Jean-Gabriel Bréhéret

6700 yr sedimentary record of climatic and anthropogenic signals in Lake Aydat (French Massif Central)

A 19 m long sedimentary record retrieved in Lake Aydat (French Massif Central) covers the last 6700 yr at a high resolution. A multiproxy approach (density, magnetic susceptibility, XRF, Rock-Eval, pollen and non-pollen palynomorph analyses and a historical archives study) was used to characterise and propose a model of sedimentation. The high deposition rate results from the combined effects of the high suspension load of the river, autochthonous production and the narrow shape of the incised fluvial valley dammed by a lava flow c. 8550 years ago. Two contrasted periods (6700±200 to 3180±90 cal. BP, and 1770±60 cal. BP to present) were characterized. The lower unit (mid Holocene) displays a fine and regular lamination and holds a single, major, flood deposit. This unit is capped by an erosive mass-wasting deposit triggered c. 1770±60 cal. BP. The upper unit (late Holocene) is made of organic-rich and fine-grained faintly laminated sediment, with numerous interbedded flood deposits and diatom blooms. The sedimentation was principally controlled by climatic forcings until c. 1100 cal. BP, accompanied by detrital events linked to human activities around the lake. Then, a more detrital input attested by numerous and recurrent flood deposits can be linked to the intensification of a persistent anthropogenic impact on the catchment. Two phases of lake eutrophication are highlighted: 1200–1130 cal. BP, as a result of increased anthropogenic pressure, and the current phase that could have started c. 150 cal. BP.

Sedimentary cannabinol tracks the history of hemp retting

Hemp (Cannabis sp.) has been a fundamental plant for the development of human societies. Its fibers have long been used for textiles and rope making, which requires prior stem retting. This process is essential for extracting fibers from the stem of the plant, but can adversely affect the quality of surface waters. The history of human activities related to hemp (its domestication, spread, and processing) is frequently reconstructed from seeds and pollen detected in archaeological sites or in sedimentary archives, but this method does not always make it possible to ascertain whether retting took place. Hemp is also known to contain phytocannabinoids, a type of chemicals that is specific to the plant. Here we report on the detection of one of these chemicals, cannabinol (CBN), preserved in a sediment record from a lake in the French Massif Central covering the past 1800 yr. The presence of this molecule in the sedimentary record is related to retting. Analysis of the evolution of CBN concentrations shows that hemp retting was a significant activity in the area until ca. A.D. 1850. These findings, supported by pollen analyses and historical data, show that this novel sedimentary tracer can help to better constrain past impacts of human activities on the environment.

Quantitative analysis of climate versus human impact on sediment yield since the Lateglacial: The Sarliève palaeolake catchment (France)

Minimum rates of solid (SSY) and dissolved (DSY) sediment yield (SY) were evaluated in t/km 2 per yr from sediments stored in the Sarliève palaeolake (French Massif Central) for seven phases of the Lateglacial and Holocene up to the seventeenth century. The catchment (29 km2), mainly formed of limestones and marls, is located in an area rich in archaeological sites in the Massif Central. The respective impacts of human activities and climate on SY were compared by quantification of human settlements through archaeological survey and palynological data. During the Lateglacial and early Holocene up to about 7500 yr cal. BP, variations in SSY and DSY rates were mainly related to climate change with higher rates during colder periods (Younger Dryas and Preboreal) and lower rates during warmer periods (Bölling-Alleröd and Boreal). However, CF1 tephra fallout induced a sharp increase in SY during the Alleröd. During the middle and late Holocene after 7500 yr cal. BP, SSY and DSY greatly increased (by factors of 6.5 and 2.8, respectively), particularly during the Final Neolithic at about 5300 yr cal. BP when the climate became cooler and more humid. After this date, at least 75% of the SSY increase and more than 90% of the DSY increase resulted from human activities, but SSY rates showed little variation during Protohistoric and Historic Times up to the seventeenth century. SSY and DSY rates and DSY/SSY ratio indicate that catchment soils began to form during the Lateglacial and Preboreal, thickened considerably during the Boreal and Atlantic, finally thinning (rejuvenation) mainly as the result of human-induced erosion during the sub-Boreal and sub-Atlantic. Increased mechanical erosion during the late Holocene also induced an increase in chemical erosion.

Un exemple de spéciation lié à l'eustatisme: l'apparition précoce de Schackoina cabri (foraminifère planctonique mésogéen)

Investigation of planktic foraminifers from the Lower Aptian of the Vocontian Basin (SE France) reveals that Schackoina cabri makes a very discrete first appearance in the Weissi ammonite zone and not in the Deshayesi zone, i.e. before the global anoxic event OAE 1a and not after as was accepted in the literature. This discovery seems to support the recent model whereby speciation of planktic foraminifers occurred in marine lowstands and their real expansion during the ensuing sea-level rise.