Holocene Elevated Sea Levels on the North ... - Wiley Online Library

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Holocene Elevated Sea Levels on the North Coast of Vietnam WILLIAM EDGAR BOYD, Southern Cross University, Australia and DOAN DINH LAM, National Centre for Natural Science and Technology of Vietnam Abstract We present evidence of elevated Holocene sea levels on the north coast of Vietnam, comprising radiocarbon dating of raised coastal geomorphological and palaeobiological features at localities on the margins of the Bac Bo (Red River) Plain. This evidence indicates that by ca. 5500 cal yr B.P. mid-tide lay approximately 5.4 m above national datum, that is 3.25 m above present local mid-tide. By c. 3500 cal yr B.P., sea level was lowering and, by ca. 2000 cal yr B.P., mid-tide lay at or above 1.5 m above present local mid-tide. It is not yet possible to infer patterns of sea-level fluctuation other than an overall lowering of elevation, although the separation of the raised and modern erosion notches does suggest two dominant mid to late Holocene periods of erosion — probably periods of sea-level still-stand or relatively slow sea-level change. The intervening period of relatively rapid sea-level change and associated reduced erosion probably dates to after ca. 2000 cal yr B.P.. KEY WORDS Holocene; sea level; radiocarbon dating; Ha Long Bay; Tam Coc; Bac Bo Plain; Red River delta; Vietnam; South China Sea

Introduction The coastal zone is a significant environmental resource in Vietnam. Increasing scientific interest in this resource includes a need to understand its geological development. Geological mapping has recorded surface and buried sediment distribution and underlying bedrock structures, but the interpretation of landscape evolution is limited by the lack of reliable regional Quaternary sea-level data. Many parts of the coastal zone are tectonically unstable. Studies in Vietnam, therefore, have had to rely on indicators of former sea levels in distant tectonically stable areas (Do Van Tu, 1985; Nguyen Duc Tam, 1989; Tran Duc Thanh, 1991; Tran Nghi, 1991; Tran Nghi

et al., 1991; Doan Dinh Lam and Boyd, 2000). This is a problem also encountered elsewhere on the Vietnamese coast (Le Duc An, 1996; Van Lap Nguyen et al., 2000). Former changes in sea level differ in timing, magnitude and periodicity throughout S.E Asia and the South China Sea, reflecting regional or local conditions. Importantly, they provide sea-level data that are often not relevant to specific study localities. Here we present radiocarbon ages of fossil shell collected from raised wave-cut notches in presently tectonically stable bedrock, to interpret a chronology of former sea-level elevations on the north coast of Vietnam, providing a basis for a Holocene sea-level curve for northern Vietnam. Australian Geographical Studies • March 2004 • 42(1):77– 88

78 There have been several impediments to establishing a sea-level, elevation-age curve for northern Vietnam. First, interpretation of the sedimentary record of the Bac Bo Plain (also known as the Red River Delta) is confounded by the effects of graben-style tectonic subsidence (Tran Dinh To et al., 2000). Sea-level history is therefore often inferred from the sedimentary sequences themselves, yielding records of relative sea-level movements and of coastal transgressions and regressions, rather than absolute sea-level changes. Second, interpretation is often by reference to absolute sea-level data from elsewhere, yielding a local implied record of absolute change (Tran Nghi et al., 1991; Le Duc An, 1996; Nguyen Quang Mien and Le Khanh Phon, 2000). The Quaternary sea-level record in the broader region is geographically variable. In nearby southern China, for example, records of former sea levels provide evidence of either a single mid-Holocene period of higherthan-present sea level (Yang Huaijen and Wang Jian, 1991) or a more complex series of rises and falls as the mid-Holocene sea level dropped from its highest elevation (Han Yousong and Meng Guanglan, 1987; Huang Zhenguo et al., 1987). Data elsewhere in S.E. Asia and the South China Sea support both general models of Holocene sea-level change (Geyh et al., 1979; Tjia et al., 1983; Sinsakul, 1992), although details, especially of timing and elevation, vary significantly (Haile, 1971; Tjia, 1977; Thommeret and Thommeret, 1978; Umitsu, 1991; Rimbaman, 1992; Kamaludin, 1993; Saito, 1995). The timing of the period of high sea level varies from ca. 6500 to 4000 14C yr B.P., and the maximum elevation lies up to 5 m above present sea level. The important point here is that this diverse record is used to interpret Holocene sealevel elevation data on the northern Vietnamese coast, creating the risk of misinterpretation due to incomparable data. Finally, past reporting of radiocarbon dating of sea-level indicators has been ambiguous, with different results being reported for the same samples, and elevations of samples and sea-level indicators being poorly recorded. In publications,

