O River Deltas, Where Art Thou? Coastal sinking stalls sediment accumulation

1:17pm, August 22, 2007
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Gradual subsidence of terrain along the western coast of Siberia since the end of the last ice age has thwarted the formation of river deltas there, a new study suggests.

When large rivers reach the sea, they slow and drop much of their sediment. As the material accumulates on the seabed, it diverts the river along other paths where it then deposits more sediment, and a classic river delta forms. Many of today's deltas began growing about 8,500 years ago, when global sea level stabilized after the last ice age ended, says Glenn A. Milne, a geophysicist at the University of Durham in England.

But large river deltas are conspicuously absent from the Arctic coast of western Siberia, says Milne. For instance, the Ob and Yenisei Rivers each carry as much water as the Mississippi does, but they meet the sea in shallow estuaries that are hundreds of kilometers long.

In contrast, the Lena River of eastern Siberia, also about the size of the Mississippi, sports a 32,000-square-kilometer delta—the largest in the Arctic, says Milne. Over the past few millennia, the eastern portion of the Lena's delta has grown seaward by as much as 200 km.

The Ob, Yenisei, and Lena each carry roughly the same amount of sediment, says Milne. Also, there are no climate differences between eastern and western Siberia that can account for the absence or presence of deltas, he adds. In the August Geology, he and his colleagues propose a new explanation for why the rivers of western Siberia lack large deltas.

During the last ice age, a broad, kilometers-thick ice sheet smothered northern Europe. The weight of that ice depressed the terrain, squeezing outward some of the underlying mantle. That in turn caused the landscape in icefree regions thousands of kilometers away to bulge upward, Milne explains. When the ice sheet melted, the ring of bulging terrain that had surrounded it—a ring that passed through the western coast of Siberia—began to subside.

The team's computer simulations suggest that eastern Siberia stopped subsiding about 3,000 years ago, allowing river deltas to accumulate there. In western Siberia, however, the terrain is still sinking about 1 millimeter each year, faster than the river sediment can accumulate to form a delta.

"This is a very elegant study," says Torbjörn Törnqvist, a geologist at Tulane University in New Orleans. "A lot of people don't really think about these issues very much."

The findings are a dramatic demonstration that subtle, ongoing changes in terrain resulting from the end of the ice age thousands of years ago can produce a dramatic effect today, says Jerry X. Mitrovica, a geophysicist at the University of Toronto. A similar effect may explain why many rivers in eastern North America haven't formed deltas, he adds.

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