. When the bottle reaches the desired depth the lanyards are released by a pressure-actuated switch, command signal or messenger weight and the caps are forced shut and sealed, trapping the seawater sample.

Lever Action Niskin

The Lever Action Niskin Bottle differs from the standard version, in that the caps are held open during deployment by externally mounted stainless steel springs rather than an internal spring or cord. Lever Action Niskins are recommended for applications where a completely clear sample chamber is critical or for use in deep cold water.

Clean Sampling

A modified version of the standard Niskin bottle has been developed for clean sampling. This is teflon-coated and uses a latex cord to close the caps rather than a metal spring. The clean version of the Levered Action Niskin bottle is also teflon-coated and uses epoxy covered springs in place of the stainless steel springs. These bottles are specifically designed to minimise metal contamination when sampling trace metals.

Deployment

Bottles may be deployed singly clamped to a wire or in groups of up to 48 on a rosette. Standard bottles and Lever Action bottles have a capacity between 1.7 and 30 L. Reversing thermometers may be attached to a spring-loaded disk that rotates through 180° on bottle closure.

RAPID Cruise D279 discrete nutrient sampling by originator Sanders

Nutrient data from D279 were collected and analysed by two originators working closely together. The full CTD sample data set of nitrate+nitrite (hereinafter nitrate), phosphate and silicate was provided by Dr Richard Sanders. Further nutrient data, including a subset of the data provided by Dr Richard Sanders, were provided by Dr Angela Landolfi. Differences between the full CTD data set and the data subset only occur in the nitrate data due to post cruise calibrations that were performed. As such, the data acquisition, onboard analysis and onboard data quality are identical. Dr Angela Landolfi also provided data from plastic carboy samples that were collected during the cruise. Data described below were supplied by Dr Richard Sanders.

Originator's data acquisition and analysis

Onboard processing and analysis

Inorganic nutrient concentrations (nitrate+nitrite hereinafter nitrate, phosphate and silicate) were measured immediately onboard using a Skalar San Plus autoanalyser according to standard colorimetric techniques (Kirkwood, 1995) with the exception that the pump rates through the phosphate line were increased by a factor of 1.5, which improves reproducibility and peak shape.

Samples were drawn from Niskin bottles into 25ml sterilin coulter counter vials and kept refrigerated at 4°C until analysis, which commenced within 24 hours. Stations were run in batches of 2-6 with most runs containing 3 or 4 stations. Overall, 34 runs were undertaken. An artificial seawater matrix (ASW) of 40 g/l sodium chloride was used as the intersample wash and standard matrix. The nutrient free status of this solution was checked by running Ocean Scientific International (OSI) nutrient free seawater on every run.

In a departure from previous methodology, a single set of mixed standards were made up at the start of the cruise and used throughout the cruise. These were made by diluting 5 mM solutions made from weighed dried salts in 1 litre of ASW into plastic 1 litre volumetric flasks that had been cleaned by soaking for 6 weeks in MQ water. This was in an effort to minimise the run to run variability in concentrations observed on previous cruises. OSI nutrient standard solutions were used sporadically during the cruise to monitor the degradation of these standards.

Data were transferred to another computer initially using a zip disk, and then after station 66 by means of a memory stick. The zip disk transfer route was unreliable and resulted in a delay between sample analysis and data work up of 8-10 stations. After station 66, data were worked up immediately. This delay had the effect that the problems with the nitrate line described below could not be evaluated in close to real time.

Data processing was undertaken using Skalar proprietary software. Generally this was straightforward, however a detailed examination of nitrate data from stations 20-60 was needed to achieve acceptable calibrations and bulk nutrient values. The wash time and sample time were 90 seconds, and the lines were washed daily with 0.25M NaOH (P) and 10% Decon (N, Si). Time series of baseline, bulk standard concentration, instrument sensitivity, calibration curve correlation coefficient, nitrate reduction efficiency and duplicate difference were compiled and updated on a daily basis.

• Problems with the analyser

  1) In the early part of the cruise on runs 1-3 (stations 2-21), the phosphate baseline suffered frequent catastrophic baseline degradations. All the samples were rerun, but duplicates could not be run as the available duplicate time was used to reanalyse samples. This was alleviated mid run by removing the flow cell and shaking it vigorously, and eliminated over the longer term by refitting some elements of the line and reducing the pull through rate. Stations 49-52 were also affected by this problem and no phosphate data are available for stations 51 and 52. Stations 71-74 were compromised by a failure of the temperature water bath. These stations were reanalysed 24 hours later using samples from salinity bottles.

