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Direct evidence for nitrogen isotope discrimination during sedimentation and early diagenesis in Lake Kasumigaura, Japan
- Kohzu, Ayato, Imai, Akio, Miyajima, Toshihiro, Fukushima, Takehiko, Matsushige, Kazuo, Komatsu, Kazuhiro, Kawasaki, Nobuyuki, Miura, Shingo, Sato, Takayuki
- Organic geochemistry 2011 v.42 no.2 pp. 173-183
- isotopes, lakes, nitrogen, organic matter, sediments, Japan
- Sediment core samples from the center of Lake Kasumigaura, Japan, were collected from 1979 to 2007 at intervals of 1month to 5yr. 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 2cm) against the vertical profile of the isotope composition of sediment core samples (15cm 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 3cm during diagenesis. The contrasting directions of N isotope discrimination during sedimentation and diagenesis suggest changing mechanisms of isotopic shift across an oxidation–reduction boundary.