The amount of river-borne carbon that is buried upon reaching the sea affects Earth's atmospheric composition. A study of rivers draining the Himalaya shows that carbon burial may occur more efficiently than was thought.
It's a popular misconception that the concentration of oxygen in Earth's atmosphere is controlled by photosynthesis. Photosynthesis is certainly the source of atmospheric O2, but the amount it produces is in almost perfect balance with the amount consumed through the respiration of living organisms. It is only when organic matter is buried in ocean sediments, and so ceases to be decomposed, that atmospheric O2 can accumulate. This burial process also reduces the levels of the greenhouse gas carbon dioxide released into the atmosphere. The exact rate of organic-matter burial is therefore a significant determinant of atmospheric composition, and thus global climate, over geological timescales.
On page 407 of this issue, Galy et al.1 bring a new perspective on how organic carbon is stored in sediments. They show that the Ganges–Brahmaputra river system, which drains the Himalaya, interacts with the sediments deposited by the rivers' waters in the Bay of Bengal — the 'Bengal fan' — so as to store organic carbon more efficiently than studies on other river systems had suggested would be the case.
The fact that organic matter can be stored at all in ocean sediments is something of a surprise. Heterotrophic organisms (those that acquire their carbon from other organic matter, rather than synthesizing it themselves from inorganic sources) operate highly efficiently in the oceans, tightly recycling carbon and nutrients in the ocean's surface layers. Less than 1% of photosynthesized organic matter makes it down to the surface of ocean sediments2, and what carbon does hit the seafloor enters a voracious ecosystem that makes short work of almost all of it3. The little burial of organic matter that does take place occurs in river deltas and at continental margins. In addition, rivers deliver more than enough carbon to the oceans to account for the total amount of carbon stored in sediments4. These two facts have long raised suspicions that terrestrially derived organic matter is a significant source of the carbon preserved in marine sediments.
But data from rivers and coastal sediments have created a more complex picture. In particular, measurements made in the world's largest river system, the Amazon, have shown that even the strongest bonds between minerals and organic matter formed in soils can be broken during transport and deposition at the river–ocean margin. This causes terrestrial carbon in mineral surfaces to be replaced, at least in part, by marine carbon5. Indeed, the fan of sediment spreading out from the mouths of the Amazon has been called a “gigantic sedimentary incinerator”6 for its efficiency at destroying both marine and terrestrial organic matter, preventing their burial at sea. There had been hints that other types of river — in particular, mountainous rivers on active tectonic margins such as those draining the Himalaya — export and store carbon differently from the Amazon7,8. But these rivers are individually small, making it difficult to place their stories in a globally significant context.
In their studies of the Ganges–Brahmaputra system, Galy et al.1 use the relationship between organic-carbon content and the ratio of aluminium to silicon in river and Bengal Fan sediments as a marker for the amount of organic matter these particles contain. They find that this relationship remains the same from the rivers' youthful stages right through to sediment deposition in the ocean (Fig. 1). Coupling this information with earlier studies using sedimentary biomarkers that showed no evidence of the storage of marine organic carbon in Bengal-fan sediments9, Galy et al.1 conclude, reasonably, that the terrestrial organic matter is stored in the Bengal fan very efficiently — in contrast to the picture of destruction at the mouths of the Amazon.
The authors attribute this efficiency to the Bengal fan's rapid sedimentation rate, which restricts the sediments' exposure to oxygen, effectively cutting off microbial decomposition of organic matter. They further argue that the driver for the rapid sedimentation is the fast erosion resulting from tectonic uplift in the Himalaya, an argument that meshes with hypotheses previously published for much smaller rivers draining tectonically active margins7.
But is this all there is to it? Those of us who study the river transport of organic matter to the ocean make the unstated assumption that carbon loading does not vary with the soil properties of the particular river basin. We assume that rocks break apart into soil minerals, soil minerals load with organic matter, and organically coated minerals are exported to rivers by essentially the same mechanisms the world over, with soil type and degree of development influencing the final storage outcome only insignificantly. Yet soil scientists know that soil mineralogy has an important part in determining the mechanisms and durability of organo-mineral bonds: for example, the effectiveness of carbon storage in soil varies greatly with the degree of soil weathering10.
The Amazon and Ganges–Brahmaputra river systems are ideal for testing the effect of river-basin soils on the storage of organic carbon in delta sediments, because they drain environments that represent two extremes of soil development. The Amazon basin is dominated by Oxisols, the most highly weathered type of soil, whereas the Ganges–Brahmaputra basin is dominated by Inceptisols, one of the least weathered soil orders. Understanding the influence of river-basin soils on the storage of carbon in ocean sediments will require the effective integration of soil data into the field of oceanography. Carbon-coated minerals travel from mountains through ecosystems to rivers and ocean sediments: perhaps understanding these particles' history will help us predict their sedimentary fate.
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Geochimica et Cosmochimica Acta (2013)