A key element of data comparison methods is the archiving of compositional data in accessible databases. One of the earliest databases designed specifically for the cryptotephra analyst was Tephrabase www. Not only does this act as a repository, but it also functions as a tool whereby several different search options can be selected. This particular database includes compositional data for Icelandic, Mexican and Eifel-sourced tephras Newton et al.
Tephra and Tephrochonology
Other databases are also available e. Such a development is clearly desirable as the geographical distribution of ash fallout never ceases to remain static. In addition to the geochemical fingerprinting, an examination of taphonomic processes in a cryptotephra study is paramount. When exploring visible tephras, several clues may indicate the influence of post-depositional reworking, for example irregularities in grain size and the thickness of the deposit e. For studies involving cryptotephras, however, the clues are often indistinct and difficult to decipher.
With no macro-features visible in the core or outcrop, the glass shards of the cryptotephra deposit are often the only indicators of real value. The shape of a shard concentration profile, geochemical inconsistencies, grain-size morphologies and vertical distribution within a sequence are among some of the key indicators within a sedimentary environment e. While these are primarily used to define the position of the isochron, they may perhaps also reveal signs of post-depositional reworking that may impair the veracity of the cryptotephra as an isochron.
Cryptotephras formed entirely from primary fallout are clearly marked by a discrete horizon of glass shards indicative of a rapid input of ash Fig. Within some records, however, a distinct peak is not always preserved and shards may be broadly scattered within a vertical profile Fig. Payne and Gehrels, ; Davies et al. Although such features bring unwanted complexities to the quest for uncomplicated and prevalent tie-points, these observations can also reveal a wealth of information on the processes operating within and around the depositional environment at the time of, and after, tephra deposition.
Schematic illustration of possible shard distribution profiles that may be observed in association with cryptotephra deposits.
Cryptotephras: the revolution in correlation and precision dating
Indicative examples of where the isochron may be placed are shown. Cryptotephra features may allow insight into taphonomic, sedimentary, anthropogenic and geomorphological processes. Even peat bog environments, where mixing and movement are thought to be negligible, are prone to vertical migration of glass shards Payne and Gehrels, In the marine environment, a complex suite of processes may disturb the preservation of a discrete cryptotephra horizon as well as impart a delay in its transport and deposition e.
High inputs of ice-rafted detritus concurrent with elevated abundances of glass shards would strongly suggest that ice rafting is the primary transport mode. In such cases, a delay in tephra deposition is anticipated due to the storage of tephra in ice sheets before calving Brendryen et al.
A geochemically heterogeneous tephra would also indicate this secondary transport process as tephras from several eruptions may be amalgamated within the ice. A broad or dispersed zone of glass shards in a marine profile e. This approach has much to offer in the future for defining the true placement of isochrons and for employing tephras as tracers to gain insight into taphonomic processes.
Indicators of reworking have also been identified by microfacies analysis and micro-XRF core scanning techniques e. To date, this approach has been applied to visible tephras in proximal settings, giving considerable insight into landscape development, archaeological change and geomorphological processes such as cryoturbation and soil erosion Streeter and Dugmore, a , b. A multitude of stratigraphic sections, including those that are poorly developed, are required to optimize this large-scale approach. Whether cryptotephra studies can offer such detailed insight into landscape-scale processes remains to be seen, but processes at this scale should not be overlooked or decoupled from the micro-scale features of a cryptotephra deposit.
As well as extending the geographical distribution of tephra fall-out, cryptotephra investigations have also identified several new, previously unknown tephra deposits in distal settings. These discoveries augment the volcanic history and dispersal compilations derived from the proximal realm e.
What has arisen is a new task of building comprehensive frameworks, lattices and catalogues of volcanic events e. These publications outline a synthesis of known eruptive events, their respective geochemical compositions and best age estimates within a given time interval and represent a prerequisite for tephrochronological work. Geochemical signatures, stratigraphic positioning and age estimates can be considered as the essential backbone or building blocks for these frameworks Lowe et al.
The importance of frameworks is highlighted by the incredible number of new cryptotephra discoveries. Dugmore mentioned that British peat bogs may contain evidence of 12 of the key Holocene eruptions that were known to have been dispersed beyond Iceland. These have, indeed, been identified together with a further 11 cryptotephra deposits that are only known from far-field occurrences Fig.
In a recent compilation, Lawson et al. New tephra discoveries have also been gleaned from medial and proximal settings e. By now, at least 15 additional tephras have been identified in distal settings, providing invaluable information on the Icelandic volcanic history during this period e. Without a doubt, medial and distal sites have a clear role to play in reconstructing volcanic histories when proximal records may be incomplete e.
The story continues into the last glacial period with new cryptotephra results from Greenland ice cores providing an exceptional catalogue of volcanic events Fig. Given the scale of new tephra and cryptotephra discoveries and the complexities that we now face, framework papers are crucial to avoid any potential miscorrelations that may arise because of an incomplete volcanic event stratigraphy e.
