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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.