The organic carbon that runs into the oceans from rivers could be hundreds or thousands of years old. If so, aspects of our understanding of the global carbon cycle will have to change.
Dissolved organic carbon in the oceans is one of the biggest reservoirs in the global carbon cycle — it is comparable in size to all of the carbon in terrestrial plants, or to all of that in the form of CO2 in the atmosphere. The input of terrestrial organic carbon from rivers, the main source of most constituents of sea water1, could fill this marine reservoir in just a few thousands of years2 — which, according to radiocarbon dating3, is also about the average age of marine organic carbon. But although there should be lots of terrestrial-derived carbon in the ocean, geochemical studies indicate that there seems to be very little. So what happens to the riverine carbon once it enters the oceans?
On page 497 of this issue4 Raymond and Bauer address the question by presenting new data on the average ages of organic matter in rivers. There may in fact be much more terrestrial carbon in the oceans than we thought, but we cannot see it. To understand this, one has to know how geochemists usually distinguish between organic carbon from terrestrial and marine sources.
Almost all of the organic carbon on Earth is created through photosynthesis, whether on land or in water. But on land the process leaves characteristic fingerprints, which mean that this carbon should be traceable after it has entered the oceans. Many land plants synthesize certain compounds, such as lignin or tannin, which are absent in marine phytoplankton. In principle, then, detecting these biomarkers in the sea can reveal if carbon had a terrestrial origin. The other widely applied method involves measuring the ratio between the two stable carbon isotopes, 13C and 12C, in the bulk organic matter. Most land plants produce carbon that is more depleted in the heavy carbon isotope (13C) than carbon produced by marine phytoplankton, leading to higher isotopic ratios in marine than in terrestrial carbon.
When going into details, however, things can be more complicated. Biomarker concentrations in different plant materials are variable, reducing their potential for the accurate assessment of sources. And stable carbon-isotope ratios can sometimes give ambiguous results because there are exceptions to the general trend5. A broader problem arises when comparing relatively old organic material in the sea with freshly synthesized material of terrestrial and marine plants, because chemical and biological degradation may have altered the composition of the fresher material and make it difficult to trace its origin.
This is where Raymond and Bauer's study4 comes in. They analysed organic matter in the Amazon (Fig. 1) and three North American rivers by radiocarbon dating, and found that it was up to several thousand years old. This contrasts starkly with the general belief that most of the organic carbon in rivers should be relatively 'fresh'6. The particulate organic carbon (that is, the fraction retained on a filter) was especially old, the dissolved carbon being younger on average. But even the dissolved fraction revealed relatively old ages in some samples, and long-term laboratory experiments with samples from one of the rivers showed that the slow bacterial oxidation of part of this organic matter can markedly increase the average age of the carbon that remained in solution.
From these results Raymond and Bauer conclude that ageing and degradation of terrestrial organic matter in river basins and coastal zones may significantly alter its structure, distribution and quantity before it reaches the open oceans. The implication is that we simply have not been able to distinguish it from marine-generated carbon. Future studies need to test whether the old radiocarbon ages found by the authors can be confirmed in rivers from other parts of the world, however, and whether ageing can wipe out the fingerprints of terrestrial carbon. Furthermore, old average ages of riverine carbon can also be explained by the presence of small amounts of 'fossil' organic carbon7 that originated in ancient sedimentary rocks. This phenomenon may be restricted to rivers draining these kind of rocks, and that has to be tested as well.
But it remains possible that dissolved organic carbon in the sea is mainly composed of old marine carbon, and that the terrestrial component is not hidden but largely absent. What if that is the case? A process would then be needed that rapidly removes the terrestrial carbon after it enters the oceans. It is generally accepted that part of this carbon is buried in the coastal zones where sedimentation rates are high. Biological activity is also high in these waters, and some carbon may be released as CO2 to the atmosphere through biological oxidation. But the two processes do not seem to be powerful enough to remove all riverine carbon5, and an additional mechanism for its rapid withdrawal from the sea would remain on the most-wanted list in the offices of many carbon-cycle researchers.
Nevertheless, at the least the results of Raymond and Bauer4 remind us that the organic matter that runs from rivers into the sea is not necessarily identical to the organic matter of the plants and soils upstream in the river catchments. Certainly, other processes taking place on the floodplains or in the river channels, such as slowed transport caused by cycles of deposition and movement downstream, may also have to be considered to better understand the composition and fluxes of river carbon8.