Late Holocene sedimentary and environmental development of the northern Mekong River Delta, Vietnam

Quaternary International 230 (2011) 57–66

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Late Holocene sedimentary and environmental development of the northern Mekong River Delta, Vietnam ˇ

Ulrike Proske a, *, Till J.J. Hanebuth a, Jens Gro¨ger a, Bu`i Pha´t Diem b _

a

Faculty of Geosciences, University of Bremen, Klagenfurter Strasse 2, 28359 Bremen, Germany b Provincial Museum Long An, QL 1A, Tan An, Vietnam

a r t i c l e i n f o

a b s t r a c t

Article history: Available online 4 December 2009

A sedimentological and palynological study of three sediment cores from the northern Mekong River Delta shows the regional sedimentary and environmental development since the mid-Holocene sea level highstand. A sub- to intertidal flat deposit of mid-Holocene age is recorded in the northernmost core. Shoreline deposits in all three cores show descending ages from N to S documenting 1) the early stages of the late Holocene regression and 2) the subsequent delta progradation. The delta plain successions vary from floodplain deposits with swamp-like elements to natural levee sediments. The uppermost sediments in all cores show human disturbance to varying degrees. The most intense alteration is recorded in the northernmost core where the palynological signal together with a charcoal peak indicates the profound change of the environment during the modern land reclamation. The sediments from at least one of the three presented cores do not show a ‘‘true’’ delta facies succession, but rather estuary-like features, as also observed in records from southern Cambodia. This absence is probably due to lack of accommodation space during the initial phase of rapid delta progradation which impeded the development of ‘‘true’’ delta successions as shown in cores from the southern Mekong River Delta. Ó 2009 Elsevier Ltd and INQUA. All rights reserved.

1. Introduction During the mid-Holocene, an elevated sea level is recorded throughout SE Asia and is linked with the early phases of the megadelta development in the area (Stanley and Warne, 1994; Hori and Saito, 2007; Tamura et al., 2009). In the western South China Sea this highstand commenced ca. 5500–6000 calibrated 14C years before present (cal a BP), reached ca. 3 m above present level and persisted for ca. 2000 years before the sea level smoothly declined to the modern level (curve A in Fig. 1, Tanabe et al., 2003a). The course of the sea level in the Mekong River Delta area, however, shows different features. The highstand of ca. 2.5 m commenced around 6000 cal a BP and lasted for ca. 1000 years before the sea level decreased very slowly to its present level (curve B in Fig. 1, Ta et al., 2001, 2002a, 2002b). During the phase of elevated sea level, the coastline in the Mekong River Delta area was located in southern Cambodia (Nguyen et al., 2000; Tamura et al., 2009). The subsequent lowering of sea level enabled the progradation of the Mekong River Delta during which time the position of the coastline shifted rapidly.

* Corresponding author. Tel./fax: þ49 421 218 65205. E-mail address: [email protected] (U. Proske). 1040-6182/$ – see front matter Ó 2009 Elsevier Ltd and INQUA. All rights reserved. doi:10.1016/j.quaint.2009.11.032

Early studies from the 1970s attempted to date the various steps of the coastline evolution (Fontaine, 1970, 1971, 1972; Fontaine and Delibrias, 1973, 1974). However, since they are based on only a few radiocarbon-dated locations, these reconstructions remain tentative. To date, only the study by Nguyen et al. (2000) provides an overview of the coastline progradation through time. Recent studies focus on the general delta development and show that the typical depositional succession of the delta consists of a muddy prodelta, a sandy to silty delta front, a sandy to silty subtidal to intertidal facies and a subaerial delta plain facies (Ta et al., 2002a, 2002b; Tanabe et al., 2003b). These major units can be subdivided into more locally restricted deposits such as beach ridges, marsh land, swamps and levee zones (Fig. 2) which underlines the dynamic nature of the delta system (Nguyen, 1993; Nguyen et al., 2000). Today, major parts of the delta plain are widely used for agriculture (Hori, 2000). This extensive cultivation started in the early 19th century when Vietnamese settlers seized the land from the Khmer people (Tanaka, 2001). During this reclamation, large parts of the indigenous vegetation including the vast Myrtaceae (predominantly Melaleuca leucadendra) stands were cleared (Duc, 2006). The outer parts of the delta are mainly inhabited by mangroves which were cleared to great extends in the second half of the 20th century (Hong and San, 1993). The modern mangrove

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Fig. 1. Sea level curves for the South China Sea. The smaller inset graph shows the post-glacial sea level rise (Hanebuth et al., 2000) with the grey box marking the time span of interest. A) The detailed course of sea level in the western South China Sea including the late post-glacial sea level rise, the pronounced mid-Holocene sea level highstand and the subsequent fall towards the modern level (Tanabe et al., 2003a). B) The course of sea level for the Mekong River Delta area showing the mid-Holocene sea level highstand being less distinct than observed in the western South China Sea (Ta et al., 2002a, 2002b).

