The role of marine diagenesis of tephra in the carbon cycle

Lead Research Organisation: University of Southampton
Department Name: School of Ocean and Earth Science

Abstract

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.

Planned Impact

Although this research contained within this proposal will primarily be of interest to the academic community, it does have some aspects that may attract some interest from the non-academic community.
The presence of tephra layers on the seafloor creates conditions that enhance organic carbon preservation in underlying sediments. Hence, there may have been periods and places in the geologic record when this process aided in the production of economic hydrocarbon deposits. Indeed, there was a PETROMAKS proposal in 2008 that incorporated some aspects of this hypothesis in developing a hydrocarbon exploration strategy on the Norwegian and Greenland margins. There have also been several papers published that have examined the relationship between hydrocarbon occurrence in the Green Tuff Basin (Japan) and volcanic activity. In addition, Prof RJ Davies (Durham University) is leading a project (Reservoir or seal? Porosity, permeability and rock properties in hyaloclastites and associated volcaniclastic facies) that is related to this area of research. We currently have a CASE PhD student (Colette Couves) in the geochemistry group of SOES who is sponsored by BG Group to examine the association of hydrocarbon deposits with igneous rocks and they have indicated an interest in our research. In addition, SOES is developing links with Petrobras who have also expressed an interest in this area.
Oxidation of organic carbon deposited on the sea floor is one of the important mechanisms by which biologically-fixed carbon dioxide is returned to the atmosphere. Geo-engineering proposals involving perturbation of Earth system processes have already been invoked to mitigate carbon dioxide in the atmosphere and global warming. For example, it has been suggested that the oceans could be seeded with nutrients to enhance removal of atmospheric carbon dioxide by the biological pump, and there have been schemes suggested to seed the upper atmosphere with sulphate aerosols to reflect solar radiation. If our hypothesis that tephra can enhance the preservation of organic carbon in marine sediments is proved correct, one could envisage covering areas of the sea floor with fresh volcanic material. The areas to choose would be those where there are high fluxes of organic carbon concentrations but currently relatively low burial efficiencies. This mechanism of geo-engineering has a number of advantages over the sulphate aerosols solution. It is simply enhancing a process that is occurring naturally anyway, it would directly lower atmospheric CO2 (rather than cooling the stratosphere that can lead to major regional climatic changes), fresh tephra is widely available in many areas of the world (efficient carbon sequestration in rocks can only take place in relatively restricted areas of ultramafic exposure), it would not need constant replenishment (unlike the sulphate aerosol solution) and it there would not be a sudden return to greenhouse conditions once the application ceased (unlike the sulphate aerosol solution). Of course there are ethical considerations, but these are no more exceptional than other geo-engineering solutions that have already been proposed.

Publications

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Cassidy M (2015) Origin of Basalts by Hybridization in Andesite-dominated Arcs in Journal of Petrology

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Manners HR The Role of Marine Diagenesis of Tephra in the Carbon Cycle in IODP post cruise meeting - Martinique January 2015

 
Description This study aims to determine whether the marine diagenesis of volcanogenic products (tephra) can play a significant role in the carbon cycle. To achieve this aim, the extent and mechanism by which Corg preservation in volcanic sediments is dependent on the composition of tephra layers and surrounding sediments from IODP Sites in the Caribbean (U1396) and Bering Sea (U1339) was investigated. Preliminary results (wt% total organic carbon data) suggest that there is preferential preservation of organic carbon close to and within tephra layers. To investigate this further, lipid extraction was conducted using modified Bligh and Dyer extraction for selected U1396 sediment (55) and tephra (10) samples. In nearly all cases the total organic extract (TOE) of tephra is markedly different to the sediments surrounding it, with different classes of compound identified in each respectively. Preliminary data suggest that amides and other nitrogen-based compounds appear to be preferentially preserved in the tephra. This enhanced organic nitrogen preservation is most likely the result of comlexation with reactive iron species generated during diagenesis of the tephra. Importantly, we see this enhanced amide preservation in our oldest tephra layers at IODP340, extending back 4.5 Myr.
These findngs suggest that volcanic-rich sediments, which are generally avoided by organic geochemists searching for biomarkers, may actually represent an important site for enhanced preservation of otherwise labile organic compounds.
Exploitation Route None as yet. None as yet.
Sectors Environment