Haflidason and colleagues outlined one of the first frameworks of Icelandic tephra deposits encompassing the last ka. Examples from very early issues of JQS include Thompson et al. Such comments reflect the continued impact of tephrochronology on wide-ranging Quaternary and archaeological studies. As distal sites yield a complex record of previously unknown volcanic events, such frameworks are regularly refined and revised e.
Some protocols have been developed that help to assess the integrity of cryptotephra deposits within the marine environment and assess their value as potential isochrons Griggs et al. Future work may need to consider a similar toolkit for those working in the terrestrial realms. Over the last 50 years, tephrochronology has been applied to address a whole range of scientific questions from palaeoseismology to environmental reconstructions e.
The advent of cryptotephra research in Europe has widened the possibilities, allowing far-reaching objectives to be tackled in novel geographical areas. These are summarized as four key themes in Fig.
Synthesis of the four main applications of cryptotephra studies, including indicative references. Where an age has been assigned to a tephra deposit, this age is particularly valuable for building and improving chronological models via tephrochronology. Tephra age estimates can form the sole basis of a model e. Alternatively, tephra age estimates can be used to test the model constructed by other dating techniques e. These applications depend entirely on the quality of the age obtained for a tephra deposit using tephrochronometry. Ages can be derived via direct e.
An age for the latter may be obtained by high-precision radiocarbon dating or be based on the stratigraphic position of a tephra within incremental records, such as varve or ice-core records e. One particular noteworthy development is the increased adoption of a Bayesian modelling approach to improve and refine the best age estimate for a tephra deposit that is bracketed by a series of high-precision radiocarbon ages. These have been particularly important for both well-known and newly identified cryptotephras that are not preserved within annually resolved records.
For instance, Wohlfarth et al. Further south, Blockley et al. The same approach is also particularly valuable for comparing tephra age estimates derived from two independent timescales, such as the Greenland ice-core chronology GICC05 and the INTCAL calibration curves e.
Coeval tephra deposits traced between different ice cores can also supplement a chemostratigraphic matching approach for synchronizing Greenland records to a master chronology Rasmussen et al. Unquestionably, successful use of tephra deposits for building and improving chronologies depends entirely on the characterization and correct identification of the source event. Evidently, a miscorrelation and importing the wrong age estimate would introduce significant errors into the chronological model. A further benefit of tephrochronology is the opportunity presented to constrain marine reservoir ages.
Several studies have successfully constrained the changes in the marine radiocarbon reservoir in different periods showing complex spatial and temporal variations e. For instance, Thornalley et al. These investigations have largely focused on well-defined and visible horizons. Very few studies, however, have explored the use of cryptotephra deposits for constraining the reservoir changes e.
Austin and Hibbert, With significant developments in the identification of discrete cryptotephra deposits in the marine environment, there is considerable scope to employ these horizons to constrain the reservoir changes during the glacial period assuming that an independent or terrestrial-based calendrical age estimate is also available. A key motivation for cryptotephra work in recent years has been the quest for constraining rapid climatic events and to integrate disparate palaeoclimatic records to assess the degree of climatic synchroneity between different components of the climate system.
A landmark paper that exemplifies what tephrochronology has to offer was recently presented by Lane et al. By tracing the Vedde Ash in cryptotephra form within the varved record of Meerfelder Maar, Lane et al. Climatic amelioration during the Younger Dryas linked to the resumption of the thermohaline circulation was constrained to years before the Vedde Ash in Germany, but 20 years after this Vedde Ash isochron in western Norway. Such well-defined and widespread tephra deposits preserved within annually resolved records are somewhat rare but incredibly valuable and demonstrate the opportunities for adopting a differential dating approach to constrain the time between tephra deposition and a rapid climatic event.
This synchronous response is well constrained and highlights the powerful approach of combining tephra isochrons with annually resolved records. A study by Macleod et al. A further advance may well emerge in the future where rapid climate events preserved within ice and a varved depositional environment are bracketed by two or more common tephra isochrons. As more and more new tephras are identified in the Greenland ice-core records in close association with abrupt climatic transitions, this is a realistic prospect Davies et al. Where annually resolved records are unavailable, however, tephra or cryptotephra deposits continue to represent powerful tie-points during an assortment of time periods.
Although synchronous changes were apparent in the Norwegian Sea area, pollen stratigraphic changes suggested that these lagged behind the climatic optimum experienced in continental Europe. During the last glacial period, Austin et al. A further correlation by Austin et al.
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Such interpretations would be difficult without the tephra isochron to provide the chronostratigraphic constraint. Finally, asynchronous changes are identified by Langdon and Barber during the mid-Holocene period, whereby seven Scottish peat bogs are anchored by the Glen Garry and Hekla-4 tephras. Bog surface wetness indices from these sites are presented on a time-slice reconstruction for each tephra deposit.
Tephrochronology and cryptotephrochronology offer much promise for facilitating a time-slice approach in a range of different time periods, but depend entirely on tracing common tephras and cryptotephras in a range of well-characterized proxy records. Moreover, with recent reports of transatlantic tephra transport, European aviation authorities may well need to cast their eyes beyond the usual volcanic suspects Jensen et al.