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

化学风化是岩石溶解在自然酸性雨水中的过程。这是因为大气中的二氧化碳溶解在雨水中形成碳酸，雨水与岩石相互作用使它们溶解。溶解的二氧化碳被捕获在河流和海水中，作为碳酸氢盐（存在于所有天然沃茨中，例如矿泉水），在那里稳定存在数千年或数万年，然后以碳酸钙（如石灰石）的矿物形式永久储存，并以石灰石的形式沉积在海洋中。岩石溶解或化学风化是全球碳循环中的一个主要过程，人们认为这种岩石的陆地化学风化以及随后的碳酸钙形式的碳埋藏，作为控制碳循环的反馈，从而控制了地球历史上的气候。不同岩石的溶解速率不同，硅酸盐矿物的溶解导致大气中碳的永久性下降，而石灰石的溶解虽然快得多，但只能在1000年内减少碳。这一问题之所以重要，是因为河流输送了大量的碳（约占目前人类活动造成的大气二氧化碳增加量的四分之一）。然而，科学家们最近的研究对上述简化版本的河流如何在碳循环中发挥重要作用提出了质疑。锁定在岩石（如富含有机物的页岩或石灰石）中的碳可以在化学风化过程中释放回大气，这代表了化石燃料燃烧的自然等价物。在亚马逊盆地，古老的有机物在沉积物运输过程中被氧化，向大气中释放二氧化碳。在扬子江（中国）和麦肯齐盆地（北美），少量的硫酸（由黄铁矿等含硫矿物氧化释放）溶解石灰石，将二氧化碳从古老的岩石中释放到大气中。那么，河流是大气中二氧化碳的净汇还是净源呢？在环境变化的背景下，显然需要更好地了解与风化有关的碳通量。我们现在已经开发了量化所有这些过程的方法，但这必须在全球范围内进行。最好的方法是在世界上最大的河流上工作，因为这些河流代表了一些最大的碳通量，而我们不知道这些通量是来自大气还是来自大气的事实表明我们对地球表面碳循环运作的知识严重不足。我们选择了世界上最大的三条河流作为碳运输的案例研究，即东南亚的伊洛瓦底江、萨尔温江和湄公河。这些河流总共输送了全球河流总碳通量的14%，约占英国碳排放量的一半，但对这些流域的研究很少，其影响在很大程度上是未知的。碳的转移最终会释放二氧化碳，还是这些河流流域充当了碳的汇？我们建议通过使用一系列同位素来限制这些盆地中的现代碳预算，这将告诉我们古代碳是否从岩石中释放出来，或者来自大气或生物圈的现代碳是否正在被消耗。我们将在2年的时间内对河流进行采样，但这里的一个关键问题是，在较长的时间内，两年的时间有多大代表性。我们将解锁河流沉积物档案，以确定在更长的千年时间尺度上平均的碳通量，以全面了解这些流域的碳转移。

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.