The Lost East China Plain

The shore at the edge of the shelf

Eastern China faces one of the widest continental shelves on Earth. From the modern coast to the Okinawa Trough, the seafloor descends so gently that large changes in horizontal geography follow from ordinary glacial changes in vertical sea level. The Bohai Sea averages only about eighteen metres deep. The Yellow Sea averages roughly fifty-five. The shelf beneath the East China Sea continues hundreds of kilometres farther before it falls toward deep Pacific water.

At the glacial lowstand, regional relative sea level lay about 120 metres below present, with an uncertainty of roughly ten metres; global mean sea level fell lower, near 134 metres at 21,000 BP. At either scale, the modern Bohai, Yellow and East China shelf seas were absent. The coastline ran close to the outer shelf, far southeast of the present mouths of the Yellow and Yangtze rivers.

Sand bodies between modern depths of about 155 and 122 metres mark palaeocoastal zones along that margin. On the exposed shelf behind them, rivers cut channels, winds worked the dry surface and soils formed over earlier marine deposits. The Shandong and Korean peninsulas were no longer separated by the Yellow Sea: a broad lowland and river system occupied the space between them, while the deeper outer margin defined the remaining sea.

The contrast across Korea was stark. West and south of the peninsula, shallow seabed became a vast plain. East of Korea, the shelf is narrow and the deep Japan Sea, also called the East Sea, remained water. It was not exposed land and it was not covered by a continental ice sheet.

Two rivers cross the lost plain

The plain can be followed underground. Cores and high-resolution seismic profiles reveal poorly sorted river sands, buried channel forms, peat, tidal sediment and marine mud. Their sequence records rivers moving across exposed land and then retreating as salt water returned.

The Yellow River crossed the North China Plain into the region of the modern Bohai. In the western Bohai area, Liangyong Zhou and colleagues traced fluvial sediments from the last glacial maximum into the early Holocene. Their mineral composition points mainly to the palaeo-Yellow River. The river did not occupy one deeply cut gorge: its deposits spread over a surface with an extremely gentle northeastward slope, closer to a vast aggrading plain. Farther south, channels also carried Yellow River sediment toward the Yellow Sea trough during glacial stages.

The Yangtze crossed the East China Sea shelf. Its mouth migrated with the coast, and its lowstand delta lay toward the shelf edge. Buried valleys and deltaic bodies preserve the route even where later tides planed the surface and marine mud covered it. At 10,000 BP, with sea level about forty-five metres below present, the estuary lay roughly 400 kilometres southeast of its modern position.

These rivers did not simply lengthen along straight modern bearings. They avulsed, joined tributaries and responded to shifting gradients, leaving branching routes now buried beneath transgressive sand and mud. Two of Asia's great rivers once ran across land where ships now cross open shelf seas.

The sea returns in stages

Guangxue Li and colleagues divide the shelf's last transformation into three broad sedimentary worlds. From about 23,000 to 15,400 BP, lowstand rivers and coastal deposits occupied the exposed shelf. From about 15,400 to 7,000 BP, transgression moved landward across it. After about 7,000 BP, highstand currents, tidal flats, deltas and mud belts built the recognizable modern system.

The middle stage did the geographic work. Ocean water first advanced across the outer East China Sea shelf. It reworked old river and coastal deposits into broad sand sheets and tidal ridges. As water deepened, wave and tidal energy migrated landward. An environment could change from floodplain to marsh, from marsh to estuary, and from estuary to inner shelf while the coast continued past it.

Because the shelf is uneven, the advance did not draw one tidy line from south to north. Deeper channels filled first. Ridges remained as islands or shoals. The East China Sea expanded while the central Yellow Sea still held long valleys. The northern Yellow Sea then connected to marine water before the shallow Bohai basin beyond its strait.

By 10,000 BP the East China Sea had crossed much of its shelf, but the Yangtze's estuary still lay hundreds of kilometres from today's mouth. In the north, river and swamp environments persisted later. By 7,000 BP the transgressive system yielded to the highstand world: the modern current pattern took shape, and sediment began accumulating in the deltas, coastal mud belt and tidal flats that now blur the old topography.

The making of the Bohai Sea

The Bohai basin lay behind a narrow gate. Today the Bohai Strait runs between the Shandong and Liaodong peninsulas and opens into the northern Yellow Sea. At the lowstand, the basin behind it was not a remnant ocean. Cores from the central Bohai preserve river-mouth or swamp environments before marine conditions arrived.

