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Beach Sediments from Northern South China Sea Suggest High and Oscillating Sea Levels During the Late Holocene
EARTH SCIENCE FRONTIERS Volume 16, Issue 6, November 2009 Online English edition of the Chinese language journal Cite this article as: Earth Science Frontiers, 2009, 16(6): 138–145.
RESEARCH PAPER
Beach Sediments from Northern South China Sea Suggest High and Oscillating Sea Levels During the Late Holocene YU Kefu , CHEN Tegu South China Sea Institute of Oceanology, Chinese Academy of Sciences, Guangzhou 510301, China
Abstract: Understanding the history of the late Holocene sea level change is critical for predicting the potential effects of future sea level changes. The coral reef region of the Leizhou Peninsula, in the northern coast of the South China Sea, is an ideal site for studying the Holocene sea level history because of its tectonic stability and the possession of numerous prehistoric sea level indicators. The paleobeach sedimentary records from the Leizhou Peninsula indicated a higher-than-present sea level stand, from 1.7 to 1.2 Cal. ka BP (or 14C ages from 2.1 to 1.7 ka BP), which was punctuated by a short-term drop in the sea level at ~1.5 Cal. ka BP. At 1.2 Cal. ka BP, the sea level was at least 128 cm higher than the present level. After that time, the sea level fell significantly resulting in the retreat of the coastline by ~210 m and the formation of the modern beach-sand bank geomorphic system. Additional evidence from the coral reef records at this site suggested that similar to the climate change, the sea level also oscillated in the millennial-, centennial-, and interdecadal-scale cycles over the mid-late Holocene. Key Words: sea-level; late Holocene; oscillation; beach sediments; coral reef; South China Sea
1
Introduction
Accurate reconstructions of the Holocene sea level change are critical for understanding the nature of modern sea levels, and for anticipating the magnitude and potential rates of future sea level changes. Globally, an estimated 37% of the population lives within ~100 kilometers of the coast lines[1], while in China alone, ~41% of the population and ~60% of its ķ economy resources occur in coastal regions . The recent rise in sea level, possibly an indirect result of anthropogenically-induced global warming, have submerged large tracts of land and significantly eroded vast stretches of coastline, along with an increased frequency of storms, which have the potential to severely impact the economy. The Intergovernmental Panel on Climate Change (IPCC) predicted that the global sea level will rise by 0.18 m to 0.59 m, on an average, within the next 100 years[2] . At the mouth of the ķ
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Zhujiang River, at the northern South China Sea, the sea level is predicted to rise up to 30 cm by 2030 and 50 cm by 2050[3]. On account of such predictions, studies on Holocene sea level processes have long been the focus of global change research. There is growing evidence to suggest that some periods of the Holocene have experienced higher sea levels than the present ones. Additionally, regional differences in the magnitude of prehistoric sea level changes have also been identified. However, there are still several issues of Holocene sea level research that require further investigation, such as: (1) the timing, rates of change, and duration of previous high sea level stands; (2) the timing of the origin of present day sea levels; (3) the nature of previous sea level changes (e.g., oscillation versus progressive, stepwise rising or falling). Knowledge of such processes is critical for predicting the nature of a hypothesized future sea level change and for decision-making by the governments on the strategies for coastal development, utilization, and protection. Reconstructions of the Holocene sea level history should be a
Received date: 19-Aug-2009; Accepted date: 04-Nov-2009.
Corresponding author. E-mail: [email protected] Foundation item: Supported by the Chinese Academy of Sciences Innovation Program (No. kzcx2-yw-318), the National Natural Science Foundation of China (Nos. 40830852 and 40572102), the Chinese Ministry of Science and Technology Project (No. 2007CB815905), and an Australian Research Council Discovery Project (No. DP0773081). Copyright © 2009, China University of Geosciences (Beijing) and Peking University, Published by Elsevier B.V. All rights reserved. DOI: 10.1016/S1872-5791(08)60110-4
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continual focus of climate change research in the future. Since the Academicians of the Earth Science Department, of the Chinese Academy of Sciences, investigated the deltaic areas of the Zhujiang, Changjiang, and Huanghe Rivers, and the Tianjing city in 1993, sea level change research has invited widespread concerns in China. Nearly 100 projects have been granted by various departments and more than 200 research articles and at least 10 monographs (collections) have been published. The main outcome of such studies broadly cover six aspects: (1) the study of sea level indicators in the geological past; (2) trends in sea level change during the instrumental survey period; (3) short- and long-term predictions on sea level change; (4) influence of sea level rise on the coastal environment and social economy; (5) analyses of sea level characteristics from the Topex / Poseidon (T / P) satellite data; and (6) reconstruction of the Holocene sea level history. Although there have been significant advances in our understanding of the processes of sea level change, key issues of research remain unsolved, including a broader understanding of long-term and high-precision observations on crustal deformation and long-term and consistent tidal gauge records. High-precision dating and high-resolution interpretation of past sea level indicators have the potential to solve such problems. The Leizhou Peninsula, in the northern South China Sea, is an ideal region for Holocene sea level research. The local fringing reef possesses numerous sea level indicators (such as in situ corals, microatolls, reef profiles, and beach sediments) and the region had been tectonically stable over the Holocene time. The elevations and ages of in situ massive corals at this site reveal multiple sea level highstands during the Holocene[4-6]. The Goniopora coral profile[7,8] also from this site, suggests that the sea level was cyclically rising between 7500 and 7000 years ago. At 7.3 Cal. ka BP, the sea level was at the same height as the present level. By 7.0 Cal. ka BP, the sea level was 1.8 m higher than the present level. Coral microatolls from this site indicate that the sea level during the period 7050–6600 Cal. a BP was about 171 to 219 cm higher than the present level, with at least four centennial-scale fluctuations in amplitude, to about 20–40 cm[9]. This article will provide new evidence from the beach sediment at the same area to show the higher-than-present sea level and its oscillation over the later part of the Holocene.
