Journal of Asian Earth Sciences 29 (2007) 545–557 www.elsevier.com/locate/jaes Recent sedimentation and sediment accumulation rates of the Ba Lat prodelta (Red River, Vietnam) G.D van den Bergh a,Ô, W Boer a, M.A.S Schaapveld b, D.M Duc c, Tj.C.E van Weering a a Royal Netherlands Institute for Sea Research (NIOZ), P.O Box 59, 1790 AB Den Burg, Texel, The Netherlands b Shell Todd Oil Services Ltd., Private Bag 2035, 4620 New Plymouth, New Zealand c Ha Noi University of Science, 334 NguyenTrai, Thanh Xuan, Ha Noi, Viet Nam Received September 2003; received in revised form 21 February 2005; accepted March 2006 Abstract The Ba Lat River is the major distributary of the Red River system in North Vietnam To assess the recent to subrecent depositional processes in the Ba Lat prodelta, a detailed sediment analysis was conducted Bottom samples were collected during two Weld surveys, one in the dry season (winter) and one in the wet season (summer) A steep frontal prodelta slope is characterized by very rapid sedimentation (tens of cm per year) of muddy sediments under inXuence of the turbid river plume Beyond direct inXuence of the river plume the bottom slope decreases and bottom transport by the prevailing southward directed currents becomes important Coarse-grained tempestites alternate with the dominating muddy sediments Downcore changes in the 234Th activities indicate that the subaqueous delta progrades to the southwest, with erosion and reworking of older sediments occurring north of the present outlet The southwestward progradation is also encountered in the trend of 210Pb activities indicating that this process has continued for at least 100 years Avulsion of the Ba Lat outlet in 1973 has led to a decrease in sedimentation rates north of the Ba Lat outlet © 2006 Elsevier Ltd All rights reserved Keywords: Recent sedimentation; Red River Delta; Vietnam; 210Pb dating; 234Th analysis; Prodelta; Tempestites Introduction The Ba Lat is the main distributary of the Red River system in North Vietnam (Fig 1) The coastal plain near the Ba Lat outlet has accreted over a distance of 23 km during the last 500 years, and is build up by an alternation of fossil beach-spit systems with back barrier swamp deposits in between (Thanh et al., 1997) The river discharge follows a clear seasonal pattern reXecting the variation in rainfall under the constraint of a monsoonal climate Besides seasonal diVerences, the inter-annual variation in suspended sediment transport varies between 30 and 120 million tons per year passing Son Tay Environmental changes, both anthropogenic as well as natural induced changes, have a * Corresponding author Tel.: +31 222 369 394; fax: +31 222 319 674 E-mail address: gertb@nioz.nl (G.D van den Bergh) 1367-9120/$ - see front matter © 2006 Elsevier Ltd All rights reserved doi:10.1016/j.jseaes.2006.03.006 high potential to aVect the coastal zone of the Ba Lat Delta The main Ba Lat channel debauched at its present position before 1938 and after 1973 From 1938 until 1971 it entered the sea 10 km more to the north, when during a severe Xood in August 1971 its location shifted to a position south of the present outlet During the typhoon Kate in 1973 the frontal sand barrier broke through and the main outlet started to occupy its present position (Thanh et al., 1997) There are indications that the frequency of typhoons aVecting the coast of Vietnam has increased during the second half of the 20th century (Thanh et al., 1997) Furthermore, the construction between 1979 and 1994 of the Hoa Binh Dam in one of the three major tributaries of the Red River (Fig 1A), has nearly halved the average annual suspended sediment concentration at Son Tay gauging station (van Maren, 2004) These are some of the factors that are expected to inXuence the densely populated coastal zone of the Red River in the coming decades, and the development 546 G.D van den Bergh et al / Journal of Asian Earth Sciences 29 (2007) 545–557 Fig (A) Map of Vietnam with location of the study area and (B) map of the study area showing a sub-division of the Ba Lat prodelta (various shaded zones), based on the acoustic study of van den Bergh et al (2006) Also shown are the positions of bottom sampling stations Hatched area represents the zone of maximum accumulation, where the most recent acoustic unit has a thickness in excess of m of sustainable coastal zone management becomes increasingly necessary Integrated coastal zone studies provide useful information not only in documenting modern terrestrial and marine environments, but also in understanding erosion patterns within geological and historical contexts (Milliman et al., 1987) Our main objective within the framework of the Red River Delta Research Program was to gain an understanding of the sedimentological processes that govern the development of the present Ba Lat prodelta The study is based on the analysis of bottom samples recovered from the Ba Lat prodelta The sedimentological and geochemical imprints in the sedimentary record were analyzed in order to reconstruct the processes that resulted in the present conWguration of the prodelta on a 100-years time scale Analysis of the downcore 210Pb activity has been performed on a number of gravity cores, to assess spatial variability in recent accumulation