Connecting with tephras: principles, functioning, and applications of tephrochronology in Quaternary science

Tephrochronology is a unique method for linking and dating geological, palaeoecological, palaeoclimatic, or archaeological sequences or events. The method relies firstly on stratigraphy and the law of superposition, which apply in any study that connects or correlates deposits from one place to another. Secondly, it relies on characterising and hence identifying or ‘fingerprinting’ tephra layers using either physical properties evident in the field or those obtained from laboratory analysis, including mineralogical examination by optical microscopy or geochemical analysis of glass shards or crystals (e.g., Fe-Ti oxides, ferromagnesian minerals) using the electron microprobe and other tools. Thirdly, the method is enhanced when a numerical age is obtained for a tephra layer by (1) radiometric methods such as radiocarbon, fission-track, U-series, or Ar/Ar dating, (2) incremental dating methods including dendrochronology or varved sediments or layering in ice cores, or (3) age-equivalent methods such as palaeomagnetism or correlation with marine oxygen isotope stages or palynostratigraphy. Once known, that age can be transferred from one site to the next using stratigraphic methods and by matching compositional characteristics, i.e., comparing ‘fingerprints’ from each layer. Used this way, tephrochronology is an age-equivalent dating method. Even if a tephra layer is undated, or if it is dated imprecisely, it nevertheless provides an isochron or time-plane that allows the sequence in which it is found to be correlated with other sequences where it occurs. Herein lies the unique power of tephrochronology: deposits and their palaeoarchival evidence are thus able to be connected and synchronized  positioned precisely on a common time scale  using the tephra layer as a stratigraphically fixed tie-point, even where the tephra is poorly or undated. In this situation, the age scale is best envisaged as a length of elastic that can be stretched or contracted when numerical ages are obtained, or age precision is improved, whilst the tephra’s stratigraphic juxtaposition with respect to the enclosing deposits and associated archival data remains fixed on the ‘elastic’. When the tephra age is known, however, that age can be applied directly to the sequence where the tephra has been newly identified. This is because tephra layers are erupted over very short time periods (volcanic eruptions typically last for only hours or days to perhaps weeks or a few months or so at most), and thus each represent an instant in time, geologically speaking (Lowe, 2011). A tephra layer from a powerful eruption can be spread widely over land, sea and ice, hence forming a thin blanket that has exactly the same age wherever it occurs (unless it has been reworked). For example, the Icelandic Fugloyarbanki tephra, identified in the NGRIP ice core from Greenland, has been dated at 26,740 ± 390 (1) calendar (cal.) years before AD 2000 on the basis of multi-parameter counting of annual layers in NGRIP (Davies et al., 2008). It forms a widespread marker horizon or isochron in marine deposits in the North Atlantic and on the distant Faroe Islands between Iceland and Scotland. Thus palaeoarchives at these widely separated localities are now able to be connected precisely. Moreover, the extent of the radiocarbon marine reservoir effect in this region at the time can be examined using the Fugloyarbanki tephra as an independent time-plane.