2 Location, elevation, age, and depositional characteristics of the paleobeach sedimentary profile The outcrop of the paleobeach sedimentary profile (Fig. 1a, 20°15.113´N, 109°55.338´E) occurs ~210 m landward from the modern beach (Fig. 1b), located outside Fangpo Village, Jiaowei suburb, Xuwen County. The upper 75 cm of the outcrop emerges above the modern land surface (Fig. 1c), with
the sediment layers dipping seawards at an angle of 6°–7°. We had excavated to a depth of 214 cm, but did not reach the bed rock. Thus, the total depth of the paleobeach profile is ~ 289 cm. The upper 50 cm is cemented, with the remaining lower portion unconsolidated. According to their grain size and the composition of the sediments the profile can be subdivided into nine distinct horizontal layers. The features of each layer are shown in the figure (Fig. 2). Along with this beach profile there is an ancient sand bank (Fig. 1a). The ancient sand bank together with the beach sediment sequence constitutes the ancient beach–sand bank geomorphic framework resembling the modern geomorphological structure (Fig. 1b). The modern beach contains numerous widespread washed-up coral branches and fragments. However, there is no evidence of the significant coral debris layers within the deep-excavated modern beach profile, indicating an obvious sedimentary environment difference between the paleobeach profile and the modern one. Elevation surveying indicates that the surface of the paleobeach sediments is 379 cm above the 1956 AD national sea level benchmark (3K2128; 16 km east of the study site, at an elevation of 7.9 m) for the Yellow Sea, and 133 cm above the modern tidal datum plane (or the low spring tide level) for the study site[6]. The mean sea level of the study site is 184 cm above the tidal datum plane and 51 cm above the 1956 Yellow Sea benchmark. Considering that a 2 m tidal difference exists at the study site, the surface of the Holocene beach sediment is calculated to be 328 cm above the mean sea level, or 128 cm above the high tide level (Fig. 3). The paleobeach sediments consist of quartz sand and various biogenic debris, which are dominated by corals, molluscan shells, and foraminifera. Layers 2a, 2b, 4a, and 4b (Fig. 1d, 1e) have a very high biological composition, and are generally very similar to the modern beach sediments of the coral reef islets, if the terrestrial quartz composition in these layers is excluded (Fig. 1e, 1f). The coral debris is dominated by Acropora branches, and to a lesser degree, debris of massive corals. The foraminifera in the sediment are poorly preserved, but are dominated by Ammonia annectens, Cellanthus craticulatus, and Elphidium hispidulum, with Quinquaeloculina sp., Guttulina pacifica and Amphistegina madagascariensis also present. Such forms are most closely associated with inshore to shallow sea environments. Thin sections were made of the cemented biological debris quartz sand sediments from layers 2a, 3, 4a, 5, and 6, respectively. Microscopy revealed that their cementation materials were calcsparites (Fig. 1g), with grain sizes generally between 0.02 to 0.05 mm. The materials filling the pore spaces of foraminifera and corals were also composed of calcsparites (Fig. 1h). Some needle-shaped aragonite biological debris was undergoing a transitional state toward crystallization of the calcite (Fig. 1i). The type and texture of the cementation materials indicated that lithification of the
YU Kefu et al. / Earth Science Frontiers, 2009, 16(6): 138–145
Fig. 1
Paleobeach sedimentary profile at Fangpo Village, Leizhou Peninsula, and the micro-characteristics of the deposited biological debris
a. Paleobeach (or late Holocene beach) and sandy-bank; b. Modern beach and sandy-bank; c. Late Holocene beach stratigraphic profile; d. Coral reef debris from the 4b layer; e. The small coral gravel and coral sand from the 2b layer; f. Quartz sand of the third layer overlain by coral sand of the 2b layer; g. Calcsparite cement between the sediment grains; h. Foraminifera deposited in the late Holocene beach sediment with calcsparite cement and filling in its pores; i. Biological debris with incomplete transformation from aragonite to calcite
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Fig. 2
Ages and the sedimentary characteristics of the paleobeach sediment profile
Fig. 3
Relative locations of the paleobeach-bank and modern beach-bank systems
beach sediment in the profile was due to precipitation from meteoric waters, and therefore, it could not be defined as “beachrock”, which generally shows aragonite cement. Coral branches and molluscan shells from layers 2b, 3, 4a and 6 were selected for conventional radiocarbon dating. The results (Fig. 2) indicated that the beach sediment profile had formed between (2135 ± 65)–(1665 ± 70) a BP (1 ı), or 1710–1235 Cal.a BP (calibrated with Stuvier et al’s correction procedure[10]).