in all stages of the delta evolution (Nguyen et al., 2000). The study by Nguyen et al. (2000) implies that especially in the northern delta, mangroves were common until at least 2000 cal a BP, marking prolonged brackish conditions along rivers and streams. Apart from a few reports on the present state of the indigenous vegetation (e.g. Hong and Schmidt, 1974; San, 1993; Duc, 2006; van Loon et al., 2007) studies about the general botanical evolution of the Mekong River Delta are currently not available. Hence, studies from Cambodia (Sin, 1990; Maxwell, 2001; Bishop et al., 2003; Penny, 2006; Penny et al., 2006) and Thailand (Somboon, 1990; Boyd and McGrath, 2001; Penny, 2001; Penny and Kealhofer, 2005) have been used here to understand the development of the regional, native flora. The northern part of the Mekong River Delta has not yet been a major focus for geological research as the delta deposits are relatively thin and diverse (Nguyen et al., 2005). Investigation of the sediments from this area is aimed at closing the gap between the well studied successions in Cambodia and the southern, outer delta in order to understand the early stages of the delta development. The palynological record presented here is the first of its kind that also includes evidence for the onset of the modern human impact in the area. 2. Regional setting

communities consist mainly of various species of Avicennia, Bruguiera, Ceriops, Rhizophora and Sonneratia (Hong and San, 1993; Duc, 2006) and are heavily degraded (Blasco et al., 2001). However, mangrove pollen have been identified in sedimentary successions throughout the delta, suggesting that mangroves were widespread

The Mekong River Delta forms the mouth of the Mekong and Bassac Rivers as well as their distributaries (Fig. 2) and extends over an area of 62 520 km2 today (Nguyen et al., 2000). The modern, large sediment discharge of these rivers leads to delta

Fig. 2. Maps of the study area. A) The greater Mekong River Delta with its two major rivers, the Bassac and the Mekong. The smaller inset map displays the position of the delta within Southeast Asia. The black square marks the area shown in b) including the studied sites. B) The facies distribution and geomorphological features in the study area according to Nguyen et al. (2000). The stars indicate the coring locations of GGM07-02, TA07-01 and GGN07-01.

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progradation of about 13–14 m/year into the South China Sea and partially into the Gulf of Thailand (Ta et al., 2002b). The tidal amplitude in the South China Sea is in the range of 3.0–3.5 m (Mekong River Commission, 2005) which controls the hydrological settings throughout the delta. In addition, the delta plain is strongly affected by the monsoonal seasons. During the wet summer monsoon mainly the inner and northern delta is inundated up to 4 m for 2–4 months/year (Mekong River Commission, 2005), However, during the dry winter monsoon the main part of the delta dries out and saline waters reach more than 50 km inland (Kolb and Dornbusch, 1975). In addition the influence of the high tide can be detected up to 230 km upstream due to low river runoff (Wolanski et al., 1996). The north-eastern part of the delta including the study area is characterised by a large swampy depression, the Plain of Reeds or Dong Thap Muoi, in which large parts are not elevated above modern sea level. This basin-like structure is fringed to the north by up to 3 m high remnant terraces of late Pleistocene age. To the south the Mekong River and its broad levee systems border the plain and impede the drainage of flood waters causing the long and deep inundation of the area in the rainy season. The Vam Co Tay River, as the major waterway among numerous artificial channels, connects the Plain of Reeds region with the South China Sea. Together with its floodplain it forms the main geomorphologic feature in this area (Fig. 2). Additional geomorphologic elements such as relict beach ridges/sand dunes have been recorded (Fig. 2). However, their appearance in the landscape is ambiguous and their age is still poorly understood. Among the

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recognised sub-surface elements, various channels filled with peaty successions, which possibly represent mangrove deposits, are very common in the area. These channels are believed to have been active tidal creek systems during the mid-Holocene sea level highstand (Nguyen et al., 2000). 3. Material and methods 3.1. Core characteristics The cores GGM07-02 (GGM), TA07-01 (TA) and GGN07-01 (GGN) are three out of 30 sediment cores that were hand drilled (Edelmann auger) during two campaigns to the northern Mekong River Delta in March 2005 and 2007. 3.1.1. Core GGM The drilling site of the GGM core (10 45.1060 N, 105 42.6610 E) is located at the edge of a swampy area near the village Go Gao Mieu in Long An province (Fig. 2). The visual core description of the 410 cm long sediment core revealed that it consists of black (106–97 cm) to greyish brown (main parts of the core) and dark yellowish brown (top of the core) silty clay (Fig. 3). Furthermore, abundant bivalve shells (410–296 cm) embedded in muddy sediments were detected. Abundant mica (around 290 cm) appears in organic-rich sediments. Most organic matter is uniformly distributed in the sediment. Additionally, between 106 and 97 cm organic matter is accumulated into a peaty layer. Numerous concretions as well as mottles are spread throughout the upper part of the core;

