Research

Aerial image of the Rio Bermejo (northern Argentina) showing the primary influxes and effluxes of organic carbon in the river system. photo: Dirk Sachse

Are rivers net sources or sinks of atmospheric co2?

Lowland alluvial rivers regulate the flux of particulate organic carbon (POC) between uplifted mountain ranges, productive lowland forests, and ocean basins, thus playing a key role in the global carbon cycle.

During fluvial transit, POC may be oxidized and emitted to the atmosphere as CO2, or it may escape oxidation and get buried in sedimentary deposits over geologic timescales.

Large uncertainties are associated with these oxidation and burial fluxes because source-to-sink transit times and the mechanisms regulating fluvial POC reactivity are largely unknown.

HYPOTHESIS
Two key processes regulate OC cycling in rivers:

Sediment transit velocity

Transit velocity is the rate at which sediment and particulate organic carbon (POC) are transported downstream. The slower the transit velocity, the more time POC spends in floodplain storage.

2. Mineral protection

POC decomposition rates may be significantly reduced if organic compounds are bound to short-range-order (SRO) mineral phases and metal oxyhydroxides.

Image above: Wagai et al., 2015Photo left: Kristen Cook

A natural flume experiment

We tested this hypothesis in a river system that acts as a natural flume. The Rio Bermejo in northern Argentina flows nearly 1300 km from the Andean mountain front to the Rio Paraguay with no tributaries, allowing us to isolate the effects of river transit on organic carbon composition. We sampled suspended sediment depth profiles at 6 stations from the mountain front to the river outlet and analyzed its geochemical composition.

The mean sediment transit time through the Rio Bermejo is ~8500 yr, with an average transit velocity of 0.14 km/yr.

Repasch et al., 2020, JGR Earth Surface

Evidence for organic matter protection by secondary minerals

Radiocabon content of bulk POC correlates negatively with the abundance of secondary oxyhydroxide mineral phases. POC can be preserved for millennia in those grain coatings.

Repasch et al., 2021, Nature Geoscience

Two POC pools

1) Slow-cycling, mineral-bound
2) Fast-cycling, labile

nanoSIMS images showing where carbon (yellow) is located in river sediment. These data suggest two phases: mineral-associated (left) and labile, discrete organic matter particles (right).

Estimating POC turnover during fluvial transit

Blair and Aller, 2012

Using the time-averaged transit velocity of sediment and POC, and what we know about the decomposition rates of organic carbon, we developed a new framework to determine the POC oxidation flux during source-to-sink transit.

Because it is composed of a mixture of labile and refractory organic matter, fluvial POC exhibits a range of values for k. We can use radiocarbon data to determine values of k for different POC pools.

In a given year, the Rio Bermejo exports more POC to the Rio Paraguay than is oxidized during transit, making the Rio Bermejo a net organic carbon exporter.

Are most rivers net carbon sinks?

Despite long floodplain sediment residence times up to 20 kyr and frequent channel-floodplain exchange, the erosional flux of POC from lowland floodplain vegetation into the river outpaces the slow rate of POC oxidation during transient storage.

Different hydroclimatic and geomorphic conditions would be needed to change the Rio Bermejo from a net OC sink to a net CO2 source to the atmosphere.

We can address this question by applying the framework presented here to rivers globally.

A caiman basking on a raft of woody debris in the Rio Bermejo. Large woody debris like this contributes to the river's massive carbon export. photo: Marisa Repasch

References

Blair, N., & Aller, R. (2012). The fate of terrestrial organic carbon in the marine environment. Annual Review of Marine Science, 4(1), 401–423. https://doi.org/10.1146/annurev-marine-120709-142717

Newbold, J. D., Mulholland, P. J., Elwood, J. W., & O’Neill, R. V. (1982). Organic carbon spiralling in stream ecosystems. Oikos, 38, 266–272.

Repasch, M., Wittmann, H., Scheingross, J.S., et al. Sediment transit time and floodplain storage dynamics in alluvial rivers revealed by meteoric 10Be (2020). Journal of Geophysical Research: Earth Surface. https://doi.org/10.1029/2019JF005419

Repasch, M., Scheingross, J.S., Hovius, N. et al. Fluvial organic carbon cycling regulated by sediment transit time and mineral protection. Nat. Geosci. 14, 842–848 (2021). https://doi.org/10.1038/s41561-021-00845-7

Repasch, M., Wittmann, H., Scheingross, J. S., Sachse, D., Szupiany, R., Orfeo, O., et al. (2020). Sediment transit time and floodplain storage dynamics in alluvial rivers revealed bymeteoric10Be. Journal of Geophysical Research: Earth Surface,125,e2019JF005419. https://doi.org/10.1029/2019JF005419

Wagai, R., Kajiura, M., Asano, M., & Hiradate, S. (2015). Nature of soil organo-mineral assemblage examined by sequential density fractionation with and without sonication: Is allophanic soil different? Geoderma, 241–242, 295–305. https://doi.org/10.1016/j.geoderma.2014.11.028

Research conducted during my PhD was supported by: