Paleo Environment Reconstruction Of Zhi Dan Yuan Sluice Dating From Yuan Dynasty In Shanghai And Discussion On Controlling Factors Of Its Construction And Disuse

The Zhi-Dan-Yuan sluice site,located in Shanghai was discovered during real estimate development and construction,in May,2001.It is reported that the sluice was constructed by an irrigationist named Renfa Ren in Yuan Dynasty and is about 700 years old.The sluice site was evaluated as one of the ten new-findings of Environment Archaeology of China reported in 2006 for its magnitude dimension, skillful construction and perfect conservation.The finding of the Zhi-Dan-Yuan sluice provides some new stuff for the study of human-environment relationship in Shanghai area during the past 1500 years.And the sluice is also an important visual evidence for studies of river evolvement,conservancy projects and shipping development in this area.With the theory of environmental archaeology,geography and other correlative subjects,this thesis tends to explain the Holocene environment evolution of the Taihu dish-like depression with high resolution and reconstruct the palaeo hydrology of the Palaeo Wusongjiang River since 1500 years ago.The results can further explain why the Zhi-Dan-Yuan sluice was constructed and why it was deserted only decades of years after its construction.Our present study is chiefly based on the sediments taken by one borehole(E) and five profiles(A,B,C,F,G)from the site.The buried depth of the borehole and five profiles is:6.7～25 m(E),4.3～5.1 m(A),5.1～6.7 m(B),4.5～7.05 m(C), 4.7～6.5 m(F),5.02～6.27m(G)respectively.The forming time of the deposition base of E approximated to early Holocene.The profile A and B were formed before the construction of the Zhi-Dan-Yuan sluice,instead the profile C,F,G were formed in river channel after the sluice was constructed.The samples,obtained from the sediments of the borehole and the five profiles,are taken for experiments in the laboratory,including grain size analysis(totally 195 samples),and foraminifer analysis(totally 166 samples).Eight samples were selected for AMS(Accelerator Mass Spectrum)dating in the Quaternary Dating Laboratory of Peking University. Sixty-six samples were selected for magnetic fabric analysis in Research Center of Nanjing Institute of Geology and Mineral Resources.The sediments of the Holocene borehole E,at a buried depth below 17m, primarily composed of clayey silt and characterized by a lot of Fe-oxides and carbonized plants,deposited about 7.5kaBP according to the results of radiate carbon dating,indicate a coastal plain and tidal flat depositional environment.The stratigraphic lithology and the results of AMS-¹⁴C dating and foraminifer analysis demonstrate that the sediments,buried between 13～17m,are deposited during Mid-Holocene.Estuary-shallow marine depositional environment dominated during 7～4kaBP.Since 4000kaBP,the coastline advanced towards the sea quickly,due to the relative stable sea level and the southeast migration of Yangtze delta.During this period the sedimentary environment shifted towards tidal flat and delta facies.And then,the riverine process by Palaeo Wusongjiang River prevailed in the study area during the past 2000 years.Obviously,the construction and disuse of the Zhi-Dan-Yuan sluice are correlated with the evolution of Palaeo Wusongjiang river.The values of magnetic fabric parameters P,F in profile B are higher than 1.06, while all of L and q are lower than 1.02 and 0.3 respectively in the same profile.It can be inferred that riverine process dominated in profile B and its hydrodynamics is relative strong and stable.The character of magnetic fabric parameters of profile A suggests that tidal incursion occurred,though riverine process prevailed.Through comparing the magnetic fabric parameters of profile A and B,we find both the hydrodynamics and the stability of the sedimentary environment declined gradually. The character of magnetic fabric parameters of profile G,similar to profile B with relative higher P,F(basically higher than 1.06)and lower L,q(less than 0.4), suggests relative strong and stable hydrodynamics.On the contrary,the F,L of profile F fluctuate around 1.02 with opposite tendency,while the q fluctuates around 0.5,even sometimes higher than 0.7.The differences of the magnetic fabric parameters between profile F and G suggest strong and stable riverine depositional