The Sunda and Sahul continental platform: Lost land of the Last Glacial Continent in S.E. Asia

Quaternary International, Vol. 29/30, pp. 12%134, 1995. Copyright © 1995 INQUA/Elsevier Science Ltd Printed in Great Britain. All rights reserved. 1040-6182/95 $29.00

Pergamon 1040-6182(95)00015-1

THE S U N D A AND SAHUL CONTINENTAL PLATFORM: LOST LAND OF THE LAST GLACIAL CONTINENT IN S.E. ASIA W.S. Hantoro,* H. Faure,t R. Djuwansah,* L. F a u r e - D e n a r d t and EA. Pirazzoli:~ *Research and Development Centre for Geotechnology, Indonesian Institute of Sciences LIPI, Jl Cisitu Sangkuriang 21/154 D, Bandung 40135, Indonesia t Laboratoire de Gdologie du Quaternaire-CNRS, Luminy Case 907, 13288 Marseille Cddex 9, France ~.Laboratoire de Gdographie Physique du CNRS, 1 Place A. Briand, 92190 Meudon-Bellevue, France

Global climate change is the most significant phenomenon that may control the global variations of sea level over the coming thousands of years. During the alternate glacial and interglacial periods, ice-cap melting and ice accumulation in the high latitudes change the ocean water volume, which causes the sea level oscillations. For the longer periods, the change of sea level is due to the change of the basin volume following basin uplift or subsidence and the tectonic opening of the ocean floor due to plate motion. Some maximum glacial periods were marked by the very low sea level, about 125 m below the present sea level during the last glacial maximum, drying up and exposing the continental platform that was quickly covered by humid lowland tropical forest. The following rapid sea level rise due to the melting of the ice cap submerged the continent, transferring most of the carbon to the atmosphere. During the very low sea level, the deep pass Indian-Pacific Ocean Gateways remained open, allowing the global ocean current to go through the corridor between the two exposed platforms, Sunda in the West and Sahul in the East of the Indonesian Archipelago. Data obtained from these platforms will be important in order to understand the global climatic pattern from the Last Glacial Maximum (LG.M. 18,000 BP) which was followed by a rapid sea level rise.

INTRODUCTION

and interglacial phases. The sea level of the L.G.M. of isotopic stage 2 might be at -115 _+40 m below the present Indonesia, also called the Maritime Continent, consists of sea level (Faure et al., 1993). thousands of islands, so that it has a long coastal plain with The Sunda and Sahul, the two large shallow continental a large lowland area which is subject to sea level changes. platforms (about-125 m depth) of the west and eastern parts of Two large shallow and stable continental platforms, Sunda the Indonesian Island Arc (Fig. 1), are equatorial regions, with and Sahul, cover more than one-third of the archipelago area. only slight seasonal air-temperature fluctuation, though perhaps As a tectonically active area, the island arc experiences more during the glacial period. As a tropical continent, during differential vertical deformation, manifested by the the low sea level phases of early Holocene, this area might have subsidence of basins or the sinking of coastal plains, but also been covered by a large flat swampy low flood plain and a the presence of uplifted marine terraces (Hantoro, 1992). coastal tropical (wet) forest. During the glacial maximum Some evidence suggests that a slight (climatic) change in according to palaeovegetation maps (Van der Kaars, 1991) the southeast Asia may induce the genesis and propagation of continental platform might have been covered by woodlands global (climatic) events (Hantoro, 1993b). The major control and open forest (Fig. 2). During sea level rise, it then on South Asian circulation is the regular shift of the highly successively changed into a muddy-sandy or coral reef mobile Indonesian Low, producing seasonal monsoons carbonate bank of the near-shore and a shallow marine (Moore, 1993). The change of physicochemical aspects of environment during the interglacial high sea stand (Fig. 3). In the sea surface (temperature, salinity, etc.) due to terms of biomass or terrestrial carbon storage this means that anthropogenic effects in this area (coastal and marine each square metre of the present-day platform could bear pollution, gas emission, reduction of the green land-covers, between 13.5 and 43 kg of organic carbon in its vegetation and change of the surface, hydrological balance, change soil (Adams et al., 1990). Two million square kilometres of such of albedo, etc.) may help to bring about an unusual change land could lose around 60 Gt of carbon during sea level rise. of seasonal circulation. Natural processes (volcanic Some consequences following the change in the exposure eruption, flood, etc.) may strengthen the local unusual of the continental platform surface cover might be: climate change. (1) A change of the total amount of solar energy reflected, that also absorbed and transferred to biomass stocks in land; BACKGROUND HYPOTHESES (2) A variation in the primary production (photosynthesis) of the sea and also in its carbon flux; With the local rheological effect, the lowest sea level (3) Changes in terrestrial biomass stocks; reached during the L.G.M. varied from -106 _+ 11 to -148 _+ (4) Exchange variations of the terrestrial, marine and 16 m (Hantoro, 1992). The important implication is a atmospheric carbon and oxygen; significant change of the continental platform surface, from (5) Changes of the hydrological balance (ground and land with vegetation cover to sea water between the glacial surface water, evapotranspiration, rainfall); 129

