Alteration of 15 N / 14 N and 13 C / 12 C ratios of plant matter during the initial stages of diagenesis: Studies utilizing archaeological specimens from Peru

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Scientific paper

The stable carbon and nitrogen isotope ratios of plants extracted from Peruvian archaeological sites, ranging in age from 400 to 4000 years B.P., have been measured. The 13 C and 15 N values of prehistoric plants that were carbonized prior to deposition are similar to those of modern plants which have the same carbon dioxide fixation or nitrogen assimilation mechanisms. In contrast, the 15 N values of prehistoric plants that were not carbonized are generally 10-20%. and as much as 35%. more positive than those of their modern counterparts. The 13 C and 15 N values of different parts of prehistoric uncarbonized plants differ by as much as 8%. and 21%. respectively, whereas the same parts of modern plants have 13 C or 15 N values that lie within 2%. of one another. SEM analysis etiminated the possibility that diagenetic alteration of the isotope ratios of the prehistoric uncarbonized plants was caused by the adsorption of particulate soil matter. Isotopic analysis of the organic residues that remained after the prehistoric uncarbonized plants were treated to remove fractions that might have been added in the depositional environment indicated that the alteration of the 13 C values resulted from the adsorption of humic and fulvic acids, whereas the diagenetic shifts of the nitrogen isotope ratios were not caused by adsorption of 15 N -enriched organic or inorganic forms of nitrogen from the soil. Our observations do not permit us to speculate on a mechanism that accounts for the 13 N -enrichment of the prehistoric uncarbonized plant remains, although the excellent state of preservation of these samples suggests that isotopic fractionation during microbial decomposition was not the cause. The observation that the isotope ratios of the prehistoric carbonized plants are similar to those of their modern counterparts indicates that it will be possible to separate carbonized plants residues into three groups--legumes, non-leguminous C 3 plants or C 4 and CAM plants--based on their 13 C and 15 N values. This method should provide useful information to archaeologists working in environments in which carbonized plant remains, often burned beyond morphological recognition, are the only archaeobotanical remains which are recovered.

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