Australian Geographical Studies it is frequently unclear exactly what material or stratigraphic evidence have been dated, and how this relates to a modern datum or to any former sea level. Consequently, the resulting interpretation of former sea-level change has often been unsatisfactory and, in some cases, questionable. In one case, for example, a 10 m sea-level drop is postulated to separate sea-level highs during the middle and late Holocene (Tran Nghi et al., 1991), a postulated sea-level change unlikely to find support elsewhere in the world. There is clearly a need for a regionally-specific sea-level curve, based on accurate elevational and chronometric data from tectonically stable sites, that can apply directly to the northern part of the Vietnamese coast. The geographical setting and geological evidence We report here the first chronometric evidence for the timing and elevation of the mid-Holocene high sea level and subsequent sea-level lowering in the region, as the start of a dependable and regionally-relevant Holocene sea-level curve. Our evidence is from the radiocarbon analysis of the remains of fossil oyster (Ostrea sp.) shells (specifically, fossil areas of attachment), cemented in growth position to near-vertical-sea cliff rock faces and cave roofs in tectonically-stable localities. The significant fossil shell material lies at elevations higher than present tidal range, indicating a relative difference between present sea level and the former sea levels. Nguyen The Thon (1994) argues, using geomorphological evidence (Holocene shoreline deposits, river-mouth configurations and relief values), that Ha Long Bay has been stable for at least the last 10 000 years. His argument is supported by recent tectonic studies, by staff of the Institute of Geology, who use nearby Hon Dau Island (35 km to the southwest) as a key stable datum locality. This situation contrasts with parts of the neighbouring mainland, where Tran Dinh To and Nguyen Trong Yem (1991) record subsidence at 1–2 mm yr−1 . The region is characterised by Miocene-Pliocene uplift of Carboniferous limestone in Ha Long Bay (Quang Ninh © Institute of Australian Geographers 2004

Holocene Elevated Sea Levels on the North Coast of Vietnam Province) and Triassic limestone at Tam Coc (Ninh Binh Province) (Figure 1), and subsequent weathering and erosion resulting in the present karst landscapes at both localities. The Ha Long Bay karst is drowned by the sea, and the shoreline of about 100 islands was surveyed by boat. A nearby mainland outcrop of this karst was also examined at Quang Hanh. The Tam Coc karst is now inland, with the former marine bay comprising the valley and tunnel caves of the Ngo Giang River. The limestone cliff base typically has wavecut notches reflecting wave erosion at the modern (Ha Long Bay only) and two former (higher) sea levels (Figure 2). The modern notch, present throughout the Bay, is typically a multiple feature with a deep under-cut in the lower tidal range. The top of this notch lies at c. 3.9 m above Vietnamese national datum. The national datum is the lowest annual tidal level at the tide gauge at nearby Hon Dau Island, and is

Figure 1

the datum used for tidal tables published by the Marine Hydrometeorological Centre. The present tidal range lies at 0.4–3.7 m above national datum, and thus the top of the notch lies just above local average high water mark. The lower raised notch, widespread in Ha Long Bay and present at Tam Coc, comprises a single or double notch, a partitioned under-cut or an etched surface. The notch lies at c. 4.0–7.0 m above national datum, representing a sea level some 3 m higher than present. The higher raised notch (in places a pair of notches) is less common in Ha Long Bay but ubiquitous at Tam Coc. It lies at various elevations up to c. 9 m above national datum. The fossils used in this study are the remains of the cemented areas of attachment of oysters (Black, 1970). They are found in growth positions on vertical or near vertical rock faces and overhangs or cave roofs. In Ha Long Bay, living oysters are presently abundant at the bases of

Location map, showing the sampling sites discussed in this paper.