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(6) A variation in the carbonate (reef and shallow marine) production, dissolution and distribution; (7) A variation in the local or regional climate due to the change of the seasonal temperature regulation, wind (monsoon system) and global ocean circulation through the Indonesian archipelago; (8) A change in the balance between erosion and sedimentation (placer and mineral trap deposits), variation of soil mineral alteration and laterization efficiency; (9) Eastward migration of Eurasian flora and fauna through the island arc; (10) And the question that still remains: did tectonic and volcanic activities in this area affect the sea level history of this area? Together with different rheological character, hydroisostasy effects and vertical tectonic deformations in this active area, the sea level changes have a spatially variable impact on the coastal environment. RESULTS

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Research activities related to the tectonic and global change study have been carried out in different areas along

the Sunda and Banda arc. Some absolute chronologies for the Quaternary limestone series have been established (Hantoro, 1992). On the basis of the study of the 475 m uplifted coral reef terraces at Cape Laundi-Sumba and the 600 m marine terraces at Kabola-Alor, the neotectonic history (Pirazzoli et al., 1991, 1993; Hantoro, 1992; Hantoro et al., 1993a) and the sea level variation curve can be established for the last million years for the Savu sea area (Hantoro, 1992). The Sunda and Banda arc shows a differential uplift rate during the Pleistocene (Hantoro et al., 1993b). The Holocene sea level curve and the near future relative sea level rise for western and central Indonesia has been established based on the emerged coral reef study (Hantoro, 1992, Jouannic et al., 1992, Hantoro et aL, 1992). The different relative high sea level stands (eustasy) found in the islands might be influenced by the different basin rheological effect or the Holocene tectonic activity (Hantoro et al., 1993a, b). Identification of natural resources and the potential risk for specific development in the coastal area has been carried out within the framework of studies on neotectonics, sea level and climate global change (Hantoro et aL, 1993c). Physicochemical sea surface data from the last glacial period to recent time can be obtained by studying the coral

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The Sunda and Sahul Continental Platform skeletons, or the bottom sediment cores of the deep sea and the epicontinental platform. This study needs to result in a continuous high-resolution record of some aspects of the regional climate variations (sea surface temperature, annual rainfall, wind and ocean current, etc.) that could influence the global climate system. The methods used for the study are isotope geochemistry (~13C, ~180, etc.), magnetostratigraphy, radiometry, sedimentology, palynology, modelling, etc. Measurements and recordings should be submitted to the data system to complete the information in addition to obtaining current data on some lithospheric, oceanographic or atmospheric change parameters that are used to study the global sea level and climatic changes. DISCUSSION Geodynamic study of the boundary evolution of the three plates reveals that there has been no significant change in the East Sunda and Banda Arc geographical and geological setting since the Pleistocene to Recent time. Though this is still in discussion, the (Upper) Miocene paleomagnetic data suggest that present Flores and Sumba Islands have not changed position since that time (Yokoyama e t al., 1980). Quaternary stratigraphic sequences show that in the volcanic inner arc, continuing volcanic activities were responsible for deposition of ejecta in the marine environment that produced, in some places, a reefal-volcanic material mixed deposit. The coral reef limestone lies conformably on this deposit by forming marine terrace series. In the non-volcanic outer arc islands, the Quaternary sequence reveals that the marine environment was influenced by the vertical uplift inducing the coral reef terrace formation. The deep corridor of the Banda and Sulawesi Strait remained open during the high

