Benthic Foraminiferal Assemblages And Their Environmental Implications In Surface Sediments Of The Yangtze River Estuary And Adjacent Sea Areas

The Yangtze River Estuary(YRE)and its adjacent Hangzhou Bay are located in the eastern edge of Eurasia,and are typical sea-land interaction zones in the middle latitudes and have been greatly affected by human activities year by year.Dissolved oxygen(DO)is an important physical and chemical parameter of water,which determines the growth level of organisms and is an important reference item for the pollution degree of sea water.Serious hypoxia has been found in many estuaries around the world,and the YRE is one of the most serious areas in the world.As a common used microfossil,benthic foraminifera is an important proxy index for the inversion of palaeo-environmental changes,and previous studies have shown that some species of benthic foraminifera have a good tolerance to hypoxia,which can be used as indicators of hypoxia seawater.Changes of benthic foraminifera show a significant periodic pattern during the geological evolution,which is considered to reflect DO in the YRE since the Holocene.In this paper,108 surface sediment samples were taken from the YRE and adjacent areas.Based on the statistics of benthic foraminifera species and planktonic foraminifera,combined with environmental parameters such as sediment grain size and TOC,the main factors affecting the distribution of foraminifera species and the differences of foraminifera assemblages under different environmental conditions were discussed.And in view of the YRE hypoxia phenomena,conclude that local LOFA(Low-oxygen foraminiferal assemblages)applies to the YRE ancient hypoxic event reconstruction,to explore coastal shelf ecosystem response to human activities and climate events.The main conclusion are as follows:(1)The study area was dominated by benthic foraminifera with small individuals,and planktonic foraminifera are rare.The abundance and diversity of foraminifera increased with the water depth,and the highest abundance was concentrated in the Yangtze Estuary subaqueous delta margin and pre-delta area,which were mainly affected by water depth,salinity,submarine topography and ocean current.(2)The benthic foraminiferal assemblages in different areas of the study area showed different assemblages: 1.The Hangzhou Bay Area is characterized by Eistominella naraensis-Ammonia convexidorsa-Elphidium magellanicum assemblage,which represents the nearshore shallow estuarine environment affected by strong tidal currents.2.The area in the north of the YRE is characterized by the Nonion akitaense-Ammonia compressiuscula-Ammonia pauciloclata assemblage,which indicates the nearshore characteristics influenced by low temperature coastal current.If the content of porcelaneous group and arenaceous group increases simultaneously,it can indicate a poor trophic environment under the influence of strong hydrodynamics.3.The Yangtze River Estuary Area is characterized by the Eistominella naraanensis-Bolivina cochei-Ammonia ketienziensis assemblage,which indicates the high organic matter flux and low dissolved oxygen environment outside the YRE,and the distribution depth of the assemblage is generally over 30 m.(3)Epistominella naraensis,Bolivina spp.and Bulimina spp.in the YRE have a good tolerance to hypoxic environment,and can be used as the LOFA of the Yangtze estuary.Compared with reflecting the intensification of hypoxic environment caused by human activities in the last 100 years,LOFA of the YRE has a better indication for the paleoclimatic cycles in geological history since the Holocene.