© Institute of Australian Geographers 2004

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Holocene Elevated Sea Levels on the North Coast of Vietnam the vertical or near vertical rock faces, where they grow at and below the mid-tide level. The highest measured position for modern oyster growth in these localities within the study area is at 2.5 m above datum, and oysters can be seen to be growing to below low tide. These oysters and their growth environment are directly analogous to the past environments being studied here. The fossil remains, where they exist above the present tidal range, are taken to represent former elevated sea levels. These fossils indicate former mid- to sub-tidal conditions, and so provide a minimum measure of the elevation of former mid-tide. Fossil remains of oyster areas of attachment are cemented to the rock face at some localities in the lower parts of both the lower raised notch and the higher raised notch, as well as at equivalent elevations on some rock faces. The fossils are more abundant on the lower than the upper features. Radiocarbon dates and inferred elevations of former sea levels The radiocarbon dates of 15 samples (Figure 3, Table 1) provide evidence for two periods of former higher-than-present relative sea level. Dates Wk-8261, Wk-8263, Wk-8264 and Wk8266 are all from the higher raised notch, and indicate that this notch was formed during preHolocene times. By analogy with global sea level curves (Chappell and Shackleton, 1986; Zazo, 1999), this most probably represents a last interglacial high sea level. The variation in elevations of the samples yielding these dates, and in particular the high elevation (c. 10 m above datum) of shells on a cliff face at Quang Hanh (not illustrated in Figure 3), implies local tectonic instability since the last interglacial high sea level of ca. 125 000 years ago. An interpretation of an interglacial high sea level is broadly supported by the >40 000 year ages of three of the samples. However, one sample (Wk-8264) provides a date of 32 960 ±

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380 14C yr B.P., a date that sits uncomfortably with the global sea level curve, on which there is no evidence for a higher-than present Pleistocene sea level at that time. The sample was large enough to provide a reliable radiocarbon measurement (A. Hogg, Director, University of Waikato Radiocarbon Dating Laboratory, personal communication, May 2001). On the other hand, the sample could be partially recrystallised, thus explaining the apparently young age, although recrystallisation would be undetectable in calcitic oysters. It is not possible to identify whether the radiocarbon dates of other old samples are also younger than their true age. More importantly, however, given the age-elevation patterning of the eleven Holocene ages for the younger samples (see below), it seems unlikely that there is a significant problem with recrystallisation of these latter samples. The elevations of the Holocene samples at Ha Long Bay samples were surveyed to local tide level and linked to national datum through local tide tables. The Tam Coc Holocene sample elevations were surveyed to local river water level and linked to national datum through land survey data. This appears to be the first study in Vietnam that accurately ties elevated sea-level indicators to the Vietnamese national datum, and thus we consider our data to be the first true and accurate measures of former sea-level elevation in the region. This is a most important aspect of this relatively straightforward study. By analogy with present patterns of oyster growth in the region, and specifically at the bases of the cliffs surveyed and sampled, sample elevations are taken to represent former positions at or below mid-tide. Importantly, sets of samples from the same localities (Wk-8255, Wk-8256 and Wk-8257; Wk-8258, Wk-8259 and Wk-8260) indicate progressively younger dates with lowering elevation, implying a lowering sea level and suggesting that the dated materials represent contemporary upper limits of shell growth.

Figure 2 Examples of the raised wave-cut notches (marked by the arrows) in Ha Long Bay (A, Hon Hang Dinh; B, Han Rom Duoi; C, E, F, Dao Dau Go) and Tam Coc (D). The upper arrow in each, except D, lies at around 6 m above water level; at Tam Coc (D), the upper arrow lies at c.2.5 m above water level.


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Table 1 Radiocarbon analyses of raised fossil oyster shell samples, Ha Long Bay (Quang Ninh Province) and Tam Coc (Ninh Binh Province), northern Vietnam. Location is given as local sample site name, general area and GPS reading taken in November 1999. Age is presented as radiocarbon years ( 14C yr) before present (B.P.) and as calibrated calendar years (cal yr) B.P., using Oxcal v.3.4 and adjusting reported calendar years A.D./B.C. to cal yr B.P. taking 1950 as present. The elevations at Ha Long Bay and Quang Hanh are surveyed above local sea level and tied to the Vietnamese national datum (lowest annual tide level at Hon Dau Island) through the published tide tables. The elevations at Tam Coc are surveyed to local river water level, surveyed at 2.9 m above national datum. All radiocarbon measurements, calculations and calibrations were conducted at the Radiocarbon Dating Laboratory, University of Waikato, New Zealand, following standard practices at that laboratory, and are presented here as reported by that laboratory. Radiocarbon laboratory code

Elevation (m above Vietnamese national datum)

MC13 (per mill)

% modern

Age (14C yr B.P.)