133

sea-stand of the Quaternary interglacial period, allowing the global ocean current to pass through. But can it be estimated that the deep global ocean circulation continued without any major disturbance during the low sea-stand of the Quaternary glacial period (Fig. 4)? It can be assumed that, since the Quaternary period, the global ocean circulation has had an important contribution to the global climatic change by keeping the dynamic ocean circulation passing through the Indian-Pacific Gateways. The changes of the land mass surface due to the sea level oscillation in the surrounding gateway's area should have induced a significant additional effect to the climatic change. The Quaternary tectonic activity, however, could have had an important influence on the regional climatic change because of the volcanic eruptions, of which emitted gas and ash could have raised the greenhouse effect or reduced the insolation budget. CONCLUSION The dynamic interactions of tectonic, climatic and eustatic changes in the Indonesian island arc have played a significant role in the global climatic change during Quaternary glacial-interglacial cycles. The Sunda and Sahul continental platform was covered by the sea during the post-glacial transgression. This has a significant role in the global change of the climatic system. One of these changes concerns the global carbon cycle. Each square metre of terrestrial land bearing 10-20 kg of organic carbon in vegetation and soil is replaced by 1 m 2 of oceanic platform bearing about the same amount of carbon in carbonate of shells and corals. This significant modification in the nature of carbon storage has consequences on the ocean and atmosphere carbon variations. Because the carbonate precipitation implies an equal quantity of carbon

FIG. 4. The Greatoceanconveyorbelt (AfterBroecker, 1987;Broeckeret al., 1990)keepspassingthe Pacific-IndianOceanGatewaysduringthe low seastand of the glacial periods. (Thehatched arrow delineates the area of Pacific-IndianOcean gateways.)

134

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shifted from dissolved oceanic inorganic carbon to atmospheric CO2. It is now necessary to get more data on the actual environment of the emerged continental plateau during the Last Glacial Maximum and on the timing and extent of the area invaded by the post-glacial transgression. Further research and new sets of data will help us to gain a better understanding of the dynamic mechanisms behind the sea level oscillation and the emerged-submerged epicontinental platform in this Pacific-Indian Ocean gateway that may induce an effect on the global climate change. Some data should come from the tropical shallow epicontinental platforms (Sunda and Sahul) that bear important information to understanding glacial and interglacial climate changes, and perhaps these two platforms have a significant role on stabilizing or promoting climatic fluctuation. The data should comprise the following: bathymetry, bottom features and lithological covers, structure and rheology of the platform; coastal line evolution and rate of sea level rise between 17,000 and 7000 BP, total carbon content per square metre of tropical forest and of marine carbonates, and anthropological trace, palaeoannual rainfall, etc. A database of dated indicators of the submergence must be established. Based on these data, it will be possible to make a better long-term development plan of the coastal area. ACKNOWLEDGEMENTS We appreciate the permission of the Chairman of the RDCG LIPI to present the results of a national report. We would like to thank Li Sumantri, Djulaeha and Djoko Trisuksmono who helped to complete the data and the figures of this manuscript.

REFERENCES Broecker, W.S. (1987). The biggest chill. Natural History Magazine, October, 74-82. Broecker, W.S., Bond, M., Bonani, G. and Wolfli, W. (1990). A salt oscillator in the glacial North Atlantic? The concept. Paleooceanography, 5, 4, 469-477.

al.

