Impact of fractional probability distributions on statistics of hydraulic conductivity,dynamics of groundwater flow and solute transport at a low-permeability site

The fractional Brownian motion (fBm) and fractional Lévy motion (fLm) can easily describe the geometry and the statistical structure of hydraulic conductivity (K) for real-world. However, the fBm and fLm models have not been systematically evaluated when building the K field for a low-permeability site. In this study, both the fBm and fLm are used to simulate the low-K field at NingCheGu (NCG), Tianjin, China. Groundwater flow and solute transport are then computed using MODFLOW and MT3DMS, respectively, and the influence of the fBm/fLm models for K on groundwater flow and solute transport is discussed. Results show that the fLm fits better the statistics of the low-K medium than fBm, and the random logarithmic K (LnK) field generated by fLm is more stable because the resultant LnK field captures more of the measured properties at the field site than that generated by fBm. In contrast, the LnK generated by fBm is more likely to form both high-K channels and low-K barriers. The fBm therefore predicts more extreme behaviours in flow and transport, including the preferential flow, low-concentration blocks and solute retention. The overall groundwater renewal period and solute travel time for the fLm simulation are slightly shorter than those for fBm. The impacts of the fLm and fBm models on the statistics of the resultant LnK fields and the dynamics of groundwater flow and solute transport revealed by this study shed light on the selection and evaluation of the fractional probability distribution models in capturing the K fields for low-K media.     

The Afragola settlement near Vesuvius,Italy: The destruction and abandonment of a Bronze Age village revealed by archaeology,volcanology and rock-magnetism

Public works in progress in the Campanian plain north of Somma-Vesuvius recently encountered the remains of a prehistoric settlement close to the town of Afragola. Rescue excavations brought to light a Bronze Age village partially destroyed and buried by pyroclastic density currents (PDCs) of the Vesuvian Pomici di Avellino eruption (3.8 ¹⁴C ka BP) and subsequently sealed by alluvial deposits. Volcanological and rock-magnetic investigations supplemented the excavations.Careful comparison between volcanological and archaeological stratigraphies led to an understanding of the timing of the damage the buildings suffered when they were struck by a series of PDCs. The first engulfed the village, located some 14 km to the north of the inferred vent, and penetrated into the dwellings without causing major damage. The buildings were able to withstand the weak dynamic pressure of the currents and deviate their path, as shown by the magnetic fabric analyses. Some later collapsed under the load of the deposits piled up by successive currents. Stepwise demagnetization of the thermal remanent magnetization (TRM) carried by potsherds embedded in the deposits yields deposition temperatures in the order of 260–320 °C, fully consistent with those derived from pottery and lithic fragments from other distal and proximal sites. The fairly uniform temperature of the deposits is here ascribed to the lack of pervasive air entrainment into the currents. This, in turn, resulted from the lack of major topographical obstacles along the flat plain.The coupling of structural damage and sedimentological analyses indicates that the currents were not destructive in the Afragola area, but TRM data indicate they were still hot enough to cause death or severe injury to humans and animals. The successful escape of the entire population is apparent from the lack of human remains and from thousands of human footprints on the surface of the deposits left by the first PDCs. People were thus able to walk barefoot across the already emplaced deposits and escape the subsequent PDCs. The rapid cooling of the deposits was probably due to both their thinness and heat dissipation due to condensation of water vapour released in the mixture by magma–water interaction.     

Statistical Analysis of Palaeomagnetic Data from the Last Four Centuries: Evidence of Systematic Inclination Shallowing in Lava Flow Records

The main objective of this work is to compare directional (declination and inclination) volcanic and archaeomagnetic data for the last four centuries (~1600–1990) with the historical geomagnetic predictions given by the GUFM1 model which spans from 1590 to 1990. The results show statistical agreement between archaeomagnetic data and directions given by the geomagnetic field model. However, when comparing the volcanic data with the model predictions, marked inclination shallowing is observed. This systematically lower inclination has already been observed in local palaeomagnetic studies (Italy, Mexico and Hawaii) for the 20th century, by comparing recent lava flows with the International Reference Geomagnetic Field (IGRF) model. Here, we show how this inclination shallowing is statistically present at worldwide scale for the last 400 years with mean inclination deviation around 3° lower than the historical geomagnetic field model predictions.     

An updated catalogue of Greek archaeomagnetic data for the last 4500 years and a directional secular variation curve

We present an updated compilation of Greek directional archaeomagnetic data for the last 4.5 millennia. The data set comprises 89 directions from archaeological artefacts and volcanic rocks. Most of the data come from the Late Bronze Age (1700-1400 BC) that is the flourishing period of the Minoan civilization in Crete, while parts of the classical (480-323 BC), Hellenistic (323-31 BC) and Roman (146 BC-330 AD) periods are also well covered. The dataset has been analysed using the Bayesian approach for curve building and a directional secular variation (SV) curve for Greece is proposed. Comparisons with regional and global model predictions show a general agreement even though some discrepancies are observed for some time intervals. The new curves together with the previously published intensity SV curve for Greece, also using the Bayesian approach, form a homogeneous set and enrich our knowledge of the full geomagnetic field vector variation in Greece during the last millennia.     

Palaeotemperature estimation of the pyroclastic deposit covering the pre-Minoan palaeosol at Megalochori Quarry, Santorini (Greece): Evidence from magnetic measurements

Thermal remanent magnetization analyses were carried out on ceramic fragments and lithic clasts embedded in the first pumice fall deposits of the Minoan eruption. The aim of this study is to estimate the equilibrium temperature after deposition of these pyroclastic fall deposits and their thermal effect on the pre-Minoan surface. A total of 30 samples from 22 independent ceramic fragments and 20 samples from 14 lithic clasts have been studied. Samples were collected from the Megalochori Quarry, located at the southern part of Santorini island. Stepwise thermal demagnetization reveals that the ceramics were mostly re-heated at temperatures around 140–180°C; in few ceramics a higher temperature component is also present, probably related to the original heating or the use of the ceramics before the eruption. Thermal demagnetization of the lithic clasts shows similar results with slightly higher re-heating temperatures, around 180–240°C. The estimated temperatures represent the equilibrium temperatures obtained after the deposition of the pumice fall and show that the pyroclastic fall deposits at a distance of around 6 km from the eruption vent maintained a temperature high enough to re-heat the buried ceramics at temperatures around 140–180°C.