|Environmental Changes on the Coasts of Indonesia (UNU, 1980, 53 pages)|
Indonesia consists of about 13,700 islands, with an intricate coastline whose length has been estimated as just over 60,000 kilometres by Soegiarto (1976). The islands show considerable diversity of coastal features, related partly to contrasts in the geology and geomorphology of the hinterland and the bordering sea floors, and partly to variations in adjacent marine environments. In terms of global tectonics the Indonesian archipelago occupies the collision zone between the Indo-Australian, Pacific, and Eurasian plates. It is a region of continuing instability, marked by frequent earthquakes and volcanic eruptions. Its mountain ranges are areas of Cainozoic uplift, augmented by large volcanic constructions, and its bordering seas are underlain by unstable shelf areas, especially towards the Java Trench, the subduction zone that lies to the south of the Indonesian island-arc.
Indonesian coastlines show the effects of past and present tectonic instability, volcanic eruptions, and changes of sea level. There have been upward and downward movements of the land, often accompanied by tilting or faulting; outpourings of volcanic lava and ash have influenced coastal features both directly and indirectly; and vertical movements of sea level have resulted in complicated sequences of emergence and submergence of island coastlines. Many characteristics of Indonesian coastal landforms are related to their development under tropical -especially humid tropical-conditions, and it will be useful first to consider these features briefly.
Although much of Indonesia lies within the humid tropical zone, some parts have sub-humid and even semi-arid climates. Climatic characteristics are determined largely by the position of the Intertropical Convergence zone (ITC), a zone of unstable air and heavy rainfall which migrates north and south over Indonesia, crossing the equator in May and November each year, and reaching latitudes of about 15 south in January. Indonesian climatic stations generally show a more pronounced wet season when the Intertropical Zone of Convergence is to the south, when westerly winds prevail; the dry season occurring after it migrates away to the north, and winds move around to the south-east. Generally the southern part of the archipelago has a smaller mean annual rainfall than the rest of the country, partly because of a reduction in the water content of westerly winds as air masses move to the east, and partly because of the influence of drier air brought in from the Australian region by winds from the southeast during the dry season. In additon, afternoon showers caused by local intensive heating are common, and may occur even in the dry season. The pattern of rainfall is influenced by the orographic factor, notably where moist air is forced upwards as it moves eastwards across mountain ranges, particularly in Sumatra, Java, and Irian Jaya. The wettest coastal areas are thus found to the west of the ranges, and the relatively dry areas in the "rain shadows" to the east (Sukanto 1969).
Winds are generally light to moderate, the most vigorous being the south-easterlies in the dry season. The cyclones of northern Australia and the typhoons of the South China Sea do not reach Indonesia, although waves generated by these disturbances are occasionally transmitted into Indonesian coastal waters.
In terms of Koppen's classification, most Indonesian coastal regions are in Category A, with mean temperatures in the coolest month of at least 18°C., but a few sectors have a sufficiently long and dry winter season to be placed in the semi-arid category BS. Truly humid tropical coasts (Af), with a mean rainfall of at least 60 millimetres in the driest month, are extensive around Sumatra and Kalimantan, in southern Java, much of Sulawesi, and the islands to the east. They give place to monsoonal lam) climates, with a short dry season compensated by a large annual rainfall, along the north coast of Java (Jakarta, Semarang, Bangkalan) and in several minor sectors around Sulawesi, including Ujung Pandang; and to somewhat drier savanna (Aw) climates in the rain shadow areas of northeastern Java (Surabaya, Pasuran) and the islands to the east, and around Timor (Dill, Kupang), which is much influenced by dry air masses arriving from Australia. Sectors dry enough to warrant semiarid (BS) classification are limited, but occur on the north coasts of Lombok and Sumba. Much more information from coastal stations is necessary before climatic sectors around the Indonesian archipelago can be delimited accurately.
The interior uplands record substantially higher rainfall than most coastal regions, so that river systems carry a very large runoff from the high hinterlands.
Landscapes in humid tropical environments are subject to the intense chemical and associated biological weathering of rock formations that proceeds under perennially warm and wet conditions. This has led to the formation of deep mantles of decomposed rock material, mainly silt and clay, and in places these are up to 30 metres thick. Away from coastal cliff exposures, natural rock outcrops are rare: they are found locally on resistant sandstones, some limestones, and recently formed lava flows.
