Environment-marine ecosystem association in East Asia : biogeographical and paleoecological approaches using microfossil ostracodes and foraminifera

Abstract

Marine microfossils (i.e. small organisms with excellent fossil records) such as benthic ostracodes and foraminifera are known for their large spatiotemporal coverage and high abundance in various marine (broadly defined as shallow marine, estuarine, brackish and deep sea) environments. They have been increasingly used as model systems to spatially and temporally reveal interrelationships between environment and marine benthic organisms. Although our understanding of both the modern and historical biospheres in East Asia has advanced over past decades, several relatively poorly investigated, yet significant questions remain: (1) How do Taiwanese ostracodes relate to ostracodes in the China Seas? (2) How have the growing anthropogenic activities in the Yangtze River basin affected the benthic fauna at the Yangtze River discharge area? (3) On a decadal to centennial scale, could oceanic proxies provide new insights into the spatial variability of the East Asian Summer Monsoon strength over the past millennium? (4) On a millennial to multimillenial scale, how did the Northwestern Pacific deep-sea fauna respond to the (abrupt) climate changes during the last glacial period and the Holocene? This study primarily aims to address these research gaps using benthic ostracode and foraminiferal records. Modern shallow marine ostracode compositions and diversity were analyzed from 40 sediment samples in an area covering the East China Sea, the Taiwan waters and the northeastern South China Sea. The results indicated that the Taiwanense ostracodes have a close association with the Chinese ostracodes. Linear regression analysis revealed that ostracode diversity was significantly correlated to water depth (positive) and surface production (negative) while species composition was correlated with water depth, surface production, seasonality of production and salinity. 29 sediment samples in the Yangtze River estuary and its adjacent East China Sea were collected to examine changes of ostracode species composition and diversity from the 1980s to the 2010s. The 2010s ostracodes showed relatively higher diversity and density. Percent abundance of relatively offshore, oceanic species prominently increased towards landward direction, probably reflecting a response to inshore intrusion of saline water. A sediment core was retrieved in the outer lake of Hwajinpo to reconstruct EASM variability during past millennium using foraminiferal and ostracode records. Four EASM strengthening Events 1–4 at A.D. 1900, 1550, 1450 and 1250, respectively, were identified. Events 2 and 3 are largely restricted in the coast of East Asia, and Events 1 and 4 could be pervasive both in coastal region and continental China. Millennial and multimillennial (orbital) scale deep-sea ostracode records were reconstructed using a sediment core in the central Okinawa Trough. The results showed that the variations of in situ deep sea ostracode composition exhibited a precession-period of ~21, 000 years, which was also shared by variations of total organic carbon contents (i.e., a surface production proxy). This suggested potential responses of deep-sea benthic fauna to flux of organic matter in the Okinawa Trough. This study improved our understanding of marine environment-biotic interrelationships in East Asia. More importantly, it could facilitate further investigations on a broad range of disciplinary (including biogeographical, paleoceanographical and paleoclimate) research.