Seawater reached the northern Yellow Sea at about 11,600 BP and the Bohai region around 10,500 BP. Offshore sites turned marine before inland ones. Basal marine and brackish layers become younger westward from the strait toward the Yellow River delta, the signature of a sea advancing into a shallow basin.

This is a breach in the geographic sense, not proof of a single destructive torrent. Once relative sea level rose high enough for tides to pass the Bohai Strait, marine water occupied low channels and expanded into the basin. The first connection may have changed currents and salinity quickly at a particular site, while the western shoreline continued moving for millennia. By roughly 9,700 BP, basal peat on the western Bohai coast places relative sea level near seventeen metres below present; later transgression carried the coast farther inland.

At the mid-Holocene maximum, parts of the western Bohai shoreline lay tens of kilometres west of the modern coast. That fact seems backward until sediment enters the story. Since then, the Yellow River and other rivers have delivered enormous volumes of silt, building deltas seaward. The modern coastline is not the farthest reach of the Holocene sea. It is also land made afterward.

Pulses and pauses

The global curve rose fastest around Meltwater Pulse 1A, approximately 14,500 to 14,000 BP. East China shelf deposits independently record rapid rise in the following interval. A six-core study by Jiang Dong and colleagues estimates an average rate of about 30.5 millimetres a year from 14,000 to 12,850 BP, followed by a pronounced slowdown during the Younger Dryas cold interval.

Around 11,620 BP, one inner-shelf site changed sharply from tidal flat to nearshore subtidal water. Six East China Sea cores, however, record a smooth regional rise after the Younger Dryas rather than a shelf-wide Meltwater Pulse 1B. The local coast crossed a threshold quickly even as the wider sea continued advancing without a comparable jump.

Marine algae and foraminifera record another pronounced local rise around 8,200 BP, followed by stable inner-shelf conditions around 7,540 BP. The East Asian transgression had a long direction but an uneven tempo.

A coastline made of moving sediment

Sea level alone cannot reproduce every ancient eastern China shoreline. The Yellow and Yangtze rivers carry sediment onto one of the world's great shelf systems. Tides redistribute it. Deltas compact under their own weight. Tectonic motion and the slow response of Earth's crust to vanished ice alter local relative sea level. The surface being flooded moves while the water rises.

The difference is measurable. On the central west coast of Korea, basal peat about 10,300 years old records relative sea level near fifteen metres below modern. At Gunsan Bay, only about 140 kilometres south, a roughly contemporary indicator lies near twenty-eight metres below. Dong-Yoon Yang and colleagues attribute much of the contrast to tectonic structure, glacial isostatic adjustment and the preservation or erosion of channel deposits. A single global curve cannot be exact at both sites.

By 5,000 BP, sea level approached its Holocene highstand while deltas and tidal flats were actively building. No single shoreline recovers the exact edge of every ancient delta. Three shelf seas had replaced the plain, while the Yellow and Yangtze had begun making new land inside them.

Fast flooding and sudden disasters

Flooding occurred at radically different speeds. Meltwater pulses unfolded over centuries. Seawater could turn a low channel tidal much faster once it crossed a sill. Typhoons, river floods, storm surges, earthquakes and tsunamis struck in hours or days. Each left a different geological signature.

Eastern China's enormous rivers add another distinction. A Yellow River flood can destroy settlements and shift a channel, and a channel shift can move a delta from one sea to another. Yet it does not create the water volume of the Yellow Sea. A tsunami can run far across a low coast and leave an anomalous sand layer, then withdraw. It does not maintain marine mud deposition for the next ten thousand years. Persistent seas require persistent relative sea level above the drowned surface.

The ocean rose because land ice melted. It advanced in faster and slower phases and crossed local thresholds that made the spatial response abrupt. Sudden hazards occurred within that transformation and could have made particular days disastrous, but no single disaster created the three new seas.

Chronology

The plain beneath three seas

Bohai, Yellow and East China are three names over one drowned shelf. Their depths differ, their currents differ and their modern coasts were rebuilt by different rivers. Yet at the glacial lowstand they belonged to a continuous lowland extending far beyond the present shore.

The sea did not erase the plain cleanly. Buried valleys still guide groundwater and sediment. Old river sand lies beneath tidal ridges and marine mud. The Yellow and Yangtze continue to build deltas across the edge of their former country, making coast even as the drowned shelf records how far they once ran.

Rising seas drowned the plain beneath the Bohai, Yellow and East China seas, shortening great rivers and remaking the coast of China and Korea.