Tephrochronology: principles, functioning, application

Tephrochronology is a unique method for linking and dating geological, palaeoecological, palaeoclimatic or archaeological sequences or events. The method relies firstly and fundamentally 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

Ages on weathered Plio-Pleistocene tephra sequences, western North Island, New Zealand

Using the zircon fission-track method, we have obtained five ages on members of two strongly-weathered silicic, Pliocene-Pleistocene tephra sequences, the Kauroa and Hamilton Ash formations, in western North Island, New Zealand. These are the first numerical ages to be obtained directly on these deposits. Of the Kauroa Ash sequence, member K1 (basal unit) was dated at 2.24 ± 0.29 Ma, confirming a previous age of c. 2.25 Ma obtained (via tephrochronology)from K/Ar ages on associated basalt lava. Members K2 and K3 gave indistinguishable ages between 1.68 ± 0.12 and 1.43 ± 0.17 Ma. Member K12, a correlative of Oparau Tephra and probably also Ongatiti Ignimbrite, was dated at 1.28 ± 0.11 Ma, consistent with an age of 1.23 ± 0.02 Ma obtained by various methods on Ongatiti Ignimbrite. Palaeomagnetic measurements indicated that members K13 to K15 (top unit, Waiterimu Ash) are aged between c. 1.2 Ma and 0.78 Ma. Possible sources of the Kauroa Ash Formation include younger volcanic centres in the southern Coromandel Volcanic Zone or older volcanic centres in the Taupo Volcanic Zone, or both. Of the Hamilton Ash sequence, the basal member Ohinewai Ash (HI) was dated at 0.38 ± 0.04 Ma. This age matches those obtained by various methods on Rangitawa Tephra of 0.34-0.35 Ma, supporting correlation with this Whakamaru-caldera derived deposit. The origin of the other Hamilton Ash beds is unknown but various younger volcanic centres in the Taupo Volcanic Zone are possible sources. The topmost member, Tikotiko Ash (H6-H7), is estimated to be aged between c. 0.18 and 0.08 Ma. Various silicic pyroclastic deposits documented in North Island and in marine cores may be co-eval with members of the Kauroa Ash and Hamilton Ash sequences on the basis of their age

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

Connecting and dating with tephras: principles, functioning, and application of tephrochronology in Quaternary research

Tephrochronology, the characterisation and use of volcanic-ash layers as a unique chronostratigraphic linking, synchronizing, and dating tool, has become a globally-practised discipline of immense practical value in a wide range of subjects including Quaternary stratigraphy, palaeoclimatology, palaeoecology, palaeolimnology, physical geography, geomorphology, volcanology, geochronology, archaeology, human evolution, anthropology, and human disease and medicine. The advent of systematic studies of cryptotephras – the identification, correlation, and dating of sparse, fine-grained glass-shard concentrations ‘hidden’ within sediments or soils – over the past ~20 years has been revolutionary. New cryptotephra techniques developed in northwestern Europe and Scandinavia in particular and in North America most recently adapted or improved to help solve problems as they arose, have now been applied to sedimentary sequences (including ice) on all the continents. The result has been the extension of tephra isochrons over wide areas hundreds to several thousands of kilometres from source volcanoes. Taphonomic and other issues, such as quantifying uncertainties in correlation, provide scope for future work. Developments in dating and analytical methods have led to important advances in the application of tephrochronology in recent times. In particular: (i) the ITPFT (glass fission-track) method has enabled landscapes and sequences to be dated where previously no dates were obtainable or where dating was problematic; (ii) new EMPA protocols enabling narrow-beam analyses (<5 um) of glass shards, or small melt inclusions, have been developed, meaning that small (typically distal) glass shards or melt inclusions <~10 um in diameter can now be analysed more efficaciously than previously (and with reduced risk of accidentally including microlites in the analysis as could occur with wide-beam analyses); (iii) LA-ICPMS method for trace element analysis of individual shards <~10 um in diameter is generating more detailed ‘fingerprints’ for enhancing tephra-correlation efficacy (Pearce et al., 2011, 2014; Pearce, 2014); and (iv) the revolutionary rise of Bayesian probability age modelling has helped to improve age frameworks for tephras of the late-glacial to Holocene period especially

A late Pleistocene long pollen record from Lake Urmia, NW Iran

A palynological study based on two 100-m long cores from Lake Urmia in northwestern Iran provides a vegetation record spanning 200 ka, the longest pollen record for the continental interior of the Near East. During both penultimate and last glaciations, a steppe of Artemisia and Poaceae dominated the upland vegetation with a high proportion of Chenopodiaceae in both upland and lowland saline ecosystems. While Juniperus and deciduous Quercus trees were extremely rare and restricted to some refugia, Hippophaë rhamnoides constituted an important phanerophyte, particularly during the upper last glacial sediments. A pronounced expansion in Ephedra shrub-steppe occurred at the end of the penultimate late-glacial period but was followed by extreme aridity that favoured an Artemisia steppe. Very high lake levels, registered by both pollen and sedimentary markers, occurred during the middle of the last glaciation and upper part of the penultimate glaciation. The late-glacial to early Holocene transition is represented by a succession of Hippophaë, Ephedra, Betula, Pistacia and finally Juniperus and Quercus. The last interglacial period (Eemian), slightly warmer and moister than the Holocene, was followed by two interstadial phases similar in pattern to those recorded in the marine isotope record and southern European pollen sequences