3 Late Holocene sea level highstand indicated by the paleobeach sediment profile Leizhou Peninsula and the surrounding fringing reefs have
been tectonically stable since mid-Holocene. Additionally, the modern coral reef geomorphologic framework was basically formed by ~6700 Cal. a BP[5]. Thus, theoretically, the in situ corals from this area might accurately record the Holocene sea level history of the region[4-6]. Previous studies on early-middle Holocene sea level history, using corals from this site, demonstrated patterns of sea level changes that occurred contemporaneously with peripheral tectonically stable regions such as the Malaysia-Thailand Peninsula[11] and the Penghu Islands[12], at the northeastern South China Sea. Thus, consistency of results between these regions demonstrated the credibility of using corals to reconstruct the sea level history at the Leizhou Peninsula. Here, we extend our study about the history of the past sea level fluctuations as recorded in the late
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Holocene-beach sediments of the fringing reef at the Leizhou Peninsula. 3.1
The elevation of the late Holocene sea level
Beach sediment is generally considered to be less useful in reconstructing past sea levels than coral reefs and microatolls, as those may provide a more accurate evidence of the sea level position and are easier to be dated using readily available U-series dating techniques. However, beach sediments have the advantage of being able to clearly indicate the position of paleo-coastlines, and commonly possess numerous calcareous biogenic debris for 14C dating, with a longer chronological span and multiple regularly preserved depositional cycles. Therefore, beach sediments are still good recorders for the marking of the past sea level, which is significant for indicating the changes in sea level and coast lines. The Holocene stratigraphic profile of the Leizhou Peninsula is characterized by obvious horizontal sedimentary layers, poorly-sorted sediments, and shallow-sea (or inshore) originated calcareous organisms, which are all typical of modern beach environments. Although we are not able to precisely identify at which relative part of the ancient beach this sediment profile was originally deposited, even if it was at the high-tide line of the ancient beach, it is still 128 cm above that mark. Therefore, the sea level 1235 years ago (i.e., layer 2b) was at least 128 cm higher than the present level. Considering the thickness of the Holocene beach sediment profile, it is evident that the sea level was at least 104 cm and 80 cm higher than the present sea level, at 1500 and 1700 years ago, respectively. The U-series dating of the in situ massive corals from the fringing reef at this site also indicated that the sea levels at (1915 ± 15) Cal. a BP (2 ı) and (1511 ± 23) Cal. a BP (2 ı) were higher than the present level, which confirmed the credibility of the sea level records from the beach sediments[4]. Unfortunately, the accurate elevations relative to the present sea level were not well surveyed, especially as there was no microatoll (it can accurately indicate the altitude of the sea level) being found, therefore, it was not able to provide further sea level information. Previous studies based on the ages and elevations of massive corals in the study area suggested that the sea level was 2.9–3.8 m higher than the present during the mid-late Holocene (2500–1500 Cal. a BP)[4,6]. However, contrastingly, more recent evidence from the microatolls suggested that the sea level was only about 171–219 cm above the present level, 6600 years ago[9], suggesting that the previous estimation was not reliable. Therefore, the paleobeach sediment profile provides useful sea level elevation information on the Leizhou Peninsula during the late Holocene. Generally, there are few reports on the late Holocene sea level altitude in mainland China, because many potential geological, geomorphological indicators (such as beachrocks
and beach sediments) are not well-cemented (or consolidated) and have been destroyed as a result of anthropogenic activities. In addition, even if these potentially useful, late Holocene sea level indicators do exist, they are usually difficult to date using the currently available radiometric and optical dating methods; the younger the sample is, the greater is the difficulty in dating. A previous report[13] on the Luhuitou fringing reef at Sanya, Hainan Island, has indicated a ~1 m sea level highstand over the period 2.1–1.6 ka BP, which is roughly consistent with our estimation, based on the beach profile at Leizhou Peninsula. Further evidences from the surrounding areas of the South China Sea indicate higher late Holocene sea levels, broadly matching our results in timing and elevations. For example, beachrock and other sea level indicators from the tectonically stable Malaysia–Thailand Peninsula suggest that the sea level was 0.7–1.2 m higher than the present one, around 2000 years ago, but then dropped to +0.2 to +0.4 m about 1500 years ago, and then progressively fell down to the present level[11]. Radiocarbon 14C ages of shells and corals from the tectonically stable Penghu Islands indicate that the sea level was 1.3 to 1.5 m above the present one, around (1890 ± 50)–(2020 ± 40) a BP[12]. Similarly, 14C ages of oysters from the Tioman Island (Malaysia) indicate that the sea level was 1.1 m above the present one around (1900 ± 90) a BP[14]. A calibrated 14C age of a massive coral from Phuket, southern Thailand, indicates that the sea level around (2210 ± 57) a BP was at least 0.34 m above the present level (determined on the relative position of the dated massive corals to the modern coral)[15]. 14C ages of beachrock from the Ryukyu Islands, southwestern Japan, indicate that the sea level between (1640 ± 65)–(1970 ± 95) a BP was 0.1–1.7 m higher than the present[16]. 14C ages of beachrock from Hong Kong suggests that it formed at (1660 ± 75) a BP at a position of 2.03 m above the present mean sea level[17]. Thus, differences do exist in both sea level elevations and timing of sea level highstands among those different records. This is probably because different sea level indicators have varying effects that indicate sea level elevations and the precision of dating may also be different. Improvements in determining elevational accuracy and the dating precision of sea level indicators will be a major focal point of future research. Several other studies across the globe also show higher-than-present sea levels during the late Holocene. The Coral reefs from Polynesia[18] and the Great Barrier Reef[19,20] indicate a high sea level stage from 2.1 to 1.5 ka BP (14C ages). 14 C ages from the Australian coast suggest that the sea level was 2 m above the present level, 1500 years ago[21]. The Polynesian coral reefs indicate that the sea level was 0.9 m higher than the present 1200 years ago[22]. Many 14C ages of coral reefs in the Pacific also demonstrate that the sea level was 1–1.5 m above the present between 1500–2000 a BP[23]. Coral reefs from the Christmas Island (which is close to the equator) show that the sea level was (0.6 ± 0.2) m above the
