Geochemistry of modern carbonaceous sediments overlain by a water mass showing photic zone anoxia in the saline meromictic Lake Kai-ike, southwest Japan: I. Early diagenesis of organic carbon, nitrogen, and phosphorus

The early diagenetic effects of organic matter (OM) decomposition in carbonaceous sediments overlain by an anoxic water body were investigated using a sediment core (KAI4) from the saline meromictic Lake Kai-ike, in southwest Japan. Lake Kai-ike, a current model for past anoxic ocean environments, is characterized by development of photic zone anoxia and a “bacterial plate,” which create permanent density stratification in the water. Over the last ~ 40 years, rapid decreases in organic carbon (Corg; ~ 55%), nitrogen (~ 60%), and reactive phosphorus (ΔP; ~ 85%) contents accompanied by ~ 1‰ increase (positive shift) in the stable isotope compositions of Corg (δ13Corg), which is attributed to a preferential loss of low-δ13Corg components during early diagenesis, are reflected with increasing depth of the sediments. The molar Corg/ΔP and N/ΔP ratios increase with depth, from initial values that are very close to those of the Redfield ratio. During the early diagenesis of OM degradation, the decomposition rate of P is much faster than that of Corg and N. In contrast, the molar Corg/N ratios are rather insensitive to various biogeochemical reactions, confirming their utility as a proxy for source OM. Although the stable isotope compositions of nitrogen (δ15N) in sedimentary OM are useful indicators of OM sources, correlations to the above-mentioned parameters for the KAI4 core remain unclear. We emphasize that the OM present in ancient carbonaceous sediments or sedimentary rocks was most likely subject to very rapid (< 40 years) decomposition after its initial burial, and that the original Corg, N, and P contents at the time of deposition could have been substantially higher than the preserved (measured) contents. The same cautions may be applied to the δ13Corg and δ15N values of carbonaceous sediments and sedimentary rocks, which also undergo a small degree of positive shift (~ 1‰) during early diagenesis. To better reconstruct the paleoenvironmental conditions for redox-stratified oceans and lakes using the geochemistries of sediments or sedimentary rocks (i.e., Corg, N, and P), one must take into account how rapid OM decomposition during early diagenesis affects the related biogeochemical proxies. This study has implications for the geochemical reconstructions of past anoxic environments, such as those during the Cretaceous OAEs (oceanic anoxic events) and the early Earth.