Clay minerals suspended in seawater binds sedimentary organic carbon to their mineral surfaces. But the quantity of carbon that is bound and the source of that carbon very much depends on the clay mineral in question.
A research team from ETH Zurich and Tongji University have shown this by studying sediments in the South China Sea.
Rivers discharge a constant supply of sediment into the world’s oceans. This sediment is largely composed of various clay minerals – the products of rock weathering – and organic compounds of plant origin that have decomposed in soils. These two components end up in rivers as a result of erosion.
On its way to the oceans, organic matter in sediments binds with clay minerals to form clay-humus complexes. Once they reach the sea, these complexes sink to the seabed, where they are buried by other sediments. This captures the carbon contained in these complexes, removing it over geological timescales from the atmosphere and from the pools of carbon standing in rapid exchange with Earth’s surface.
This is why clay minerals, also known as phyllosilicates, are extremely important for the global carbon cycle: some 90 percent of the organic carbon sequestered in the seabed around the continents is related to reactions between organic material and various minerals. A variety of phyllosilicates are responsible for a particularly large share because their small size and their geometry mean they have a particularly high specific surface area and can bind large quantities of carbon.
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