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present (1680 ± 95) Cal. a BP[24]. Beachrock from South Africa suggests that the sea level was 1.5 m above the present 1610 years ago[25]. The 14C ages of vermetid shells[26] and pollen[27] from Brazil indicate a sea level highstand around 2080 a BP. The 14C ages of in situ biological data from the Florida coastline suggest that the sea levels were 0.5 m higher than the present between 2000–1000 a BP[28]. 3.2
Late Holocene sea level oscillation
The sedimentary characteristics of the Leizhou Peninsula Holocene paleobeach deposit can be further divided into two sedimentary cycles, with the first cycle (from layer-7 to layer-4a) covering 1.7–1.5 Cal. ka BP and the second cycle (from layer-3 to layer-2a) covering 1.4–1.2 Cal. ka BP. Both cycles coarsen upwards and have coral debris and coral branches clustered in the top layer. These data indicate progressive sea level rises, with increased sedimentary dynamism. Along with the sea level rises, the corals grew in abundance on the neighboring reef flat and provided a proximal source of coral branches and coral debris deposited within the beach sediments. After completion of both the cycles, the biological component of the sediments decreased dramatically. For example, there is nearly no biological component at all in layer-3 and layer-1. The absence of biological components within these layers suggests that there was no biological source near the beach, which is very unusual for a coral reef beach. The only possible interpretation for this absence is that the neighboring organism-enriched reef flat was totally covered by a quartz sand sheet that had originated from the land, possibly as a result of storm / typhoon influences (e.g., see Fig. 3b in reference[29], which shows part of a modern reef flat covered by a sand sheet after Typhoon Krovanh on 25-08-2003) during the time when the sea level fell and the reef flat was exposed. Coral Sr/Ca and į18O records[30] from the study site showed that there was a cool period around 1500 Cal. a BP when the decadal-scale sea surface temperature (SST) was 22.6°C (2.2°C lower than that in the 1990s) and decadal-scale winter SST was 16.5°C (3.8°C lower than that in the 1990s). However, even if the low SST disturbed the growth of corals, it could not have prevented the transport of the biological debris (such as molluscan shells, foraminifera, and coral debris) from the reef flat to the beach. Therefore, the beach sediment profile clearly suggested that the sea level rose between 1710 and 1235 Cal. a BP, but with a short-term lowering around 1490 Cal. a BP (or ~1500 Cal. a BP). However, the amplitude of the sea level lowering remains unclear. There is still an intense debate with regard to the nature of the changes in the Holocene sea level, which is, oscillating versus smooth rises and falls. The most widely known oscillation curve for the mid- to late-Holocene is the Fairbridge Curve (published in 1961)[31]. The classic Holocene smooth (non-oscillating) sea level curve model is the Shepard
Curve (published in 1964)[32]. Most publications in China have supported the Fairbridge sea level oscillation curve (such as Zhao’s[33] and Fang’s[34] publications). In contrast, evidence from the Great Barrier Reef support the Shepard’s non-oscillating curve for the mid- to late-Holocene sea level changes[35]. On the basis of a detailed study of the bio-geomorphological zonation of a fringing reef at the Leizhou Peninsula, northern South China Sea, Yu et al[5] proposed that multiple millennial-scale sea level highstands of ~2 m above the present sea level occurred over the mid- to late-Holocene. New evidence[9] from microatolls at the same reef show that the sea level was also oscillating on centennial-scales. For example, there were four centennial-scale fluctuations occurring between 7110–6990, 6990–6890, 6890–6760 and 6760-? Cal. a BP, when the sea level was 171 to 219 cm higher than the present level. A detailed study[8] of a well-developed Goniopora reef profile from the same region demonstrates that the overall sea level rise during 7500–7000 Cal. a BP was punctuated by multiple interdecadal-scale sea level lowering events (which resulted in the emergence and erosion of the Goniopora coral heads, in several stratigraphic layers). Our beach sediment profile from this site provides new evidence on centennial-scale sea level oscillations. 3.3
Later Holocene sea level falling
The late Holocene beach sediment profile is 210 m landward from the modern beach (Fig. 3), which obviously indicates that the coastline has retreated by 210 m since its formation ~1235 Cal. a BP. The timing of post 1235 ka sea level and the coastline retreat, and origins of the present-day coastline, are still unknown. Nie at al[6] have reported a 14C age of 1170 Cal. a BP or 700 Cal. a BP for a massive coral that is regularly exposed at low tide, thus, suggesting that the sea level was still higher than the present level, around 700 years ago. All the above-mentioned data show that the coral reefs at Leizhou Peninsula have a great potential to reveal the critical details of the Holocene sea level changes.
4
Conclusions
Study on a late Holocene beach sediment profile at the Leizhou Peninsula suggests that from 1.7–1.2 Cal. ka BP (or 14 C ages from 2.1 to 1.7 ka BP) the sea-level rose significantly, but was punctuated by a short-term sea level lowering around ~1.5 Cal. ka BP, corresponding to climate cooling. The sea level at 1.2 Cal. ka BP was at least 128 cm higher than the present level. Sea levels fell some time after 1.2 Cal. ka BP resulting in the retreat of the coastline by about 210 m and the formation of the present beach—sandy bank system. In combination with the other periods of the Holocene sea level change evidences from the coral reefs at this site, the data suggest that the sea levels were similar to the climate,
YU Kefu et al. / Earth Science Frontiers, 2009, 16(6): 138–145
oscillated at millennial-, centennial- and interdecadal-scale cycles during the mid-late Holocene. The Leizhou Peninsula has a great potential to further reveal important details of the Holocene sea level change.
years, Penghu Islands, Taiwan Strait. Quaternary Research, 1996, 45(3): 254–262. [13] Chen J R, Chen X S, Zhao X T, et al. Study of sea level in Holocene change at Luhuitou area of Hainan Province. Marine Geology Studies, 1991, 3(71-86): 127–134.
Acknowledgments
[14] Tjia H D, Fujii S, Kigoshi K. Holocene shorelines of Tioman Island in the South China Sea. Geologie en Mijnbouw, 1983,
The authors would like to thank Profs. Liu Tungsheng and Zhong Jinliang, who both passed away over last years for participating in the fieldwork of this project; Profs. Zhou Guoqing, Qin Guoquan, Sha Qingan, and Wang Sumin for helping in the samples’ microscopic observation, foraminifera identification, and in discussions; and Dr. Gilbert J Price for language polishing.
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RESEARCH PAPER
Beach Sediments from Northern South China Sea Suggest High and Oscillating Sea Levels During the Late Holocene YU Kefu , CHEN Tegu South China Sea Institute of Oceanology, Chinese Academy of Sciences, Guangzhou 510301, China
Abstract: Understanding the history of the late Holocene sea level change is critical for predicting the potential effects of future sea level changes. The coral reef region of the Leizhou Peninsula, in the northern coast of the South China Sea, is an ideal site for studying the Holocene sea level history because of its tectonic stability and the possession of numerous prehistoric sea level indicators. The paleobeach sedimentary records from the Leizhou Peninsula indicated a higher-than-present sea level stand, from 1.7 to 1.2 Cal. ka BP (or 14C ages from 2.1 to 1.7 ka BP), which was punctuated by a short-term drop in the sea level at ~1.5 Cal. ka BP. At 1.2 Cal. ka BP, the sea level was at least 128 cm higher than the present level. After that time, the sea level fell significantly resulting in the retreat of the coastline by ~210 m and the formation of the modern beach-sand bank geomorphic system. Additional evidence from the coral reef records at this site suggested that similar to the climate change, the sea level also oscillated in the millennial-, centennial-, and interdecadal-scale cycles over the mid-late Holocene. Key Words: sea-level; late Holocene; oscillation; beach sediments; coral reef; South China Sea
1
Introduction
Accurate reconstructions of the Holocene sea level change are critical for understanding the nature of modern sea levels, and for anticipating the magnitude and potential rates of future sea level changes. Globally, an estimated 37% of the population lives within ~100 kilometers of the coast lines[1], while in China alone, ~41% of the population and ~60% of its ķ economy resources occur in coastal regions . The recent rise in sea level, possibly an indirect result of anthropogenically-induced global warming, have submerged large tracts of land and significantly eroded vast stretches of coastline, along with an increased frequency of storms, which have the potential to severely impact the economy. The Intergovernmental Panel on Climate Change (IPCC) predicted that the global sea level will rise by 0.18 m to 0.59 m, on an average, within the next 100 years[2] . At the mouth of the ķ
http://www.cas.ac.cn/htm/Dir/2007/02/07/14/73/46.htm
Zhujiang River, at the northern South China Sea, the sea level is predicted to rise up to 30 cm by 2030 and 50 cm by 2050[3]. On account of such predictions, studies on Holocene sea level processes have long been the focus of global change research. There is growing evidence to suggest that some periods of the Holocene have experienced higher sea levels than the present ones. Additionally, regional differences in the magnitude of prehistoric sea level changes have also been identified. However, there are still several issues of Holocene sea level research that require further investigation, such as: (1) the timing, rates of change, and duration of previous high sea level stands; (2) the timing of the origin of present day sea levels; (3) the nature of previous sea level changes (e.g., oscillation versus progressive, stepwise rising or falling). Knowledge of such processes is critical for predicting the nature of a hypothesized future sea level change and for decision-making by the governments on the strategies for coastal development, utilization, and protection. Reconstructions of the Holocene sea level history should be a
Received date: 19-Aug-2009; Accepted date: 04-Nov-2009.
Corresponding author. E-mail: [email protected] Foundation item: Supported by the Chinese Academy of Sciences Innovation Program (No. kzcx2-yw-318), the National Natural Science Foundation of China (Nos. 40830852 and 40572102), the Chinese Ministry of Science and Technology Project (No. 2007CB815905), and an Australian Research Council Discovery Project (No. DP0773081). Copyright © 2009, China University of Geosciences (Beijing) and Peking University, Published by Elsevier B.V. All rights reserved. DOI: 10.1016/S1872-5791(08)60110-4
YU Kefu et al. / Earth Science Frontiers, 2009, 16(6): 138–145
continual focus of climate change research in the future. Since the Academicians of the Earth Science Department, of the Chinese Academy of Sciences, investigated the deltaic areas of the Zhujiang, Changjiang, and Huanghe Rivers, and the Tianjing city in 1993, sea level change research has invited widespread concerns in China. Nearly 100 projects have been granted by various departments and more than 200 research articles and at least 10 monographs (collections) have been published. The main outcome of such studies broadly cover six aspects: (1) the study of sea level indicators in the geological past; (2) trends in sea level change during the instrumental survey period; (3) short- and long-term predictions on sea level change; (4) influence of sea level rise on the coastal environment and social economy; (5) analyses of sea level characteristics from the Topex / Poseidon (T / P) satellite data; and (6) reconstruction of the Holocene sea level history. Although there have been significant advances in our understanding of the processes of sea level change, key issues of research remain unsolved, including a broader understanding of long-term and high-precision observations on crustal deformation and long-term and consistent tidal gauge records. High-precision dating and high-resolution interpretation of past sea level indicators have the potential to solve such problems. The Leizhou Peninsula, in the northern South China Sea, is an ideal region for Holocene sea level research. The local fringing reef possesses numerous sea level indicators (such as in situ corals, microatolls, reef profiles, and beach sediments) and the region had been tectonically stable over the Holocene time. The elevations and ages of in situ massive corals at this site reveal multiple sea level highstands during the Holocene[4-6]. The Goniopora coral profile[7,8] also from this site, suggests that the sea level was cyclically rising between 7500 and 7000 years ago. At 7.3 Cal. ka BP, the sea level was at the same height as the present level. By 7.0 Cal. ka BP, the sea level was 1.8 m higher than the present level. Coral microatolls from this site indicate that the sea level during the period 7050–6600 Cal. a BP was about 171 to 219 cm higher than the present level, with at least four centennial-scale fluctuations in amplitude, to about 20–40 cm[9]. This article will provide new evidence from the beach sediment at the same area to show the higher-than-present sea level and its oscillation over the later part of the Holocene.