The natural vegetation cover is tropical rain forest, with a dense canopy and a thick organic litter that protects the ground from the direct erosive effects of heavy rainfall. A subsurface network of roots also binds and stabilizes the upper part of the weathered mantle. This luxuriant vegetation tends to hold the weathered mantle in place, but on steep slopes the rapid runoff that occurs during heavy rain may wash away surface material even where the vegetation is dense, and landslides and mudflows frequently scar the forested hillsides.
Runoff is thus typically laden with fine-grained sediment, silt and clay produced by weathering, but in steep areas the streams incise their valleys and derive sand, or even gravel, from the less-weathered underlying rock formations. Coarser sediment is also derived from the lava and ash produced by volcanic eruptions. On steep volcanic slopes lahars are formed, when torrential rainfall saturates and mobilizes masses of pyroclastic debris, which flow down into the valleys. Streams also derive sand and gravel when previously constructed volcanic structures are dissected by runoff.
The combination of steep elevated hinterlands of deeply weathered rock, recurrently active volcanoes, and frequent heavy rainfall produces large river systems that carry substantial quantities of sediment down to the coast. Deposition of this material has built extensive deltas and broad coastal plains, especially in Java, Sumatra, Kalimantan, and Irian Jaya. The lithology of outcrops within each catchment determines the nature of the weathered mantles and strongly influences the composition of sediment loads carried downstream by the rivers. As Meijerink (19771 has shown, the sediment volumes per square kilometre per year from catchments dominated by sedimentary formations are much greater than those from volcanic catchments (Table 11. Where sandy material is carried down to the coast it is reworked by waves and deposited as beach formations along shorelines adjacent to river mouths: the most extensive of these are on the south coast of Java. Silts and clays are incorporated in tidal mudflats and coastal swamps, and deposited in lowlyingareas on and around river deltas.
TABLE 1 Influence of Lithology on Sediment Yields, as Observed for a One-Year Period (based on Meijerink 1977)
|Catchment||Drainage area km²||Sediment yield ton/km² /year||Source|
|Ciliwung||130||250 - 375||Rutten|
|Banyuputih||225||750 - 1,000||Rutten|
|Brantas||10,000||875 - 1,500||Rutten|
|Citarum||73,000||800 - 1,200||Modified after Soemarwoto|
|Cimanuk||3,000||1,000 - 2,000||Rutten|
|Tandjum||210||750 - 1,000||Rutten|
|Cilamaya||225||2,500 - 3,500||Rutten|
|Lusi||860||2,500 - 3,500||Rutten|
|Serayu||700||3,500 - 4,500||Rutten|
|Jragung||101||4,000 - 6,250||Rutten|
|Pengaron||41||9,250 - 12,500||van Dijk|
Runoff has undoubtedly been accelerated and sediment yield increased in many parts of Indonesia as the result of the modification or removal of the natural vegetation cover (Table 21. Many formerly forested areas now carry more open plantations, or have been cleared for cultivation or grazing. Soil erosion has become a widespread phenomenon, and the material lost from deforested terrain augments the loads carried downstream in the river systems. Moreover, accelerated runoff increases the frequency and extent of river flooding in the valleys and out over the coastal plains and deltas. These effects are most evident in densely populated and intensively utilized regions, particularly in Java, but a similar sequence of events can be demonstrated in other parts of Indonesia.
TABLE 2 Runoff from Small Plots under Different Vegetation on Java (based on Meijerink 19771
a s l
|Mean annual Rainfall||Runoff in % of rainfall during observation period|
|Bare soil||4 stations, W Java||25 - 55|
|< 21/2 years||Monggot||150||2,200||2.8||7.5||2.8|
|Bomboo grass||4 stations,||-||-||-||10 - 20||-|
|W Java||-||5 - 10||-|
Although much of the sediment supplied to the coast under humid tropical conditions has been delivered by rivers, material has also been derived from the erosion of cliffs along the shore, and from the sea floor. Sediment has been carried onshore from shallow coastal waters to many parts of the world's coastline, and it is likely that the deltas and coastal plains around Indonesia incorporate in their stratigraphy marine sediments that originated in this way. For example, marine clays are known to underlie parts of the extensive swampy lowlands of south-eastern Sumatra.