rates on a »100 years time scale Analysis of excess 234Th has been applied on box core samples, in order to determine seasonal diVerences in deposition, re-suspension, and mixing-processes of the surface sediments Other parameters that were analyzed are the composition, grain-size distributions and organic carbon and nitrogen contents of the sediments A study on acoustic facies analysis and prodelta geometry is presented elsewhere in this volume (van den Bergh et al., 2006) For the hydrodynamical and climatic conditions governing the delta development the reader is referred to van Maren and Hoekstra (2004, 2005) and van Maren et al (2004) Delta setting Based on the acoustic study carried out by van den Bergh et al (2006), a morphogenetic subdivision of the study area was made (Fig 1B) This subdivision consists of: (1) delta front, (2) prodelta, (3) Gulf of Tonkin Shelf, and (4) erosional shoreface zones The delta front fringes the delta plain and is marked by a slope break between and m water depth at the transition with the prodelta The delta front has not been sampled in the course of this study The prodelta forms a relatively steep muddy slope that merges in to the shelf of the Gulf of Tonkin around the 30 m isobath The recent prodelta deposits are recognizable on the acoustic proWles as a dark band of high reXectivity with multiple sub-bottom reXectors, which become weaker and gradually converge with the bottom reXector in oVshore direction Based on bottom gradients, bottom proWle, and the relative position with respect to the Ba Lat River mouth, the prodelta can be subdivided into: (A) the Northern Prodelta, (B) the Frontal Prodelta, and (C) the Southern Prodelta (Fig 1) The Frontal Prodelta, located adjacent to the Ba Lat mouth, is only km wide and has the steepest bottom gradient of »6.5 m/km between 12 and 23 m water depth Towards the South the prodelta rapidly widens to more than 20 km across and bottom gradients decrease to »1.5 m/km The Southern Prodelta is detached from the coast Towards the North the prodelta widens as well, but near coastal bottom gradients remain relatively steep (»4.5 m/km) The Northern Prodelta is also detached from the coast by a 2–3 km wide erosional shoreface G.D van den Bergh et al / Journal of Asian Earth Sciences 29 (2007) 545–557 Methods In 2000 Weld campaigns where conducted in February– March and July–August, during the dry and wet monsoon, respectively Bottom sampling stations were selected based on a preliminary study of the shallow penetrating acoustic proWles (van den Bergh et al., 2006) A gravity corer with a length of m and a diameter of cm was used Gravity coring stations are shown in Fig 1, and a list of all cores with their coordinates is presented in Table At the Weld station cores were split, photographed and described macroscopically Magnetic susceptibility was measured at cm intervals with a handheld Bartington MS2E1 surface sensor Sub-samples with known volume were taken at 5-cm intervals for standard Dry Bulk Density (DBD) measurements The top part of each core was more intensely sampled for 210 Pb analysis Then the cores were covered with plastic foil and sealed in plastic and stored horizontally at a temperature of 5–7 °C The intact core halves were shipped to the NIOZ in the Netherlands for X-ray photograpy and further analysis (granulometry, XRF, Corg and C/N ratios) The XRF Cortex-corescanner, is a non-destructive, semi-quantitative logging instrument for major element determination (Jansen et al., 1998) The elements Fe and Ca were analyzed at 1-cm intervals Stereo X-ray photographs were made of selected intervals Based on a study of the photographs, additional sample levels for grain-size and, for cores and 15, Corg and C/N ratio analyses, were selected besides standard intervals of cm 547 Grain-size analyses were performed on 15 cores using a Coulter LS 230 analyser Approximately 0.1 g of sediment was weighted in glass beakers and mixed with 15 ml of tap water The samples were put in an ultrasonic bath for minutes Then the sample was passed through a mm sieve and measured under continued ultrasonic treatment Organic carbon and nitrogen contents were determined on samples from cores and 15, using a Carlo Erba NA1500 series Nitrogen Carbon Sulphur Analyser Sample treatment and analysis was according to the method of Verardo et al (1990) The data are presented as weight percentages of organic carbon and nitrogen versus depth and C/N versus depth Measurements for 210Pb analysis were made following the methods outlined in Boer et al (2006) Coarse-grained intervals, recognized by the dry bulk densities or macroscopic descriptions, were omitted for 210Pb analysis The best model Wts through the data points were calculated using the Constant Flux and Constant Sedimentation (CF–CS) model (Appleby and OldWeld, 1992; Boer et al., 2006) For gravity cores 6, 10 and 15 the supported 210Pb activity, as deWned by 226Ra, was determined by analyzing the 226Ra activity using gamma spectrometry, according to the method outlined in van den Bergh et al., 2003) In several cores with very high accumulation rates, supported 210Pb activities were not reached at the base of the core If no 226Ra-based supported activities were available, model Wts were obtained with pre-deWned