Nghiên cứu vùng đới bờ đồng bằng sông mê kông và thềm lục địa liền kề halocene hiện tại phục vụ cho phát triển bền vững

HQi thao khoa hpc - Hgp tac Quflc tg digu tta, nghign cihi titi nguygn va m6i truimg bign TO STUDY COASTAL ZONE OF MEKONG DELTA AND ADJACENT SHELF IN HOLOCENE-RECENT FOR SUSTAINABLE DEVELOPMENTS Phung van Fhach' Karl Stattegger^, Nguyen Trung Thinh', Klaus Schwarzer^, Biii Vift Dung', Daniel Unverricht^ Nguyen Ti^n Hai', Rik Tjallingii^ 1-lnstilute of Marine Geology and Geophysics (IMGG); Address: 18- Hoang Qudc Viet, Cdu Gidy, Hd Ngi Email: pvphach@yahoo.com 2- Institute of Geology, Kiel University (CA U), Germany Abstract: Vietnam-German scientific collaboration on marine investigation since 1996 has been achieved very good results and it can be served as a good example in intemationgl cooperation for Eastem Vietnam Sea research The collaboration was supported by high ranking agreements between Viet Nam and Germany These agreements have played an important legal role to accelerate efforts of scientists of two countries to carry out their joint projects on Eastern Vietnam Sea researches In the framework of Vietnam MOST (Ministry of Science and technology) and German BMBF (Ministry of Education and Research) agreement, a Joint project "Holocene Coastal Evolution, Sea-Level Fluctuations, Terrigenous Sedimentation and Sediment Dynamics on the Continental Shelf between the Mekong Delta and Nha Trang, SE-Vietnam" The counterparts are Institute of Marine Geology and Geophysics (IMGG) from Viet Nam and Kiel University (CAU) from Germany The joint project had been implemented in two phases: the phase I (2003-2006) and phase // (2006-2009) Investigations in the field of coastal and marine geology within the joint German-Vietnamese project aim lo (i) reconstruct the spatial-temporal history of the hitherto, only sparsely known as coastal evolution in SEVielnam during the Holocene period; (ii) to analyse modem coastal and shelf dynamics; and (iii) to compare the Holocene coastal evolution with the present coastal processes in order to (iv) evidence and estimate actual changes in the coastal depositional system After completing research in the larger regional framework of shelf and shore in SE Vietnam, the investigations in Phase II were focused on the youngest and ongoing period of the Mekong Delta evolution The results of investigation of the South East Shelf architecture, the outbuilding of Mekong Delta, the sediment transport from the Mekong distributaries to Ca Mau Peninsula and sedimenl accretion around it must Phung van Ph4ch 215 Woricshop: •tatenutJomJCooEggiffi-"" 'nvestiaation and Research of Marine NaUual Resomce and EnvimitM;^ help to improve the predictability of the future evolution of this complex delta system and the adjacent shelf Our results have supported planning strategies for integrated coastal zone management with respect to sustainable development L INTRODUCTION Vietnam has aheady begun experiencing tiie effects of sea level rise The most dangerous threat that Viet Nam may be suffered will happen when the several natural hazards that origin from global changes combine with anthropogenic impacts (hydroelectric dams, exploitation of coastal forests, exploitation of minerals ) at once If it happens, it will accelerate unexpectable consequences According to Vietaam's Initial National Communication to the UNFCCC (United Nations Framework Convention on Climate Change), coastal sea level is expected to be up 33 centimeters by 2050,45 centimeters by 2070 and meter by 2100 The Mekong and Red River deltas, two of the world's largest river deltas, will lie sttongly affected They believed that a one-meter rise in the sea level along the coast would potentially resuh in ;i 5% land loss and an 11% population displacement in Vietnam (World Bank 2007) Scientists have stiidied the vulnerability of populations and ecosystems to natural and anthropogenic hazards In recent years, researchers have focused on assessing biophysical and socioeconomic vulnerabilities within an ecosystem to assess the impacts of potentially hazardous conditions such as sea level rise Therefore, reconstmction of the history of sea level changes, Mekong Delta evolution and