Beaches of sand and gravel are extensive around the coasts of Indonesia, especially near the mouths of rivers delivering this kind of material, adjacent to cliffs of sandstone or conglomerate, and along shorelines to the rear of fringing coral reefs. Beach sediments of volcanic origin are typically black or grey; those of coralline origin white or yellow. Quartzose sands are of very localized occurrence in relation to quartz-arenite outcrops along the coast and within hinterland river catchments.
Sandy beaches are typically backed by swash ridges, and multiple beach ridges occur on sectors that have intermittently prograded. Coastal dunes are poorly developed in the humid tropics generally, and in Indonesia they occur only on a few sectors, notably in southern Java, where the fluvially nourished beaches near Yogyakarta are backed by dune topography, and locally in southwestern Sumatra. Beach ridges and dunes carry a woodland formation dominated by Casuarina, Pandanus, Calophyllum, Inophyllum, and Barringtonia species, usually with planted or self-seeded coconut palms. Many beaches show evidence of erosion with backshore cliffing and undercutting of vegetated terrain but this is not the case where there is a continuing supply of sandy, or gravelly, sediment to maintain or prograde the shoreline.
Shorelines of depositional coasts are typically sandy or swampy, and in the humid tropics swampy sectors are usually occupied by mangroves, which colonize the upper part of the inter-tidal zone. Once established, mangroves can protect the coast from wave scour and may promote the accretion of sediment to build up new depositional terrain to high-tide level. On accreting shores, the mangroves spread forwards, and as deposition attains high-tide level nipa palms, or rain forest, or freshwater swamp vegetation, move in from the rear. The constructive and protective value of mangroves is often demonstrated where they have died back, or been cut down, exposing the substrate which is then rapidly eroded by wave scour.
Steep and Cliffed Coasts
Cliffed coasts are relatively rare in the humid tropics, partly because of the great extent of deltas and coastal plains formed by deposition in front of hilly, or mountainous, terrain; partly because of the protection of fringing and barrier reefs of coral on many sectors; and partly because of the general absence of strong wind and wave action in these environments. Where high country extends to the coast it terminates in steep coastal slopes with features similar to those of valley-sides inland, usually with a dense vegetation cover extending down almost to high-tide level. The slope base has often been undercut by wave action, which has removed weathered material to expose rocky outcrops or boulder accumulations on the shore. Because of this basal undercutting, landslides occur frequently on steep coastal slopes.
Steep coasts of this kind are extensive in Indonesia, especially around Sulawesi and the islands to the east. On the other hand, cliffs have developed along sectors of the southern coasts of Sumatra, Java, and the islands east to Sumba, which are exposed to the relatively strong wave action generated across the Indonesian Ocean.* They are best developed on sandstones, limestones, and outcrops of volcanic rock. While most cliffs are formed by wave attack, some are due to catastrophic changes such as earthquakes and volcanic eruptions. The explosive eruption of Krakatau in Sunda Strait in 1883 left high cliffs cut in volcanic materials on the residual islands.
Rocky shore outcrops are subject to intense physical, chemical, and biological weathering. Limestone outcrops become intricately pitted and honeycombed (Plate 4), and similar disintegration can be seen on sandstones and volcanic rocks exposed to the action of surf and spray, salt corrosion, solution by rain water and percolating ground water, and the effects of wind scour. Such features are well known on humid tropical coasts, but little attention has been given to them in Indonesia.
Reefs built up by coral and associated organisms occur extensively in Indonesian waters, especially in the Flores and Banda Seas (Darwin 1842, Davis 1928, Molengraaf 1929, Kuenen 1933, Umbgrove 1947). Coral growth requires clear warm water, with temperatures that do not fall below 18 C., and salinity within the range 27 to 38 parts per thousand. Such conditions are widely satisfied in the seas around Indonesia, the chief exceptions being off the mouths of rivers, where salinity is diluted and the sea made turbid by the discharge of suspended sediment loads. Thus coral reefs are sparse in the shallow, often muddy seas south and west of Kalimantan, and in the sediment-laden waters off the deltaic coastline of northern Java. Nevertheless, there are scattered coral reefs in Jakarta Bay, in the clearer water seaward of the muddy areas off river mouths.
