Earth's weathering reactor: carbon source or sink over short and long time-scales?

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Chemical weathering is the process by which rocks dissolve in rainwater, which is naturally acidic. This is because atmospheric carbon dioxide dissolves in rain to form carbonic acid, and the rainwater interacts with rocks making them dissolve. The dissolved carbon dioxide becomes trapped in river and seawater, as bicarbonate (present in all natural waters such as mineral water for example), where it resides stably for thousands, or tens of thousands of years, and is then stored permanently in a mineral form as calcium carbonate (like limescale) and deposited as limestone in the oceans. Rock dissolution or chemical weathering is a major process in the global carbon cycle and it is thought that this terrestrial chemical weathering of rocks, and subsequent burial of carbon as calcium carbonate, acts as the feedback which has controlled the carbon cycle and thus climate over Earth history. Different rocks dissolve at different rates and the dissolution of silicate minerals results in a permanent drawdown of atmospheric carbon, whereas the dissolution of limestones, although much faster, only draws down carbon for 1000s of years. The reason this matters is that rivers transport a significant amount of carbon (about a quarter of the present increase in atmospheric carbon dioxide due to anthropogenic activities). However, recent research by scientists has called into question the above, simplified version of how rivers play an important role in the carbon cycle. Carbon locked up in rocks (such as shales rich in organic matter or limestones) can be released back to the atmosphere during chemical weathering, which represents the natural equivalent of fossil fuel burning. In the Amazon basin, ancient organic matter becomes oxidised during sedimentary transport, releasing carbon dioxide to the atmosphere. In the Yangtze (China) and Mackenzie Basins (North America), small amounts of sulphuric acid (released by the oxidation of sulphur-bearing minerals such as pyrite, or 'fools gold') dissolves limestone, releasing carbon dioxide from ancient rocks to the atmosphere. So are rivers a net sink for carbon dioxide from the atmosphere or a net source? In the context of environmental change there is a clear need to better understand carbon fluxes associated with weathering. We have now developed methods to quantify all these processes, but this must be done at a global scale. The best way to do this is to work on the largest rivers in the world, as these represent some of the largest fluxes of carbon and the fact that we don't know if these fluxes are TO or FROM the atmosphere represents a serious deficit in our knowledge of the operation of the carbon cycle at Earth's surface. We have selected three of the largest rivers in the world as case studies for carbon transport, the Irrawaddy, Salween and Mekong from SE Asia. Combined, these rivers transport about 14% of the global total riverine flux of carbon, or about half the UK's carbon emissions, but there is so little work on these basins that their impact is largely unknown. Does the transfer of carbon end up releasing carbon dioxide, or do these river basins act as a sink for carbon? We propose to constrain the modern carbon budgets in these basins by using a series of isotopes, that will tell us if ancient carbon is being released from rocks, or whether modern carbon derived from the atmosphere or biosphere is being consumed. We will conduct our sampling of the rivers over a 2-year period, but a key question here is how representative is a two-year period of the longer term. We will unlock the archive of river sediments to determine carbon fluxes averaged over longer, millennial time-scales to comprehensively understand carbon transfer in these basins.