Old Sundaland

A continent hiding in shallow water

The Sunda Shelf is the submerged edge of continental Asia. It stretches through the modern Gulf of Thailand, along both sides of the Malay Peninsula, under the southern South China Sea and across the Java Sea. Most of it is astonishingly shallow. A fall of a few dozen metres exposes wide belts of seabed; the full glacial fall reveals a landmass larger than many countries.

At about 21,000 years before present, regional reconstructions put sea level near 116 metres below its modern height. Edlic Sathiamurthy and Harold Voris calculated that approximately 2.37 million square kilometres of shelf then lay above the sea. The present islands did not merely grow broader beaches. The Malay Peninsula ran through Sumatra into Java and eastward to Borneo across continuous low country. Singapore stood well inland. Bangka, Belitung and the Natuna Islands were uplands inside the plain.

The exposed land was not topographically grand. Its power came from width. Till Hanebuth and Karl Stattegger describe a shelf as much as 800 kilometres across, with an exceptionally low gradient and enormous buried valleys. On steep coasts, a vertical rise can move the shore only a short distance. Here, a few metres could send salt water far across river flats. Modern bathymetry preserves the shape of that old vulnerability: the Java Sea averages only tens of metres deep, and broad parts of the Gulf of Thailand are shallower than the deeper valleys cut across them.

Sundaland also stopped at a real eastern boundary. The deep channels of Wallacea remained water even at the lowstand. Borneo and Java joined Asia; Sulawesi did not. Bali joined Java; Lombok remained across a strait. The old continent enlarged the western half of island Southeast Asia without erasing the deep-water geography that separated it from Sahul.

The rivers beneath the seas

Valleys, sediments and ancient coastal deposits survive beneath the shelf. Along the North Sunda palaeovalley, terrestrial soils give way upward to intertidal plants, mangrove peat, estuarine mud and fully marine sediment: an advancing coast preserved layer by layer.

Four great systems organized the country. The Siam system extended the Chao Phraya drainage south through the Gulf of Thailand. The Malacca system occupied the long trench between the Malay Peninsula and Sumatra. The North Sunda, sometimes called the Molengraaff River, gathered water from Sumatra and western Borneo and crossed what is now the southern South China Sea. The East Sunda system drained northern Java and southern Borneo through the modern Java Sea. Holocene mud now buries many of their exact channels.

The North Sunda system gives the clearest section through the drowning. At the lowstand, rivers incised the exposed shelf and carried sediment toward a coast near the outer margin. As the sea rose, valleys filled with estuarine and mangrove deposits. After about 13,500 years ago, flooding removed upper parts of the great catchment from the river one by one. Sediment supply to the outer shelf fell sharply because the sea now stood between the old tributaries and their former mouth.

That reversal is the essence of Sundaland. A river network once joined freshwater worlds now divided among Thailand, Malaysia, Indonesia and Borneo. Flooding did more than cover the channels. It cut a single drainage into separate rivers that now end on opposing coasts. The Kapuas of Borneo, rivers of eastern Sumatra and streams of the Malay Peninsula became members of different islands and different seas because the shared country between them went under.

The advancing coast

The first millennia after 21,000 BP were slow. Sea level rose from about −116 metres at 21,000 BP to −114 metres at 19,000 BP. Over two thousand years, the vertical change was only two metres and the coast barely moved. Sundaland did not disappear in one post-Ice Age deluge.

After 19,000 BP, the water climbed more decisively. By 14,600 BP it stood around −96 metres. At first the coast moved through the lower ends of valleys and around enclosed depressions. Then connected marine water spread onto the broader platform. Shorelines became estuaries; estuaries widened into gulfs.

The rise was uneven. Long intervals of shoreline retreat were broken by Meltwater Pulse 1A, the loss of river catchments and the drowning of the last land bridges.

Sixteen metres in three centuries

Between about 14,600 and 14,300 years ago, Sunda Shelf cores record the great acceleration called Meltwater Pulse 1A. Sea level rose from about 96 metres below present to about 80 metres below present: sixteen vertical metres in approximately three centuries. The average is more than five metres per century. Across low-gradient country, the horizontal retreat could be immense.

This was catastrophic in the ordinary human meaning of relentless loss. A child could grow old in a landscape whose coast had materially changed. Estuaries pushed up valleys, groundwater became brackish and storms began from a nearer shore. Several generations could watch entire districts pass from dry ground through marsh and tidal flat into open water.

Meltwater Pulse 1A was not a tsunami. Collapsing ice sheets added water to the world’s oceans over centuries, not in one crest racing across Sundaland. Earthquakes, eruptions, storm surges and tsunamis struck as separate local hazards, especially along the Sunda Arc. Terrestrial soils give way upward to mangrove peat, estuarine mud and marine sediment rather than one universal impact bed.

After the pulse, the sea continued rising. It reached about −64 metres by roughly 13,200 BP and about −50 metres by 11,600 BP. The rates changed, the direction endured. The greatest transformation came from the conjunction of rapid vertical rise and a surface so flat that water could translate height into distance.

How the islands appeared

The late thresholds are visually quieter than Meltwater Pulse 1A and geographically decisive. Around 11,000 years ago, broad land connections still linked the Malay Peninsula with Sumatra, Java and Borneo. Between approximately 11,000 and 9,000 BP, those low corridors drowned. A continent became an archipelago.

The sequence was not one clean snap. Salt water first occupied the lowest valleys and depressions. Straits narrowed the remaining corridors. Peninsulas became long islands before smaller channels cut them apart. The Java Sea opened through drowned river country between Java and Borneo. The Strait of Malacca took possession of its palaeovalley. Water advanced through the Gulf of Thailand while the shoreline withdrew toward the Chao Phraya basin. The South China Sea crossed the North Sunda drainage until its old tributaries ended on separate shores.

By about 9,000 BP, the principal land bridges had gone. Sea level still lay below its modern mark, yet the governing outline was recognizably modern: Sumatra west of Malacca, Java south of the Java Sea, Borneo isolated to the east. Subsequent rise widened the straits, flooded deltas and completed the shallow seas. In Sathiamurthy and Voris's regional stages, the sea reached modern mean level around 6,070 BP and continued toward a mid-Holocene highstand. By 5,000 BP, regional relative sea level stood slightly above modern height, though individual coasts did not move in unison.

Island formation altered more than travel. Freshwater populations that had occupied one drainage were divided among rivers on different landmasses. Terrestrial animals lost routes across the plain. Ocean circulation entered spaces formerly occupied by soil and rivers. Yet water also made new coasts and short crossings. The geography of connection changed from roads across land to routes across straits.

Slow inundation and sudden disaster

Sundaland changed at three different rates. Shorelines retreated across sixteen thousand years. Meltwater pulses and breached sills transformed districts within centuries or less. Tsunamis, storm surges, river floods and eruptions struck in hours, days or seasons.

Melting ice supplied the 116 vertical metres of water that drowned Sundaland; low valleys and sills made some local changes abrupt.

Chronology

The archipelago is the remainder

Modern Southeast Asia begins with islands and treats the water between them as permanent. The longer view reverses figure and ground. The Gulf of Thailand, Java Sea and southern South China Sea are the flooded middle of a former landmass. Malacca is a drowned valley. The islands are surviving heights, and their opposing rivers are fragments of older basins.

Sundaland disappeared at different rates: a long near-standstill, a centuries-long pulse, the capture of river valleys, the narrowing of corridors and the final conversion of roads into straits. Rising seas drowned the plains of Sundaland and made islands of Borneo, Java and Sumatra.