Other factors inhibiting reef growth include active vulcanicity, where coastal waters are frequently invaded by lava and ash, as around Manada-tua, the active volcano off northern Sulawesi, and the deposition of large quantities of sediment off coasts where the slopes of recent or active volcanoes are undergoing mass movements downslope or rapid dissection by streams. Examples are Ternate, Tidore, and Makian in the Moluccas, Gunong Ija, south of Flores, Sangeang, north-east of Sumbawa, and Ruang, north of Sulawesi. In each case the reefless island shores are bordered by beaches of black volcanic sand and gravel, or cliffs and rocky shores of lava, with only scattered coral growth in the nearshore zone.
Tectonic instability has also interfered with reef develop" meet, either by maintaining an unstable substrate, or by raising coral growths and reef formations out of the sea as emerged features (Verstappen 1960). Subsidence, leading to submergence of reefs, may stimulate upward growth, as in the classical sequence whereby a fringing reef developed along an earlier coastline becomes an outlying barrier reef off a submerged coast, and where reefs fringing high islands become encircling reefs enclosing a lagoon with a partly submerged central island (an "almost-atoll") and eventually, with further subsidence, atolls surrounding a lagoon on the site of a completely submerged island. Davis (1928) and Molengraaf (1929) quoted examples of each of these features from the Indonesian region. Gunong Api is an island in the Banda Sea with an encircling fringing reef; Goram, to the north-east, has a ring of reefs around a lagoon with a central island; and there are numerous atolls built up to present sea level from subsided foundations in deep water south of Sulawesi: Kalukalukuang, Postillion, Sabalana, Sapuka, Paternoster, and Zandbuis are good examples. Major barrier reefs include the Great Sunda Reef, which rises from submerged shelf margins south-east of Kalimantan, the reef east of Sulawesi, and the similar reefs off the south-west coast of Sumatra, which curve out towards the islands off Batu and Banjak. Submerged reefs have also been charted in sea areas west of Irian Jaya.
Emerged reef features are widespread in the Indonesian region. They are found in northern Sumatra, along the south coast of Java, and especially around Sulawesi and along the islands east of Bali, notably Sumbawa and Timor. They are common in eastern Indonesia, particularly in the Banda Sea, where there are uplifted atolls such as Manowolko and Matabello. Kafiau, off Irian Jaya, is an uplifted almost-atoll, with rimming hills of reef limestone around a hollow that encircles an interior upland, while Salajar, south of Sulawesi, is the emerged half of a tilted atoll, with gentle slopes to the west and a steep descent to the east, where a volcanic foundation is exposed. Muna Island off Sulawesi, Pomana Island off Nusatenggara, and Satengar to the north of Sumbawa are reef patches that have been raised out of the sea by earth movements to form islands. There are also many islands with bordering stairways of uplifted reef terraces, Timor being the largest and most elevated.
In addition to uplifted reefs, coralline islands include low sand cays deposited on offshore reefs and more complex depositional islands, with coarser remnants of coral shingle on the side exposed to stronger wave action, sand cays to leeward, and intervening shallow lagoons, in which mangroves may grow. It is possible that the coralline sediment which forms these low islands is the outcome of erosion of coral reefs that had been built up during phases of slightly higher sea level, and were exposed to wave attack when the sea subsequently fell. Material is also broken from the submerged slopes of coral reefs, especially during tsunami, or periods of strong wave action, when the delicate structures of staghorn coral are dislodged and thrown up onto the reef as cylindrical fragments of coral shingle. Another source of coral shingle is the debris generated when a reef is quarried for limestone. Larger coral boulders known as "negro-heads" littered the reef-fringed shores of Sunda Strait after the tsunami that resulted from the Krakatau explosion in 1883; one of them had a volume of 300 cubic metres. The tsunami from the Paloweh eruption in 1928, drove similar large coral boulders up onto the nearby shores of Flores.