the adjacent shelf evolution in Holocene and to resume the tendency of the future changes is an important scientific task The Phase II of CAU and Fig L The stady area [5, 6] IMGG collaboration had been focused on tiie Mekong delta We investigated the late Holocene progradalion of the delta onto the shelf, the delta-shelf tiansition zone, tiie sedimentfransportfi-om Uie Mekong river distributaries to Ca Mau Peninsula, and acttial sediment accretion enveloping its southem shore Investigations in the field of coastal and marine geology within the joint viemamese-German project aim at the reconstruction of the hitherto, only sparsely 216 Phung Van Phadi Hoi Uiao khoa hoc - Hgp tac QuOc tg digu tra, nghign ctru Hi nguygn va mfli ttufmg bign known as Shelf evolution in South East Vietnam Shelf during Late PleistoceneHolocene and at the assessment of ongoing hydrodynamic, sediment dynamic and morphodynamic coastal and shelf processes So that we could understand the sediment dynamic processes acting in this highly complex delta system what will allow and improve the predictability of its future evolution Figure shows the area of interest and some data collected on it II THE PRINCIPAL TASKS OF THE JOINT PROJECT (2006-2009) AIM TO Reconstruction of the influence of short- and longer-term sea-level fluctuations on delta evolution/architecture, namely outbuilding and shoreline migration as natural framework and boundary conditions of the ongoing changes Survey of the modem hydrodynamic and sediment-dynamic processes: influence of river-discharge, tides and waves on sediment dispersion, transport and deposition: - At the transition from the delta to the open shelf - In the distributaries of the delta - For the transport paths from the Mekong River distributaries to Ca Mau Peninsula - At Ca Mau Peninsula with its spit system III THEMATIC ISSUES (1) Important geomorphologic structures identified by remote sensing (satellite images and aerial photographs) of the Mekong Delta and the adjacent shelf; (2) Bathymetry, hydro- and morphodynamics of the delta front and prodelta area; (3) Sediment transport and accretion in tiie marine part of the delta; (4) Water and sediment discharge from the Mekong River distributaries; (5) Holocene sea-level change and delta evolution with special regard to the evolution and growth of the Cua Dai/Cua Tieu distributary and the Ca Mau Peninsula; (6) Recent changes in the deltaic forcing in response to global warming and sealevel rise (7) Effects of (1) to (6) on infrastructure and on the use of aquatic and mineral resources These thematic issues help in solving important scientific problems concerning the reconstruction of delta evolution and actual growth High-resolution analysis of sedimentary archives and of ongoing physical processes had allowed an analysis of change with extrapolations to improve the predictability of the future evolution of the Mekong Delta An extensive data-base has been provided for integrated coastal zone management options PhCing van Phdch 217 Woriishop: International Coopeiation on Invesdaation and Research of Marine Nanjtal Resoutce and Environing- The joint project has been involved a number of scientists from both Viet Nan and Germany, as well from Poland As result of the projects several undergraduaj smdents had fiilfilled tiieir thesises in Germany and Vietnam IV SOME MAIN RESULTS The late Pleistocene-Holocene sedimentary architecture of the Southeast (SE) Vietaam shelf has been revealed on the basis of sequence stratigraphic concepis Five seismic units and three main bounding surfaces are indentified forming the fifth-order sequence on the SE Vietaam shelf The lowstand systems tract consists of two depositional systems: prograding delta wedge, and an incised-valley system formed by fiuvial incision The lowstand wedge was primarily developed on the modem outer shelf (120-150 m) with thickness of ~ 50 m (figure 2) The paleochannel system is fraced by seismic profiles from 20 to 60 m of modem water depth and seems to be govemed by the shelf morphology (figure 3) The northen incised-channel branch off Vtmg Tau shows narrow and deeply V-shape which is probably resulted from the high-gradient morphology of the shelf