23 October 2019
The role of landslide dams in depositing carbon in marine sediments
As concern grows about the threats of global heating, there is increased interest in understanding how, when and how much carbon is stored in the oceans. One mechanism is the transport of wood from uprooted trees through the river system and out to sea. This may be a key pathway through which carbon is stored, and thus understanding the mechanisms through which this can occur is important.
In a paper just published in PNAS, (Lee et al. 2019), a team has looked at cores drilled into the Bengal Fan, the largest sedimentary deposit on Earth, which stretches 2,000 km from the mouth of the Ganges River. They have been particularly interested in the presence of wood in these marine deposits. The results are intriguing – the team found very distinct woody layers in the sediments, even in those drilled in 3,700 m water depth, at distances of hundreds of kilometres from land. Perhaps most interestingly, they found one layer, dated to about 50,000 years ago, that contained abundant pristine pieces of conifer, which typically grow at about 3 km above sea level. The implication is that large volumes of wood are able to travel in concentrated pulses from high in the mountains out to the deep ocean. This raises a question as to how this can occur.
Lee et al. (2019) propose that the most likely mechanism is very high discharge events – i.e. catastrophic floods that carry large volumes of woody material in a single pulse. The authors propose that the most likely cause of such floods would be the collapse of either landslide or glacial dams. An article on Eurkalert, informed by one of the researchers, Dr Sarah Feakins (associate professor of Earth sciences at USC Dornsife College of Letters, Arts and Sciences) described how this might happen:
The trees likely were uprooted during the last ice age by a massive release of water from the breach of a natural dam created by a glacier, landslide or similar land feature. In what must have been a surge of water, the trees rode rivers thousands of miles from Nepal through Bangladesh and into the Bengal Fan.
We know that these large, catastrophic events are critically important in creating the landscapes in active mountain chains. The discovery that these extreme events are also important in the ways in which the carbon cycle operates is now becoming clear. Landslides generated by a large earthquake release vast numbers of trees from the hillsides. Very often this wood is then trapped behind landslides dams. The breaching of these dams provides a mechanism through which that wood can transported out to the deep ocean, and buried on the sea floor.