Algal structures (mainly of Lithothamnium) locally surmount coral reefs and are better developed on the southern coasts of Indonesia, which are subject to surf generated by ocean swell and south-easterly wave action. Algae thrive in wellaerated surf, and tend to build rims awash at high tide and exposed at low tide along the outer edges of fringing reefs in these relatively high-energy environments. By contrast, fringing reefs bordering inner sea areas in Indonesia generally lack algal rims and are built out at slightly lower levels.
Processes in Coastal Waters
The shaping of these various coastal features has been much influenced by the wave regime in Indonesian coastal waters. A strong swell transmitted from the Southern Ocean moves in from the south-west to the south coasts of Sumatra and Java. Farther east it becomes attenuated and weaker, having crossed the broad north-west shelf of Australia, but it can still be detected on Timor. A Pacific swell moves southwards through the Philippine Sea to reach the north coast of Irian Jaya, weakening as it diffuses through the Moluccas. Otherwise, wave action in Indonesian coastal waters is determined by local winds. Between April and November south-easterly winds are dominant over sea areas south of Indonesia and waves from this direction are important along the south-facing coasts of Java, Bali, Lombok, and Sumbawa, and on the south coast of Timor. At this season, winds over the Java Sea are easterly to northeasterly, and there are lighter breezes from various directions in the equatorial zone to the north. In the wet season the winds over Indonesian waters are gentler and more variable but typically westerly (Table 3).
TABLE 3 Prevailing Wind Directions at Selected Coastal Stations in Indonesia
|Semarang - Maritim||W||W||W||NW||E||N||E||SE||E||NW||NW||NW|
|P.G. Bone Arasoe||N||W||N||N||S||SE||SE||SE||SE||SE||SE||SE|
With the exception of the southern shores of Sumatra, Java, and the Lesser Sunda islands, which receive a southwesterly ocean swell and relatively strong southeasterly wave action in the winter, the coasts of Indonesia are exposed only to low-wave energy. As has been noted, the tropical cyclones which produce short-term high-energy conditions on the coasts of northern Australia, and around the South China Sea, do not reach Indonesia, although waves generated by these disturbances are transmitted into Indonesian coastal waters south of Java and the Lesser Sunda islands, including Timor, and into the sea between western Kalimantan and the islands south-east from Singapore.
Tidal movements in Indonesian waters result from impulses arriving from the Pacific and Indian Oceans. One wave moves into the Straits of Malacca from the north-west, augmenting its range and generating strong currents as the configuration narrows. Another arrives from the South China Sea, diverging through the narrow waters west of Kalimantan, and producing an interacting system with the Malaccan tides south of Singapore. Tides from the Pacific Ocean advance through the South Philippine Sea to the north coast of Irian Jaya, and penetrate the straits around the Moluccas, while tides from the Indian Ocean move through the waters south of Java to the Timor and Arafura seas, augmenting in the coastal waters south of Irian Jaya. Within the Java and Banda seas there are minor but complex tides, the patterns being related to deep basins within the intricate configuration of the eastern Indonesian archipelago.
Spring tide ranges (Fig. 1 ) are a metre or less on the south west coast of Sumatra, but they increase to more than 3 metres in the narrows of the Straits of Macassar. On the south coast of Java they are 1 to 1.5 metres, but less than 1 metre on the north coast, except in the Straits of Madura where Surabaya has 1.7 metres. They are up to 1 metre on the south-west coast of Kalimantan, and somewhat larger (up to 2.8 metres) on the east coast. Sulawesi has small tide ranges, exceeding 1 metre only on the north coast and in Teluk Bone, the southern gulf. Tide ranges of about 1 metre are typical in eastern Indonesia, but they exceed 2 metres on the south coasts of Sumba, Flores, and Timor as the result of augmentation through the Timor Sea, where the Australian coast to the south records tide ranges that locally exceed 10 metres. North-east of the Arafura Sea tide ranges of more than 5 metres occur in estuarine inlets along the southern coast of Irian Jaya, where tidal bores are generated, moving upstream as steep waves as the tide rises.
Tidal oscillations are also complicated by wind action. Northeast winds over the China Sea build up the water level south of Singapore by as much as 0 5 metres between January and March, while south-east winds raise winter sea levels a similar amount along the southern coasts between Timor and Java.