2 Location, elevation, age, and depositional characteristics of the paleobeach sedimentary profile The outcrop of the paleobeach sedimentary profile (Fig. 1a, 20°15.113´N, 109°55.338´E) occurs ~210 m landward from the modern beach (Fig. 1b), located outside Fangpo Village, Jiaowei suburb, Xuwen County. The upper 75 cm of the outcrop emerges above the modern land surface (Fig. 1c), with
the sediment layers dipping seawards at an angle of 6°–7°. We had excavated to a depth of 214 cm, but did not reach the bed rock. Thus, the total depth of the paleobeach profile is ~ 289 cm. The upper 50 cm is cemented, with the remaining lower portion unconsolidated. According to their grain size and the composition of the sediments the profile can be subdivided into nine distinct horizontal layers. The features of each layer are shown in the figure (Fig. 2). Along with this beach profile there is an ancient sand bank (Fig. 1a). The ancient sand bank together with the beach sediment sequence constitutes the ancient beach–sand bank geomorphic framework resembling the modern geomorphological structure (Fig. 1b). The modern beach contains numerous widespread washed-up coral branches and fragments. However, there is no evidence of the significant coral debris layers within the deep-excavated modern beach profile, indicating an obvious sedimentary environment difference between the paleobeach profile and the modern one. Elevation surveying indicates that the surface of the paleobeach sediments is 379 cm above the 1956 AD national sea level benchmark (3K2128; 16 km east of the study site, at an elevation of 7.9 m) for the Yellow Sea, and 133 cm above the modern tidal datum plane (or the low spring tide level) for the study site[6]. The mean sea level of the study site is 184 cm above the tidal datum plane and 51 cm above the 1956 Yellow Sea benchmark. Considering that a 2 m tidal difference exists at the study site, the surface of the Holocene beach sediment is calculated to be 328 cm above the mean sea level, or 128 cm above the high tide level (Fig. 3). The paleobeach sediments consist of quartz sand and various biogenic debris, which are dominated by corals, molluscan shells, and foraminifera. Layers 2a, 2b, 4a, and 4b (Fig. 1d, 1e) have a very high biological composition, and are generally very similar to the modern beach sediments of the coral reef islets, if the terrestrial quartz composition in these layers is excluded (Fig. 1e, 1f). The coral debris is dominated by Acropora branches, and to a lesser degree, debris of massive corals. The foraminifera in the sediment are poorly preserved, but are dominated by Ammonia annectens, Cellanthus craticulatus, and Elphidium hispidulum, with Quinquaeloculina sp., Guttulina pacifica and Amphistegina madagascariensis also present. Such forms are most closely associated with inshore to shallow sea environments. Thin sections were made of the cemented biological debris quartz sand sediments from layers 2a, 3, 4a, 5, and 6, respectively. Microscopy revealed that their cementation materials were calcsparites (Fig. 1g), with grain sizes generally between 0.02 to 0.05 mm. The materials filling the pore spaces of foraminifera and corals were also composed of calcsparites (Fig. 1h). Some needle-shaped aragonite biological debris was undergoing a transitional state toward crystallization of the calcite (Fig. 1i). The type and texture of the cementation materials indicated that lithification of the
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Fig. 1
Paleobeach sedimentary profile at Fangpo Village, Leizhou Peninsula, and the micro-characteristics of the deposited biological debris
a. Paleobeach (or late Holocene beach) and sandy-bank; b. Modern beach and sandy-bank; c. Late Holocene beach stratigraphic profile; d. Coral reef debris from the 4b layer; e. The small coral gravel and coral sand from the 2b layer; f. Quartz sand of the third layer overlain by coral sand of the 2b layer; g. Calcsparite cement between the sediment grains; h. Foraminifera deposited in the late Holocene beach sediment with calcsparite cement and filling in its pores; i. Biological debris with incomplete transformation from aragonite to calcite
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Fig. 2
Ages and the sedimentary characteristics of the paleobeach sediment profile
Fig. 3
Relative locations of the paleobeach-bank and modern beach-bank systems
beach sediment in the profile was due to precipitation from meteoric waters, and therefore, it could not be defined as “beachrock”, which generally shows aragonite cement. Coral branches and molluscan shells from layers 2b, 3, 4a and 6 were selected for conventional radiocarbon dating. The results (Fig. 2) indicated that the beach sediment profile had formed between (2135 ± 65)–(1665 ± 70) a BP (1 ı), or 1710–1235 Cal.a BP (calibrated with Stuvier et al’s correction procedure[10]).
