Correlating weathered, microphenocryst-rich, intermediate tephra:An approach combining bulk and single shard analyses from the Lepué Tephra, Chile and Argentina

Chemical correlation of intermediate tephra deposits using microanalytical data is problematic because (i) the phenocryst content of their component glass shards affects major and trace element analyses (ii) bulk chemistry can be affected by variations in mineral/lithic components across the fall-out, and (iii) weathering readily alters their composition. All of these problems affect the Lepué Tephra, a prominent marker horizon extensively distributed across the Los Lagos Region of Chile and the Chile-Argentina frontier in north-western Patagonia, which was erupted from Volcán Michinmahuida at c. 11000 cal a BP. Weathering of terrestrial cover-bed deposits in this hyper-humid depositional environment leaves only a few occurrences of the tephra which contain fresh glass shards for microbeam analysis, but their highly phenocrystic nature makes data interpretation difficult. Equally, leaching of mobile elements during weathering causes considerable compositional changes across the fall-out region and is evident in bulk sample analyses. Elements such as the REE and Y, generally regarded as immobile, show marked mobility. Within the REE, the development of M-type tetrad effects and positive Ce-anomalies result from a combination of dissolution/leaching of the REE from the bulk sample and retention by co-precipitation of Ce4+ on Fe-oxyhydroxides in this high-rainfall, hyper-humid, oxic environment. Chemical correlation of the Lepué Tephra is thus not straightforward. However, by careful consideration of the data for a limited range of elements, chemical correlation can be achieved using elements which (i) are incompatible in magmatic systems (and thus their ratios are unaffected by the presence of phenocrysts in single glass shard microbeam analysis) and (ii) are not mobilised in these weathering conditions. These elements are Zr, Hf, Nb, Ta and Th. Their ratios (i) allow for the comparison of single grain and bulk sample analyses, extending the geographic range over which data can be compared for the Lepué Tephra, (ii) provide a robust chemical correlation of this weathered, intermediate tephra deposit, enabling correlation even where elements traditionally considered immobile (REE, Y, and U) have been significantly mobilised, and (iii) allow the Lepué Tephra to be distinguished from other local tephra deposits. This combined analytical approach enables tephras that have been variably weathered to become useful marker beds over much wider geographical areas than previously feasible, thereby enhancing their tephrochronological application in Quaternary research

Stratigraphy and chronology of the Stent tephra, a c. 4000 year old distal silicic tephra from Taupo Volcanic Centre, New Zealand.

Tephrostratigraphic and chronologic studies in two areas of the North Island have identified a previously unrecorded, thin, distal silicic tephra derived from the Taupo Volcanic Centre. In Taranaki, three radiocarbon ages of the uncorrelated tephra are consistent with the independent radiocarbon chronology obtained from enveloping Egmontsourced tephras. In western Bay of Plenty, where the uncorrelated tephra is also directly dated, it is overlain by Whakaipo Tephra (c. 2.7 ka) and underlain by Hinemaiaia Tephra (c. 4.5 ka). From these sites in Taranaki and western Bay of Plenty, seven radiocarbon dates obtained on the uncorrelated silicic tephra yield an error-weighted mean age of 3970 ±31 conventional radiocarbon years B.P. The ages on the uncorrelated tephra (informally referred to as Stent tephra) from both areas are statistically identical but significantly different from those on both Waimihia and Hinemaiaia Tephras. occurrence of Stent tephra in Taranaki, c. 160 km upwind from the postulated source area, and in western Bay of Plenty, suggests that it represents the product of a moderately large plinian eruption. Until recently, its validity as a discrete eruptive event had been problematical, because a near-source equivalent deposit between Waimihia and Hinemaiaia Tephras was not recognised in the Taupo area. However, a revised stratigraphy proposed by C. J. N. Wilson in 1993 for eastern sectors of the Taupo area shows that multiple tephra layers were erupted from Taupo volcano between c. 3.9 and 5.2 ka. Of these newly recognised layers, unit-g--the product of a moderately large eruption (>0.15 km3) at c. 4.0 ka--is tentatively correlated with Stent tephra. Other eruptive units recognised by Wilson are either too old or too small in volume to be considered as likely correlatives

An 18,000 year-long eruptive record from Volcán Chaitén, northwestern Patagonia:Paleoenvironmental and hazard-assessment implications

