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

Abstract

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 paleoenvironments. 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 Verneuilinulla 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.

Introduction

Living benthic foraminifera have a short life cycle (∼3 months to 2 years) (Murray, 1991), so the organisms are able to respond rapidly to environmental changes, with changes in diversity and the composition of foraminifera assemblages. As for other benthic marine species, the distributions of living benthic foraminifera are limited by multiple environmental parameters at different time and space scales (Murray, 2006). Such rapid adaptive responses of living benthic foraminifera fauna assemblages to temperature and salinity are well documented (e.g. McLusky and Elliott, 2004, Murray, 2006). Benthic foraminifera responses to various marine environmental parameters such as dissolved oxygen (DO) conditions (Alve, 1995, Gooday et al., 2000, Langezaal et al., 2003), substrates (Hottinger, 1984, Hottinger, 1990), the flux of organic matter (e.g. Epistominella exigua, Uvigerina paregrina, Eponides pusillus, Bolivina pacifica) (Corliss and Emerson, 1990, Corliss, 1991, Rathburn and Corliss, 1994, Jorissen et al., 1995, Gooday et al., 2000), and tides and currents (Murray, 2006) have been reported. Consequently, dead assemblages of benthic foraminifera give us time-averaged faunal compositions, which integrate different seasonal conditions of changing inhabited environmental parameters on a limited spatial scale (Duros et al., 2012). However, factors related to population dynamics, seasonal variability in standing stocks, and interspecific differences in the reproduction rates of benthic foraminifera assemblages, or postmortem/taphonomic processes in the burial of the foraminiferal fossil shells, can create considerable differences between the living and dead assemblages (Jorissen and Wittling, 1999, de Stigter et al., 1999, Gooday, 2003).

Numerous marine cruise surveys for studying benthic foraminifera assemblage distributions and their relationships to modern environmental parameters have been conducted in the Bohai Sea (BS) and North Yellow Sea (NYS) based on dead assemblages from surface sediment samples collected during cruises (Sun et al., 2009, Wang et al., 2009, Wang et al., 2011). Few investigations have examined the living benthic assemblages and such studies have usually been restricted to intertidal areas (Li et al., 2009). To date, there has been no extensive survey of living benthic foraminifera assemblages in the BS and NYS, a dynamic physical oceanographic component of the western Pacific marginal seas. In order to provide a data base for more robust usage of benthic foraminifera assemblages as a tool for paleoenvironmental reconstructions in the marginal western Pacific, we here report, for the first time, the results from seasonal marine cruise investigations of the distribution patterns of living and dead (thanatocoenose) benthic foraminifera collected from the surface sediments of the BS and NYS during May and November, 2012 (Fig. 1). The investigations had the following objectives: (1) mapping the seasonal distributions of living and dead benthic foraminifera assemblages in the BS and NYS; (2) identifying any major differences between the dominant faunal distribution or compositional patterns from different seasons, and between living/dead assemblages; (3) exploring the relationships among the dominant living assemblages and multiple environmental parameters based on redundancy analysis (RDA); and (4) based on all the above, advancing our understanding of the possible factors causing any differences between the living and dead assemblages, with important implications for future applications of benthic foraminiferal fossil shells in paleoenvironmental reconstructions.