Fig. 3. Columnar section of the sediment cores GGM, TA and GGN in metres below land surface vs. grain size. The interpreted facies units have been listed on the right side of each core. The 14C-age data are given on the left side of the cores (in cal a BP if not stated differently).

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whitish to straw yellow concretions (from 183 to 125 cm along root channels and from 83 cm to the top of the core along root channels or as mottling) and darker, browner concretions (from 40 to 26 cm). 3.1.2. Core TA The drilling site of the TA sediment core (10 31.4320 N, 106 23.1890 E) is close to the town of Tan An in Long An province on the margin of the Vam Co Tay River floodplain (Fig. 2). The main part of the 340 cm long core consists of brownish to greyish clay. Dark grey to bluish silty fine sand occurs from 340 to 196 cm. Organic remains appear throughout the whole core, mainly as small blackish spots. From 238 cm to the top numerous brownish – yellow concretions and mottles have been observed (Fig. 3). 3.1.3. Core GGN The sediment core GGN (1014.3830 N, 106 21.0960 E) was taken in Ben Tre province close to the Ham Luon River and its levee (Fig. 2). The 230 cm long core is composed of clayey sands and silty to sandy clays of brownish to greyish colour. Abundant mica appears from 70 to 170 cm and large organic remains are limited to 170–120 cm core depth. Variously coloured concretions and mottles have been observed from 120 cm to the top (Fig. 3).

3.2. Methods Following the core description and based on the observed features, 35 (GGM), 16 (TA) and 10 (GGN) samples were taken. One half of each sediment sample was freeze-dried and the bulk of the dried samples were wet-sieved at 63 mm to separate the sand fraction from the clay and silt fraction. The sand fraction was further sieved into sub-fractions (sieves 63, 125, 250, 500, 1000 mm). The sand sub-fractions were analysed with a binocular microscope. The remaining part of the dried sample was ground and used for measuring the total carbon content (TC, LECO combustion, Table 1). Radiocarbon dating was carried out on organic fragments (wood/plant remains) and carbonate shells (Table 2). Calibration of the raw 14C data was accomplished by Calib version 5.0.1 (Stuiver and Reimer, 1993) using the terrestrial record for the organic fragments (intcal04) and the marine record for the shells (marine04). In order to achieve a more precise calibration of the marine material, an ocean specific, local reservoir effect based on two calibration points for southern Vietnam has been calculated in Calib and used for the calibration (Southon et al., 2002; Dang et al., 2004). However, all the dated carbonate shells originate from organisms with shallow water habitats, which still leaves some uncertainties to the local reservoir age. This study uses the 2srange of all samples, as it showed more stable results in the relevant time span than the 1s -range (Table 2). The preparation for pollen analysis of core GGM was carried out at the Department of Palynology and Climate Dynamics, University of Go¨ttingen, Germany. From each sample, 0.5 cm3 of the raw, cooled material was processed with the standard techniques including HF treatment and acetolysis (Faegri and Iversen, 1989). Prior to the chemical treatment, one Lycopodium tablet was added to each sample in order to calculate the pollen concentration (grains/cm3). After the treatment the residue of each sample was fixed on slides with glycerine gelatine, and at least 200 pollen grains/slide were counted. As the focus of this study is mainly on mangrove pollen (indicating coastal/brackish environments), the results were simplified into the groups ‘‘mangrove’’ and ‘‘other’’. The identification of pollen was accomplished by reference images (Roubik and Moreno, 1991; APSA Members, 2007). Additionally, the abundance of charcoal particles was listed semi-quantitatively.

