Sediments taken from the ocean floor more than a thousand miles from land include pieces of wood. Remarkably, some of these come from far inland, having already made epic journeys even before they reached the coast. The discovery suggests keeping rivers free-flowing may have unrecognized benefits for the climate.
Dr Sarah Feakins of the University of Southern California collected sediment cores from the Bengal Fan, which stretches 2,000 kilometers (1,250 miles) from the mouth of Ganges River, making it the world's largest active sediment deposit. In cores that reach 800 meters (0.5 miles) into the sea bed, she frequently found pieces of wood, some 19 million years old.
Much of the wood comes from coastal species, but Feakins also made more surprising discoveries. "We found pristine pieces of conifers," Feakins said in a statement. "These trees grow 2 miles above sea level, up in the Himalayas."
Although different climatic conditions may have allowed these trees to grow at a lower altitude, Feakins believes they were always mountain-dwellers in this region.
In Proceedings of the National Academy of Sciences, Feakins and co-authors argue the trees from which this wood came must have been swept downriver by enormous floods following catastrophic events.
The most likely explanation, the authors believe, is that glaciers or landslides created natural dams, and when these burst they released walls of water that brought down everything in their paths. Even large trees were swept all the way to the Indian Ocean, and then thousands of kilometers out to sea. The abundance of wood laid down 50,000 years ago suggests a particularly large event at that time.
Just such a dam was created a century ago by an earthquake on the other side of the Himalayas, and there are fears if another earthquake should breach it hundreds of thousands of people living in the valley below could be killed.
We already know animals sometimes cross oceans to flourish in new lands by rafting on logs or branches washed down rivers. The arrival of monkeys in the Americas being the most notable example. However, the possibility of origins so far inland had not been well considered.
Most importantly, Feakins' discovery could alter thinking about the way carbon is sequestered. When trees die they usually burn or are broken down by bacteria and fungi. Although much of the carbon is stored in the soil for a period, eventually it is released into the atmosphere to warm the planet.
We know rivers dump organic material on the seafloor where it is buried before it can decompose and release its carbon. Feakins' work shows the Ganges-Brahmaputra system, and perhaps other big rivers as well, sequester much more carbon than allowed for by estimates that ignored wood's contribution.
Damming major rivers could interfere with this carbon burial, Feakins notes, and undermine a natural check on global heating.