Chemical and isotopic alteration of organic matter during early diagenesis: Evidence from the coastal area off-shore the Pearl River estuary, south China

In addition to the kinetic isotopic fractionation associated with the biodegradation of OM, formation and degradation of bacterial biomass contributed significantly to the observed change of δ13C and δ15N during diagenesis. Although the bacteria biomass was believed to be rich in 13C relative to the substrate, bacteria biosynthesis also produced 13C-rich and 13C-poor fractions, and the subsequent biodegradation preferentially decomposes the 13C-rich compound classes and the 13C-rich compounds in a specific class as well, which made the δ13C of remaining organic matter similar to the substrate in the sediment. On the other hand, the low δ15N of the POM and sedimentary OM relative to the fresh plankton was resulted from the addition of 15N-depleted biomass that was possibly generated by the preferential uptake of 15N-depleted ammonium during bacterial growth.