Evidence for bias in C/N, δ13C and δ15N values of bulk organic matter, and on environmental interpretation, from a lake sedimentary sequence by pre-analysis acid treatment methods

There is a known bias in C/N, δ13C and δ15N values of organic matter (OM) due to pre-analysis acid treatment methods. We report here, for the first time, the results of a pre-analysis acid treatment method comparison of measured C/N, δ13C and δ15N values in bulk OM from a sedimentary sequence of samples to illustrate this bias. Here we show that acid treatment significantly reduces the accuracy (between method biases) and precision (within method bias) of C/N, δ13C and δ15N values of OM, suggesting a differential response of sample OM between methods and sample horizons, and in some cases inefficient removal of inorganic carbon. We show that different methods can significantly influence environmental interpretation in some of our sample horizons (i.e. interpretation of aquatic vs. terrestrial OM source; C3 vs. C4 vegetation). Specifically, there are unpredictable and non-linear differences between methods for C/N values in the range of ∼1–100; δ13C values in the range of 0.2–6.8‰ and; δ15N values in the range of 0.3–0.7‰. Importantly, these ranges are mostly much greater than the instrument precision (defined as the standard deviation of replicate analysis of standard reference materials; for this study, ±0.5 for C/N values, ±0.1‰ for δ13C values and; ±0.1‰ for δ15N). The accuracy and precision of measured C/N, δ13C and δ15N values of bulk OM is not just dependent upon environmental variability, but on acid pre-treatment, residual inorganic carbon and organic matter state and composition. Collectively, this makes the correlation between samples prepared in different ways, including those from down-core reconstructions, highly questionable.

► Comparison of pre-analysis acid treatment methods on bulk organic matter.
► Significant differences in C/N and δ13C within and between acid treatment methods.
► Biases in C/N values of ∼1–100; δ13C values of 0.2–6.8‰; δ15N values of 0.3–0.7‰.
► Accuracy and precision of measured data is highly non-linear and unpredictable.