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Direct evidence for nitrogen isotope discrimination during sedimentation and early diagenesis in Lake Kasumigaura, Japan |
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| Authors: | Ayato Kohzu Akio ImaiToshihiro Miyajima Takehiko FukushimaKazuo Matsushige Kazuhiro KomatsuNobuyuki Kawasaki Shingo MiuraTakayuki Sato |
| Institution: | a Water and Soil Environment Division, National Institute for Environmental Studies, Ibaraki 305-8506, Japan b Ocean Research Institute, The University of Tokyo, Tokyo 164-8639, Japan c Graduate School of Life and Environmental Sciences, University of Tsukuba, Ibaraki 305-8572, Japan |
| Abstract: | Sediment core samples from the center of Lake Kasumigaura, Japan, were collected from 1979 to 2007 at intervals of 1 month to 5 yr. We evaluated the degree of modification in N isotope composition during sedimentation and diagenesis. We estimated the degree of isotope discrimination during diagenesis by comparing historical changes in N isotope composition of the surface sediment (top 2 cm) against the vertical profile of the isotope composition of sediment core samples (15 cm depth). The degree of 15N enrichment during sedimentation appeared to be significant under the preferential N decomposition that occurred in the periods with low C/N ratio values of suspended particulate organic matter. We documented 15N depletion in sediment deeper than approximately 3 cm during diagenesis. The contrasting directions of N isotope discrimination during sedimentation and diagenesis suggest changing mechanisms of isotopic shift across an oxidation-reduction boundary. |
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