Calibrated calendar age at 95.4% probability (cal yr B.P.)

Wk-8255 Wk-8256 Wk-8257

Hon Cau Ngu (location 1), Ha Long Bay, 20°53′00″N 107°10′30″E 4.85 − 0.4 ± 0.2 53.7 ± 0.6 4990 ± 90 B.P. 4.55 0.7 ± 0.2 60.0 ± 0.4 4100 ± 50 B.P. 4.05 − 0.2 ± 0.2 62.1 ± 0.4 3820 ± 50 B.P.

5560 – 5050 B.P. 4290 – 3980 B.P. 3890 – 3630 B.P.

Wk-8258 Wk-8259 Wk-8260

Hon Cau Ngu (location 2), Ha Long Bay, 20°53′00″N 107°10′30″E 4.90 − 0.4 ± 0.2 60.4 ± 1.1 4050 ± 140 B.P. 4.25 0.0 ± 0.2 66.5 ± 0.5 3280 ± 60 B.P. 3.50 0.6 ± 0.2 75.3 ± 0.5 2280 ± 60 B.P.

4450 – 3650 B.P. 3270 –2940 B.P. 2010 –2940 B.P.

Wk-8261 Wk-8262

6.80 4.80

Non Dao Gieng Cut, Ha Long Bay, 20°52′32″N 107°16′54″E 0.2 ± 0.2 0.1 ± 0.0 >40 000 BP 1.1 ± 0.2 55.2 ± 0.4 4770 ± 60 B.P.

Wk-8263 Wk-8264

7.05 7.85

Hon Hang Dinh, Ha Long Bay, 20°56′30″N 107°05′00″E − 1.3 ± 0.2 0.6 ± 0.1 >40 000 B.P. − 2.1 ± 0.2 1.7 ± 0.1 32 960 ± 680 B.P.

No calibration 5230 – 4850 B.P. No calibration No calibration

Wk-8265 Wk-8266

5.4 c. 10

Quang Hanh, 20°59′46″N 107°14′08″E − 1.6 ± 0.2 57.7 ± 0.5 4420 ± 70 B.P. − 0.5 ± 0.2 0.5 – 0.0 >40 000 B.P.

4790 – 4410 B.P. No calibration

Wk-8267

5.4

Tam Coc (location 1), 20°13′30″N 105°55′57″E − 7.1 ± 0.2 53.4 ± 0.4 5040 ± 60 B.P.

5550 – 5270 B.P.

Wk-8268

5.05


5740 – 5550 B.P.

Wk-8269

4.0


2930 –2660 B.P.

The data from Ha Long Bay and Tam Coc are used to define our preliminary elevation-age sea-level curve for this region (Figure 4). By ca. 5500 cal yr B.P., mid-tide lay at around 5.4 m above national datum, that is around or just under 3.25 m above present local mid-

tide: a difference between the modern and mid-Holocene sea levels of +3.25 m is proposed. By ca. 3500 cal yr B.P., sea level was lowering, and by ca. 2000 cal yr B.P., midtide lay at or above 1.5 m above present local mid-tide. © Institute of Australian Geographers 2004

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Figure 3 Sketch profiles of the rock faces at sample sites in Ha Long Bay (the islands of Hon Cau Ngu, Hon Dao Gieng Cut and Hon Hang Dinh) and inland locality of Tam Coc. The profile of the rock face at Quang Hang was not surveyed. The sample positions are identified by the radiocarbon laboratory codes. The indicated water levels are the local tidal levels (at the Ha Long Bay sites) and river level (Tam Coc) at the times of sampling. Samples Wk-8268 and Wk–8269 were collected in tunnel caves, and the others from open exposed cliff faces.

