Reading the tale of an ancient river

Ocean-floor sediment near England holds material deposited there during the last ice age by what was then Europe’s largest river system. A new analysis suggests that the waterway began carrying substantial amounts of precipitation and meltwater from European ice sheets at least 20,000 years ago, a few millennia earlier than scientists had previously suspected.

At the height of the last ice age, thick layers of ice covered much of northern Europe. What today is the English Channel was then a broad river valley, says Guillemette Ménot, a paleo-oceanographer at Université Aix-Marseille III in Aix-en-Provence, France. Much of the water draining from European rivers would have flowed into that valley’s waterway, which scientists have dubbed the Channel River.

Ménot and her colleagues analyzed sediment drilled from the seafloor about 250 kilometers southwest of the English Channel, a spot that would have been just offshore from the Channel River delta. They found that slightly more than 20,000 years ago, the sediment began to include organic substances typically found in the cell walls of soil bacteria, a sign that the material came from land via the Channel River.

That initial surge of land-derived organic material in the sediment peaked about 19,500 years ago, the researchers report in the Sept. 15 Science. After a short hiatus, the river again brought large amounts of organic material to the region between 18,500 and 17,000 years ago. These two periods of increased river runoff probably represent the initial stages of melting of the European ice sheets, says Ménot. The sediment record at the seafloor site doesn’t show the later stages of ice sheet melting, probably because the Channel River delta moved northeast as sea level rose.

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