supported Table List of Red River Delta gravity cores sampled in 2000 Core Sampling date (UTM zone 48Q) Easting Northing 8b 10 11 12 13 15 16 18 21 22 23 25 26 27 29 30 31 32 33 March 12 March 12 March 12 March 13 March 13 March 13 March 13 March 13 March 18 March 18 July 22 August August August July 22 July 22 July 22 August August August August August August August August 27 August 27 672225 671893 670983 655824 652584 648589 647683 645558 674960 676499 675985 666934 658934 654725 673363 675408 687998 656885 651707 662166 665910 681869 683876 678874 678113 676988 2232724 2233551 2234186 2216655 2219909 2223832 2225116 2227512 2249079 2239790 2233428 2223748 2222861 2226651 2228744 2254562 2251230 2224829 2212073 2214096 2220296 2244712 2244781 2251021 2225223 2227083 a Core not located along acoustic transect: water depth is estimated Water depth (m) Core length (cm) 26 25 21,3 24,6 21.3 16 13 10,7 20 27.5 29.5 25 22 17 29.5 17.5 28 20 30a 28.5 27.5 29.5 30 22 32 31.5 69 162 155 161 115 Sandy bottom Sandy bottom §15 stiV clay 74 86 105 29.5 141 111 147.5 121.5 Sandy bottom 139 131.5 135 56 141.5 141.5 167.5 34 25 548 G.D van den Bergh et al / Journal of Asian Earth Sciences 29 (2007) 545–557 activities of 36.4 and 41.3 Bq kg¡1, based on the minimum and maximum 226Ra-based supported activities measured in other cores The development of a surface mixed layer (SML) was not always evident in core proWles In case of doubt solution Wts were forced both with and without SML, and the calculated variables resulting from the various solutions are presented as a range in tables and Wgures In four cores (2, 10, 12 and 18) vertical grain-size Xuctuations appeared to result in highly irregular 210Pb activity plots, due to rather variable speciWc surface areas in these samples In these cores 210Pb activities were measured on the 2 m, is indicated hatched in Fig 1B This area of major accumulation covers the steep frontal delta slope and a large proximal area of the Southern Prodelta Gravity cores 3, 15, 16 and 23 are located in this area of major accumulation Stations 1, 2, and 12 cover the more distal areas of the Frontal Prodelta Stations 6–8, 13–14, and 24–27 are located in the distal parts of the Southern Prodelta At stations and 8B stiV, Wne-grained sandy sediment was retrieved in the core catcher Cores 10–11, 21 and 29–31 penetrate the Northern Prodelta Several short cores (stations 22, 18, 32 and 33) sampled the surface sediments of the Gulf of Tonkin Shelf At station 22 coring failed but some coarse sandy material was recovered in the core catcher Station 18 is located in an oVshore depression, which is bounded to the west by a SSW-NNE trending fault with surface expression at the bottom and to the east by a convex ridge (Fig 1B) The erosional coastal zones were not sampled, but are clearly revealed on the acoustic proWles (van den Bergh et al., 2006: Fig 3A) At station 9, located between the Hai Hau coast and the Southern Prodelta, multiple coring attempts resulted in the retrieval of only 15 cm of stiV, wellconsolidated mud The acoustic proWle that runs along this station shows inclined sub-bottom reXectors that are truncated by the bottom reXector, indicating erosion of older prodelta deposits At 12 stations boxcores were retrieved, either during the dry season, the wet season, or during both seasons (Fig 1B) The boxcore retrieved at station during the dry season at 15.5 m water depth showed fresh elongated parallel scours of several mm deep at the surface, indicating erosion by strong bottom currents 4.1 Sediment characterization Data compilations of lithology and various analyzed parameters are shown in Figs 2A–C and Muddy sediments from the Ba Lat prodelta can be easily distinguished from the contrasting sandy deposits covering the Gulf of Tonkin Shelf The modern prodelta deposits have a reddish brown color, whereas the sandy shelf de posits have a greenish gray color, are coarser-grained, and contain abundant shell debris The prodelta sediments contain quartz, feldspar and mica as major grain types in the coarse silt and Wne sandy fractions Silt-sized detrital grains are mostly covered with a reddish brown coating of Fe-oxides XRD analysis on bulk samples indicated the presence of 14 Å clay minerals, mostly chlorite Kaolonite is probably also present in the clay fraction, besides hematite Calcite shows very weak intensities on the XRD diagrams The contrasting color diVerence between the prodelta muds and the sandy shelf deposits is caused by relatively low amounts of Fe-oxides in the latter The XRF measurements demonstrate the relatively low amount of Fe minerals in these sandy shelf deposits (e.g Fig 2B: base of core 18), and the low Fe contents also correlate well with very low magnetic susceptibility values No hematite could be demonstrated to be present in the sandy shelf samples Instead, pyrite is more prominently present in these sediments At stations along the distal margin of the prodelta, where cores penetrated the muddy prodelta deposits into the underlying shelf sands, the transition between both sediment types was always found gradual and marked by a 10– 20 cm thick interval of heavily bioturbated and mottled sediments The prodelta deposits consist predominantly of muddy sediments with a few thin (