The 2008 eruption of Volcán Chaitén (VCha) in northwestern Patagonia was the first explosive rhyolitic eruption to have occurred within a century and provided an unprecedented scientific opportunity to examine all facets of the eruption ranging from magma rheology/ascent rates to ash-fall effects on biota and infrastructure. Up to very recently it was thought that the latest eruption prior to the 2008 event occurred c. 9750 cal. a BP. Although a number of researchers have recognised additional eruptive products, but their stratigraphy, age, and geochemical attributes have not been systematically described and/or recorded. In this study, we provide a detailed examination of andic cover-beds and tephra-bearing lake sequences located both proximally and distally to VCha, which record a series of hitherto unknown rhyolitic eruptive products and place all previous observations firmly within a coherent stratigraphic framework. Through major- and trace-element glass shard geochemistry we are able to confidently verify eruptive source. A total of 20 discrete tephra beds are recognised, with at least 10 having widespread areal distributions and/or depositional imprints broadly comparable to, or greater than, the 2008-tephra event. This record indicates that VCha has been continuously but intermittently active as far back as the end of the Last Glacial Maximum (c. 18,000 cal a BP) with two dominant, genetically related magma types and an intermediary ¿mixed¿ type. Before this the eruptive record has been largely obscured and/or erased by widespread Andean piedmont glaciation. However, based on the tempo of VCha activity over the last c. 18,000 years, older VCha eruptives can be anticipated to occur as well as future hazardous explosive events. The new eruptive inventory will ultimately be useful for correlating equivalent-aged sequences and refining long-term eruptive tempo as well as corresponding temporal changes in magmatic evolution

Age and context of the oldest known hominin fossils from Flores

Recent excavations at the early Middle Pleistocene site of Mata Menge in the So\u27a Basin of central Flores, Indonesia, have yielded hominin fossils1 attributed to a population ancestral to Late Pleistocene Homo floresiensis2. Here we describe the age and context of the Mata Menge hominin specimens and associated archaeological findings. The fluvial sandstone layer from which the in situ fossils were excavated in 2014 was deposited in a small valley stream around 700 thousand years ago, as indicated by 40Ar/39Ar and fission track dates on stratigraphically bracketing volcanic ash and pyroclastic density current deposits, in combination with coupled uranium-series and electron spin resonance dating of fossil teeth. Palaeoenvironmental data indicate a relatively dry climate in the So\u27a Basin during the early Middle Pleistocene, while various lines of evidence suggest the hominins inhabited a savannah-like open grassland habitat with a wetland component. The hominin fossils occur alongside the remains of an insular fauna and a simple stone technology that is markedly similar to that associated with Late Pleistocene H. floresiensis

An integrative geochronological framework for the pleistocene So'a basin (Flores, Indonesia), and its implications for faunal turnover and hominin arrival

Flores represents a unique insular environment with an extensive record of Pleistocene fossil remains and stone artefacts. In the So\u27a Basin of central Flores these include endemic Stegodon, Komodo dragons, giant tortoises, rats, birds and hominins, and lithic artefacts that can be traced back to at least one million years ago (1 Ma). This comprehensive review presents important new data regarding the dating and faunal sequence of the So\u27a Basin, including the site of Mata Menge where Homo floresiensis-like fossils dating to approximately 0.7 Ma were discovered in 2014. By chemical fingerprinting key silicic tephra originating from local and distal eruptive sources we have now established basin-wide tephrostratigraphic correlations, and, together with new numerical ages, present an update of the chronostratigraphy of the So\u27a Basin, with major implications for the faunal sequence. These results show that a giant tortoise and the diminutive proboscidean Stegodon sondaari last occurred at the site of Tangi Talo ∼1.3 Ma, and not 0.9 Ma as previously thought. We also present new data suggesting that the disappearance of giant tortoise and S. sondaari from the sedimentary record occurred before, and/or was coincident with, the earliest hominin arrival, as evidenced by the first records of lithic artefacts occurring directly below the 1 Ma Wolo Sege Tephra. Artefacts become common in the younger layers, associated with a distinct fauna characterized by the medium-sized Stegodon florensis and giant rat Hooijeromys nusatenggara. Furthermore, we describe a newly discovered terrace fill, which extends the faunal record of Stegodon in the So\u27a Basin to the Late Pleistocene. Our evidence also suggests that the paleoenvironment of the So\u27a Basin became drier around the time of the observed faunal transition and arrival of hominins on the island, which could be related to an astronomically-forced climate response at the onset of the Mid-Pleistocene Transition (MPT; ∼1.25 Ma) leading to increased aridity and monsoonal intensity

