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Nitrogen and oxygen isotope measurements of nitrate from CTD bottles during cruise AMT16

Originator's Protocol for Data Acquisition and Analysis

This data originates from analyses on samples collected from CTD bottles between 20 May and 29 June 2005. Samples were taken from a meridional cross section from the Northen Gyre through the equatorial upwelling zone to the Northen Gyre. Samples were also taken from the euphotic zone.

Water samples were collected from selected bottle depths and selected CTD casts. Approximately 60 ml of sample was drawn into clear acid-cleaned Nalgene flasks, tightly capped and frozen at -20 °C.

The 15N/14N and 18O/16O isotope ratios were analysed using the denitrifier method (see Casciotti et al., 2002 and Sigman et al., 2001 for detailed methods) The method of measurement of the isotopic composition of nitrate is based on the isotopic analysis of nitrous oxide (N2O) generated from nitrate in denitrifying bacteria that lack N2O reductase activity. Firstly, the denitrifying culture is prepared; organic nitrogen in a tryptic soy broth and ammonium is added to the culture medium to provide nitrogen for assimilation. Cultures are incubated for six to ten days on a reciprocol shaker. To convert the samples to nitrous oxide, samples are centrifuged then decanted and alliquotted into 20 ml headspace vials. Each vial is purged with N2 gas. The water sample is then injected into each vial and incubated overnight. Extraction and isotopic analysis of nitrous oxide is carried out by stripping N20 from each vial using a helium gas carrier then purifying the sample and analysing its isotopic composition using an isotope ratio mass spectrometer.

References Cited

Casciotti, K.L., Sigman, D.M., Hastings, M.G., Bohlke, J.K., Hilkert, A. 2002. Measurement of the oxygen isotopic composition of nitrate in seawater and freshwater using the denitrifier method. Analytical Chemistry 74, 4905-4912, doi:10.1021/ac020113w.

Sigman, D.M., Casciotti, K.L., Andreani, M., Barford, C., Galanter, M., Bohlke, J.K. 2001. A bacterial method for the nitrogen isotopic analysis of nitrate in seawater and freshwater. Analytical Chemistry 73, 4145-4153.

Instrumentation Description

Not applicable for this data set.

BODC Data Processing Procedures

Data were submitted in an Excel spreadsheet (AMT16_Nitrate_isotope_data_Casciotti_9Nov2009.xls) and archived under BODC's accession number WHC090227. Nutrient data were also supplied but not loaded with this dataset as these data have been previously supplied. The standard deviation of samples were provided and loaded with the dataset. Sample metadata (CTD cast, bottle number and depth) were checked against information held in the database and against the cruise CTD bottle firing logsheets. There were a number of depth discrepancies between the originators depth-bottle number combinations and those on the CTD logsheets for CTD029t. The bottle numbers were considered correct and bottle number 2 was loaded against 500 m (CTD log) instead of 300 m (originator's file), bottle number 3 was loaded against 300 m (CTD log) instead of 150 m (originator's file) and bottle number 4 against 150 m (CTD log) instead of 100 m (originator's file). There was no bottle fired at 100 m during CTD029t. For CTD043t, there was a difference between the intended firing depth (recorded on the CTD logsheets) and the actual depth (contained in the Seabird bottle files) of 11 m . In addition, CTD027t was provided with a typo in the latitude, 14.255°S, which should have been 4.255°S. 109 CTD samples were loaded in total.

The isotopic enrichment data were provided in parts per thousand which is consistent with the BODC parameter code units, so no conversion was applied.

The data were reformatted and loaded in BODC's samples database under Oracle Relational Database Management System. Data were marked up with BODC parameter codes and loaded into the database. Individual samples were matched through rosette sampling bottle and depth.

A parameter mapping table is provided below;

Data Quality Report

BODC was not advised of any specific quality issues with the data. The data were checked visually; no values were flagged by BODC.

Problem Report

Not relevant to this data set.