In addition to these regular tidal and seasonal alternations there are irregular surges generated by earthquakes and volcanic eruptions in the Indonesian region. These tsunami are occasionally devastating, causing shoreline erosion, displacing material from coral reefs, overwashing beaches, and flattening mangrove fringes. The most severe tsunami so far recorded was that generated by the Krakatau explosion in 1883, when waves reached up to 30 metres on the adjacent shores of Sunda Strait, washing away the lighthouse on Java Head. Lesser surges were then experienced all round the coasts of Indonesia. In 1979, massive landslides occurred on the coasts of Lomblen and Nusatenggara as the result of a tsunami.
Changing Levels of Land and Sea
So far we have dealt mainly with processes effective with the sea at its present level, but a study of the coastal features of Indonesia soon encounters evidence that the sea has stood at different levels in relation to the land at various times in the past. On some sectors there are emerged coral reefs and terrace features that formed when the sea stood at a relatively higher level; others show the effects of submergence, in the form of drowned mouths or embayments over drowned lowlands. Some of the changes are due to eustatic movements of the sea surface, resulting from world-wide changes in ocean volume; others to uplift or depression of the land margin as the result of earth movements, which have been particularly marked in the Indonesian region (Katili and Tjia 1969, Tjia 1968, Umbgrove 1949, Verstappen 1974, Zen and Sudarmo 1977). Until the sequence of eustatic movements has been established, it will not be possible to elucidate the extent to which these various vertical changes are due to land movements independent of sea-level oscillations. Meanwhile it should be noted that terraces along river valleys or on coastal sectors are not necessarily due to land uplift; they may be due, at least in part, to eustatic lowering of sea level.
World-wide sea-level oscillations occurred during Quaternary times, the sea falling to relatively low levels during the Glacial phases of the Pleistocene and rising to relatively high levels during the Interglacial phases. The fact that the sea floor of the Sunda Shelf region was exposed to sub-aerial denudation during low-sea-level phases, its topography retaining valley patterns shaped by river action, and that its submergence resulted from the world-wide sea-level rise that accompanied late Pleistocene deglaciation was first perceived by Molengraaff and Weber (1920). East of Sumatra the islands of Bangka and Billiton show Quaternary terraces 50,25, and 6 to 8 metres above present sea level (Tjia et al. 1977), which probably correspond with the higher sea levels attained during the Pleistocene Interglacial phases.
Twenty thousand years ago, during the Last Interglacial, world sea level was at least 100 and perhaps as much as 140 metres lower than it is now. At that stage Java, Sumatra, and Kalimantan were areas of higher country rising from the broad plains of the emerged sea floor on an enlarged Malaysian Peninsula: according to Verstappen (1975b) its land area increased by about 3 million square kilometres. To the south-east, an enlarged Australia was linked with New Guinea by way of the emerged Torres Strait land bridge. In between, Sulawesi and the eastern Indonesian archipelago persisted as slightly larger islands amid deeper seas and straits (Fig. 2). A deep channel extended down through the Straits of Macassar between Kalimantan and Sulawesi, and on through the gap between Bali and the island of Lombok. This became known as "Waliace's Line" after Alfred Russel Wallace reported in 1856 that it formed a biogeographical boundary between Asian and Australasian faunas which could not migrate across this late Pleistocene strait. Later research has shown that the division is, in fact, a broader zone, the sea area between the enlarged Malaysian and Australian land masses, the biota of the eastern Indonesian archipelago being transitional in character between that of Malaysia and Australia (Hooijer 1975).
According to Verstappen (1975b) the climate of the Indonesian region was cooler and drier (with longer dry seasons) and also windier during the late Pleistocene low sealevel phase, but the uplands remained humid tropical, and carried rain forest. Currents in the Indonesian seas were modified and disrupted, and although the temperature of the sea fell 4 to 5 C., it remained warm enough for coral growth, the waters being less turbid because of reduced rainfall and runoff. Verstappen questioned the correlation of river valley incision with low sea-level phases of the Pleistocene, on the grounds that Interglacial climate and vegetation were also conducive to stream entrenchment, especially in the hinterland, where the snow line was lowered several hundred metres and vegetation zones became vertically compressed, protective rain forest giving place to less dense vegetation on many slopes. However, it should be remembered that relatively high sea levels in Interglacial phases set downward limits to valley incision, and that in coastal regions there has been aggradation of valleys previously cut down to lower, glacial phase, sea levels, as well as submergence and infilling of their former extensions across the neighbouring sea floor.