3 Late Holocene sea level highstand indicated by the paleobeach sediment profile Leizhou Peninsula and the surrounding fringing reefs have
been tectonically stable since mid-Holocene. Additionally, the modern coral reef geomorphologic framework was basically formed by ~6700 Cal. a BP[5]. Thus, theoretically, the in situ corals from this area might accurately record the Holocene sea level history of the region[4-6]. Previous studies on early-middle Holocene sea level history, using corals from this site, demonstrated patterns of sea level changes that occurred contemporaneously with peripheral tectonically stable regions such as the Malaysia-Thailand Peninsula[11] and the Penghu Islands[12], at the northeastern South China Sea. Thus, consistency of results between these regions demonstrated the credibility of using corals to reconstruct the sea level history at the Leizhou Peninsula. Here, we extend our study about the history of the past sea level fluctuations as recorded in the late
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Holocene-beach sediments of the fringing reef at the Leizhou Peninsula. 3.1
The elevation of the late Holocene sea level
Beach sediment is generally considered to be less useful in reconstructing past sea levels than coral reefs and microatolls, as those may provide a more accurate evidence of the sea level position and are easier to be dated using readily available U-series dating techniques. However, beach sediments have the advantage of being able to clearly indicate the position of paleo-coastlines, and commonly possess numerous calcareous biogenic debris for 14C dating, with a longer chronological span and multiple regularly preserved depositional cycles. Therefore, beach sediments are still good recorders for the marking of the past sea level, which is significant for indicating the changes in sea level and coast lines. The Holocene stratigraphic profile of the Leizhou Peninsula is characterized by obvious horizontal sedimentary layers, poorly-sorted sediments, and shallow-sea (or inshore) originated calcareous organisms, which are all typical of modern beach environments. Although we are not able to precisely identify at which relative part of the ancient beach this sediment profile was originally deposited, even if it was at the high-tide line of the ancient beach, it is still 128 cm above that mark. Therefore, the sea level 1235 years ago (i.e., layer 2b) was at least 128 cm higher than the present level. Considering the thickness of the Holocene beach sediment profile, it is evident that the sea level was at least 104 cm and 80 cm higher than the present sea level, at 1500 and 1700 years ago, respectively. The U-series dating of the in situ massive corals from the fringing reef at this site also indicated that the sea levels at (1915 ± 15) Cal. a BP (2 ı) and (1511 ± 23) Cal. a BP (2 ı) were higher than the present level, which confirmed the credibility of the sea level records from the beach sediments[4]. Unfortunately, the accurate elevations relative to the present sea level were not well surveyed, especially as there was no microatoll (it can accurately indicate the altitude of the sea level) being found, therefore, it was not able to provide further sea level information. Previous studies based on the ages and elevations of massive corals in the study area suggested that the sea level was 2.9–3.8 m higher than the present during the mid-late Holocene (2500–1500 Cal. a BP)[4,6]. However, contrastingly, more recent evidence from the microatolls suggested that the sea level was only about 171–219 cm above the present level, 6600 years ago[9], suggesting that the previous estimation was not reliable. Therefore, the paleobeach sediment profile provides useful sea level elevation information on the Leizhou Peninsula during the late Holocene. Generally, there are few reports on the late Holocene sea level altitude in mainland China, because many potential geological, geomorphological indicators (such as beachrocks
and beach sediments) are not well-cemented (or consolidated) and have been destroyed as a result of anthropogenic activities. In addition, even if these potentially useful, late Holocene sea level indicators do exist, they are usually difficult to date using the currently available radiometric and optical dating methods; the younger the sample is, the greater is the difficulty in dating. A previous report[13] on the Luhuitou fringing reef at Sanya, Hainan Island, has indicated a ~1 m sea level highstand over the period 2.1–1.6 ka BP, which is roughly consistent with our estimation, based on the beach profile at Leizhou Peninsula. Further evidences from the surrounding areas of the South China Sea indicate higher late Holocene sea levels, broadly matching our results in timing and elevations. For example, beachrock and other sea level indicators from the tectonically stable Malaysia–Thailand Peninsula suggest that the sea level was 0.7–1.2 m higher than the present one, around 2000 years ago, but then dropped to +0.2 to +0.4 m about 1500 years ago, and then progressively fell down to the present level[11]. Radiocarbon 14C ages of shells and corals from the tectonically stable Penghu Islands indicate that the sea level was 1.3 to 1.5 m above the present one, around (1890 ± 50)–(2020 ± 40) a BP[12]. Similarly, 14C ages of oysters from the Tioman Island (Malaysia) indicate that the sea level was 1.1 m above the present one around (1900 ± 90) a BP[14]. A calibrated 14C age of a massive coral from Phuket, southern Thailand, indicates that the sea level around (2210 ± 57) a BP was at least 0.34 m above the present level (determined on the relative position of the dated massive corals to the modern coral)[15]. 14C ages of beachrock from the Ryukyu Islands, southwestern Japan, indicate that the sea level between (1640 ± 65)–(1970 ± 95) a BP was 0.1–1.7 m higher than the present[16]. 14C ages of beachrock from Hong Kong suggests that it formed at (1660 ± 75) a BP at a position of 2.03 m above the present mean sea level[17]. Thus, differences do exist in both sea level elevations and timing of sea level highstands among those different records. This is probably because different sea level indicators have varying effects that indicate sea level elevations and the precision of dating may also be different. Improvements in determining elevational accuracy and the dating precision of sea level indicators will be a major focal point of future research. Several other studies across the globe also show higher-than-present sea levels during the late Holocene. The Coral reefs from Polynesia[18] and the Great Barrier Reef[19,20] indicate a high sea level stage from 2.1 to 1.5 ka BP (14C ages). 14 C ages from the Australian coast suggest that the sea level was 2 m above the present level, 1500 years ago[21]. The Polynesian coral reefs indicate that the sea level was 0.9 m higher than the present 1200 years ago[22]. Many 14C ages of coral reefs in the Pacific also demonstrate that the sea level was 1–1.5 m above the present between 1500–2000 a BP[23]. Coral reefs from the Christmas Island (which is close to the equator) show that the sea level was (0.6 ± 0.2) m above the
