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

Abstract

In sediment trap materials or suspended particulate organic matter, δ13C values must be averaged over at least several years to determine δ13C 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 13C enrichment of up to 1.5‰ was sometimes observed, especially in the periods of relatively oxidative conditions. Copyright 2011 by the American Geophysical Union.