François Demory

Early Pleistocene Presence of Acheulian Hominins in South India

Dates from a site in southeast India imply an early migration of Homo through Eurasia about 1.1 to 1.5 million years ago. South Asia is rich in Lower Paleolithic Acheulian sites. These have been attributed to the Middle Pleistocene on the basis of a small number of dates, with a few older but disputed age estimates. Here, we report new ages from the excavated site of Attirampakkam, where paleomagnetic measurements and direct 26Al/10Be burial dating of stone artifacts now position the earliest Acheulian levels as no younger than 1.07 million years ago (Ma), with a pooled average age of 1.51 ± 0.07 Ma. These results reveal that, during the Early Pleistocene, India was already occupied by hominins fully conversant with an Acheulian technology including handaxes and cleavers among other artifacts. This implies that a spread of bifacial technologies across Asia occurred earlier than previously accepted.

The effect of hydrostatic pressure up to 1.61 GPa on the Morin transition of hematite‐bearing rocks: Implications for planetary crustal magnetization

We present new experimental data on the dependence of the Morin transition temperature (TM) on hydrostatic pressure up to 1.61 GPa, obtained on a well-characterized multidomain hematite-bearing sample from a banded iron formation. We used a nonmagnetic high-pressure cell for pressure application and a Superconducting Quantum Interference Device magnetometer to measure the isothermal remanent magnetization (IRM) under pressure on warming from 243 K to room temperature (T0). IRM imparted at T0 under pressure in 270 mT magnetic field (IRM270mT) is not recovered after a cooling-warming cycle. Memory effect under pressure was quantified as IRM recovery decrease of 10%/GPa. TM, determined on warming, reaches T0 under hydrostatic pressure 1.38–1.61 GPa. The pressure dependence of TM up to 1.61 GPa is positive and essentially linear with a slope dTM/dP = (25 ± 2) K/GPa. This estimate is more precise than previous ones and allows quantifying the effect of a pressure wave on the upper crust magnetization, with special emphasis on Mars.

RATE AND PROCESSES OF RIVER NETWORK REARRANGEMENT DURING INCIPIENT FAULTING: THE CASE OF THE CAHABON RIVER, GUATEMALA

Deeply incised river networks are generally regarded as robust features that are not easily modified by erosion or tectonics. Although the reorganization of deeply incised drainage systems has been documented, the corresponding importance with regard to the overall landscape evolution of mountain ranges and the factors that permit such reorganizations are poorly understood. To address this problem, we have explored the rapid drainage reorganization that affected the Cahabón River in Guatemala during the Quaternary. Sediment-provenance analysis, field mapping, and electrical resistivity tomography (ERT) imaging are used to reconstruct the geometry of the valley before the river was captured. Dating of the abandoned valley sediments by the 10Be-26Al burial method and geomagnetic polarity analysis allow us to determine the age of the capture events and then to quantify several processes, such as the rate of tectonic deformation of the paleovalley, the rate of propagation of post-capture drainage reversal, and the rate at which canyons that formed at the capture sites have propagated along the paleovalley. Transtensional faulting started 1 to 3 million years ago, produced ground tilting and ground faulting along the Cahabón River, and thus generated differential uplift rate of 0.3 ± 0.1 up to 0.7 ±0.4 mm · y−1 along the rivers course. The river responded to faulting by incising the areas of relative uplift and depositing a few tens of meters of sediment above the areas of relative subsidence. Then, the river experienced two captures and one avulsion between 700 ky and 100 ky. The captures breached high-standing ridges that separate the Cahabón River from its captors. Captures occurred at specific points where ridges are made permeable by fault damage zones and/or soluble rocks. Groundwater flow from the Cahabón River down to its captors likely increased the erosive power of the captors thus promoting focused erosion of the ridges. Valley-fill formation and capture occurred in close temporal succession, suggesting a genetic link between the two. We suggest that the aquifers accumulated within the valley-fills, increased the head along the subterraneous system connecting the Cahabón River to its captors, and promoted their development. Upon capture, the breached valley experienced widespread drainage reversal toward the capture sites. We attribute the generalized reversal to combined effects of groundwater sapping in the valley-fill, axial drainage obstruction by lateral fans, and tectonic tilting. Drainage reversal increased the size of the captured areas by a factor of 4 to 6. At the capture sites, 500 m deep canyons have been incised into the bedrock and are propagating upstream at a rate of 3 to 11 mm · y−1 while deepening at a rate of 0.7 to 1.5 mm · y−1. At this rate, 1 to 2 million years will be necessary for headward erosion to completely erase the topographic expression of the paleovalley. It is concluded that the rapid reorganization of this drainage system was made possible by the way the river adjusted to the new tectonic strain field, which involved transient sedimentation along the rivers course. If the river had escaped its early reorganization and had been given the time necessary to reach a new dynamic equilibrium, then the transient conditions that promoted capture would have vanished and its vulnerability to capture would have been strongly reduced.

