Paleoceanography in Northwest Pacific during Early Albian: Global Carbon Cycle and the Related Ocean Environments

The major components (organic carbon and carbonate) and inorganic elements are analyzed in the sediments from the Okusakainosawa and Chirashinaigawa sections in the middle to north Hokkaido, Japan. It is estimated that the Yezo basin was located on a continental slope at the middle latitude of the northwest Pacific from late Aptian to early Albian. Total organic carbon contents vary from 0.30 to 1.12 wt. %, averaging 0.79 wt. % in the Okusakainosawa section, and from 0.32 to 0.95 wt. % in the samples of the Chirashinaigawa section. The black shale is not recognized at the studied area. The mean C_organic/N atomic ratios of samples reveal about 20, which shows that most of the organic matter was terrigenous in origin at the Okusakainosawa section, while the organic matter is a mixture of marine and terrestrial organic carbon in the Chirashinaigawa section. Namely, terrestrial organic matter had been a main contributor in both sections and labile organic matter, which could reduce dissolved oxygen content very little in the bottom water, hardly deposited during early Albian. On the other hand, both sections are characterized by very low occurrence of carbonate due to dissolution through the water column and/or on the seafloor. As the cause of low content of marine organic carbon in both sections, the primary records of carbonate preservation were affected by seawater chemistry rather than by local rain of organic matter. Based upon the organic carbon content at the Tethys, the severe dissolution of carbonate is attributed to the degraded intermediate and deep seawater rather than the released carbon dioxide by the local rain. Hence, there is some possibility that the intermediate-deep water at the northwest Pacific Ocean was one of the oldest during mid Cretaceous. Nevertheless the dissolved oxygen in the bottom water had not been completely consumed during early Albian from the behavior of Mn in the sediment. In contrast, dissolved oxygen is completely consumed in the northwest Atlantic although all carbonate is well preserved. If any possibility is adopted, big differences exist between the north Pacific and Atlantic during early Albian. Actually, the organic rich strata were only deposited at the limited area in the Pacific as compared with the Atlantic. It is suggested that the Pacific seldom experienced the reduced conditions spatially and/or temporally in contrast to the Tethys Sea. These observations point out one possibility that the Pacific and Atlantic had each original chemical composition of intermediate and deep water and different conveyor belt thermohaline circulation may exist at least during early Albian.