Fronts from integrodifference equations and persistence effects on the Neolithic transition

We extend a previous model of the Neolithic transition in Europe [J. Fort and V. Méndez, Phys. Rev. Lett. 82, 867 (1999)] by taking two effects into account: (i) we do not use the diffusion approximation (which corresponds to second-order Taylor expansions), and (ii) we take proper care of the fact that parents do not migrate away from their children (we refer to this as a time-order effect, in the sense that it implies that children grow up with their parents, before they become adults and can survive and migrate). We also derive a time-ordered, second-order equation, which we call the sequential reaction-diffusion equation, and use it to show that effect (ii) is the most important one, and that both of them should in general be taken into account to derive accurate results. As an example, we consider the Neolithic transition: the model predictions agree with the observed front speed, and the corrections relative to previous models are important (up to 70%)

Anisotropic dispersion, space competition, and the slowdown of the Neolithic transition

The front speed of the Neolithic (farmer) spread in Europe decreased as it reached Northern latitudes, where the Mesolithic (huntergatherer) population density was higher. Here, we describe a reaction diffusion model with (i) an anisotropic dispersion kernel depending on the Mesolithic population density gradient and (ii) a modified population growth equation. Both effects are related to the space available for the Neolithic population. The model is able to explain the slowdown of the Neolithic front as observed from archaeological data

Front spreading in population dynamic models. Theory and application to the Neolithic transition

This thesis presents population dynamics models that can be applied to predict the rate of spread of the Neolithic transition (change from hunter-gathering to farming economics) across the European continent, which took place about 9000 to 5000 years ago. The first models in this thesis provide predictions at a continental scale. We develop population dynamics models with explicit kernels and apply realistic data. We also derive a new time-delayed reaction-diffusion equation which yields speeds about a 10% slower than previous models. We also deal with a regional variability: the slowdown of the Neolithic front when reaching the North of Europe. We develop simple reaction-diffusion models that can predict the measured speeds in terms of the non-homogeneous distribution of pre-Neolithic (Mesolithic) population in Europe, which were present in higher densities at the North of the continent. Such models can explain the observed speeds.

The “right way” of Hermetism: disputes on Gnostic’s, Pagan’s and Christian’s (re-)visions of the Hermetic phonomenon in late antiquity

This paper aims to understand the phenomenon of Hermetism though the perspective of its process of reception and reproduction in society. It will explore the phenomenon that was the transformation of the Hermetica into a social discourse. The so-called technical and philosophical Hermetica are texts. A text is the result of a production: it is composed by men, and addressed to men. It is important to consider the intentions and values present in a text’s production, and to understand that its process of reception and reproduction in society are, in fact, complex and dynamic.

Evaluating the efficacy of planktonic foraminifer calcite and 18O data for sea surface temperature reconstruction for the Late Miocene

This study examines the efficacy of published δ18O data from the calcite of Late Miocene surface dwelling planktonic foraminifer shells, for sea surface temperature estimates for the pre-Quaternary. The data are from 33 Late Miocene (Messinian) marine sites from a modern latitudinal gradient of 64°N to 48°S. They give estimates of SSTs in the tropics/subtropics (to 30°N and S) that are mostly cooler than present. Possible causes of this temperature discrepancy are ecological factors (e.g. calcification of shells at levels below the ocean mixed layer), taphonomic effects (e.g. diagenesis or dissolution), inaccurate estimation of Late Miocene seawater oxygen isotope composition, or a real Late Miocene cool climate. The scale of apparent cooling in the tropics suggests that the SST signal of the foraminifer calcite has been reset, at least in part, by early diagenetic calcite with higher δ18O, formed in the foraminifer shells in cool sea bottom pore waters, probably coupled with the effects of calcite formed below the mixed layer during the life of the foraminifera. This hypothesis is supported by the markedly cooler SST estimates from low latitudes—in some cases more than 9 °C cooler than present—where the gradients of temperature and the δ18O composition of seawater between sea surface and sea bottom are most marked, and where ocean surface stratification is high. At higher latitudes, particularly N and S of 30°, the temperature signal is still cooler, though maximum temperature estimates overlap with modern SSTs N and S of 40°. Comparison of SST estimates for the Late Miocene from alkenone unsaturation analysis from the eastern tropical Atlantic at Ocean Drilling Program (ODP) Site 958—which suggest a warmer sea surface by 2–4 °C, with estimates from oxygen isotopes at Deep Sea Drilling Project (DSDP) Site 366 and ODP Site 959, indicating cooler than present SSTs, also suggest a significant impact on the δ18O signal. Nevertheless, much of the original SST variation is clearly preserved in the primary calcite formed in the mixed layer, and records secular and temporal oceanographic changes at the sea surface, such as movement of the Antarctic Polar Front in the Southern Ocean. Cooler SSTs in the tropics and sub-tropics are also consistent with the Late Miocene latitude reduction in the coral reef belt and with interrupted reef growth on the Queensland Plateau of eastern Australia, though it is not possible to quantify absolute SSTs with the existing oxygen isotope data. Reconstruction of an accurate global SST dataset for Neogene time-slices from the existing published DSDP/ODP isotope data, for use in general circulation models, may require a detailed re-assessment of taphonomy at many sites.

Croft Moraig and the chronology of stone circles

The Perthshire stone circle of Croft Moraig was excavated 40 years ago and is usually taken to illustrate the classic sequence at such monuments in Britain. A timber setting, accompanied by a shallow ditch, was replaced by two successive stone settings. The pottery associated with the earliest construction was dated to the Neolithic period. A new analysis of the excavated material suggests that, in fact, the ceramics are Middle or Late Bronze Age. They provide a terminus post quem for at least one of the stone settings on the site. Further study of the evidence suggests an alternative sequence of construction at Croft Moraig, involving a change in the axis of the monument. It seems possible that other stone and timber circles were equally late in date and that their period of use in Britain and Ireland may have been longer than is generally supposed.

Interpreting the response of a dryland river system to Late Quaternary climate change

A U-series calcrete chronology has been constructed for three Late Quaternary terrace units, termed the D1, D2 and D3 terraces in age descending order, from the Rio Aguas river system of the Sorbas basin, southeast Spain. The D1 terrace formed between 30,300 +/- 4400 year BP and 12,140 +/- 360 year BP, correlating well with the Last Glacial Maximum when rates of sediment supply would have increased greatly, because of higher rates of weathering, reduced vegetation cover and weak soil development. The D2 terrace formed between 12,800 +/- 1100 year BP and 9,600 +/- 530 year BP, correlating well with the Younger Dryas event. The D3 terrace could only be poorly constrained to the early Holocene and no unequivocal cause could be assigned to this period of aggradation. The sedimentology and geomorphology of the D2 terrace suggests, however, that the aggradation of this unit was a response to diapirism/karstic processes occurring within the underlying Messinian gypsum strata and the subsequent damming of the Aguas system. Therefore, despite its coincident occurrence with the Younger Dryas, aggradation of the D2 terrace is unrelated to climate change. The style of this response, controlled predominantly by the characteristics of the underlying bedrock, makes correlating the terrace record of the Aguas with other systems in the Mediterranean unreliable. This study, therefore, highlights the problems of correlating fluvial sequences in regions of variable tectonics, climatic history and bedrock geology and emphasises the need to properly understand the main controls on individual fluvial systems before any attempt is made to correlate their depositional histories. (C) 2004 Elsevier Ltd. All rights reserved.