Sediments capable of storing organic carbon for hundreds of thousands of years

Land plants use atmospheric carbon dioxide (CO2) and transform it into organic carbon for growth. Once the plants and the living things that consume them die, most of the organic carbon is returned to the atmosphere, but a small part is transported by rivers to the ocean, where it accumulates in marine sediments. Burial and storage of this continental-derived organic carbon in marine sediments can reduce Earth’s atmospheric carbon dioxide levels, and thus regulate its climate cycle, over geological-scale time periods, generally longer time intervals. hundred thousand years.

The largest accumulation site of organic carbon in today’s oceans is found in deltaic zones, some of which are composed of large accumulations of sediment. The analysis of modern deltaic sediments, however, only provides information of a short instant, in geological terms, of the temporal and spatial characteristics of these complex sedimentary environments, a fact that complicates the calculation of their efficiency to bury organic carbon in the long term. term.

In a new study, with the participation of the Autonomous University of Barcelona (UAB), it has been possible to determine the volume of organic carbon stored in a deltaic sedimentary succession accumulated in the Upper Cretaceous, 75 million years ago, in the Magellan Basin, In the south of Chile. To calculate carbon burial rates on a geological scale, the researchers have reconstructed the dimensions of the delta and quantified the organic content of the rock samples, combining this information with the dating of the rocks, carried out by them in previous work.

The results show that up to almost 100 megatonnes of organic carbon of continental origin were stored in these sediments over a period of approximately 100,000 to 900,000 years, at an annual burial rate of 2-16 tonnes per km²/year. These values ​​are of an order of magnitude similar to organic carbon burial rates in modern deltas, such as the Amazon River.

Satellite image of the mouth of the Amazon River, in Brazil. Of all the large river deltas on our planet, the Amazon River is the one that contributes the largest amount of organic carbon of continental origin to the sea. But until now the efficiency of deltaic sediments to store organic carbon in the long term was not known. (Public domain image, obtained from NASA’s World Wind software.)

“In this study we show that deltaic zones have been, are and probably will be great natural stores of continental organic carbon on our planet and, therefore, important climate regulators throughout geological time periods”, says Miquel Poyatos. “The governments and institutions of countries with deltas in their territory have the need to protect, maintain and restore them, especially in the current context of climate change, rising sea levels and loss of sediment due to reservoirs”, concludes the researcher from the UAB.

The study is titled “High rates of organic carbon burial in submarine deltas maintained on geological timescales.” And it has been published in the academic journal Nature Geoscience. (Source: UAB. CC BY-NC 4.0)