There is morphological evidence for terraces of Quaternary age both above and below present sea level, probably marking stillstands in sea level relative to the land. Tjia (1975) listed several former coastline levels in Malaysia and parts of Indonesia, but the determination and dating of such features is still largely speculative, especially in the tectonically unstable Indonesian region. In terms of environmental changes, interest centres on the Holocene sequence of land- and sea-level changes, during and since the world-wide marine transgression that began about 18,000 years ago, accompanying climatic amelioration and the partial melting of polar and upland ice sheets.
This Holocene marine transgression brought the sea up to approximately its present level about 6,000 years ago, since when there have been only minor oscillations. The evidence for Holocene changes of land and sea level in Indonesia and Malaysia has been examined in a number of papers by Tjia (e.g., Tjia 1970a, 1975; Tjia et al. 1972, 1975,1977), who has put forward an oscillating Holocene sea-level curve, based largely on Malaysian evidence, with several peaks of sea level up to 3.5 metres above the present, diminishing in amplitude through to the modern level (Tjia et al. 1977). There is certainly good evidence of a higher Holocene sea level along the east coast of peninsular Malaysia, where oyster beds are found up to 3.5 metres above present sea level, but on Indonesian coasts the evidence is more variable, indicating the complicating effects of upward and downward movements of the land. Thus, on the west coast of Sulawesi at Pangkajene, molluscs between 4,000 and 5,000 years old have been found in position of growth attached to Eocene limestone cliffs 5.75 to 6.50 metres above present sea level: even if these grew during an episode of higher Holocene sea level, they must have been uplifted to attain their present height.
Emerged shoreline features of Holocene age have been widely observed on Indonesian coasts by Tjia (Fig. 3) and have been deduced in recent work in the Straits of Malacca (Geyh et al. 1979) and at points along the north coast of Java, but it is not clear which of them formed in relation to a higher sea-level stand on stable coast sectors and which owe their present elevation, at least partly, to tectonic uplift. West of Kalimantan there are coastal terraces up to 2 metres above present sea level on the islands of Tambelan and Bunguran. On Tambelan dead corals attached to rock outcrops just above sea level and Tridacna specimens found above their normal growth level have been dated within a range of 5,200 to 5,500 years BP (Haile 1970).
Evidence of rapid Holocene uplift at rates of up to 10 millimetres/year has been deduced from radiocarbon dating of coral reef terraces in Sulawesi, on Timor and Sumbawa, and bordering the islands of Tomea, south-east of Sulawesi, and Selu, off Yamdena. Saubi Island, east of Madura, and Satengar Island, a coral cay north of Sumbawa, also show Holocene uplift, while the tilted reef terraces on Biak Island north of Irian Jaya indicate transverse displacement in the course of uplift that has continued into Holocene times (Tjia et al. 1974). Active subsidence is less easily demonstrated, but it is very likely that isostatic subsidence is in progress in the areas occupied by large deltas, where the massive sediment loads depress the underlying crust. Sectors of deltaic shoreline not maintained or advanced by river-mouth deposition and longshore accretion are likely to develop inlets and embayments as submergence accompanies subsidence.
Much more detailed research is necessary to establish the location and extent of the various raised shorelines in Indonesia, and to separate the effects of sea-level oscillations from those of tectonic uplift or depression. Attempts to establish average rates of uplift or subsidence in Indonesia, or over regions within Indonesia (Tjia 1970b, Tjia et al. 1974) are of little value because the regional pattern is so complex and variable, but measurements of rates of vertical movement at specific sites (cf. Chappell and Veeh 1978) are of much interest as a basis for deciphering patterns of differential uplift and subsidence.
