Construction of volcanic records from marine sediment cores: A review and case study (Montserrat, West Indies)

Abstract

Detailed knowledge of the past history of an active volcano is crucial for the prediction of the timing, frequency and style of future eruptions, and for the identification of potentially at-risk areas. Subaerial volcanic stratigraphies are often incomplete, due to a lack of exposure, or burial and erosion from subsequent eruptions. However, many volcanic eruptions produce widelydispersed explosive products that are frequently deposited as tephra layers in the sea. Cores of marine sediment therefore have the potential to provide more complete volcanic stratigraphies, at least for explosive eruptions. Nevertheless, problems such as bioturbation and dispersal by currents affect the preservation and subsequent detection of marine tephra deposits.Consequently, cryptotephras, in which tephra grains are not sufficiently concentrated to form layers that are visible to the naked eye, may be the only record of many explosive eruptions.Additionally, thin, reworked deposits of volcanic clasts transported by floods and landslides, or during pyroclastic density currents may be incorrectly interpreted as tephra fallout layers, leading to the construction of inaccurate records of volcanism. This work uses samples from the volcanic island of Montserrat as a case study to test different techniques for generating volcanic eruption records from marine sediment cores, with a particular relevance to cores sampled in relatively proximal settings (i.e. tens of kilometres from the volcanic source) where volcaniclastic material may form a pervasive component of the sedimentary sequence. Visible volcaniclastic deposits identified by sedimentological logging were used to test the effectiveness of potential alternative volcaniclastic-deposit detection techniques, including point counting of grain types (component analysis), glass or mineral chemistry, colour spectrophotometry, grain size measurements, XRF core scanning, magnetic susceptibility and X-radiography. This study demonstrates that a set of time-efficient, non-destructive and high-spatial-resolution analyses (e.g. XRF core-scanning and magnetic susceptibility) can be used effectively to detect potential cryptotephra horizons in marine sediment cores. Once these horizons have been sampled, microscope image analysis of volcaniclastic grains can be used successfully to discriminate between tephra fallout deposits and other volcaniclastic deposits, by using specific criteria Page | 3 related to clast morphology and sorting. Standard practice should be employed when analysing marine sediment cores to accurately identify both visible tephra and cryptotephra deposits, and to distinguish fallout deposits from other volcaniclastic deposits.

The relationship between eruptive activity, flank collapse, and sea level at volcanic islands: A long‐term (>1 Ma) record offshore Montserrat, Lesser Antilles

“…The volcaniclastic turbidite and tephra fall deposits are less easily distinguished from each other, as both can comprise normally graded sand and silt [ Trofimovs et al ., ]. Their discrimination requires grain size and component analysis to identify the type of event deposit [ Cassidy et al ., , 2015].…”

“…Volcaniclastic turbidites are deposits from high‐energy, erosive flows that may entrain bioclastic material and preexisting volcaniclastic sediments. Therefore, although some parts of a volcaniclastic turbidite can be well‐sorted, they are likely to be less well sorted than tephra fall deposits [ Cassidy et al ., ]. Volcaniclastic turbidites are defined here as comprising <30% bioclasts, with a Folk and Ward [] sorting coefficient >0.5 (phi).…”

“…For each sample, approximately 400 grains were point‐counted using an area counting method. Componentry classes follow Le Friant et al ., [] and Cassidy et al ., []: (1) vesicular pumice clasts; (2) nonvesicular andesite; (3) altered lithic clasts; (4) crystal and glass fragments; (5) mafic scoria clasts; and (6) bioclasts. Grain size and componentry data are summarized in supporting information Tables S1 and S2, respectively (see supporting information Figures S1–S3 for photos).…”

Identifying cryptotephra units using correlated rapid, nondestructive methods: VSWIR spectroscopy, X‐ray fluorescence, and magnetic susceptibility

“…[]). Furthermore, if tephra has similar physical characteristics (e.g., texture, color) to the host sediment, they can be difficult to identify using simple sedimentological analyses [e.g., Cassidy et al ., ].…”

“…[] for a detailed discussion); however, their integration or optimization has not always been fully explored. Here we combine measurement of magnetic susceptibility, reflectance spectroscopy, and X‐ray fluorescence (XRF) core scanning and evaluate their potential for (crypto)tephra identification in Integrated Ocean Drilling Program (IODP) core Site U1396 (Figure ), which from previous regional studies [e.g., Le Friant et al ., ; Cassidy et al ., ] is expected to contain an extensive record of visible and subvisible tephra, and cryptotephra.…”