By contrast, the wide and low-gradient shelf off modem Mekong delta and Camau peninsular have created shallow incised-channels on the exposed self The transgressive systems tract (TST) is mostly preserved from erosional processes in the incised-valleys where its thickness reaches up to 15-25 m On the exposed shelf and the interfluvial, tiie TST shows a thin layer mostly consisted of sand overlymg the sequence boundary SBl The highstand systems tract (HST) is primarily consisted of the thick prograding mud clinoforms of the modem Mekong subaqueous delta distiibuted at modem water of 0-30 m (figure 4) Toward the middle and outer shelf, tiie HST represents a very thm layer (0 to centimeters) mixing wifli the lower tiansgressive sediments Local shelf morphology and sh-ong hydrodynamic conditions resuh in differences between the SE Vietaam shelf and the tiieoretical sequence-stiatigraphic model (figure 5) Fig.2 Profile SO140-7-2 shows the lowstand wedge on the outer modem shelf [I] Phung Van Phacli H$i thao khoa hgc - Hgp tic Qu6c te di^u tra, nghien cihi tM nguySn v i moi tnr&ng bien Fig Example of northern incised-valley with different distributaries[l] I " ! m i l m>hi« imiirrr '^ ''''"" iiii.«i, i Fig Part of seismic profile showing thick highstand wedge deposit [1] _ 1111 ™, , »™™, Fig Late Pleistocene-Holocene sequence stratigraphic model for SE Vietaam shelf (Bui Viet Dung, 2007)[1] Infilling of the Mekong incised valley initiated prior to 13.3 ka BP when the deglacial sea level rise reached the shallow and lowgradient part of the southeast Vietaamese shelf Flooding of the incised valley is accurately marked in this valley by a Vietaamese shelf Flooding of the incised valley is accurately marked in this Phung Van PhSch 219 Woikshop: "International Cooperation on hivestigation and Reseaich of Marine Naniral Resouite and Enviromnm^ valley by a sharp tiansition from fluvial to marine-dominated sedimentation The tiansgressive boundary preserved in the Mekong incised-valley sediments is hi good agreement with tiie sea-level changes obtained from multiple East Asian suband mter-tidal sediment records The relatively constant sea level rise indicated by East Asian sea-level history between 13.0 and 9.5 ka BP resulted in near complete infilling of the valley by aggradation of fluvially dominated sediments Backstepping of the river mouth strongly reduced sedimentation rates after flooding of the valley and even affected the sediment supply to deeper parts of the shelf Initially, filling of the valley abraptly changed from fluvial to marine conditions, but progressively changed into more dynamic estaarine conditions at the final stage of infillmg after 9.5 ka BP This caused signiggicant sediment reworking and erosion, and compUcated accurate dating of the younger valley-fill sediments The valley tiansformed into an estaarine channel that probably connected an inland bay located around cores BT2 and DTI (Ta et al., 2001,2005) [9] with the shelf Nevertheless, the Mekong valley infilling and coastal deposits hi the Cambodian lowlands indicate a rapid and vast flooding of the shelf between 9.5 ka and 8.5 ka BP This fast flooding between 9.5 and 8.5 ka BP can be ascribed to the low shelf gradient and a strong acceleration of the deglacial sea-level rise from 34 to mbsl corresponding to MWPIC After this sea-level jumb, the Mekong river mouth stabilized in the Cambodian lowlands, where the Mekong River Delta initiated between and 7.5 ka BP Fig Sediment deposition and sediment transport pathways under the influence of northeast monsoon (Nguyin Tmng Thanh, 2009)[7] The dominant deposition of sediments on the Mekong subaqueous delta is divided into five main areas including: the sandy deposition is dominating in front of the Mekong River moutiis and the area between Ganh Hao river mouth and Bo De river mouth; the muddy deposition is dominating along the SoctiangBaclieu coast and the area from Hon Khoai Island to the west of Ca Mau Peninsula; the deposition of both sand and mud occurs ftom Bo De to Hon Khoai Island The sediment accumulation rates in different areas of the Mekong subaqueous delta have significant difference and are different from tiie sediment accumulation rates