Pollen-based temperature and precipitation records of the past 14,600 years in northern New Zealand (37°S) and their linkages with the Southern Hemisphere atmospheric circulation

Regional vegetation, climate history, and local water table fluctuations for the past 14,600 years are reconstructed from pollen and charcoal records of an ombrogenous peatbog in northern New Zealand (38°S). A long-term warming trend between 14,600 and 10,000 cal. yr BP is punctuated by two brief plateaux between 14,200-13,800 and 13,500-12,000 cal. yr BP. Periods of relatively drier conditions are inferred between 14,000-13,400 and 12,000-10,000 cal. yr BP, while a long-term wet period is observed between 10,000 and 6000 cal. yr BP. The last 7000 years feature relatively stable temperatures, a long-term drying trend that culminates with persistent drier conditions over the last 3000 years and cyclical f luctuations in the bog\u27s water table and fires. Present-day climate controls and comparisons with other climate reconstructions from New Zealand, the Southern Hemisphere mid-latitudes and the tropical Pacific suggest that complex and temporally variable teleconnections exist between northern New Zealand and the Southern Hemisphere low- and high-latitude circulation

Thermal history of volcanic debris flow deposits on the eastern flanks of Mt. Taranaki, New Zealand: Implications for future hazards

We use palaeomagnetic methods to decipher the thermal histories of a succession of massive to weakly stratified debris flow deposits (Ngatoro and Te Popo formations) of late Holocene age located on the eastern lower flanks of Mt. Taranaki/Egmont Volcano, western North Island, New Zealand. Results from two sites, Vickers Quarry and Surrey Road Quarry, both c. 9.6 km from the present-day summit, enable us to distinguish between clast incorporation temperatures of about 400 °C and emplacement temperatures between 150 and 200 °C, consistent with observation of superficial charring and desiccation of outer podocarp-hardwood tree trunks at Vickers Quarry. Analysis of palaeomagnetic directions and lithofacies architecture suggest that these deposits were likely initiated as a closely-spaced succession of block-and-ash flows (BAFs) that rapidly cooled as they descended the volcano flanks. Radiocarbon chronology and the widespread occurrence of a palaeosol between the products of the preceding Inglewood eruptive phase, c. 3.4 cal. ka B.P., and the overlying Ngatoro Formation suggest that these two events are temporally unrelated. Certainly, there is no field evidence of contemporaneous explosive volcanic activity that might be related to the emplacement of Ngatoro Formation. However, we suggest that these low-temperature deposits might either relate to collapse of a small emergent lava dome or a cooling dome remnant, possibly emplaced in the aftermath of the Inglewood eruption. How collapse was initiated remains uncertain: the remnant dome may have been rendered unstable by volcano-tectonic or tectonic seismic events and/or by adverse meteorological events. Nevertheless, this study demonstrates that edifice collapse events generating potentially hazardous debris flows can occur independent of specific eruptive activity

Far-flown markers

Tephras are fragmentary materials that are blasted explosively into the air during volcanic eruptions. Distributed throughout Zealandia, tephras provide useful markers for connecting and dating land surfaces, sediment layers, and archaeological sites

John A. Westgate - Global tephrochronologist, stratigrapher, mentor

The growth of tephra studies and tephrochronology as a field of research in the last four decades owes an enormous debt to the contributions of John Westgate. John has been one of the clear leaders of this field over this time. Through a career spanning some 40 years, he has been instrumental in the development of research techniques that have led to the global expansion of tephra studies. In particular, he has stressed a multi-proxy approach to tephrochronology, requiring the careful application of glass composition, petrology, and the understanding of field settings and stratigraphy in the development of robust tephrostratigraphic frameworks (Fig. 1 and Fig. 2). In turn, these techniques and records have helped to solve some of the most complex and longstanding problems in the geosciences, geoarchaeology, and other disciplines. John has had a major impact in all countries where he has worked, where he has been a committed researcher, leading and encouraging in his usual fashion and always publishing high-quality and groundbreaking papers that will be used for generations to come. Here we provide a brief summary of John's exceptional career, documenting his very substantial influence on tephra studies worldwide

Far-flung markers

Tephras are fragmentary materials that are blasted explosively into the air during volcanic eruptions. Distributed throughout Zealandia, tephras provide useful markers for connecting and dating land surfaces, sediment layers, and archaeological sites