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

Abstract

There is a known bias in C/N, δ 13 C and δ 15 N values of organic matter (OM) due to preanalysis acid treatment methods. We report here, for the first time, the results of a preanalysis acid treatment method comparison of measured C/N, δ 13 C and δ 15 N values in bulk 16 OM from a sedimentary sequence of samples to illustrate this bias. Here we show that acid 17 treatment significantly reduces the accuracy (between method biases) and precision (within 18 method bias) of C/N, δ 13 C and δ 15 N values of OM, suggesting a differential response of sample OM between methods and sample horizons, and in some cases inefficient removal of 20 inorganic carbon. We show that different methods can significantly influence environmental 21 interpretation in some of our sample horizons (i.e. interpretation of aquatic vs. terrestrial OM 22 source; C 3 vs. C 4 vegetation). Specifically, there are unpredictable and non-linear differences between methods for C/N values in the range of ~ 1-100; δ 13 C values in the range of 0.2-6.8 ‰ and; δ 15 Nvalues 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 δ 13 C values and; ± 0.1 ‰ for δ 15 N). The accuracy and precision of measured C/N, δ 13 C and δ 15 N values of bulk OM is not just dependent upon environmental variability, but on acid pretreatment, 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.

Historic carbon burial spike in an Amazon floodplain lake linked to riparian deforestation near Santarém, Brazil

“…A homogenized portion was acidified (10 % HCl following the procedures outlined in Naidu et al, 2000) to remove carbonate material, then dried and ground to powder for OC, nitrogen (N), δ 13 C and δ 15 N analyses using a flash elemental analyzer coupled to a Thermo Fisher Delta V IRMS (isotope ratio mass spectrometer). The δ 15 N results and the C / N ratio results should be interpreted with caution, based on this pre-treatment method, which may be influenced by the composition of the bulk organic matter (Brodie et al, 2011). Working standards were used (glucose, 10.7 ppt and urea, 41.3 ppt) to calibrate for δ 13 C. A pair of standards was measured with every 20 samples.…”

Nitrogen isotopes in bulk marine sediment: linking seafloor observations with subseafloor records

“…The combusted material is dominated by marine organic matter at most locations, although there is significant contamination by clay-bound inorganic (Kienast et al, 2005) and terrigenous nitrogen ) in some locations. Given the known problems with acidification of samples (Brodie et al, 2011), we flagged samples that had been acidified where possible, though acidification is not always reported. Reported errors for the bulk combustion method are generally better than ±0.3 ‰ for replicates.…”

Sedimentary geochemical record of human-induced environmental changes in Huanggaihu Lake in the middle reach of the Yangtze River, China

“…For the carbon isotope ratios of sedimentary organic matter (δ 13 C org ), the samples were treated with HCl (5%) to remove carbonate (Brodie et al, 2011). Afterward, the samples were centrifuged, rinsed repeatedly with deionized water until neutral pH was achieved, and freeze dried.…”