Geochemical analyses of biogenic opal, total organic carbon (TOC), total sulfur (TS), and stable sulfur isotopic composition were conducted on the middle Eocene section of the ACEX cores obtained by IODP Expedition 302. The analyzed %TS contents were high in all the examined intervals, indicating sufficient sea water was present in the deep layer of the paleo Arctic basin in contrast with the low salinity surface waters determined by freshwater siliceous microfossils. From the high %TS in Lithological Unit 1/6, the extent of sea water supply from outside of the Arctic basin appeared to have increased after 45 Ma. The euxinic condition of the bottom water is suggested by the TOC-TS diagram. The anoxic environment was brought about by the estuarine type circulation pattern and salinity stratification. The light sulfur isotope composition (delta (super 34) S) indicates the microbial sulfate reduction in an open system. The trend of delta (super 34) S shift toward lower values with time is an indication of the history that the water exchange between the Arctic Ocean and the outside basin became progressively extensive. The high %TS, the plotted pattern of TOC vs TS, and the extremely low sulfur isotopic values are unique and not comparable with the modern analogous such as the Black Sea. These results illustrate that the middle Eocene Arctic Ocean retained characteristically unique environments on the geochemical aspect.