Biomolecular characteristics of an extensive tar layer generated during eruption of the Soufrière Hills volcano, Montserrat, West Indies

Abstract

On December 26th, 1997, a violent pyroclastic density current (PDC), resulting from an eruption of the Soufriere Hills volcano, occurred on the island of Montserrat. About 4 km2 of the area devastated by the PDC was covered partially by a blackened tar layer, of about 1–4 mm depth. Organic geochemical evidence strongly supports the hypothesis that the tar layer was created from soil and associated vegetation, probably in the form of a combustion cloud generated by the PDC. Vegetation, soil and tar were investigated using solid state 13C nuclear magnetic resonance spectroscopy (NMR), reflectance microscopy and scanning electron microscopy (SEM) to provide a general overview of the organic matter (OM) composition and morphology. Parallel investigations of the lipid, carbohydrate and lignin contents using gas chromatography (GC) and GC–mass spectrometry (GC/MS) were also undertaken. Solid state 13C NMR revealed that the majority of the OM in both vegetation (∼70%) and soil (∼40%) was cellulose and hemicellulose, with the remainder comprising predominantly aromatic, N-alkyl/methoxyl and alkyl structures. Contrastingly, the two tar layers were shown to be composed of 44% and 52.5% aromatic moieties, attesting to their thermogenic origin. Analysis of solvent extractable lipids revealed an n-alkane distribution in the tar layer that correlated with those observed for the vegetation and soil, maximising at C33. Moreover, a higher relative abundance of the C22 and C32n-alkanols was observed in the same tar layer compared with that obtained from the soil. This indicated that both vegetation and soil were major contributors of OM to the tar layer. Overall, the two tar layers were shown to contain lipids in differing states of preservation, indicating a higher temperature of formation for one of the layers, resulting from large, local temperature variation within the PDC. Analysis of total hydrolysable carbohydrates confirmed the large loss of these components in the tar layers, corroborating the information obtained from 13C NMR. This almost total loss of carbohydrates and concomitant increase in aromatised components in the tar layers is indicative of a PDC temperature >325 °C. CuO oxidation afforded lignin derived products for each sample, the relative distributions of which revealed that significant oxidation had occurred in the tar layers. Further information obtained from the tar layers using reflectance spectroscopy enabled the minimum temperature of the PDC to be constrained to 325–370 °C, possibly rising as high as 425 °C.