²¹⁰Pb, ¹³⁷Cs Tracing And Dating On Bohai Bay Coast: Modern Sedimentation And Implication

The study area was chosen on the west coast of Bohai Bay,40 short undisturbed cores(1～2m long) were used to study the modern sedimentation.The results show that different types of the ²¹⁰Pb and ¹³⁷Cs activity-depth profiles were identified,and the relationship between ²¹⁰Pb and ¹³⁷Cs activity and clay content was discussed,furthermore,the spatial distribution character of the modern sedimentation was analysised quantitatively.Three types of ²¹⁰Pb activity-depth profiles were identified,which were Ideal-decaying-type, 2) Wiggling with approximate equivalent amplitude,and 3) Episodic-event-influencing-type. Three types of the ¹³⁷Cs activity-depth profiles were identified,which were:A) Discontinuous ¹³⁷Cs curves,B) No peak can be identified continuous curves,and C) at least one peak can be identified relative ideal continuous ¹³⁷Cs curves.Theoretically,nine types of ²¹⁰Pb and ¹³⁷Cs activity-depth profiles(1A-1C,2A-2C and 3A-3C) were existed.However,six types of ²¹⁰Pb and ¹³⁷Cs curves from the nine types were discovered on Bohai Bay,which were(1) 1B,(2) 2B,(3) 2C, (4) 3A,(5) 3B,and(6) 3C.Moreover 3B was most commonly in the study area,which represented that intertidal zone were episodic events(storm surge) influencing environments with tidal currents,waves,and mixing activities;Moreover,the records of the storm surge during the last 50 years and ²¹⁰Pb activity-depth profiles supported this results.The relationship between the activity and clay content was studied,and the results showed that 1) no obvious relationship between the clay content and ¹³⁷Cs activity,2) linear relationship between the clay content and ²¹⁰Pb_exc activity was obvious.Furthermore,the liner equation between the clay content and ²¹⁰Pb_exc activity was constructed(R²=0.87),which provided the basic data for standardizing the ²¹⁰Pb_exc activity-depth profiles.Based on this equation,the ²¹⁰Pb_exc activity-depth profiles of 10 cores were standardized,which showed that the "low activity of ²¹⁰Pb_exc" was not caused by the change of clay content.At mean time,the inventory and sedimentation rates were discussed,and the results showed that the sedimentation rates increased with the inventory,but no obvious equation between them.The intertidal zone and parts shallow sea area from Huanghua Harbor to Dashentang on Baohai Bay,modern sedimentation rates showed different spatial distribution characteristics.The littoral currents from NW to SE was cut off by the seawall of Huanghua Harbor,which led to the sedimentation rates in less than～4km away from shoreline in the north of the Harbor were obviously higher than the southern area of the Harbor.5km far away from the shoreline,the sedimentation rates were higher in southern area of the harbor.Moreover,the site of HHG6 was a sedimentation center,the sedimentation rate was 1.36cm/a,which mean that sediments supplying were enough.Moreover,HHG6 was near the mouth of the two breakwater of the Harb in 2003,so port channel siltation will happen when the wind direction was similar with the breakwater direction.The sedimentation rates of the supratidal zone of Laolangtuozi in the north Huanghua Harbor was 2～3cm/a.The sedimentation rates decreased from Laolangtuozi to D-L1,and increased from D-L1 to Daxinpu village,till to Nanpaihe Count,where the sedimentation rate was 2.32～2.74cm/a.The sedimentation rate of Mapengkou was also 2.32～2.74cm/a.Considering the C4 and Q1 taken near Mapengkou,the sedimentation rates result showed that it was a modern deposition center on the intertidal zone from Nanpaihe Count to Mapengkou.The sedimentation rates of the supratidal zone on the south Haihe River were higher than northern.The sedimentation rates on the supratidal zone from Mapengkou to Dagu Pollution River showed decreased trend, and the sedimentation rates were all higher than lcm/a;the sedimentation rates of supratidal zone from Chengtougu to Dashentang showed decreased trend,and the sedimentation rates were all less than 0.84cm/a.There are four rivers in the study area into the sea,and high sedimentation rates happened to estuary except Dakouhe estuary on Bohai Bay.The cores taken from the estuary of Duliujianhe River,Dagu Plution River and Beitang Channel show that the sedimentation rates were all higher than lcm/a.The sedimentation rate of Beitang Channel was higher than 3.17cm/a, where was a sedimentation center.The sedimentation rates of Chengtougu fault in the north Haihe River were 0.30～0.34cm/a, which were lower than the supratidal zone next to it.The close sedimentation rates on both side of the fault meaned that no obvious sign to judge the fault active during the last fifty years.The highest latitude of modern living oyster reef——Dashentang living oyster reef——the shallow sea area next to Dashentang Village,the sedimentation rates near Dashentang live oyster reef were 0.75～0.85cm/a,which were higher than the supratidal zone next to it.Maybe,this sedimentation rates was caused by the construction of the living oyster reef.Compared with the growth rate of the oyster reef calculated by a colleague of our research group,the sedimentation rates were lower, which means that the Dashentang living oyster reef was still in the construction period.Finally,the use of modern sedimentation on geo-environmental changes of Bohai Bay was introduced and sumarized.In this paper,the results of the past were carefully studied,and a systematic summary had been done.Based on it,through the addition of 28 ²¹⁰Pb and ¹³⁷Cs data,a systematic study of modern sedimentation rate of intertidal zone and shallow sea of Bohai Bay were done for the first time,and gained the sedimentation rates(0～4cm/a).The discovery of this value,make up for the impact of(muddy) coastal processes and trends of the modern six basic elements——existing land surface elevation,modern sedimentation rates,plans to prevent environmental degradation,land subsidence,sea-level rise and extreme events——modern sedimentation rates.Based on these results,many programs have been done,such as "Tianjin intertidal zone land resources evaluation and selection","environmental stability evaluation of Huanghua Harbor","evaluation of fault activity","evaluation of the ecosystem of the living oyster reef",and so on.