Table 1 TC-values for all three presented cores. GGM

TA

GGN

Depth [cm]

TC [wt.%]

Depth [cm]

TC [wt.%]

Depth [cm]

TC [wt.%]

12 35 51 63 66 79 88 94 102 114 122 138 154 170 179 189 197 208 212 232 239 247 258 266 280 299 328 348 373 382 403

1.5 1.4 2.8 2.7 2.6 1.4 4.1 4.0 6.9 4.7 4.0 3.2 2.0 2.0 1.7 2.0 1.5 1.4 1.4 1.8 1.9 2.0 1.8 2.2 2.7 1.9 2.7 2.8 2.4 2.2 3.2

29 52 77 82 127 149 199 209 218 238 249 270 298 335

1.5 0.3 0.2 0.2 0.2 0.2 0.3 0.2 0.2 0.5 0.5 0.9 0.8 1.0

30 53 68 104 118 137 149 161 185 229

1.2 0.3 0.3 0.3 0.1 0.5 1.3 10.7 0.6 0.4

4. Results 4.1. Sedimentary properties and age control 4.1.1. Core GGM The complete core predominantly consists of siliciclastic material with a grain size < 63 mm (Fig. 3). This mud accounts for at least 80 wt.% of the total sediment except for the samples at 403, 382 and 280 cm core depth where the portion of mud is slightly less. The TC-values range between 1 and 7 wt.% (102 cm) with a mean value of about 2.5 wt.% (Table 1). The shells from the basal part of the core could be identified as Placuna placenta, an oyster that has its habitat on muddy sediments in the intertidal to shallow subtidal zone (Nguyen, 2005). The three radiocarbon ages for this core are 6310– 6660 (328 cm), 3980–4180 (235 cm) and ca. 100–150 (88 cm) cal a BP, whereby the last date can be expressed as ‘‘the early to mid19th century’’.

Table 2 Radiocarbon ages for the three presented cores. Sample [cm core depth]

Uncalibrated age [a BP]

Specific reservoir correction DR

Calibrated age Dated (2s-range) material early to mid-19th century A.D. 3980–4180 a BP 6310–6660 a BP 4440–4820 a BP 1930–2120 a BP

GGM – 88

108  0

–

GGM – 235

3740  40

–

GGM – 328

6020  40

TA – 335

4420  35

GGN – 149

2050  35

70 23 70 23 –

   

30 56 30 56

Plant remain Plant remain Oyster shell Carbonate shells Wood

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4.1.2. Core TA From 340 to ca. 200 cm the core is composed of either very fine to fine sand (around 218 cm) or very sandy mud (50–70 wt.% mud). The remaining part of the core consists of >90 wt.% mud. The TC-values range between 0.2 (major parts of the core) and 1.5 wt.% (top of the core) with a mean value of 0.5 wt.% (Table 1). From 340 to 270 cm abundant, well-preserved shells and shell fragments from bivalves, gastropods, crustaceans, echinoderms, ostracods and foraminifera (mainly Asterorotalia sp.) are recorded. The radiocarbon age determined at 335 cm is 4440–4820 cal a BP. 4.1.3. Core GGN The majority of the sediment is composed of muddy fine to very fine sand (sand portion between 60 and 75 wt.%) except for the uppermost 55 cm as well as the sediments around 160 cm which consist of sandy mud (between 58 and 71 wt.% mud). The TC-values range between 0.1 (118 cm) and 10.7 wt.% (161 cm) with a mean value of 0.6 wt.% (Table 1). Around 156–170 and 145–148 cm core depth two peaty layers occur. Mica is reported from the base of the core up to 68 cm being mainly biotite and muscovite. The radiocarbon age at 149 cm is 1930–2120 cal a BP. 4.2. Pollen analysis Fig. 4 illustrates the distribution of mangrove pollen throughout core GGM. The term ‘‘mangrove’’ is used for Rhizophoraceae,

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Avicennia and Sonneratia since the latter two accounted only for 3% of the total pollen count. In order to show the expected maximum range of the counted data the 95%-confidence interval was calculated after Birks and Gordon (1985) (Fig. 4, grey band). Throughout large parts of the core the mangrove pollen account for w60–w80% of total pollen. Only the samples at 122, 114 and 102 cm show minimum values in occurrence (40, 19 and 12% respectively). The pollen concentration values are relatively low, less than 100 000 grains/cm3 from the base up to ca. 180 cm core depth whereas the upper part of the core is characterised by relatively high and varying values (ca. 112 000–575 000 grains/cm3, Fig. 4). The highest value is recorded at 66 cm with ca. 971 000 grains/cm3. The samples at 122, 114 and 102 cm show a remarkably low mangrove pollen signal. This is a unique signal in the core implying a major change in composition at this depth. Thus the pollen content of these samples was studied in more detail. The pollen from the grassland/swamp markers Poaceae and Cyperaceae show an increase of 9–17%. Parallel to this signal, the pollen signal from Myrtaceae (presumably M. leucadendra) decreases from 6 to 1% of total pollen sum. However, the extreme diversity of pollen in these samples and the lack of comparable palynological reference material precluded any further detailed specification at the time. Thus, apart from the mentioned families, a large portion of the pollen had to be summarised as ‘‘other forest/grassland pollen’’ (not shown).

