Interpreting holocene sea-level changes using fossil diatom assemblages and pyrite formation in the sedimentary environment of the Fujii area, Okayama Plain, Japan

Pyrite sulfur and fossil diatom analysis, coupled with the radiocarbon dating, provides important information regarding the sedimentary environment and past sea level fluctuations. Since this knowledge is beneficial in predicting future climate change patterns and sea level variations, the present study examines the paleo sea-level changes of Fujii, Okayama Prefecture, Japan, using fossil diatom assemblage and sedimentary pyrite formation. Based on the pyrite sulfur content, a marine environment has been developed at a depth of 1.8 meters around 6,745 ± 80 years BP. This result has been verified with the finding of high pyrite sulfur percentage and the presence of marine diatoms. Based on the paleo salinity variations defined by concentrations of pyrite sulfur, a sea level rising period has been observed from 11,035 ± 160 years BP to 6,745 ± 80 years BP (from 2.4 m to 1.8 m depth), and it perfectly aligns with the sea level changes in the early Holocene period. Additionally, the diatom assemblage in this depth change from freshwater (i.e. Navicula sp., Melosira sp.) to marine (i.e. Melosira architecturalis Brun, Nitzschia sp.), supporting the findings. Finally, the reducing pyrite concentration above 1.8 m reflects the reduced marine influence due to receding sea levels and the introduction of more freshwater, which is supported by the presence of more freshwater diatoms.