Chemical alteration of tephra in the depositional environment: theoretical stability modelling

The study of the chemical stability of vitreous material in aqueous media is well‐established. There has to date been little consideration of the implications of variations in the chemical durability of tephra in Quaternary tephrochronology. Chemical alteration can take the form of cationic leaching from the matrix, or complete destruction of the silica network, either of which could constrain the ability to chemically identify distal tephra. Here we apply established models of vitreous durability to the published chemical analyses of a large number of Icelandic tephras in order to predict their relative durabilities under equivalent conditions. This suggests that some important tephras have relatively poor chemical stability, and that rhyolitic tephras are, in general, more stable than basaltic. We conclude that tephras should be expected to show predictable differential chemical stability in the post‐depositional environment. Copyright © 2003 John Wiley & Sons, Ltd.     

The lateglacial Quercus expansion in the southern European Alps: rapid vegetation response to a late Allerød climate warming?

A pollen‐inferred vegetation shift, from pioneer birch–pine woodland to mixed pine–summergreen oak forests, in the southern Alpine forelands, is commonly attributed to a centennial‐scale warming that occurred between the Gerzensee Oscillation (GO) and the Younger Dryas. Two microtephra layers bracketing the Younger Dryas onset (the Laacher See Tephra and the Vedde Ash) improve the chronology at Lago Piccolo di Avigliana (northern Italy) and allowed accurate correlation with Central European records where the GO is clearly detected. We used pollen percentages, pollen accumulation rates (PARs) and plant macrofossils to assess the population dynamics of Quercus, and leaf‐cuticle analysis for a better taxonomic identification of Quercus. Our results indicate that the species that was locally present was probably Quercus robur. PARs suggest that the population expansion started as early as the Bølling and followed an exponential increase through time. We attribute this gradual shift to increasing summer temperatures and longer growing seasons which contrast with a gradually decreasing temperature trend as recorded in Greenland ice cores and in Central Europe. Breaks or set‐backs in the PAR record may indicate the biotic response to minor Lateglacial cooling events of different life‐history stages in the Quercus population. Copyright © 2011 John Wiley & Sons, Ltd.     

A revised chronology for the adoption of agriculture in the Southern Levant and the role of Lateglacial climatic change

This paper re-examines the chronology and environmental context for the transition to agriculture in the Southern Levant, seen as the likely starting point for the adoption of agriculture in Europe and the Near East. The role in this process of abrupt late Quaternary climate change has been discussed widely, but limitations on the archaeological and palaeoenvironmental chronologies have led to varying interpretations. Here we attempt to clarify the situation by first testing the available radiocarbon database for the archaeological transitions from the Natufian through to the PPNA. We apply internationally accepted radiocarbon quality assurance procedures and find that a significant number of the published dates fall bellow acceptable standards. The cleaning process significantly clarifies and constrains the reported time ranges for the Natufian, Late Natufian and PPNA. We then apply the new IntCal09 calibration curve and Bayesian calibration methods, using the archaeological phasing to constrain the data and calculate the most likely timing of the transitions between each phase. We then compare the onset and duration of archaeological phases to data representing the key Northern Hemisphere climatic transitions, using the new GICC05 Greenland Ice core timescale and the timing of transitions between wet and dry phases in the southern Levant from published high precision isotopic analyses of Speleothem data. The results of this exercise present the currently best available chronology for these events and suggest that during the second part of the Lateglacial interstadial, drying of the southern Levant may have triggered the transition to the Late Natufian, when hunter-gatherer communities resorted to a more mobile lifestyle. The Late Natufian culture appears to have disappeared from the southern Levant during the Younger Dryas, as drying intensified. There is then a gap in well dated evidence for human occupation until a reappearance of humans at the onset of the Pre-Pottery Neolithic A (PPNA) period at the beginning of the Holocene. Thus the onset of the Holocene can be hypothesised to be the driver behind the onset of the Neolithic in this region.     