Fig. 4. Pollen data for core GGM. The mangrove pollen distribution is given relative to the total pollen counts. The grey bar represents the 95%-confidence interval. The total pollen concentration is given in 104 pollen/cm3 sediment. For explanation of the columnar section see Fig. 3.

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From the base of the core to up to w170 cm charcoal particles are common. In the upper part of the core almost no charcoal is recorded except at 79 cm core depth where charcoal is remarkably abundant (Fig. 4). 5. Discussion 5.1. Facies development in the northern Mekong River delta 5.1.1. Core GGM 5.1.1.1. Unit Ga: sub- to intertidal flat (410–298 cm). This ca. 110 cm thick unit shows muddy sediments being rich in organic carbon with comparatively low pollen concentration values and a strong signal from mangrove pollen (Fig. 3, 4). Sediments with these characteristics are described from mangrove-fringed coasts of South America (Behling et al., 2001; Behling and da Costa, 2004). Therefore, they are typical of an intertidal environment with at least close-by dense mangrove communities. The abundant appearance of in-situ preserved shells from the oyster P. placenta underlines this facies determination. The radiocarbon age of 6310– 6660 cal a BP at 328 cm core depth implies that this tidal flat was active during the early phase of the mid-Holocene sea level highstand (curve B in Fig. 1). 5.1.1.2. Unit Gb: upper shoreface to shoreline (298–265 cm). This unit is ca. 30 cm thick and shows a slightly decreased but still high mangrove pollen signal. The short-term occurrence of mica and the increase in sand from ca. 10 to 20 wt.% around 280–290 cm core depth suggest a change of the sedimentary environment (Fig. 3). The mica is probably part of the fluvial suspended load whereas the sand could have been part of the fluvial bed load (Nguyen et al., 2000). However, the deposition of mica requires calm hydrological conditions (e.g. during high tide) whereas sand can only be moved under higher water energy (e.g. during rising tide). This implies a combined influence of tidal movement and river outflow suggesting an intertidal system near a river mouth. Deposits with similar characteristics have been described from the upper tidal flat facies in cores from the southern part of the Mekong River Delta (Nguyen et al., 2000; Ta et al., 2002a). The absence of P. placenta shells could indicate that the water depth was too shallow for these organisms, which would imply that this sediment represents the upper shoreface to shoreline with mangroves near-by. 5.1.1.3. Unit Gc: floodplain with swamp-like elements (265–95 cm). This unit of ca. 180 cm thickness consists of relatively uniform mud with a high TC-content reaching its maximum value at the top of the unit. The mangrove pollen signal shows a gradual decline up to ca. 170 cm with a subsequent rapid drop towards the top of the unit. This drop coincides with the peaty layer around 100 cm core depth. The total pollen concentration values increase parallel to the drop of the mangrove pollen signal. The decrease of the mangrove pollen signal implies the decline in mangrove abundance at the site, however, the values remain high. As suggested by Nguyen et al. (2000) mangrove communities may have been still present along the Vam Co Tay River at the time which would provide a close source for the pollen in the record. Furthermore, the majority of pollen in this part of the core consists of Rhizophoraceae, which are known for high pollen production and extensive distribution (Muller, 1959; Grindrod, 1988). Therefore, the high mangrove pollen abundance at this stage in the record could document the close-by presence of mangroves but at the site itself brackish conditions did not necessarily prevail. From 150 cm core depth to the top of the unit, the mangrove pollen signal shows a rapid decline indicating the disappearance of the close-by mangrove communities and thus the cessation of

