Small and mighty fjords are carbon sink hotspots

Fjords are major global sites of carbon sequestration, according to a study published online in Nature Geoscience. Although previous studies have revealed high rates of carbon burial in the sediments of individual fjords, it had been assumed that they make a small contribution to carbon burial globally because they cover a tiny proportion of the world's oceans.

Fjords are steep-sided marine inlets left behind after glaciers retreat. They are found along coasts world-wide at high latitudes, and tend to receive large amounts of sediments from the rapid erosion of coastal mountains. Since fjords also tend to be deeper than the surrounding ocean, they effectively trap sediments and terrestrial carbon transported from the land surface, and the rapid accumulation of sediments prevents the carbon from being mineralized.

Richard Smith and colleagues compiled estimates of river sediment delivery rates and measurements of carbon in sediments from nearly every fjord system in the world. Their analysis suggests that although fjords cover just 0.1% of the surface area of oceans globally, they represent 11% of the carbon buried in marine sediments each year.

In an accompanying News & Views, Richard Keil writes that the carbon accumulation rates in fjord sediments “are at least five times faster than other marine systems, making fjords the oceanic ‘hot spot’ for organic carbon burial.”