The role of marine diagenesis of tephra in the carbon cycle

每年火山喷发大约10亿吨的火山灰到大气中。因为大多数火山都是在大陆边缘和岛屿上发现的，所以大部分这些物质最终都进入了海洋。因此，据估计，太平洋中大约四分之一的沉积物来自围绕这片海洋的火山爆发的产物，但人们对这些物质落到海底后的情况知之甚少。火山灰（或火山灰）不是一种惰性物质。它的表面积与体积之比非常高，它的化学成分是这样的，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，在海底堆积的火山灰层中，由于火山颗粒表面的铁氧化，在沉积物-水界面几毫米内，它们完全耗尽了沉积物孔隙水中的溶解氧。沉积物中氧气的快速消耗是非常不寻常的，因为通常只有在沉积物中有机物浓度非常高的地方才会观察到这种情况，例如在河口的浅水和大陆架上。当白热菌在海洋中积累时，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成二氧化碳。这一点很重要，因为这种二氧化碳的来源返回到大气中有助于调节地球的气候，而且有证据表明，在遥远的过去，地球上的大规模火山爆发与剧烈冰川作用的开始有关。虽然我们可以通过对单个火山周围沉积物的研究（例如我们在加勒比地区所做的研究）对这一过程对海水化学的全球影响做出一些估计，但很可能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洋与温暖的热带海洋）将产生不同的影响。因此，我们计划研究一系列不同类型的火山喷发到海洋的几个地区。由于大多数石油和天然气矿床最终来源于海洋沉积物中有机碳的保存，因此我们的研究也有可能帮助石油公司在未来找到新的勘探目标。此外，还有几项关于我们如何实施地球工程来减缓大气中二氧化碳浓度增加的研究。这些解决方案中有许多都涉及相当大的费用和对环境的潜在危害，通过在海底区域散布麻黄（一种丰富、廉价、可再生和自然产生的材料）来隔离碳可能是危害最小和最昂贵的替代方案之一。

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.