Spatial distribution of organic and sulfur geochemical parameters of oxic to anoxic surface sediments in Beppu Bay in southwest Japan

Abstract

In order to elucidate the organic and sulfur geochemical processes of surface sediments associated with oxic/anoxic conditions, we have examined the relationship between the spatial distribution of sedimentary δ 13C, δ 15N, TOC, TN, TS, C/N ratio, and C/S ratio and environmental variables in Beppu Bay, Japan. The results of the principal component analysis and redundancy analysis of these geochemical parameters revealed two dominant patterns in their variations. The first pattern is characterized by variations in δ 13C, δ 15N, and C/N, which are closely related to the distance between the river mouth and sampling sites. This pattern is attributed to the mixing of organic materials from two sources—C 3 land plants and marine phytoplankton—rather than various alternation processes such as diagenesis and eutrophication. The second pattern is characterized by variations in TOC, TN, TS, and C/S. The RDA results showed that these geochemical parameters are closely related to the water depth and DO. Meanwhile, a simple correlation analysis also showed a good correlation of TOC, TN, TS, and C/S with bottom temperature and Eh. In Beppu Bay, a significant negative correlation is observed between the depth and the dissolved oxygen concentration and Eh, indicating that the deeper bottom environments have lower oxygen availability and are more reductive. According to these observations and some findings of previous studies, the spatial variations in TOC and TN are controlled by decomposition rates. This is in contrast to those in TS and C/S, by rate of pyrite formation. Both these rates are possibly influenced by temperature or oxic/anoxic conditions. The RDA results revealed that the two patterns in the variations of the geochemical parameters exhibit a perpendicular relation with respect to RDA axes. This indicates that the two geochemical groups are controlled only by their main factors. It is possible that TOC, TN, TS, and C/S are sensitive to temperature or oxic/anoxic conditions in Beppu Bay; in contrast, δ 13C, δ 15N, and C/N are less sensitive to these conditions.