Why is it that not all carbon is preserved in the same way in coastal sediments?

As part of the CarboNium and CABESTAN, projects, researchers studied a coastal system comprising an intertidal mudflat and an adjacent salt marsh in Aiguillon Bay. These two habitats, subject to the same tidal, swell and sediment supply conditions, provide an ideal natural laboratory for understanding how the nature of organic matter influences its preservation in sediments. To this end, the team combined several complementary approaches (isotopic, elemental and molecular analyses) to trace the sources of the organic matter and its behaviour with depth.

The results show that the composition of organic matter directly determines its fate. In mudflats, organic matter is mainly of marine and microbial origin. Being fresher and more readily biodegradable, it decomposes about three times faster than in salt meadows. Conversely, salt meadows retain a higher proportion of organic matter derived from plant tissues, particularly woody compounds that are more resistant to degradation.

This difference is all the more striking given that the two habitats have comparable rates of sediment accumulation. In other words, it is not only the quantity of buried sediment that explains carbon sequestration, but also the quality of the organic matter incorporated into it.