Temporal Changes in Holocene Ostracode Faunas in North Pacific Intermediate Water

Abstract

The North Pacific Intermediate Water (NPIW), defined as the water mass with salinity minima at depths between ca. 300 and 800 m, is widely distributed around the North Pacific Ocean. It is influential in the dynamics of the North Pacific subtropical circulation, thus it is important to comprehensively understand the NPIW behavior. However, its behavior throughout geological time is not well understood, in particular, for the low-latitude ocean. Therefore, we investigated temporal changes in paleoenvironments under the influence of NPIW in the low-latitude ocean during the last 15,000 years on the basis of high-resolution statistical analyses of the fossil Ostracoda (Crustacea) as an effective environmental indicator. For the investigation, we used a sediment core obtained from the southern part of Makassar Strait, the eastern Java Sea (site: 70GGC, water depth: 482 m, latitude: 3°34′S, longitude: 119°23′E). In the process, more than 100 ostracode species were recorded in the study sequences. To reveal the transition of the bottom water environment, we investigated fossil ostracode faunal composition and species diversity changes during the deposition of the study site. The faunal composition drastically changed from ca. 7,500 to 6,000 years BP. The species diversity rapidly increased between ca. 13,000 and 11,000 years BP. According to a previous study, using high-resolution proxy records from sediment cores in the Java Sea, intermediate water temperatures at 500 m depth decreased after ca. 6,000 years BP. Therefore, it is possible that ostracode faunal changes were associated with decreasing intermediate water temperatures. Moreover, the peak of the ostracode species diversity between ca. 13,000 and 11,000 years BP coincided with the Younger Dryas cold event (YD). The abrupt change in the species diversity in this interval appears connected to the YD. We conclude that the ostracode assemblage at this site is sensitive to climatic change and is very useful for the reconstruction of ocean environments of NPIW throughout geological time.