The multiple chronological techniques applied to the Lake Suigetsu SG06 sediment core,central Japan

The varved sediment of Lake Suigetsu (central Japan) provides a valuable opportunity to obtain high‐resolution, multi‐proxy palaeoenvironmental data across the last glacial/interglacial cycle. In order to maximize the potential of this archive, a well‐constrained chronology is required. This paper outlines the multiple geochronological techniques being applied – namely varve counting, radiocarbon dating, tephrochronology (including argon–argon dating) and optically stimulated luminescence (OSL) – and the approaches by which these techniques are being integrated to form a single, coherent, robust chronology. Importantly, we also describe here the linkage of the floating Lake Suigetsu (SG06) varve chronology and the absolute (IntCal09 tree‐ring) time scale, as derived using radiocarbon data from the uppermost (non‐varved) portion of the core. This tie‐point, defined as a distinct (flood) marker horizon in SG06 (event layer B‐07–08 at 1397.4 cm composite depth), is thus derived to be 11 255 to 11 222 IntCal09 cal. years BP (68.2% probability range).     

Evidence for the presence of the Vedde Ash in Central Europe

As part of ongoing research to find distal tephra, two lacustrine cores were analysed for the presence of volcanic ash layers, not visible to the naked eye: Soppensee, in Switzerland, and Rotmeer, in Southern Germany. The Laacher See Tephra (12,900 ka BP) is present as a visible layer in both sites. In both cores we found a discrete tephra horizon, with similar morphologies, in the middle of the biostratigraphic units equivalent to the Younger Dryas stadial. The vitreous components of these two tephra layers are geochemically identical. Comparison of the geochemical, stratigraphical, and chronological data from both sites, strongly suggest that the tephra can be attributed to the Icelandic Vedde Ash, a widely distributed horizon found throughout the North Atlantic and Northern Europe. Our results indicate that a precise and direct correlation between the Greenland ice cores and Central European sequences is now possible, based on a co-located tephra layer. This means that there is now the potential, to independently test climate synchronicity between Greenland and Europe, as far south as the Alps.     

Granite ages within the Archaean Pilbara Block,Western Australia

Within the Pilbara Block of Western Australia, a complex of migmatite, gneissic and foliated granite near Marble Bar is intruded by a stock of younger massive granite (the Moolyella Granite) with which swarms of tin‐bearing pegmatites are associated. The age of the older granite has been determined by the Rb‐Sr method as 3,125 ± 366 m.y., and that of the Moolyella Granite as 2,670 ± 95 m.y. Initial Sr⁸⁷/Sr⁸⁶ ratios suggest that the older granite is close to primary crustal material, but that the Moolyella Granite consists of reworked material. It probably formed by partial remelting of the older granite.     

Pilbara galenas: Precise isotopic assay of the oldest Australian leads; model ages and growth-curve implications

Precise Pb isotopic data are reported for 19 galenas and 2 other minerals from the Archaean Pilbara District of Western Australia. Repeated measurements on the Broken Hill Standard Ore show good agreement with published calibrations from two other laboratories, and an overall 95 percent confidence level within ±0.05 percent per mass unit, whether fractionation correction is by the double-spike procedure or by use of a simple mass-dependent factor. Two stratiform deposits yield the most primitive ratios (²⁰⁶Pb/²⁰⁴Pb=11.89; ²⁰⁷Pb/²⁰⁴Pb=13.69; ²⁰⁸Pb/²⁰⁴Pb=31.79) and model ages in good agreement with the 3.45 Ga age on zircons from volcanics correlatable with the host-rocks. Other minor galena occurrences, with quartz-vein association, appear to be younger, with position along the growth curve surprisingly consistent with the age sequence predicted from field observation. The data also extend the time-range of previously-reported trends in apparent U/Pb of the conceptual Pb source, and reveal new features of the corresponding Th/U systematics: a generally-observable trend in the Archaean of Australia, Southern Africa and West Greenland is that the source Th/U seems significantly higher than is allowed for in any of the current Pb-growth models. In detail, the sample-sample variations in indicated source parameters are in many cases correlatable with the nature of the neighbouring rocks and emphasise that, whereas Pb isotope rations in the galenas of huge deposits may well serve as a convenient sampler of overall trends in the Earth's crust, subeconomic occurrences such as these may reveal valuable information about local variations in U-Th-Pb distribution.     

