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

Lead Research Organisation: University of Cambridge
Department Name: Earth Sciences

Abstract

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.

Planned Impact

Unravelling the complexity of the carbon cycle and how climatic feedbacks operate at different time-scales is of global socio-economic importance. The Irrawaddy, Salween and Mekong are among the world's most significant rivers and are of considerable economic and ecologic importance. 1.5% of the world's population depend on these rivers for subsistence and they are home to diverse ecosystems, including the critically endangered Irrawaddy dolphin and the giant Mekong catfish. There are direct river management and water quality implications that will arise from our research, and all three river systems are on the brink of major change. Until now, damming has been relatively minor and largely limited to tributaries, however large and controversial dam projects are proposed for all three rivers over the next decade by the Sinohydro Corporation, the China Power Investment Corporation and the Electricity Generating Authority of Thailand (EGAT). Water and sediment flux data for both annual and seasonal loads are required for these large engineering projects. Drinking water quality, and particularly arsenic contamination, is a major issue in many SE Asian countries, and our water and isotopic datasets can directly inform on both the arsenic concentration and its source from weathering reactions.

For the above environmental reasons, the outcomes of our research (system knowledge, weathering rates, terrestrial carbon storage, water chemistry and sediment concentrations) will have direct impact as these provide the detailed data and framework necessary to quantify baseline water quality, sediment loads and storage, and carbon cycling behaviour.

Who will benefit and how?

Important beneficiaries of the research outlined below are engineers, river managers, environmental policy makers, schools and the public, and the developing nations' skills base.

The main academic impact of the proposed research will be on biogeochemists, geologists, hydrologists, engineers and palaeo-climatologists. They all require the type of detailed and comprehensive datasets that we will build as part of our research programme for reasons outlined in the Academic Beneficiaries section.

The IAEA is the process of establishing a Global Network on Isotopes in Rivers (GNIR) to monitor continental environmental change and PI Tipper and Co-I Hilton have acted as consultants on this. The isotopic data that we will generate will directly feed into the GNIR database.

River engineers and managers will use the quantitative water and sediment data arising from our research.

Environmental policy makers will use the water and sediment geochemical fluxes to inform management of the carbon cycle, rivers and floodplains.

SE Asian academic colleagues will acquire core research skills from the training built into our research programme.

The developing nations will use the data on water quality in order to better inform water resource decisions in the future.

Schools and the public will be informed about natural biogeochemical cycles and Earth surface processes and how they influence water quality.

We will exploit two outreach strategies. At St Andrews University we will engage the GeoBus programme which involves academics, students, and early career researchers in direct engagement with school pupils and teachers, and the public (over 35,000 pupils in 500 schools since January 2012, and much media coverage). It also facilitates engagement with the public in science centres (Our Dynamic Earth and Sensation) and at festivals (British Science Festival, September 2012). NERC originally funded GeoBus (£50k) and total funding to 2016 is now >£350k with industry sponsors. At Cambridge, Science Week, where 45,000 visits were made by members of the public including large numbers of school children in 2015, will provide an important opportunity to communicate this research.
 
Description Key findings are beginning to emerge, but it is too soon to determine what these are.
Exploitation Route Too early to say
Sectors Agriculture, Food and Drink,Education,Environment