Centurial Hypoxic Events Reflected By Benthic Foraminifera Assemblages From The Estuary Of The Yangtze River

During the rencent century,hypoxia phenomenon have been found in the major estuaries and coastal around the world.The Yangtze river is the first river in China,the hypoxia phenomena of estuary is developing toward the worsening trend over the past few decades.Although the previous studies have been done in many aspects of the Yangtze River Estuary Hypoxia area,but few works have focused on the foraminiferal studies,especially in the study of the change of hypoxia in one hundred years.Using the suitable Low-oxygen foraminiferal assemblages(LOFA)index in the study area,to analyse the foraminiferal assemblages of CDZS-38,CDZS-39 two column,combined with 210Pb dating and grain size etc.,discussed the hundred years District of the Yangtze River to the hypoxic episodes.The Yangtze River estuary hypoxia phenomena is mainly resulted from the combined effect of dissolved oxygen consumption,caused by high nutrient,and the resistance effect on the supplement of dissolved oxygen,which is caused by the halocline,a consequence of the interaction between diluted water and the upwelling current.CDZS-38,CDZS-39 two holes were all in the low oxygen environment,but the cylindrical sample were different from bottom to top.CDZS-38 hole has an obvious change trend,that is,from 1978 began to appear hypoxia phenomena obviously,getting worsen after 1994,and the two phase fluctuations are larger.CDZS-39 hole in one smooth,although has been in a low oxygen environment,but in addition to about 1994～2002 have become serious and the phenomenon of volatility,other phases hypoxia phenomena is not very prominent.The hypoxic events were reflected by this research of the Yangtze estuary area included:(1)The age when hypoxia phenomenon occured shows consistency with the result of Marine census;(2)Since the implementation of the reform and opening up policy in 1978,the increased human activities have intensified the hypoxia phenomenon of the researched area;(3)After 1980s,in the place where the two holes located,the time when low-oxygen phenomenon started and aggravated is consistent with the beginning of warming waters;(4)The extraordinary flood along the Yangtze River in 1998 inputed amounts of nutrients,and the deffusing range of Yangtze River diluted water expanded,these all accelerated the hypoxia phenomenon of Yangtze River estuary;(5)Construction and completion of China Yangtze three gorge project led to the environmental changes of estuary region,that in 1994,2002 years the two hole foraminiferal assemblages are also reflected in the changes;(6)Hypoxia phenomenon intensified in summer;(7)The influence exerted by El nino on foraminiferal abundances and indicator species percentage had obvious impact on climate.