Changes of sea level result in horizontal movements of the shoreline, which advances as emergence takes place and retreats during submergence. These effects can be offset by erosion or deposition: a rapidly eroding shoreline may continue to recede during a phase of emergence, and a rapidly accreting shoreline could continue to advance even when submergence is taking place. These possibilities are summarized diagrammatically in Fig. 4. In order to assess the nature and extent of coastal changes in Indonesia in Holocene times it would be useful to determine the alignment of the shoreline 6,000 years ago, when the marine transgression established approximately present sea level. However, this is difficult, partly because much of the evidence has been eroded or concealed by deposition, and partly because there have been minor oscillations of sea level during the past 6,000 years, accompanied by tectonic displacements of some Indonesian coastal areas within this period.
Impact of Man
The attempt to assess the extent to which environmental changes are due, directly or indirectly, to Man's activities in Indonesia is complicated by the fact that the islands have long been populated, and that it is difficult to establish what conditions would have been obtained naturally. Indeed, since the hominid Pitbecanthropus modjokertensis was living in Java at least 1.9 +0.4 million years ago (Jacob and Curtis 1971), and there is evidence of his occupation of existing coastal regions about 1.2 million years ago, Man has evidently been present during the oscillations of land and sea level discussed previously. When sea level fell during the Last Glacial phase of the Pleistocene, Man presumably occupied emerged lands seaward of the present coastline. As the sea rose, he must have retreated to the upland areas that then persisted as islands, and during the past 6,000 years he has adjusted his occupance to advances and recessions of the shoreline.
Nevertheless, the population of Indonesia was relatively small, and apparently stable, when Dutch explorers and colonists arrived in the sixteenth century. Its rapid increase during and since this phase of European influence has resulted in the locally extreme pressures of population on resources now seen, especially in northern Java. Although there was port construction in the Jakarta area at least by the fourth century AD, and coastal settlements depending on fishing and trade have certainly existed for many centuries, the impact of dense populations on the coast has been comparatively recent. The cutting of canals; the embanking, damming, and diversion of rivers; the large. scale replacement of hinterland forests by grazed and cultivated land, with consequent increases of runoff and soil erosion; the destruction of vegetation that formerly stabilized coastal dunes; and the modification of mangrove areas for agriculture and arguaculture, are all major impacts that have developed mainly within the past century. There has also been mining of tin, bauxite, nickel, and iron ores in coastal regions, and within the last decade there has been rapid expansion of oil production from wells mainly in the coastal and nearshore zones. Reference will be made subsequently to the effects of beach mining and the quarrying of coral reefs. Industrialization has proceeded in the Jakarta and Surabaya regions, and near Cilacap, alongside an estuarine embayment on the south coast of Java. In consequence, parts of the Indonesian coastline show evidence of changes on a scale much greater than those that took place in preceding centuries; indeed, throughout the period that the sea has stood at approximately its present level. And some sectors that have so far been little modified, particularly in eastern Sumatra, are beginning to receive the impacts of people arriving from densely populated areas under the Indonesian government transmigration schemes.
The most insidious form of Man's impact on the Indonesian coastal environment is pollution (Soegiarto 1975, 1976). In a broad sense this includes additional sedimentation due to soil erosion, and to mining and dredging activities. More specifically, chemicals derived from the fertilizers, pesticides, and herbicides that have been used increasingly in recent years to improve agricultural productivity, especially in rice fields, have seeped or flowed into rivers, and thence to estuaries and coastal waters, including brackish-water fishponds. The fertilizers can lead to excessive nutrient concentration, resulting in algal blooms that impoverish or destroy the habitats of fish and crustaceans; the toxic chemicals intended to kill weeds and pests can also destroy organisms that live in coastal waters. As 98 per cent of marine fish production in Indonesia is derived from traditional artisanal fisheries centred mainly in coastal and estuarine waters, this is a serious problem. It has been compounded recently by the discharge of toxic chemicals, including such heavy metals as cadmium and mercury as dissolved salts, into waters draining from industrial areas, particularly in the Jakarta Bay region. Petrochemical wastes and oil spills have also had additional adverse effects on marine ecosystems, fouling brackish-water fishponds and tainting fish caught in estuaries or nearshore waters subject to this pollution. These various forms of pollution constitute an unwelcome environmental change in the coastal environments especially near the more densely populated regions of Indonesia.
The foregoing account of factors and processes influencing the evolution of Indonesian coastal features, their present dynamics, and the impact of Man's activities sets the scene for a systematic review of our knowledge of the changing Indonesian coastline.