Examining patterns of past ash dispersal in distal cryptotephra deposits

Abstract

Well dated tephra deposits provide important chronological isochrons in Quaternary Science and numerous studies have documented distal ash beds, from past eruptions, with the aim of synchronising palaeoenvironmental or archaeological sequences. These studies also generate important information on the provenance, temporal frequency and remobilisation of tephra deposits within a catchment or region; however, this evidence is often overlooked, particularly in distal settings ‒ several hundred kilometres from the volcanic source.

As well as chronological information, tephra deposits provide a long view of volcanic ash dispersal that exceeds both instrumental and historical records. Recent studies have used these deposits to investigate wider spatial and temporal patterns in volcanic ash dispersal. To date, this approach has been confined to Europe; however, there is clearly potential to undertake further studies in regions frequently affected by ash fall. This thesis aims to investigate past changes in ash dispersal patterns using distal cryptotephra (non-visible ash) deposits. Two case studies are used to examine this: eastern North America provides an example of ultra-distal tephra dispersal and deposition, while ash clouds affecting the Falkland Islands (Islas Malvinas), South Atlantic, are likely to be strongly influenced by changes in the strength and position of the Southern Westerly Wind belt.

To maximise shard recovery rates from discrete cryptotephra deposits characterised by electron probe microanalysis, methodological experiments were undertaken to investigate the effects of the peat acid digestion protocol on diverse volcanic glass. This method has been avoided in ultra-distal settings as small highly vesicular shards are likely to be vulnerable to chemical alteration during exposure to strong acids. The results of these experiments showed samples extracted using acid digestion were geochemically indistinguishable from control samples, and therefore, acid digestion was applied throughout this thesis.

Case study I examines ultra-distal cryptotephra deposits in eastern North America. Volcanic ash fall in this region is dependent on long-distance transport processes; therefore, it is hypothesised that past changes in atmospheric circulation patterns may be recorded in the frequency or provenance of cryptotephra deposits in this region. To test this hypothesis, new cryptotephra records were developed to close spatial and temporal gaps in the existing Lateglacial and Holocene tephra framework in eastern North America. The results demonstrate a sharp increase in the frequency of cryptotephra deposits after ca. 7600 yr BP. This increase broadly coincides with early Holocene changes in atmospheric circulation, linked with the retreating Laurentide ice sheet.

Case study II investigates distal tephra deposits in the Falkland Islands with the aim of examining the effects of changes in the Southern Westerly Wind belt on tephra dispersal in the South Atlantic. Numerous cryptotephra deposits are identified, including shard peaks correlated with the Reclus R1 and Mt. Burney MB1 eruptions. However, the majority of these tephra deposits were formed of detrital glass (tephra not derived from a primary air fall event) with heterogeneous geochemical compositions and shard morphologies. This detrital glass is likely to have been remobilised from the Patagonian Steppe by aeolian processes, where tephra shards form a large component of the regional dust source. A decrease in tephra abundance is observed between 14,300–10,500 cal yr BP, which is hypothesised to be because of changes in eruption frequency, wind‒strength or taphonomic processes. In order to disentangle potential taphonomic and climatic influences on the detrital glass signal in the Falkland Islands, a new multi-proxy record of wind strength is developed from the same record. Results suggest that the Southern Westerly Wind belt strengthened in intensity during the Antarctic Cold Reversal (14,700 BP-13,000 yr BP) and moved south, away from the Falkland Islands, during the Younger Dryas time period (12,900-11,700 yr BP) and Early Holocene. These findings suggest that changes in the position of the Southern Westerly Wind belt may have reduced the frequency of tephra deposition in the Falkland Islands; however, further research is needed to test this.

In summary, case study I demonstrates the potential for wider tephra frameworks to provide evidence for changes in long term patterns of tephra dispersal, as well as reiterating the need to report ‘failed’ results where tephra is absent. Case study II provides an important example of distal transport of reworked, detrital tephra. Temporal patterns in the abundance of this detrital glass can be tentatively linked with a new record of wind strength, developed in this thesis, suggesting ash fall in the Falkland Islands may have been affected by past changes in the Southern Westerly Wind belt.

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Identifiers

ORCID for Paul Hughes: orcid.org/0000-0002-8447-382X

ORCID for Mary Edwards: orcid.org/0000-0002-3490-6682

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