Faure, H., Faure-Denard, L., Hantoro, W.S., Lezine, A.M., Pirazzoli, P.A., Thomassin, B., Velichko, A.A., Zazo, C. and Adams, J.M. (1993). Importance des changements du plateau continental entre 18,000 et 8,000 ans BP pour la modrlisation biogrochimique et climatique globale. Volume des R~sum~s PICG 253-274, Dakar, Mai 1993. Hantoro, W.S. (1992). Etude des terrasses rrcifales quaternaires soulevres entre le drtroit de la Sonde et File de Timor, Indonrsie. Mouvements Verticanx de la Crofite terrestre et variations du nivean de lamer. Ph D thesis Univ. d'Aix-Marseille II. France. Vol I, 761 p e t Vol II, 225 p. Unpublished. Hantoro, W.S. (1993b). Environment and climate changes record since the last interglacial obtained from the study of the modern and uplifted coral reef in Indonesian Island Arc: Background and strategy of the program. In: Proceedings of the IAEA consultants Meeting of the Retrospective Studies on Coral Reefs, Townsville, Australia, 17-21 May 1993. Hantoro, W.S., Jouannic, C., Pirazzoli, P.A. and Faure, H. (1992). Near future sea level trend in Sunda Strait Area (Abstract) In: Proceedings of the Seminar Programme of lOth Anniversary of lndonesia - - France Joint Cooperation on Ocean Technology and Maritime Industry, Jakarta, Indonesia, October 7-8, 1992. Hantoro, W.S., Pirazzoli, P.A., Jouannic, C., Faure, H., Hoang, C.T., Radtke, U., Causse, C., Borel Best, M., Lafont, R., Bieda, S. and Lambeck, K. (1993a). Quaternary uplifted coral reef terraces in Alor island, East Indonesia: Intermittent or Continuous uplifting. Hantoro, W.S., Pirazzoli, P.A., Jouannic, C., Faure, H., Hoang, C.T., Radtke, U., Causse, C., Borel Best, M., Lafont, R. and Bieda, S. (1993b). Quaternary Differential vertical movement along the Sunda-Banda Arc, Indonesia: uplifted coral reef terraces study. Tectonophysics. Hantoro, W.S., Soeharsono, Siregar, M.S., Gianto, T.H., Arwono, B., Bakti, D.I., Haq, F., Sasbiyanto, T., Sumantri, I. and Mulyadi, D. (1993c)~ Indentification of the Research Potential (Coral Reef Study). Report of the Research: Kerjasama Proyek lnventarisasi dan Evaluasi Sumberdaya Nasional Matra Darat-Bakosurtanal dengan Puslitbang Geoteknologi LIPI (Indonesian volume). Jouannic, C., Hantoro, W.S. and Naryanto, H.S. (1992). Late Quaternary Vertical Deformations Along Portions of the Central Sunda Volcanic Arc Southwest Coast (From Padang to Pacitan), As Inferred from Sea Level Indicators. In: Proceedings of the Seminar Programme of the l Oth Anniversary of Indonesia - - France Joint Cooperation on Ocean Technology and Maritime Industry, Jakarta, Indonesia, October 7-8, 1992. Moore, M.D. (1993). Catastrophic El Ni~os and Indonesian Droughts: A 500 year history from reef corals. Field Season Report. RDCG LIPI. (Unpublished) Pirazzoli, P.A., Radke, U., Hantoro, W.S., Jouannic, C., Hoang, C.T., Causse, C., Borel Best, M. (1991). Quaternary raised coral-reef terraces on Sumba Island, Indonesia. Science, 252, 1834-1836. Pirazzoli, P.A., Radke, U., Hantoro, W.S., Hoang, C.T., Causse, C., Borel Best, M. (1993). A one million-year-long sequence of marine terraces on Sumba Island, Indonesia. Marine Geology, 109, 221-236. Van der Kaars, W. (1991 ). Palynology of eastern Indonesian marine piston cores: A Late Quaternary vegetational and climatic record for Australasia. Palaeogeography, Palaeoclimatology, Palaeoecology, 85(3-4), 239-303.