The role of marine diagenesis of tephra in the carbon cycle

每年，火山向大气中喷发约10亿吨火山灰。因为大多数火山都是在大陆边缘和岛屿上发现的，所以这些物质中的大多数最终都会进入海洋。因此，据估计，太平洋大约四分之一的沉积物来自环绕这片海洋的火山爆发的产物，但人们对这种物质落入海底后会发生什么知之甚少。火山灰(或称火山灰)不是惰性物质。它的表面积与体积的比率非常高，而且它的化学成分使得它一进入海洋就开始与海水发生广泛的反应。例如，在对蒙特塞拉特火山岛周围海底的一项研究中，我们发现，在海底堆积了一层层的Tephra，由于结合在火山颗粒表面的铁的氧化，它们在沉积物-水界面几毫米范围内完全耗尽了沉积物孔隙水中的溶解氧。沉积物中的这种快速耗氧是非常不寻常的，因为通常只有在沉积物中有机物浓度非常高的地方才能观察到这种情况，例如在河口和大陆架的浅水中。当这种行为在海洋中积累时，这种行为的后果之一是，它有助于保存海洋沉积物中高浓度的有机碳，否则这些有机碳将被氧化为二氧化碳。这一点很重要，因为二氧化碳的这种来源返回到大气中有助于调节地球气候，而且有证据表明，地球遥远的过去的大规模火山喷发与强烈冰川的开始有关。虽然我们可以通过对一座火山周围沉积物的研究(如我们在加勒比海所做的那样)，对这一过程对海水化学的全球影响做出一些估计，但不同类型的火山物质喷发到海洋的不同部分(例如，寒冷的高纬度海与温暖的热带海)可能会产生不同的影响。因此，我们计划研究一系列不同类型的火山灰，这些火山灰已经喷发到海洋的几个区域。由于大多数石油和天然气矿藏最终来自海洋沉积物中有机碳的保存，我们的研究也可能为石油公司未来的新勘探目标提供帮助。此外，已经有几项关于我们如何进行地球工程来缓解大气中二氧化碳浓度增加的研究。其中许多解决办法涉及大量费用，但可能对环境造成潜在损害，因此，通过在海底区域铺设非洲菊(一种丰富、廉价、可再生和自然存在的材料)来封存或排放碳，可能是损害最小、成本最高的替代办法之一。

Every year volcanoes eject approximately 1 billion tonnes of ash into the atmosphere. Because most volcanoes are found around the edges of continents and on islands, most of this material ends up in the oceans. As a result, it is estimated that around a quarter of all the sediment in the Pacific is derived from the products of explosive volcanoes that surround this ocean, but very little is known about what happens to this material after it falls to the seafloor. Volcanic ash (or tephra) is not an inert material. It has a very high ratio of surface area to volume and the chemical composition of the tephra is such that it starts to undergo extensive reaction with seawater as soon as it enters the oceans. For example, in a study of the seafloor around the volcanic island of Montserrat we found that where layers of tephra accumulate on the seafloor they completely deplete the sediment pore water of dissolved oxygen within a few millimetres of the sediment-water interface as a result of oxidation of iron bound to the surface of the volcanic particles. This rapid oxygen depletion in sediments is very unusual as it is normally only observed where there are very high concentrations of organic matter in the sediments, for example in the shallow waters in estuaries and on the continental shelf. One of the consequences of this behaviour when tephra accumulates in the oceans is that it helps to preserve high concentrations of organic carbon in marine sediments that would otherwise be oxidised to carbon dioxide. This is important, because the return of this source of carbon dioxide to the atmosphere helps to regulate the Earth's climate, and there is evidence that massive volcanic eruptions in the Earth's distant past have been linked to the initiation of intense glaciations. While we can make some estimates of the global impact of this process on the seawater chemistry from studies of the sediments around a single volcano (such as we done in the Caribbean), it is likely that different types of volcanic material erupted into different parts of the oceans (e.g. cold high latitude seas versus warm tropical seas) will have different effects. Hence, we plan to study a range of different types of tephra that have been erupted into several areas of the oceans.As most oil and gas deposits are ultimately derived from the preservation of organic carbon in marine sediments, it is possible that our studies will also aid oil companies with new exploration targets for the future. In addition, there have been several studies of how we might carry out geoengineering to mitigate the increase in carbon dioxide concentrations in the atmosphere. Many of these solutions involve considerable expense at potential harm to the environment, it is possible that the sequestering or carbon by spreading tephra (an abundant, cheap, renewable and naturally occurring material) on areas of the seafloor may be one of the least damaging and expensive alternatives.