The potential distortion of sedimentary δ15N and Corg/N ratios by NH4+ and the effects of pre-analysis sample treatment
Abstract. We examined the susceptibility of δ15N-signals and Corg/N ratios in organic-rich sediments to pre-analysis sample treatment. Each sample was subjected to three different kinds of processing. For comparative purposes, the first measurement series (MS-1) was carried out on untreated sediment. In MS-2, the sediment was rinsed with distilled water. In MS-3, analyses were carried out on decalcified and rinsed material, in MS-4 the samples were decalcified without being subsequently washed. The sediment yielded conspicuously different results depending on the type of processing it was subjected to. Rinsing, irrespective of whether acidification was included or not, induced substantial modifications in δ15N accompanied by a pronounced loss of NH4+ (up to 14 wt% of the initial N-content). Molar Corg/N ratios, on the other hand, were only affected by a combination of acidification and rinsing. The discrepancies are ascribed to the influence of decomposition-derived ammonium (NH4+). In untreated sediment (MS-1), NH4+ seems to produce misleading shifts in both δ15N-signals and Corg/N ratios. Presumed mechanisms involved are as follows: Firstly, nitrogen isotopes fractionate during NH4+-volatilisation in the heating oven, where the sediment is put to desiccate. Secondly, NH4+-ions are able to escape that fractionation when adsorbed to negatively-charged SiO2-surfaces. The adsorption capacity of SiO2 increases with increasing pH of the pore water and hence with increasing carbonate content. Our findings raise serious doubts about whether untreated sediment (MS-1) can provide reliable Corg/N and δ15N-records. Pre-analysis acidification plus rinsing (MS-3) seems to eliminate the deceptive influence of NH4+-adsorption and -outgassing.