Figure 4 Mid-Holocene raised sea-level dates at Ha Long Bay and Tam Coc, northern Vietnam, showing elevation-age distributions. The ages are indicated as the reported 95.4% probability calibrated calendar dates (see Table 1 for details of calibrations). The vertical line at present represents the present tidal range; with the zone of modern oyster growth on the bases of the cliffs surveyed and sampled in the study area is also indicated. All local surveyed elevations have been converted to elevations above the Vietnamese national datum.

Discussion This study provides the basis for a regionally relevant Holocene sea-level curve for the northern coast of Vietnam. Researchers in this region © Institute of Australian Geographers 2004

will no longer, therefore, have to rely on sealevel data from distant localities with demonstrably different sea-level histories, and will be in a better position to evaluate evidence for

84 former sea levels. In the nearby Hai Phong region, for example, there is ample sedimentary evidence for a complex Holocene coastal history, some of which cannot be fully evaluated because of the lack of a secure sea-level curve (Doan Dinh Lam and Boyd, 2000). Furthermore, where sea-level curves have been constructed, such as Le Duc An’s (1996) curve for the southern half of Vietnam (using evidence from disparate sites on the central and southern coasts of Vietnam), these have relied on data from disparate locations, with no clear relationship to the elevations of present sea level (for example Van Lap Nguyen et al. 2000, for the Mekong Delta region). The sea-level curve presented here, on the other hand, is specifically tied to surveyed sea levels at apparently tectonically stable localities, and thus represents true elevational changes during the latter part of the Holocene. In terms of the details of the curve, two points are important: 1. the elevation-age curve provides a seemingly straightforward record of sea level lowering during late Holocene times, and 2. it is not possible to infer patterns of sea-level fluctuation (other than a gradual lowering). General trend of sea-level change since the midHolocene, and regional comparisons The first point is that the elevation-age sea-level curve provides a record of sea level lowering from a mid-Holocene (ca. 5500 cal yr B.P.) high (3.25 m above present local sea level) to a lateHolocene elevation of +1.5 m by around 2000 cal yr B.P.. The curve is notable for its lack of evidence of multiple peaks. There is much regional evidence for a mid-Holocene sea-level high, although detailed comparison with the curve presented here is difficult given the general lack of published detail regarding datum and sea-level elevations used in both the measurements and the inferred interpretations. It is not, therefore, always clear whether the elevations quoted represent true elevations above present sea level or above a national datum. Nevertheless, the patterning is of relevance. Other sea-level studies in Vietnam include Le Duc An’s (1996) work in southern Vietnam. He

Australian Geographical Studies suggests a mid-Holocene sea-level peak of +4 m at around 5500 years ago, while Van Lap Nguyen et al. (2000) claim that the Holocene marine transgression reached its maximum spatial extent, and thus probably its elevational maximum, at 5000 to 6000 years B.P.. Earlier work (Fontaine, 1970) reached similar, if in part inferred, conclusions of a mid-Holocene sea level high of +4.5 m at 6000 to 5000 years B.P., with sea level at +2.5 m around 4150 14C yr B.P.. It is of interest that Fontaine recorded two erosion notches, dating the lower by radiocarbon dating of oyster shell and, importantly, assuming the higher to also date from the Holocene. This assumption was based on comparison with radiocarbon dates from sediment sequences elsewhere in the Mekong delta region. Such reasoning was used in the early stages of the study reported here, based on the regional patterns of elevated sea-level evidence, but has subsequently been clearly shown to be erroneous. It may well be, therefore, that Fontaine’s data simply indicate a mid-Holocene sea level peak at some time before 4000 years ago, with the higher wave-cut erosional feature being, as in this study, a Pleistocene feature. Elsewhere in mainland southeast Asia, the Bangkok Plain provides evidence for a midHolocene period of maximum marine transgression that may represent the period of highest sea-level stand, variously reported at 6500 to 7300 years B.P. (Somboon and Thiramongkol, 1992) or 5000 to 6000 years ago (Somboon, 1988). Sinsakul (2000) suggests a sea-level high of +4 m at around 6000 years ago. At the eastern margin of the embayment, however, dates indicate a very short-lived but significantly later maximum transgression shoreline position (with no evidence for an earlier marine presence), and thus inferred sea level highstand, at around 3800–3900 14C yr B.P. (ca. 4200 cal yr B.P.) (Boyd et al., 1996; Higham and Thosarat, 1998). Tectonic effects are well recorded within (subsidence) and at the margins (uplift) of the Bangkok Plain, as is the compaction of sediments in the main part of the Plain (Thiramongkol, 1987; Somboon and Thiramongkol, 1993). Such seemingly contradictory dating may not, © Institute of Australian Geographers 2004

