Direct evidence for the alteration of 13C natural abundances during early diagenesis in Lake Kasumigaura, Japan

In sediment trap materials or suspended particulate organic matter, delta C-13 values must be averaged over at least several years to determine delta C-13 shifts during sedimentation. To overcome this problem, we studied isotopic modification of sedimentary organic carbon during early diagenesis in sediment samples collected intermittently from a site at the center of Lake Kasumigaura, Japan, over a period of nearly 30 years, beginning in 1979. We examined the degree of apparent isotope discrimination by comparing long-term changes in the carbon isotope composition of the surface sediment (top 2 cm) with the isotope composition profile from 0 to 15 cm. The downcore C isotope composition profile was close to the values calculated from the long-term changes in the isotope composition of the surface sediment, suggesting that the sedimentary carbon isotope composition was generally conserved (+/-0.6%) during early diagenesis (up to 30 years). However, at greater core depths, significant C-13 enrichment of up to 1.5 parts per thousand was sometimes observed, especially in the periods of relatively oxidative conditions.