marine/brackish conditions in the region. Simultaneously, pollen from other plants such as grasses, sedges and Myrtaceae occur, suggesting that the environmental conditions on the delta plain were now favourable for the development of the indigenous freshwater vegetation. Assuming that the Myrtaceae signal is mainly produced by pollen from M. leucadendra (the most common Myrtaceae in southern Vietnam) it underlines the predominance of freshwater since M. leucadendra cannot tolerate saline soils (van der Moezel et al., 1991). Furthermore, M. leucadendra signifies moist but not wet conditions (Morzadec-Kerfourn, 2005) indicating that on the subaerial floodplain partly swamp-like conditions were established. The pollen composition within the peaty layer at the top of the unit shows the enhancement of this signal: Myrtaceae, Cyperaceae and Poaceae account for a large portion of the total pollen count whereas the mangrove pollen signal reaches its minimum within the GGM record. This implies that the freshwater, indigenous vegetation on the floodplain has fully developed. The accumulation of organic matter into a peaty layer reflects the high production of organic material at the site (also shown in the high pollen concentration values) and favourable conditions for conservation including little or no input of siliciclastic material (e.g. only during the annual monsoonal flood). The radiocarbon age of 3980–4180 cal a BP at 235 cm indicates that these conditions commenced during the gradually falling sea level (curve B in Fig. 1). 5.1.1.4. Unit Gd: anthropogenic (95–0 cm). This unit of ca. 100 cm thickness consists of mud enriched in TC. The mangrove pollen signal shows a sudden increase contradicting that at this stage the mangrove communities should be located ca. 100 km away from the site (Nguyen et al., 2000). This signal could be produced by an unusually strong supply of mangrove pollen, the decrease of local pollen producers (enhancing the mangrove pollen signal on a relative scale) and/or the contamination of the record with sediments from mangrove deposits. These three possibilities are discussed in Section 5.3. The radiocarbon age at 89 cm core depth can be expressed as ‘‘early to mid-19th century’’ implying that this unit has been deposited relatively recently. 5.1.2. Core TA 5.1.2.1. Unit Ta: sub- to intertidal flat (340–258 cm). This unit is ca. 80 cm thick and consists of very sandy mud. Abundant shells and shell fragments from bivalves, gastropods, echinoderms, crustaceans, ostracods and foraminifera (mainly Asterorotalia sp.) indicate a diverse shallow water environment. In the southern Mekong River Delta Asterorotalia sp. appears in deposits from the delta front up to the sub- to intertidal flat (Ta et al., 2002a, 2002b) implying that this foraminifera is not indicative of a particular facies. In addition, Asterorotalia sp. is described in sediments from the outer Mahakam Delta front in Indonesia but also with the caveat that their tests can drift into the tidal flats (Lambert, 2003). The generally high diversity of the shells in this unit supports the facies determination of a sub- to intertidal flat indicating that the foraminifer tests have been drifted into this environment as implied by Lambert (2003). The radiocarbon age of 4440–4820 cal a BP at 335 cm core depth implies that this sub- to intertidal flat was active during the initial sea level regression (curve B in Fig. 1). 5.1.2.2. Unit Tb: shoreline (258–210 cm). This unit is ca. 50 cm thick, shows no plant remains or carbonate shells and consists of very sandy mud grading into muddy fine sand at the top of the unit. This coarsening upward trend and the lack of organic material are

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indicative of gradually increasing, higher energy water conditions where the settling of fine-grained siliciclastic and organic material is impeded. At that stage of the facies development, these hydrodynamic conditions suggest that these sediments represent a shoreline deposit. 5.1.2.3. Unit Tc: floodplain (210–32 cm). This unit is ca. 180 cm thick and is composed of muddy sediments. The grain size marks an abrupt transition from unit Tb into unit Tc which indicates a relatively rapid change in sedimentary environment. Sediments that consist of almost pure mud with negligible organic matter have been described from throughout the Mekong River lowland and delta as floodplain sediments (Nguyen et al., 2000; Tamura et al., 2006,, 2009). 5.1.2.4. Unit Td: anthropogenic (32–0 cm). This unit is ca. 30 cm thick and is represented by muddy sediments with abundant fresh organic matter. Furthermore, pieces of brick have been observed, and hence this unit can be assigned to the anthropogenic cover. 5.1.3. Core GGN 5.1.3.1. Unit Na: tidally-influenced fluvial environment (230– 175 cm). This unit of ca. 60 cm thickness is composed of fine sand with little organic matter. The grain size distribution implies a tidal flat deposit but the complete lack of carbonate shells is problematic for this interpretation. Tamura et al. (2009) interpret deposits with very similar characteristics as a tidally-influenced fluvial environment. Since the GGN core is located close to two large rivers the fluvial sediment input may play a bigger role at this location than at the previously described sites. Thus, the facies determination of a tidally-influenced fluvial environment seems suitable for this unit. 5.1.3.2. Unit Nb: shoreline with mangrove (175–135 cm). This unit is ca. 40 cm thick and consists of sandy mud with plenty of organic matter, numerous larger plant remains and two peaty layers. In addition, abundant large mica was observed. Deposits with these characteristics have been described from the lower Mekong River lowland as salt marshes to mangrove forests (Tamura et al., 2006,, 2009). Salt marshes appear on the landward side behind mangrove forests and can trap sediment as well as organic matter but usually no peat deposits are developed (Nguyen et al., 2000). The network of stems, roots and branches in mangrove forests, however, provides a good trap for mica, fine-grained sediments and organic matter so that peaty layers can be formed (van Santen et al., 2007). Thus, the two peaty layers as well as the substantial amount of mica and fine-grained siliciclastic material in this unit of core GGN imply that this deposit represents a shoreline with a mangrove forest. The radiocarbon age of 1930–2120 cal a BP at 149 cm core depth indicates that this shoreline was active during the final period of falling sea level (Fig. 1b). 5.1.3.3. Unit Nc: natural levee to floodplain (135–40 cm). This unit is ca. 100 cm thick and consists of a fining upward succession (fine to very fine sand into sandy mud). Large mica, either accumulated in layers (between 120 and 100 cm core depth) or uniformly distributed occurs frequently. Sandy silts to fine sands have been described from deposits of natural levees (Nguyen et al., 2000). As core GGN was drilled close to a major river with a broad levee this unit can represent the natural levee of this stream. The fining upward may indicate that this levee deposit gradually grades into a floodplain deposit. 5.1.3.4. Unit Nd: anthropogenic (40–0 cm). This unit is ca. 40 cm thick and consists of muddy sediments with abundant fresh organic