Large Zero Autocorrelation Zone of Golay Sequences and 4q4^q-QAM Golay Complementary Sequences

Sequences with good correlation properties have been widely adopted in modern communications, radar and sonar applications. In this paper, we present our new findings on some constructions of single $H$-ary Golay sequence and $4^q$-QAM Golay complementary sequence with a large zero autocorrelation zone, where $H\ge 2$ is an arbitrary even integer and $q\ge 2$ is an arbitrary integer. Those new results on Golay sequences and QAM Golay complementary sequences can be explored during synchronization and detection at the receiver end and thus improve the performance of the communication system

Marine tephrochronology: a personal perspective

This special volume on marine tephrochronology is remarkable, and timely, because it marks a concerted step towards what might be informally termed ‘phase 3’ of a revolution in Quaternary geosciences that began around 40 years ago. The 10 articles collectively represent a re-focussed examination of tephras and cryptotephras preserved in ocean sediments at various locations and the authors describe their significance for a range of subdisciplines. Eight articles provide a new understanding of the origin, distribution and ages of various tephra and cryptotephra deposits and their stratigraphic inter-relationships; how the terrestrial ages of the tephra/crypotephra deposits relate to those of enclosing sediments and inform the ongoing development of the marine radiocarbon time-scale; mechanisms for the emplacement, remobilization or bioturbation of the tephras or cryptotephras; and volcanic eruption history. Two further articles document the characterization of tephra-derived glass shards using microbeam techniques to analyse 30–40 elements from individual shards as small as 10 µm in diameter. The collection thus provides snapshots of many aspects of the latest developments and directions in tephra studies – volcanology, primary and secondary dispersal, stratigraphy, single-grain characterization, chronology – through the medium of marine sediments. My personal perspective reflects briefly on how this point was reached and identifies a few of the important milestones on the way from ‘phase 1’ to ‘phase 3’. I am privileged to write it. Marine science revolution As an undergraduate in the early-mid 1970s, I recall my first real ‘awakening’ regarding the dynamic nature of science, and of Quaternary geoscience in particular, when told about deep-sea core V28-238 from the equatorial Pacific Ocean (Shackleton & Opdyke 1973; >2650 citations, Google Scholar). Analogous to the opening notes of Beethoven's 5th Symphony, perhaps the most famous quartet of notes in history, the alpha-numerical assemblage ‘V28-

The equatorial Pacific High-Productivity Belt: Elements for a Synthesis of Deep Sea Drilling Project Leg 85 Resultspaleoceanography

Leg 85 of the Deep Sea Drilling Project operated in the eastern central Pacific in the region of the equatorial highproductivity belt. We recovered uppermost Eocene to Quaternary reference sections amenable to fine-scale stratigraphic and paleoceanographic research, using primarily the hydraulic piston corer. Four sites (572 to 575) were drilled along an east-west (about 114 to 133°W) and north-south (about 0.5 to 6°N) transect across the equatorial belt. At Site 572 an apparently complete lower middle Miocene to Quaternary sequence was recovered: the sediment sections are dominantly siliceous-calcareous oozes and chalks, and sediment accumulation rates were high (30 to 60 m/ m.y.). Sediments at Sites 573 to 575 are similar and dominated by siliceous and calcareous oozes and chalks. Sediment accumulation rates at Site 573 were generally between 10 and 35 m/m.y. The upper Eocene to Quaternary sequence is punctuated by seven hiatuses. At Site 574 a nearly complete upper Eocene to Quaternary sequence was retrieved, including a continuous Eocene to Oligocene transition. At Site 575 a lower Miocene to Quaternary section was cored. The lower to middle Miocene section is characterized by high, constant carbonate contents and sediment accumulation rates of about 20 m/m.y.; the top of the recovered section contains two hiatuses and has accumulated at rates of less than 10 m/m.y. Except for thin, basal metalliferous layers, compositional changes in Leg 85 sediments result from shifts in the relative abundances of the biogenic siliceous or calcareous components. Leg 85 sites subsided and migrated at about 0.3 cm/yr., from about 3000 m in the eastern Pacific, south of the equator, to deeper (4000 to 4600 m), more western locations at or north of the equator. The sedimentary sequences recorded regional changes in productivity (biogenic sedimentation), dissolution, and erosion associated with the equatorial belt, as well as global paleoceanographic events. The most striking regional trend is an east-west decreasing gradient in deposition of biogenic silica, prevailing from middle Miocene to Recent, which mirrors present-day surface-water productivity. A less pronounced middle Miocene to Recent latitudinal trend in deposition appears to be the result of enhanced carbonate solution to the north. The effects of deposition in the equatorial high-productivity belt have not changed since the early Miocene. Below the middle/upper Miocene boundary, the sediments have a relatively constant high carbonate content, whereas above, carbonate percentages are highly variable. The changeover level is generally marked by a hiatus and significant changes in physical, chemical, and magnetic properties of the sediments. Results of Leg 85 contributed to advances in the four elements needed for an eventual paleoceanographic synthesis: (1) a high-resolution, multidisciplinary, and integrated datum-plane scheme and time scale, with an overall resolution of 0.13 to 0.38 m.y., was established; (2) a system of correctable acoustic reflectors was delineated over vast distances, and these reflectors were calibrated against age and physical and chemical properties at Sites 574; (3) a fine-scale geochemical (stable isotope and CaCO3) and micropaleontological climatic reconstruction was developed (discontinuously) from Oligocene to Pleistocene; and (4) broadly correctable hiatuses (NH, PH), defined by previous work, were recognized at Sites 573 to 575. The interval from 8 to 9 Ma serves to outline how integration of the foregoing elements could lead to a future synthesis. In the central basin of the equatorial Pacific, the interval from 8 to 9 Ma is marked by a hiatus (NH5) in siliceous clays. A correlative hiatus occurs in carbonate oozes of the equatorial region and in the northeastern Pacific rim, and may be correlated with the Purple equatorial Pacific reflector. The interval is characterized globally by a regression, by strong carbonate dissolution, and by isotopic and micropaleontological evidence of marked cooling. Further study of these paleoceanographic phenomena in a variety of depositional environments will lead to an understanding of their relationships and functioning