on the shelf The highest sediment accumulation rates were estimated in the southeast to the west part of Ca Mau Peninsula (from Bo De mouth to Cau Mau cape) and then as the area in front of Mekong River mouths The sedimenl 220 Phung Van Phach Hfli thao khoa hoc - Hop tac Quflc tg digu In, nghign ctnj t^i nfiuyen va mfli mritng bign accumulation rates are general low in the area along the coast of Soc Trang - Bac Lieu and the area along the west coast of Ca Mau Peninsula The calculated sediment accumulation rates are 0.36 cm/year on the prodelta, southeastwards Ca Mau Peninsula and 0.14 cm/year on the shelf, southwestwards the SE Vietnam Shelf The sediment accumulation rates on the prodelta make a balance with the mean sea level rise during the past 100 years Therefore, the subaqueous delta possibly has not propagaded into the adjecent shelf during last 100 years Although the boundary may change but it is local The sediments is mainly transported southwestward under the influence of coastal hydrodynamics and turning around Ca Mau Peninsula to the north and extending now up to the coastal waters off Rach Gia forming prodelta-clinoforms enveloping Camau Peninsula and the westem border of the subaerial Mekong Delta The rapid progradalion rates of Mekong Delta seawards (13-15 m/year during last 3000 years) have decreased significantiy in last 100 years in responsible with the present rise of sea level The development of Mekong Delta is changing from seaward progradalion in the sea level fall since Middle Holocene to aggradation and under a lot of destruction in the present rise of sea level From the mid-Holocene period to now, sea level history for southern Vietnam could be reconstructed from beachrocks and beach ridges between Phan Ri Cua and Phan Rang (figure 7) After passing the modern sea level at 7200 cai yr BP the mid-Holocene sea-level highstand extended from 6700 to 5300 cai yr BP reaching its peak by 1.52 m above modem sea level around 5500 cai yr BP Sea level fell later to its modern position In o adition, the observations of notches •* on the limestone cliff in Ha Tien combinating with the other results of Holocene sea level fiuctuations from tf"'^^^ -5 many places in world confirm that s the highstand sea level in mid •^-"S"*! S Holocene on the Mekong Delta is 27 // 2.5 m higher than the present sea level The notch on limestone cliff at / 4.5 m high may be the older -„ highstand sea level which was generated in the last deglacial period (MIS5e) Fig Holocene sea-level curve for the The low delta plain and the SE-Vietnamese coast, for mean sea level adjecent areas are block and maximum spring-tide sea level based on different sea-level indicators [1,5] architecture with the formation and development of the Cuu Long River gabben depression and Cuu Long basin offshore The depression has contiuously ^£- -'-1 Phiing van Ph4ch Workshop "ln.ematiori.1 roonetation on lnves,i.a,inn and Research of Marine Natural Resource and Environmenf- subsided in the period of Quatemary-Mordem, while the two wings of the Cuu Long basin tend to lift stably Tectonic activities keep the important role in controlling the sea level changes: the sea invades and rises faster; in contrast, the sea invades slowly or reaches the balance in the elevated areas It is essential to have con'ections when the old sea levels are calculated The geomorphology of the Mekong subaqueous delta is divided into four types depending on the width of the front delta, delta slope and sediment characteristics deposited on them: the area in front of the Mekong river mouths; the area along the Soctrang-Baclieu coast; the southeast to the southwest of Ca Mau Peninsula (from Bo De river moutii to Ca Mau Cape); the area is on tiie west of Ca Mau Peninsula Monsoon regime and tides keep an important role in driving the coastal currenis in coastal waters around Mekong Delta Plain The currents and waves under the infiuence of North East monsoon is dominating in comparison with the currents and waves under the influence of South West monsoon V SUGGESTIONS Continuation of researching the reconstruction of the sediment system tracts on the South East Vietnam Shelf in relation to the sea level fluctuations after the Last