Holocene Elevated Sea Levels on the North Coast of Vietnam therefore, be problematic. It does, however, serve as a reminder of the tectonic complexity of mainland S.E. Asia, and of the need for region-specific studies. To the north, along the Chinese coast, several studies also provide potentially comparable data and important caveats. Saito (1998) provides a general Holocene sea-level curve for the East China Sea, with a sea-level peak of around +2– 3 m at around 6000 years B.P., a general curve that is reproduced in other studies, usually as the context for studies of particular and often locale-specific paleo-coastal processes. Chen and Stanley (1998), however, discuss the complexity and diversity of Holocene sea-level curves on the Chinese coast, working with a substantial record on the Yangtze delta. They draw attention to the importance of the interaction between the sea-level rise itself and the environment upon which it is impacting. They conclude that for the Yangtze delta, the Holocene sea level has rarely been higher than the present level, and if so, only for short periods. This may also apply to the Bac Bo Plain (Doan Dinh Lam and Boyd, 2000). Moreover, in a review of a substantial dating record of sea-level change along the eastern coast of China, both Han Yousong and Meng Guanglan (1987) and Yang Huaijen and Wang Jian (1991) provide ample evidence for regional diversity in the Holocene sea-level curves. Yang Huaijen and Wang Jian warn that active and regionally variable neotectonism along the coast has resulted in great difficulty in defining a regional eustatic sea-level curve. Huang Zhenguo et al. (1987) offer the most important regional comparison. They have examined the results of studies of sea-level change along the south coast of China (Hainan and the Beibu (Bac Bo) Gulf in the south, to the Minjiang River mouth in the north). By taking into account local tectonic effects, they suggest a regional eustatic curve defined by three Holocene peaks. This probably offers the most significant comparable sea-level curve to that presented here. The three sea-level peaks recorded by Huang Zhenguo et al. are +4.5 m at 5700 years B.P., +1.5 m at 2200 years B.P. and +0.6 m at 1400 years B.P. © Institute of Australian Geographers 2004

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Similar diversity is recorded more distantly in this eastern and southeastern Asian region. Japanese data, for example, indicate regional variability, with most of the highest Holocene sea levels in stable regions lying up to +5 m at around 5500 to 6000 years B.P. (Umitsu, 1991). Well to the south, on the Thai-Malay peninsula, sea level appears to have risen to a seemingly high mid-Holocene peak (+5 m) at around 5000 years ago, falling and rising to +2.5 m at 4000 years ago (Tjia, 1979, 1996; Tjia et al., 1983; Woodroffe, 2000). In northern Java, however, sea level reached a lower maximum elevation, one more comparable to that recorded in this study, of +3.5 m at 6000 to 5000 years ago (Rimbaman, 1992). Thommeret and Thommeret (1978), on the other hand, suggest a later and lower maximum sea level (+2.47 m at around 3650 years B.P.), although this may simply represent the latter part of a sea-level lowering from the sea-level peak documented by Rimbaman. Potential still-stands and variable rates of sea-level lowering The second important point to note with the sea-level curve reported here, is that it is not yet possible to infer patterns of sea-level fluctuation other than an overall lowering of elevation (cf. Islam and Tooley, 1999; Baker and Haworth, 2000). However, the separation of the lower raised notch (dated here) and the modern notch suggests two dominant mid to late Holocene periods of erosion. This situation parallels similar evidence in limestone bedrock in the far south of Vietnam (Fontaine, 1970; Van Lap Nguyen et al., 2000). These periods are likely to be either during a sea-level still-stand or, more probably, a period of relatively slow sea-level change, and were separated by a period of relatively rapid sea-level change and associated reduced erosion. The evidence here suggests that the latter period dates to, at least, after c. 3000 cal yr B.P., and probably after c. 2000 cal yr B.P. Several records elsewhere in the region support a period of rapid or significant change after that date, changes that would have resulted in cessation of erosion. In southern Vietnam