environment in profile G and unstable depositional environment dominated by both riverine and tidal processes in Profile F.According to the data of grain size analysis,we find that the sediments of Profile B,primarily composed of silt(about 60～80%),are much coarser.The coarsest grain size in Profile B can reach 50μm.Whereas the sediments of Profile A are much finer and mainly composed of clayey silt.Silt accounts for 65～80%while clay accounts for 15～30%.The mean grain size of Profile A is about 10～25μm.We can infer that the hydrodynamics of Palaeo-Wusongjiang River is quite weak and siltation in the low Wusongjiang River is remarkably serious before the construction of Zhi-Dan-Yuan sluice.The sediments of profile C,F,G,mainly composed of silt (about 80%),become finer from the bottom to top.And the mean grain size of the sediments of the three profiles is near 30μm.It is suggested that the construction of the Zhi-Dan-Yuan sluice still could not restore the original hydrological condition of the Palaeo Wusongjiang River.The siltation of the Palaeo Wusongjiang River is still serious.From the date of the sedimentation rate during the past 10ka years,we know that the sedimentation rate of the study area was 1.6m/ka during 10～2kaBP, 3.6m/ka during 2000～1450aBP,4.6m/ka during 500～1250AD,and even 7m/ka after 1250AD.According to the date above,it is known that the rate was very low during 2～10kaBP.However,since 2000aBP,the rate accelerated quickly.Especially the rate reached a very high level after the construction of the Zhi-Dan-Yuan sluice.The southward migration of the Yangtze River mouth about 1500 years ago bring plenty sediments into the Shanghai area,and lead to the accelerated deposition of the coastal plain.The incursion of the tidal current took abundant sediments into the Wusongjiang River.It is difficult to drainage the water and silt due to the morphology of dish-like depression.The sediments silted the original vast low Palaeo Wusongjiang River and estuary soon.Difficulties in freshwater drainage and incursions of the tidal current brought great inconvenience for people's daily life in Shanghai area.Consequently,irrigationists in history managed to dredge the Palaeo Wusongjiang River and construct lots of sluices in order to block the tide and drainage the silt,meanwhile to minimize the loss caused by the natural calamities. Historical records said that the situation of drainage and tide block became especially worse in Yuan Dynasty.Hence,Renfa Ren,an famous irrigationist,together with local people,constructed Zhaopu sluice,namely the Zhi-Dan-Yuan sluice now,at the junction of the Palaeo Wusongjiang River and its distributary,Zhaopu river,with the purpose to block the tide when tiding by closing the sluice and drainage the silt when ebbing by opening the sluice.Results of magnetic fabric analysis and foraminifer analysis reveal that river process is obviously dominated at the upstream of the sluice while the downstream of the sluice was effected by both river process and tide process.It can be inferred that the Zhi-Dan-Yuan sluice do performed its expected function during the early period after its construction and fulfilled the irrigationist's desire partly.However,the results also indicate that the construction of the Zhi-Dan-Yuan sluice still could not restore the original hydrological condition and still could not change the silt tendency. What' more,the sedimentary rate since the Yuan Dynasty reached extremely high, about 7m/ka.And the siltation of Palaeo Wusongjiang River became worse and worse.Above all,the abandon of the Zhi-Dan-Yuan sluice is closely correlated with the siltation of Palaeo-Wusongjiang River.Since the silt tendency could not be changed, the abandon of the Zhi-Dan-Yuan sluice is inevitable.We found that increasing siltation during the past 1500 years is induced by geomorphologic change of the Taihu drainage basin in response to the sea level rise.And we indicate that the siltation of the Wu-song-jiang waterway system is the result of this evolution.It is predicted that the sea level around Shanghai area would raise about 0.5m in the next 50 years. The Taihu drainage basin would confront much sever disasters caused by incursions of the tidal current and flood at that time.We suggest future management of the Taihu drainage basin should obey the law of geomorphologic change in association with the sea level rise.