  2) The nitrate line was very noisy between stations 22 and 60. Initially this was suspected to be due to a fault with the reagents, which were renewed several times. However, after this failed to rectify the situation, the cadmium column was repacked on two occasions. This also failed to rectify the situation and a new cadmium column was therefore fitted, which gave no problems during the rest of the cruise. Stations 22-60 were reprocessed to give bulk nutrient values in line with those from the remainder of the stations. The effect of this on data quality has yet to be systematically evaluated.

• Analyser performance

  The performance of the autoanalyser is monitored via the following parameters: baseline value, calibration curve slope, regression coefficient of the calibration curve and nitrate reduction efficiency.

  The instrument sensitivity for nitrate varied widely and unpredictably during the cruise by up to 40%. Phosphate and silicate sensitivity behaved much more reproducibly, with these parameters varying by about 10% over the 5 week period of observations.

  The quality of the calibration curves was generally good with 95% having regression coefficients better than 0.999. The reduction efficiency of the cadmium column was <100% during the early part of the cruise. The column was changed at station 66, after which the efficiency increased to approximately 100%.

  The baseline value of the instrument barely changed throughout the cruise, with the exception of phosphate, which declined after the first run from 6300 to about 5900.

References

Kirkwood, D. S., 1995. Nutrients: Practical notes on their determination in seawater. ICES Techniques in Marine Environmental Science report 17, International Council for the Exploration of the Sea, Copenhagen. 25p. ISSN 0903-2606.

BODC data processing procedures

The data were supplied in PSTAR format, which was converted to ASCII. Units were converted, if necessary, to the BODC standard parameter units.

The data were then loaded into the project database under the ORACLE Relational Database Management System without modification.

Data that lay outside the permitted range for the parameter code were flagged suspect.

Data quality

Originator's onboard data quality

• Precision of measurements

The short term precision of the measurements was evaluated by running one or two duplicate samples per station (thus 3-6 per run). The mean differences for silicate, nitrate and phosphate were 0.67, 1.63 and 2.04%, respectively. However, this conceals substantial variability in both nitrate and phosphate precision during the cruise. A group of stations from approximately 25- 60 had poor nitrate precision but the precision of the phosphate analyses improved over the course of the cruise from about 5% to about 1%.

• Internal consistency of measurements

This was evaluated by using a deep water sample taken on station 1 and was run on every station.

Nitrate concentration appeared to be invariant whereas the phosphate and silicate concentrations declined markedly over the cruise. The variability of bulk nutrient concentration from the mean is indicative of the internal consistency of the data set. For nitrate this is simple to evaluate, as the concentration appeared to be invariant. The residual concentration appears to be normally distributed and shows no significant trend over time. The absolute average residual value was 0.27 micromoles per litre or 1.2%.

For phosphate and silicate, a linear function was fitted which predicted concentration as a function of elapsed day. This regression was used to generate values for phosphate and silicate for each day and the residual difference calculated.

Both phosphate and silicate residuals appear to have a normal distribution, with silicate (and to a lesser extent phosphate) residuals displaying a sinusoidal pattern with time for unknown reasons. The mean residual values are 0.12 micromoles per litre or 1.17% for silicate and 0.03 micromoles per litre or 2.1% for phosphate.

• Accuracy of measurements

The accuracy was monitored by the use of OSI nutrient standard solutions, which need to be diluted by the user. The analysis of these standards gave values of phosphate 1.01 +/- 0.02 micromoles per litre for a nominally 1 micromolar solution, nitrate 10.9 +/- 0.13 for a nominally 10 micromolar solution and silicate 21.4 +/- 0.1 micromoles per litre for a nominally 20 micromolar solution. These imply that the nitrate and silicate results are too low by about 10% and 5% respectively. The standards used on this cruise have been retained for further investigation and a comparison with historical data will also be used to address this issue.

Project Information

No Project Information held for the Series

Data Activity or Cruise Information

Data Activity

BODC Sample Metadata Report for D279_CTD_C024

Please note:the supplied parameters may not have been sampled from all the bottle firings described in the table above. Cross-match the Sample Reference Number above against the SAMPRFNM value in the data file to identify the relevant metadata.

Related Data Activity activities are detailed in Appendix 1

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 2

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: D279_CTD_C024

Related series for this Data Activity 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.

Appendix 2: 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.