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matter. Furthermore numerous pieces of modern pottery have been observed in this unit so it can be classified as anthropogenic. 5.2. Delta development history Due to adequate radiocarbon age control, the facies of the three presented cores can be compared to cores from the river floodplain in the north and the coastal delta complex in the south (Fig. 5). In the early phase of the mid-Holocene sea level highstand (ca. 8000– 6500 cal a BP) a large intertidal flat reaching into southern Cambodia characterised the northern part of the delta (cores PSG and GGM in Fig. 5,). During the subsequent sea level fall and delta progradation, these intertidal flat deposits developed into shoreline deposits whereas tidal flat and tidally-influenced fluvial deposits formed south of these positions (cores TA and GGN). At the same time delta front deposits are recorded in the BT2 site (4.0 ka – line in Fig. 5). Around 3500 cal a BP the rate of delta progradation was reduced (Ta et al., 2001b) which led to the development of a thick succession of delta front and sub- to intertidal flat deposits at the BT2 site. Core GGN records a shoreline deposit at 2000 a BP whereas all cores north of this position show a fully developed floodplain locally with swamp-like elements (cores PSG, GGM and TA). In the course of further progradation, natural levee to floodplain deposits were formed at the GGN site whereas at the BT2 site the subaerial delta plain facies developed. The position of the coastline and the rapid transition of marine into terrestrial environments underline the fast progradation of the delta after the mid-Holocene sea level highstand. The positions and ages of the shorelines in all three cores are in good agreement with the proposed coastline development in Nguyen et al. (2000). The comparison of the three presented cores with the PSG and BT2 successions reveals that the thickness of the Holocene deposits in the northern delta is highly variable and the deposits are always thinner than those from southern Cambodia and the southern delta. For example in core PSG deposits of mid-Holocene age cover more than 200 cm in the succession whereas in core GGM only w100 cm of sediments from this period are preserved. This local effect has to be due to the morphology of the underlying deposits which, in the case of the Mekong River Delta, are sediments of late Pleistocene age. These deposits consist of very stiff greyish clays, silts and sands with numerous concretions and plant remains (Ta et al., 2002a, 2002b; Tamura et al., 2009). These sediments have been subaerially exposed during the late Pleistocene sea level lowstand and formed a gently southward dipping land surface with channels and depressions. In the northern delta no sediment successions are associated with the post-glacial sea level rise which could be due to sediment by-passing or to immediate post-depositional erosion. Due to the irregular shape of this late Pleistocene land surface the accommodation space during the mid-Holocene sea level highstand and subsequent delta progradation varied locally, which is then reflected in the varying thickness of the sediments in the northern delta (Fig. 5). Another striking result is that typical features of a prograding delta such as sediments from the prodelta or delta front have not been observed in the successions from the northern delta. These deposits are part of the successions in the southern delta (e.g. core BT2) but are completely missing in all three presented cores (Fig. 3, 5). As the base of core GGM is directly underlain by the late Pleistocene sediments, it can be excluded that these deposits are missing. Thus, it appears that the fast early delta progradation (17–18 m/year; Ta et al., 2001b) was into a shallow sea which offered little accommodation space and thus hindered the formation of prodelta and delta front deposits. Hence the deposits from the northern part of the Mekong River Delta show rather estuarylike features, as described in Tamura et al. (2009), rather than delta signatures as described in Ta et al. (2002a, 2002b).

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Fig. 5. Comparison of cores GGM, TA and GGN (this study) with core PSG (Tamura et al., 2009) and core BT2 (Ta et al., 2001). For better visual comparison both cores PSG and BT2 are shown incomplete and have been adapted to the graphic scheme of cores GGM, TA and GGN when possible (associated legend according to Tamura et al., 2009 and Ta et al., 2001b, respectively). The small inset map displays the positions of all five cores. The dotted horizontal line represents the level of the modern mean sea level in southern Vietnam. The isochrones are given in 103 years and are based on all given radiocarbon ages.