Glacial Maximum (LGM) Investigation of modern sediment accumulations and comparison of the difference between the sediment accumulation rates and the sediment facies changes in time and sparsal on the huge SE Vietnam Shelf Investigation of the Mekong subaqueous delta in detail (the transition from subaerial delta to the adjacent shelf) in order to supplement the new research results and access the tendency of the subaqueous delta in the stage of sea level rise now, the decrease of sediment amount due to the hydroelectric dams, and socialeconomic activities along the coastal delta plain Especially, Ca Mau Peninsular need to be investigated more detail and set it in the relation to other parts of the sedimentary systems of Mekong Delta To carry out the similar research programs on Red River Delta and the shelf in Gulf of Tokin on the sea level fiuctuations, evolution of the shelf, sediment dynamics, the influence of the present sea level rise on the development of Red River Delta, erosion, accumulation along the coast and the difference between two deltas in Vietnam To enhance the intemational co-operations with Germany in marine sciences and extend the collaborations with other countries such as United States of America, Russia, Japan 222 pi,,,ng Van Phach H6i thao khoa hpc - Hgp i^c Qu6c tg digu tra nghifin cim Xki nguySn v^ ni6i trudng bJSn Acknowledgments We express our gratitude to Vietnam MOST, German DFG, CAU, IFMGEOMAR, Vietnam Academy of Science and Technology (VAST), NAFOSTED and Vietnam State department of Border and local authorities of Viet Nam for their support during our study REFERENCES Bui Viet Dung, 2006 Large -Scale bedform distributions and their use as indicator for bottom flow currents patterns on the South East Vietnam shelf M.Sc Thesis Kiel Uviversity-Germany Bui Viet Dung, Schimanski Alex, Stattegger Karl, Van Phach Phung, The Tiep Nguyen, Tien Hai Nguyen, Trung Thanh Nguyen, Truong Thanh Phi (2009) Sand waves on the Southeast Vietnam shelf recorded by high resolution seismic profiles: formation and mechanism Frontier of Earth Sciences in China Michelli.M., 2008 Sea-level changes, coastal evolution and paleo-oceanography of coastal waters in SE-Vietnam since the mid-Holocene PhD Thesis at Kiel University Library, 175 p Rik Tjallingii, Karl Stattegger, Andreas Wetzel, and Phung Van Phach, 2010 Infilling and flooding of the Mekong River incised-valley system during deglacial sea-level rise Quaternary Science reviews Stattegger K Holocene evolution and actual geologic process in the Coastal Zone of South Viet Nam Proceeding of The First National wide Scientific Symposium on Marine Geology "Vietnam Marine Geology and Sustainable development" Ha Long 910/10/2008 Pp 42-54 Stattegger K., Tjallingii R., Phung Van Phach, A Wetzel, K Schwarzer 2010 Mekong Delta and adjacent areas Holocene sea-level history of SE Asia and its global implications Special Issue of Global and Planetary Change: "Land-Ocean-Atmosphere interaction in the coastal zone of South Vietnam" Nguyin Trung Thanh 2009 Surface sediment characteristics and sediment transport from Bassac River mouths to Ca Mau Peninsula (Mekong Delta) M.Sc Thesis Supervisor(s) Prof Dr K Stattegger Coastal Research Laboratory, Institute of Geosciences Christian Albrechts University, Kiel, Germany Schimansklj, A., Stattegger, K., 2005, Deglacial and Holocene evolution on the Vietnam shelf: stratigraphy, sediments and sea-level change Marine Geology 214, 365387 Ta T.K.O., Nguyen, V.L., Tateishi, M., Kobayashi, I., Saito, Y., 2001 Sedimentary facies diatom and foraminifer assemblages in a late Pleistocene-Holocene incised valley sequence from the Mekong River Delta, Bentre Province, Southem Vietnam: the BT2 core Journal of Asian Earth Sciences 20, 83-94 10 Tamura T., Saito Y., Sieng S., Ben Bunnarin, Kong, M Sim, I., Choup S., Akiba, F., 2009 Initiation of the Mekong River delta at ka: evidence from the sedimentary succession in the Cambodian lowland Quaternary Science Reviews, 327-344 Phung van Phach 223 ... Asian Earth Sciences 20, 83-94 10 Tamura T., Saito Y., Sieng S., Ben Bunnarin, Kong, M Sim, I., Choup S., Akiba, F., 2009 Initiation of the Mekong River delta at ka: evidence from the sedimentary