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present (1680 ± 95) Cal. a BP[24]. Beachrock from South Africa suggests that the sea level was 1.5 m above the present 1610 years ago[25]. The 14C ages of vermetid shells[26] and pollen[27] from Brazil indicate a sea level highstand around 2080 a BP. The 14C ages of in situ biological data from the Florida coastline suggest that the sea levels were 0.5 m higher than the present between 2000–1000 a BP[28]. 3.2
Late Holocene sea level oscillation
The sedimentary characteristics of the Leizhou Peninsula Holocene paleobeach deposit can be further divided into two sedimentary cycles, with the first cycle (from layer-7 to layer-4a) covering 1.7–1.5 Cal. ka BP and the second cycle (from layer-3 to layer-2a) covering 1.4–1.2 Cal. ka BP. Both cycles coarsen upwards and have coral debris and coral branches clustered in the top layer. These data indicate progressive sea level rises, with increased sedimentary dynamism. Along with the sea level rises, the corals grew in abundance on the neighboring reef flat and provided a proximal source of coral branches and coral debris deposited within the beach sediments. After completion of both the cycles, the biological component of the sediments decreased dramatically. For example, there is nearly no biological component at all in layer-3 and layer-1. The absence of biological components within these layers suggests that there was no biological source near the beach, which is very unusual for a coral reef beach. The only possible interpretation for this absence is that the neighboring organism-enriched reef flat was totally covered by a quartz sand sheet that had originated from the land, possibly as a result of storm / typhoon influences (e.g., see Fig. 3b in reference[29], which shows part of a modern reef flat covered by a sand sheet after Typhoon Krovanh on 25-08-2003) during the time when the sea level fell and the reef flat was exposed. Coral Sr/Ca and į18O records[30] from the study site showed that there was a cool period around 1500 Cal. a BP when the decadal-scale sea surface temperature (SST) was 22.6°C (2.2°C lower than that in the 1990s) and decadal-scale winter SST was 16.5°C (3.8°C lower than that in the 1990s). However, even if the low SST disturbed the growth of corals, it could not have prevented the transport of the biological debris (such as molluscan shells, foraminifera, and coral debris) from the reef flat to the beach. Therefore, the beach sediment profile clearly suggested that the sea level rose between 1710 and 1235 Cal. a BP, but with a short-term lowering around 1490 Cal. a BP (or ~1500 Cal. a BP). However, the amplitude of the sea level lowering remains unclear. There is still an intense debate with regard to the nature of the changes in the Holocene sea level, which is, oscillating versus smooth rises and falls. The most widely known oscillation curve for the mid- to late-Holocene is the Fairbridge Curve (published in 1961)[31]. The classic Holocene smooth (non-oscillating) sea level curve model is the Shepard
Curve (published in 1964)[32]. Most publications in China have supported the Fairbridge sea level oscillation curve (such as Zhao’s[33] and Fang’s[34] publications). In contrast, evidence from the Great Barrier Reef support the Shepard’s non-oscillating curve for the mid- to late-Holocene sea level changes[35]. On the basis of a detailed study of the bio-geomorphological zonation of a fringing reef at the Leizhou Peninsula, northern South China Sea, Yu et al[5] proposed that multiple millennial-scale sea level highstands of ~2 m above the present sea level occurred over the mid- to late-Holocene. New evidence[9] from microatolls at the same reef show that the sea level was also oscillating on centennial-scales. For example, there were four centennial-scale fluctuations occurring between 7110–6990, 6990–6890, 6890–6760 and 6760-? Cal. a BP, when the sea level was 171 to 219 cm higher than the present level. A detailed study[8] of a well-developed Goniopora reef profile from the same region demonstrates that the overall sea level rise during 7500–7000 Cal. a BP was punctuated by multiple interdecadal-scale sea level lowering events (which resulted in the emergence and erosion of the Goniopora coral heads, in several stratigraphic layers). Our beach sediment profile from this site provides new evidence on centennial-scale sea level oscillations. 3.3
Later Holocene sea level falling
The late Holocene beach sediment profile is 210 m landward from the modern beach (Fig. 3), which obviously indicates that the coastline has retreated by 210 m since its formation ~1235 Cal. a BP. The timing of post 1235 ka sea level and the coastline retreat, and origins of the present-day coastline, are still unknown. Nie at al[6] have reported a 14C age of 1170 Cal. a BP or 700 Cal. a BP for a massive coral that is regularly exposed at low tide, thus, suggesting that the sea level was still higher than the present level, around 700 years ago. All the above-mentioned data show that the coral reefs at Leizhou Peninsula have a great potential to reveal the critical details of the Holocene sea level changes.
4
Conclusions
Study on a late Holocene beach sediment profile at the Leizhou Peninsula suggests that from 1.7–1.2 Cal. ka BP (or 14 C ages from 2.1 to 1.7 ka BP) the sea-level rose significantly, but was punctuated by a short-term sea level lowering around ~1.5 Cal. ka BP, corresponding to climate cooling. The sea level at 1.2 Cal. ka BP was at least 128 cm higher than the present level. Sea levels fell some time after 1.2 Cal. ka BP resulting in the retreat of the coastline by about 210 m and the formation of the present beach—sandy bank system. In combination with the other periods of the Holocene sea level change evidences from the coral reefs at this site, the data suggest that the sea levels were similar to the climate,
YU Kefu et al. / Earth Science Frontiers, 2009, 16(6): 138–145
oscillated at millennial-, centennial- and interdecadal-scale cycles during the mid-late Holocene. The Leizhou Peninsula has a great potential to further reveal important details of the Holocene sea level change.
years, Penghu Islands, Taiwan Strait. Quaternary Research, 1996, 45(3): 254–262. [13] Chen J R, Chen X S, Zhao X T, et al. Study of sea level in Holocene change at Luhuitou area of Hainan Province. Marine Geology Studies, 1991, 3(71-86): 127–134.
Acknowledgments
[14] Tjia H D, Fujii S, Kigoshi K. Holocene shorelines of Tioman Island in the South China Sea. Geologie en Mijnbouw, 1983,
The authors would like to thank Profs. Liu Tungsheng and Zhong Jinliang, who both passed away over last years for participating in the fieldwork of this project; Profs. Zhou Guoqing, Qin Guoquan, Sha Qingan, and Wang Sumin for helping in the samples’ microscopic observation, foraminifera identification, and in discussions; and Dr. Gilbert J Price for language polishing.
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