Constantin Cocirta

SEDIMENT YIELD DURING LATE GLACIAL AND HOLOCENE PERIODS IN THE LAC CHAMBON WATERSHED, MASSIF CENTRAL, FRANCE

A sediment budget for the Late Glacial and Holocene periods was calculated for the Lac Chambon watershed which is located in a formerly glaciated temperate crystalline mountain area. It appears that over 15 500 years: (1) 69 per cent of eroded particles have been displaced by gravity processes and then stored within the watershed, compared to 31 per cent that have been displaced by running water and evacuated outward; (2) the mean mechanical erosion due to gravity processes on the slopes amounted to 16·1± 6 m and only developed on a quarter of the watershed surface, whereas the mean mechanical erosion due to running water amounted 1·24± 0·37 m and involved the whole watershed surface. The mean sediment yields due to gravity processes on slopes were 2300 ±1360, 1770± 960 and 380±100 m 3 km -3 a -1 , respectively, for basalts, and basic and acidic trachyandesites. Values of sediment yield due to running water were 49 ±15, 120 ± 36 and 79± 24 m 3 km -2 a -1 , respectively, during the Bolling‐Allerod, the Younger Dryas and the Pre-Boreal‐Boreal periods. They were 56±17 and 166± 50 m 3 km -2 a -1 during the Sub-Atlantic period before and after 1360 a BP, respectively. These values reflect variations in the natural environment and the impact of human-induced deforestation.

Petrography of alluvial sands as a past and present environmental indicator： Case of the Loire River （France）

Four sample sets of the Upper and Middle Loire river sands were analyzed in order to study the impact of natural and anthropogenic factors on their petrographic composition in space (on an 800 km stretch) and time. Composition was determined by modal analysis of three sand-size fractions using a polarizing optical microscope and calculated for each sample (“standard sand” = Sst). The watershed is composed mainly of endogenic (Massif Central) and sedimentary (southern Parisian Basin) rocks. B-set sands collected in channels for different water flows in 1996 show that Sst compositions vary by only 5 %. Present-day sands in the Upper Loire and Middle Loire have very high petrographic immaturity comparing to others worldwide fluvial sands, although bio-climatic conditions favor sand maturation by source-rock weathering in the watershed. This shows the strong impact of the Massif Central on sediment yield due to relief rejuvenation as a consequence of the formation of the Alps during the Quaternary. Fluvial sands stored during the Weichselian and the Holocene in the Middle Loire floodplain, although partly weathered since their deposition, show higher inputs from the endogenic rocks of the Massif Central than present-day deposits. This can be explained by Weichselian periglacial conditions and the development of crop farming since the Neolithic, which favored mechanical erosion, particularly in the Massif Central which is characterized by a cold, humid climate and steep slopes. The upstream-downstream change in the composition of presently deposited sand is low in the diked area. It shows however that basalt and some heavy mineral grains are vulnerable to abrasion during transport and indicates a marked sediment yield from ancient sediment stored in the floodplain. This is in line with the high incision of the river bed over the last 150 years partly due to dam construction and aggregate mining.

Chemical characterization of aquifers with the same geological origin

Physical characteristics of the Bicaz dammed lake are described as follows: water volume of 1.150x 109 m 3, 31.1 km in length, 2 km in width. The sluice is at 45 m above the maximal altitude of the lake. The Bistrita River is the main tributary of the lake: mean water contribution of 1.721x109 m3/a, mean suspended matter flow of 291000 t/a. This river drains a catchment rich in formerly mined mineralizations of polymetallic sulfides. The aim of this study is to determine if the lake can be considered as a sink toward trace elements. We analyzed major and minor elements, TOC, organic N, and some trace elements (e.g. As, Ba, Cd, Cr, Co, Cu, Mo, Ni, Pb, Sb, Sn, V, U, Zn) in filtered and bulk water samples. We worked on three locations, an upper site, a middle site, and a down site. Here we present the data for July and October 2005. Some trace elements have smaller dissolved concentrations in lake water than in the Bistrita River. Common associations can be defined in this lake such as: Co is associated with Mn; As is released in solution as Fe oxi-hydroxide is dissolved; V and U are following the same behaviour, etc. However, their distribution is not as regular as in a natural lake. The stratification of the water column at all 3 sampled sites is not as regular as expected. At the upper site (water depth of 33 m), the water column is homogeneous until 20 m, then the redox condition allows having Mn as Mn (+ II ) and Fe as Fe (+III). For some elements a perturbation can be noticed at about 10 m. At the middle site (water depth of 59 m), there is a water section richer in dissolved and particulate Fe and Mn, with a dominant particulate form. This can assess this section, in between 30 to 50 m it is more oxidizing. At the down site, the reverse figure can be seen between 30 and 50 m depth. This redox anomaly is more noticeable for elements with a high solubility difference according to their oxidation rate. Other parameters do not follow this figure. This section could be linked to the sluice drawing off. In the whole lake, the stratification is more and more visible from the up to the down stream but it is not so pronounced than in natural lakes. Water column structuring seems to be disturbed by a lateral flow, induced by the sluice drawing off.