The role of physical erosion in the weathering of fossil organic carbon: An investigation using the trace element rhenium

岩石中以化石形式储存的碳所含的碳大约是目前大气中以温室气体形式存在的二氧化碳的2万倍。在整个地球历史中，有机物一直被埋在地下，抵消了火山脱气带来的碳输入，并在缓和全球气候方面发挥了作用。然而，当岩石暴露在表面的化学风化和古代碳输入到现代环境中时，这些化石有机碳可以被氧化。众所周知，人类活动通过燃烧化石燃料正在迅速加快这一进程的速度。然而，这种情况发生的自然速率以及它在全球范围内的变化情况尚不清楚。也许更重要的是，对这种从岩石到大气的碳转移的控制很差。例如，目前尚不清楚这种碳源如何随着未来几十年陆地上不断变化的环境条件（例如降水模式）而演变。该提案解决了这些问题，旨在更好地了解地球碳循环在过去是如何运作的，并预测未来的变化。本研究将量化法国阿尔卑斯、瑞士阿尔卑斯及台湾山区河流集水区的化石有机碳风化速率。之所以选择这些河流，是因为它们的物理侵蚀速度、气候环境和基岩地质都是众所周知的，并且在不同的集水区有所不同。这些因素被认为是化石有机碳风化的重要因素，该项目将测试它们在碳转移中的具体作用。为了实现这些目标，将通过确定河流水和沉积物的地球化学组成来跟踪化石有机碳风化的产物。具体来说，将使用元素铼（Re）。稀土与岩石中的有机物结合在一起，因此当化石有机碳暴露在风化作用下时，稀土就会溶解到水中，形成河流的溶解负荷。通过比较河水中稀土元素的浓度和岩石在化学风化过程中释放的其他元素，稀土元素的来源将得到证实。更多的信息将通过测量土壤和河流沉积物中的风化固体残留物来获得。河流中的Re通量将通过排水测量来计算，并根据Re来源的限制将其转换为化石有机碳风化率。将研究的集水区的化石有机碳风化率与它们的物理侵蚀率、气候（径流和径流变异性）和基岩地质进行比较。这将首次提供碳从岩石转移的速率与控制参数之间的经验联系。这样，就有可能在更大的范围内推断这些发现，在那里这些参数是已知的，但河水的地球化学还没有被研究过。此外，还将有可能评估这种碳源在未来几十年如何随着环境条件的变化而演变。它还将允许化石有机碳风化和有机碳埋藏之间的平衡在更长的地质时期内得到更有力的评估。

The store of carbon in rock as fossil organic matter contains approximately 20,000 times more carbon than is presently in the atmosphere as the greenhouse gas carbon dioxide. Organic matter has been buried throughout Earth history, countering carbon input from volcanic degassing and playing a role in moderating global climate. However, this fossil organic carbon can be oxidised when rocks are exposed to chemical weathering at the surface and ancient carbon input into the modern environment. It is known that human activities are rapidly increasing the rate of this process by burning fossil fuels. However, the natural rate at which this occurs and how it varies throughout the globe is not well known. Perhaps more importantly, the controls on this carbon transfer from rock to atmosphere are poorly constrained. For example, it is not clear how this carbon source might evolve with changing environmental conditions (e.g. precipitation patterns) on land over the coming decades. This proposal addresses these issues, aiming to better understand how Earth's carbon cycle has functioned in the past, and to predict future changes. This research will quantify the rate of fossil organic carbon weathering in river catchments in the French and Swiss Alps, and the mountains of Taiwan. These rivers have been selected because their physical erosion rate, climatic setting and bedrock geology are all well known and vary between catchments. These are the factors that are thought to be important for fossil organic carbon weathering and the project will test their specific role on this carbon transfer. To achieve these aims, the products of fossil organic carbon weathering will be tracked by determining the geochemical composition of river water and sediment. Specifically, the element rhenium (Re) will be used. Re is bound within organic matter in rocks and so when fossil organic carbon is exposed to weathering Re is dissolved into water, contributing to the dissolved load of a river. The source of Re will be confirmed by comparing its concentration in the river water with other elements released from rocks during chemical weathering. Additional information will be acquired from measuring the solid residue of weathering in soils and river sediments. The Re flux in a river will be calculated using measurements of water discharge and this will be converted into a fossil organic carbon weathering rate using the constraint placed on the source of Re. The fossil organic carbon weathering rate in the studied catchments will be compared to their physical erosion rate, climate (runoff and runoff variability) and bedrock geology. This will provide, for the first time, an empirical link between the rate at which this carbon is transferred from rock and the controlling parameters. As such, it will be possible to extrapolate the findings over a much larger scale, where these parameters are known but the geochemistry of the river water has not been studied. In addition, it will be possible to assess how this carbon source may evolve in response to changes in environmental conditions over the coming decades. It will also allow the balance between fossil organic carbon weathering and organic carbon burial to be more vigorously assessed over longer, geological time periods.