Late Pleistocene to Mid Holocene landscape reconstruction in the western part of the Thessaloniki Plain (Greece): evidence for environmental changes, and their consequences for the settlement history

This study presents new data from four boreholes in order to investigate the palaeoenvironmental changes and landscape configuration from the Late Pleistocene to the Mid-Holocene in the western part of the Thessaloniki plain. Palaeoenvironmental and archaeological records from northern Greece document the occurrence of significant landscape changes during the Holocene. While traces of human occupation are scarce in the western Thessaloniki plain during the transition from the Late Pleistocene (Lateglacial) to the Holocene (Mesolithic period), the climatic and environmental changes involved significant and abrupt impacts on surface processes. Recently published results indicated that palaeoenvironmental transitions in the area influenced the human occupation from the early Holocene (8000/7800 cal. BP) to recent times. However, the Late Pleistocene landscape configuration has not been clearly established. A new sequence is used for analyses of grain size, loss-on-ignition, carbonate content and rock magnetic properties. In addition, a series of 6 new AMS dates from the area provide a precise chronostratigraphy. The synthesis of the environmental proxies reveals distinct palaeoenvironmental changes within a well-constructed chronostratigraphic framework of more than 10k years, which is the longest sedimentary sequence reported for central Macedonia. The Late Pleistocene subsurface occurs at ~9 m depth and clearly indicates the presence of a large alluvial fan deposited by the Aliakmon River during the Younger Dryas (12.7k-11.7kyr BP). The abrupt transition to Early Holocene sediments reveals that a significant influx of fresh water generated a limnic sedimentary environment under detrital and human stress, from pre- to post-reforestation period in the area. Anthropogenic influence is also significantly recorded mainly by the magnetic properties of the lacustrine sediments after 9000 cal. BP. From Neolithic times to the present day, the study area was periodically occupied by a freshwater lake while palaeo-soils are clearly identified by anthropogenic activities and are dated from the Early Neolithic period.

Thermoremanence acquisition and demagnetization for titanomagnetite under lithospheric pressures

The geological sources of large-scale lithospheric magnetic field anomalies are poorly constrained. Understanding the magnetic behavior of rocks and minerals under the pressures and temperatures encountered at large crustal depths is particularly important in that task. The impact of lithospheric pressure is not well known and most of the time neglected in numerical models of the geological sources of magnetic anomalies. We present thermal remanent magnetization (TRM) acquisition, and stepwise thermal demagnetization on synthetic titanomagnetite dispersed powder, within an amagnetic cell under hydrostatic pressure up to 1 GPa. TRM is measured after thermal cycling within a cryogenic magnetometer. Pressure-dependent increase in the Curie temperature (initially in the 50-70 °C range) is observed, mostly between 0.3 and 0.6 GPa, on the order of 20 K/GPa. TRM intensity also increases with pressure up to 200% at 675 MPa, although the pressure variation with temperature inside the cell complicates the interpretation.