Living and dead benthic foraminifera assemblages in the Bohai and northern Yellow Seas: Seasonal distributions and paleoenvironmental implications

摘要

Benthic foraminifera fossil shells are particularly useful in reconstructing water depth, temperature, the exported flux of organic carbon to the sea floor, and the level of bottom-water oxygenation in paleo-environments. In this study, we investigated the living (stained) and dead (thanatocoenoses) benthic foraminifera assemblages collected in surface sediment samples from the Bohai Sea (BS) and the northern Yellow Sea (NYS). The samples were collected from different seasons, i.e. 50 samples in May (spring) and 46 samples in November (autumn). Our benthic foraminifera fauna analysis of these samples shows four living assemblages in May and three in November, indicating a sensitive response to seasonal environmental changes where the fauna live in the modern setting of the BS and NYS. Our redundancy analysis (RDA) of the living assemblages and their corresponding environmental parameters indicates that in spring, the abundances of Astrononion tasmanensis, Nonionella Stella and Bulimina sp. have positive correlations with increased water depth, density and salinity, but are negatively correlated with increased FLOUR (chlorophyll). The abundances of Buccella frigid and Vemeuilinulla advena are positively correlated with higher levels of dissolved oxygen (DO) and negatively correlated with increases in temperature. The abundances of Cribrononion subincertum are positively correlated with increased turbidity (TURB) and temperature, but negatively correlated with increased DO. In autumn, there is a positive correlation between the abundances of V. advena and increased salinity. Protelphidium tuberculatum. Ammonia beccarii (vars.) abundances, however, have negative correlations with salinity. During the autumn, the benthic foraminifera species P. tuberculatum was noticeably expanded and the dominance areas occupied by Elphidium magellanicum and C. subincertum were significantly reduced in the BS; along the southern coast of the Liaodong Peninsula, the dominant Buccella frigida in the spring was replaced by V. advena in autumn. The seasonal shifting of benthic foraminifera species are local responses to the combined changes in bottom water temperature, salinity, TURB and DO in the BS and NYS, and also to the more intensified autumn/winter intrusion of the Yellow Sea Warm Current (YSWC). The dead assemblages of benthic foraminifera, which are used to reconstruct paleoenvironments, exhibited considerable destruction of agglutinated assemblages. We found that such species as V. advena, Trochammina sp., Ammoscalaria sp. and Polskiammina asiatica are missing from the dead assemblages, possibly due to postmortem taphonomic processes in the BS and NYS. Considerable destruction of agglutinated species in dead assemblages may slightly bias our estimates of paleoenvironmental parameters when they are based on benthic foraminifera fossil shells.