The occurrence of distal Icelandic and Italian tephra in the Lateglacial of Lake Bled,Slovenia

The discovery of sites preserving tephra layers from multiple volcanic centres is key to constructing a single European tephrostratigraphic framework for the Late Quaternary. Until now, the tephrostratigraphy of Europe has been divided into two halves: sites in the North Atlantic and northern Europe regions link the Icelandic, Eifel, and the Massif Central volcanic histories; whilst sites in southern Europe record the sequence of tephra layers produced by circum-Mediterranean volcanic provinces. The missing link, able to tie together these two halves, is found in the tephrostratigraphic record of Lake Bled, Slovenia.Lake Bled, in the Julian Alps, Slovenia, holds a high resolution multi-proxy palaeoenvironmental archive for the Lateglacial of south-central Europe. Cryptotephra investigations have revealed three tephra layers: two closely spaced within Younger Dryas stadial sediments and one shortly after the start of the Bølling-Allerød interstadial warming. Two of the tephra layers (Bld_T120 and Bld_T240) are of Campanian origin and are correlated to deposits of the Pomici Principali (PP) and Neapolitan Yellow Tuff (NYT) eruptions, respectively. The third layer (Bld_T122) correlates to the Icelandic Vedde Ash (VA), extending the known fallout of this widespread marker layer farther to the southeast.The Lake Bled record also allows the stratigraphic relationship and relative ages of the VA and the PP eruption to be discerned for the first time. Whilst existing numerical age estimates for these two deposits are indistinguishable within errors, their close occurrence in the same lacustrine sediment sequence shows that the VA was erupted shortly prior to the PP eruption.The tephrostratigraphy of Lake Bled developed here helps us to tie together regional volcanic stratigraphies into a broader, continental-scale lattice of sites, with the potential to allow the transfer of dates between remote sequences and the construction of relative chronologies, beneficial in particular for environmental and archaeological research.     

Improved age modelling and high-precision age estimates of late Quaternary tephras,for accurate palaeoclimate reconstruction

The role of tephrochronology, as a dating and stratigraphic tool, in precise palaeoclimate and environmental reconstruction, has expanded significantly in recent years. The power of tephrochronology rests on the fact that a tephra layer can stratigraphically link records at the resolution of as little as a few years, and that the most precise age for a particular tephra can be imported into any site where it is found. In order to maximise the potential of tephras for this purpose it is necessary to have the most precise and robustly tested age estimate possible available for key tephras. Given the varying number and quality of dates associated with different tephras it is important to be able to build age models to test competing tephra dates. Recent advances in Bayesian age modelling of dates in sequence have radically extended our ability to build such stratigraphic age models. As an example of the potential here we use Bayesian methods, now widely applied, to examine the dating of some key Late Quaternary tephras from Italy. These are: the Agnano Monte Spina Tephra (AMST), the Neapolitan Yellow Tuff (NYT) and the Agnano Pomici Principali (APP), and all of them have multiple estimates of their true age. Further, we use the Bayesian approaches to generate a revised mixed radiocarbon/varve chronology for the important Lateglacial section of the Lago Grande Monticchio record, as a further illustration of what can be achieved by a Bayesian approach. With all three tephras we were able to produce viable model ages for the tephra, validate the proposed ⁴⁰Ar/³⁹Ar age ranges for these tephras, and provide relatively high precision age models. The results of the Bayesian integration of dating and stratigraphic information, suggest that the current best 95% confidence calendar age estimates for the AMST are 4690–4300 cal BP, the NYT 14320–13900 cal BP, and the APP 12380–12140 cal BP.