First evidence of cryptotephra in palaeoenvironmental records associated with Norse occupation sites in Greenland

The Norse/Viking occupation of Greenland is part of a dispersal of communities across the North Atlantic coincident with the supposed Medieval Warm Period of the late 1st millennium AD. The abandonment of the Greenland settlements has been linked to climatic deterioration in the Little Ice Age as well as other possible explanations. There are significant dating uncertainties over the time of European abandonment of Greenland and the potential influence of climatic deterioration. Dating issues largely revolve around radiocarbon chronologies for Norse settlements and associated mire sequences close to settlement sites. Here we show the potential for moving this situation forward by a combination of palynological, radiocarbon and cryptotephra analyses of environmental records close to three ‘iconic’ Norse sites in the former Eastern Settlement of Greenland – Herjolfsnes, Hvalsey and Garðar (the modern Igaliku). While much work remains to be undertaken, our results show that palynological evidence can provide a useful marker for both the onset and end of Norse occupation in the region, while the radiocarbon chronologies for these sequences remain difficult. Significantly, we here demonstrate the potential for cryptotephra to become a useful tool in resolving the chronology of Norse occupation, when coupled with palynology. For the first time, we show that cryptotephra are present within palaeoenvironmental sequences located within or close to Norse settlement ruin-groups, with tephra horizons detected at all three sites. While shard concentrations were small at Herjolfsnes, concentrations sufficient for geochemical analyses were detected at Igaliku and Hvalsey. WDS-EPMA analyses of these tephra indicate that, unlike the predominantly Icelandic tephra sources reported in the Greenland ice core records, the tephra associated with the Norse sites correlate more closely with volcanic centres in the Aleutians and Cascades. Recent investigations of cryptotephra dispersal from North American centres, along with our new findings, point to the potential for cryptotephra to facilitate hypothesis testing, providing a key chronological tool for refining the timing of Norse activities in Greenland (e.g. abandonment) and of environmental contexts and drivers (e.g. climate forcing)

Western Mediterranean climate and environment since Marine Isotope Stage 3: a 50,000-year record from Lake Banyoles, Spain

We present new stable isotope (δ¹⁸Ocalcite and δ¹³ Ccalcite) and diatom data from a 67-m sediment core (BAN II) from Lake Banyoles, northeastern Spain. We reassessed the chronology of the sequence by correlating stable isotope data with a shorter U-series-dated record from the lake, confirming a sedimentological offset between the two cores and demonstrating that BAN II spans Marine Isotope Stages (MIS) 3–1. Through comparison with previous records, the multi-proxy data are used to improve understanding of palaeolimnological dynamics and, by inference, western Mediterranean climate and environmental change during the past ca. 50,000 years. Three main zones, defined by isotope and diatom data, correspond to the MIS. The basal zone (MIS 3) is characterised by fluctuating δ¹⁸Ocalcite and benthic diatom abundance, indicating a high degree of environmental and climate variability, concomitant with large lake-level changes. During the full glacial (MIS 2), relatively constant δ¹⁸Ocalcite and a poorly preserved planktonic-dominated diatom assemblage suggest stability, and intermittently, unusually high lake level. In MIS 1, δ¹⁸Ocalcite and δ¹³Ccalcite initially transition to lower values, recording a pattern of Late Glacial to Holocene change that is similar to other Mediterranean records. This study suggests that Lake Banyoles responds limnologically to changes in the North Atlantic ocean–atmosphere system and provides an important dataset from the Iberian Peninsula, a region in need of longer-term records that can be used to correlate between marine and terrestrial archives, and between the western and eastern Mediterranean