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Abstract

Nitrogen isotopes of organic matter are increasingly studied in marine biogeochemistry and geology, plant and animal ecology, and paleoceanography. Here, we present results of an inter-laboratory test on determination of nitrogen isotope ratios in marine and lacustrine sediments. Six different samples covering a wide range of total nitrogen content and delta(15)N values were analyzed by eight different laboratories using their routine procedures. The laboratories were asked to measure three batches with three replicates for each sample to assess accuracy and variability within and among laboratories; this permits assessment of repeatability and reproducibility, which are essential in meta analysis of the increasing database on delta(15)N values in marine sediments. The grand average delta(15)N values for individual samples ranged from 1.65-10.90 parts per thousand One laboratory exhibited an average bias of -0.27 parts per thousand compared to the mean of all other laboratories. Apart from one sample, which showed an exceptionally high overall standard deviation (OSD) of 0.51 parts per thousand, the analytical precision (1 s) averaged 0.24 parts per thousand, ranging from 0.18-0.31 parts per thousand. for individual samples. Out of the eight participating laboratories, two showed a significantly elevated within-laboratory standard deviation (WLSD) of 0.41 parts per thousand and 0.32 parts per thousand compared to an average WLSD of 0.15 parts per thousand for the other laboratories. The WLSD was inversely correlated with the ratio of peak height to peak width, which was taken as a simple measure of peak shape. Moreover, our data also revealed an inverse correlation between total nitrogen content and measurement precision. These correlations may provide guidance for improving the measurement precision of individual laboratories. Based on the results of this round robin test, we have estimated the expanded measurement uncertainty on the 2 sigma level to 0.45 parts per thousand for sediment samples with a nitrogen content >0.07 wt%. Sediment samples with lower nitrogen contents cannot be measured with sufficient precision without additional precautions and care should be taken when interpreting delta(15)N signatures and records for sediments with nitrogen concentrations <0.07 wt%. (C) 2009 Elsevier Ltd. All rights reserved.