5.3. Historic human impact All three cores document recent human activity in the uppermost sediments of their successions. Especially in core GGM, a rather profound alteration is recorded in the mangrove pollen signal of unit Gd. This signal shows an unexpected increase, which could be due to an unusually strong supply of mangrove pollen via water and wind, the decrease of local pollen producers (increase of mangrove pollen on a relative scale) and/or the contamination of the record with older mangrove deposits. An unexpectedly high supply of mangrove pollen via water has been described in studies from Venezuela (Muller, 1959), Australia (Grindrod, 1988) and Cambodia (Penny, 2006) and has been

explained with the physical characteristics of the pollen grains. Mangrove pollen (especially those from Rhizophora sp.) behave in water like fine silt which implies a slow sinking rate and a high drifting potential (Muller, 1959). Intense tidal movement in the Vam Co Tay River is observed 75 km upstream, which is within the same range as in the Mekong and Bassac River channels (Wolanski et al., 1996). In addition, large mangrove forests (mainly Rhizophora sp.) cover the coastline where the Vam Co Tay River discharges into the South China Sea (van Loon et al., 2007). Thus the supply of mangrove pollen by tidal movement in the Vam Co Tay River cannot be excluded. The second explanation for the preponderance of mangrove pollen could be a sudden decline of local pollen producers. The age

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mark of ‘‘early to mid-19th century’’ at 89 cm core depth revealed that these sediments have been deposited contemporaneously with the modern colonisation of the Mekong River Delta (Tanaka, 2001). These settlers reclaimed large parts of the delta by clearing the land, which would have led to a sharp decline of pollen producers at the site. Local burning of the vegetation was a common procedure which is recorded in the strikingly high amount of charcoal at 79 cm core depth (Fig. 4). Therefore the increase of mangrove pollen could also be due to the human induced decrease of on-site vegetation. The third possibility involves the contamination of the unit with older mangrove deposits. In the course of the settlement numerous channels have been constructed leading to the deposition of large amounts of sediment along the margins of these channels. These sediments could likely include mangrove deposits of mid-Holocene origin which then would have been distributed in the fields behind the channels (e.g. during the annual floods) and incorporated in the surrounding, in-situ sediments. Agricultural activities such as ploughing would then produce a mixed deposit containing two features, contaminated (mangrove) and in-situ (Poaceae, i.e. rice) pollen as have been observed in this part of the record. The strong fluctuations in the signal could be an indication of the repeated contamination originating from the subsequent gradual deepening of the channels. It is likely that all of these three scenarios added to the high mangrove pollen values. However, the latter explanation seems to play a major role since the increased sedimentation rate cannot solely be explained with natural processes. In addition, no eminent amounts of crop pollen were observed which would be expected if agriculture alone caused this signal. Nevertheless, the impact of settlers altering the landscape seems to be the prerequisite for this strong signal accentuating the anthropogenic character of the Gd unit. 6. Conclusions The sedimentological and palynological analysis of sediment cores GGM, TA and GGN show, that the early stages of the Mekong River Delta development can be traced in the northern part of the delta. The progression from a tidal flat environment, with local adjacent mangroves (GGM) or strong fluvial input (GGN), into floodplain with local swamp-like vegetation can be shown from the sediments in these cores. The comparison with cores from the southern Mekong River floodplain in Cambodia and the outer Mekong River Delta shows that the stratigraphic record in the northern delta is condensed, of varying thickness and appears to have an incomplete delta facies succession (e.g. prodelta and delta front are missing). Thus, at least in the very northern delta (GGM site) a tidal flat – marsh environment of estuarine character rather than a ‘‘true’’ delta was developed during the early stages of delta progradation. Furthermore the impact of the modern settlers on their environment is recorded in all three cores. Especially, the palynological composition of the uppermost meter in core GGM shows a highly disturbed signal. This record implies that the fully developed, indigenous freshwater vegetation was cleared which is also indicated by a peak in charcoal abundance at the same time. It also shows a likely contamination with older mangrove deposits that could have been incorporated into the in-situ sediments by human activity. This palynological record also documents for the first time the change from regional indigenous to anthropogenic, disturbed vegetation in the Mekong River Delta. It shows the potential and importance of palynological and human impact studies in the Mekong River Delta.

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Acknowledgments The corresponding author would like to thank Prof. Hermann Behling for the introduction into pollen analysis and the constant support. We are grateful to Dr. Andreas Reinecke from the Deutsches Archa¨ologisches Institut (DAI) in Bonn who supported us with his long-time experience during our fieldwork. Furthermore we thank all the local Vietnamese authorities which helped with the realisation of the fieldwork. Additionally, we are grateful to two anonymous reviewers whose constructive comments helped to improve the manuscript. This study is part of the DFG-funded project HA4317\2-1.

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