A continuous 5300-yr Holocene cryptotephrostratigraphic record from northern New Zealand and implications for tephrochronology and volcanic hazard assessment

Abstract

A continuous ∼5280 calendar (cal.) yr long cryptotephrostratigraphic record of a peat core from northern New Zealand demonstrates that cryptotephra studies can enhance conventional tephra records by extending the known distribution of ash fall and enabling re-assessment of volcanic hazards. A systematic sampling strategy was used to locate peaks in glass-shard concentrations and to determine loci of individual geochemical populations, and a palynological method involving spiking samples with Lycopodium spores was adapted to facilitate accurate counting of glass-shard concentrations. Using glass shard major element compositions, and a core chronology based on eight AMS 14C ages and two visible macroscopic tephra layers, Taupo Tephra (Unit Y) (1688-1748 cal. BP) and Tuhua Tephra (6800-7230 cal. BP) (2cr-age ranges), four cryptotephras were correlated with known eruptions: Whakaipo (Unit V) (2743-2782 cal. BP), Stent (Unit Q) (4240-4510 cal. BP), and Unit K (4970-5290 cal. BP), erupted from Taupo Volcanic Centre, and Whakatane Tephra (5470-5600 cal. BP) erupted from Okataina Volcanic Centre. Mixed glass populations were found in the core, most likely an artefact of post-depositional remobilization of shards vertically (both up and down) in the peat or on its surface by wind, or a result of closely spaced eruption events, or a combination of these. A secondary glass population identified within the macroscopic Taupo Tephra was tentatively attributed to either an earlier phase within that eruption or to mixing with a slightly older Taupo-derived eruptive or (less likely) a currently unknown Okataina-derived eruptive. These results indicate that, in the absence of continuous cryptotephrostratigraphic analysis, a peak in shard concentrations may not in itself be indicative of the ‘true’ stratigraphic (ie, isochronous) level of a tephra layer. For cryptotephra studies of peat cores, we recommend (1) using a detailed sampling strategy for the analysis of distal tephra-derived glass to detect and account for any mixed populations and possible vertical spread of glass shards through the peat, and (2) analysing more shards from larger samples to help ‘capture’ sparsely represented cryptic andesitic tephra deposits.

Low-resolution Australasian palaeoclimate records of the last 2000 years

“…Additional New Zealand tephras have been identified and characterised within the Common Era , and a previously unutilised tephra in the Malay Archipelago has been characterised (Alloway et al, 2017). Identification of additional eruptions, even in the form of cryptotephra (nonvisible tephra layers), is a potential source of chronological tie points where datable material for radiocarbon analyses can be scarce (Gehrels et al, 2006(Gehrels et al, , 2008. For example, the discovery of both basaltic and silicic glass shards in the Holocene sediments of Lake Keilambete, Victoria, highlights the potential of using cryptotephra to constrain chronologies and correlate records across southeastern Australia and potentially between Australia and New Zealand (Smith et al, 2016).…”

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

“…Such deposits are termed cyrptotephras, and can be notoriously difficult to identify within marine sequences. When deposited in areas where volcaniclastic sedimentation is usually absent, grains within cryptotephra deposits can be separated from the host sediment (Blockley et al, 2005;Stanton et al, 2010), and correlated chemically with their equivalent, visible tephra deposits (e.g., Gehrels et al, 2006;Bourne et al, 2010;Albert et al, 2012).…”

“…Although cores sampled hundreds of kilometres from a volcano may still record multiple eruptions from a single volcano (Gehrels et al, 2006;Lane et al, 2012), construction of more comprehensive eruption records require cores that are sampled relatively close (proximal; i.e. tens of kilometres) to the volcanic source Carel et al, 2011;Van Daele et al, 2014).…”

Using cryptotephras to extend regional tephrochronologies: An example from southeast Alaska and implications for hazard assessment

“…The moist, vegetated surfaces of mires are effective at trapping tephra particles. Some tephra particles may move several centimeters down through the peat, but the majority appear to remain trapped at the surface, forming a defined layer that accurately records the position of the mire surface at the time of eruption Gehrels et al, 2006). Most tephra particles preserved in peat undergo limited geochemical change over millennia, although exceptions may exist for some tephras and in some peat environments (Hodder et al, 1991;Pollard et al, 2003).…”

“…With only a few exceptions (Zoltai, 1988;Gehrels et al, 2006), cryptotephrochronology has not been applied outside western Europe. Cryptotephra studies have several advantages over conventional approaches.…”