86 (Le Duc An, 1996), a rapid lowering of sea levels to around or just below present sea level is recorded at around 2000 years ago. Further afield, records on both the Malay-Thai peninsula and the Chao Phraya delta indicate that by around 1500 years ago, sea level had lowered to approximately the present elevation (Tjia, 1980; Sinsakul, 2000; Woodroffe, 2000). Huang Zhenguo et al.’s (1987) South China curve also provides possible periods of non-erosion: they suggest that sea level fell below present sea level between the peaks (that is, at times between 5700 and 2200 years B.P. and notably between 2200 and 1400 years B.P.). While a period of sea-level lowering equivalent to the first period of low sea level does not appear to be supported here, the second period may well be reflected in the northern Vietnamese record reported here. Conclusions Holocene sea-level elevation curves in the east and southeast Asian region are notable for their diversity. While some of their variability undoubtedly reflects the tectonic variability of the region, some uncertainty may be related to methodology with which elevation is recorded and reported. In this study, former sea levels are explicitly recorded relative to the present sea level at the specific localities at which the geological evidence has been collected. However, the mid-Holocene maximum elevation of 3.25 m above present local sea level translates readily to around 5 m above the Vietnamese national datum. In this way, the estimate of the midHolocene maximum elevation recorded here could be favourably compared with those relatively high (ca. +5 m) elevations in Japan and the Thai-Malay peninsula (Umitsu, 1991; Tjia, 1996), the relatively low elevations (ca. +3 m) in the East China Sea and Java (Saito, 1998; Rimbaman, 1992), or intermediate elevations (ca. +4 m) in southern Vietnam, the Bangkok Plain and southern China (Le Duc An, 1996; Somboon, 1988; Huang Zhenguo et al., 1987). Likewise, the timing is regionally variable, although a lack of clarity regarding reporting

Australian Geographical Studies of dates (for example, are reported dates 14C years B.P., calibrated calendar years B.P. or rounded estimates?) hinders exact comparison. The mid-Holocene sea-level maximum is recorded as having occurred at various dates broadly comparable with the Bac Bo dates reported here (c. 5500 cal yr B.P., equivalent to c. 5000 14C yr B.P.): 5500 B.P. on the Bangkok Plain, a general date of 6000 B.P. for the East China Sea, and a more specific date of 5700 B.P. for southern China (Le Duc An, 1996; Huang Zhenguo et al., 1987; Somboon, 1988). These estimates, however, are not without apparent contradictions — the Bangkok Plain also yields dates for midHolocene inferred sea-level maxima of 6500 to 7300 B.P. and 3800 to 3900 B.P. (Somboon and Thiramongkol, 1992; Boyd et al., 1996) — reflecting the importance of site- and regionspecific conditions and histories. Nevertheless, the Bac Bo curve does appear to be at the young end of a widely recognised 6000 to 5000 B.P. age range for the mid-Holocene sea-level maximum in the east and southeast Asian region, although this study does not have any dating evidence for any earlier phase of sea-level change. Finally, while the evidence here does not contain the detail reported elsewhere for the fine detail of a fluctuating late-Holocene sea-level curve, it does imply a staged lowering of sea-level. At least one period of rapid sea-level lowering is reflected in the geomorphology — the separation of distinct wave cut notches at separate elevations — and, probably, the radiocarbon dating records. This evidence implies a date for such an event some time after 3000 cal yr B.P., probably post-2000 cal yr B.P., a date that parallels trends evident elsewhere in the region (Tjia, 1980; Huang Zhenguo et al.,1987; Sinsakul, 2000; Woodroffe, 2000). Correspondence: Associate Professor Bill Boyd, School of Environmental Science and Management, Southern Cross University, Lismore, New South Wales 2480, Australia. Email: [email protected] ACKNOWLEDGMENTS The research reported here was funded by grants awarded to Boyd by Southern Cross University, Australia. We acknowledge the permission and support for this research, of the


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