Headwater valley response to climate and land use changes during the Little Ice Age in the Massif Central (Yzeron basin, France)
Introduction
The Little Ice Age (LIA) is the last pronounced period of climate cooling, which was defined by historians and Earth Sciences as comprising stricto sensu A.D. 1550–1850 (Lamb, 1977, Grove, 1988, Acot, 2005). This episode is marked by a significant advance of Northern Hemisphere glaciers owing to long and severe winters and relatively cool and wet summers. These climatic conditions are particularly well documented by historical climatology studies through the evolution of the grape harvest dates (Dufour, 1870, Angot, 1883, Le Roy Ladurie, 1967, Chuine et al., 2004, Meier et al., 2007, Garnier et al., 2011, Daux, 2012) and the comparison of iconographic representations of glaciers (Le Roy Ladurie, 1967, Nussbaumer et al., 2012, Zumbühl and Nussbaumer, 2012). Some work has also been conducted on the geomorphological evolution of rivers and valley bottoms since the Medieval Climate Optimum (Bravard, 1989, Bravard, 2000, Rumsby and Macklin, 1996, Gob, 2005, Gob et al., 2008, Jacob-Rousseau and Astrade, 2010, Jacob-Rousseau and Astrade, 2014, Macklin et al., 2012;; Notebaert et al., 2014, Lespez et al., 2015). For example, Bravard, 1989, Bravard, 2000 has shown that the braided pattern of most rivers of the French Alps appeared after the increase of sediment production during the LIA, but sometimes with a significant time delay. This trend was not found in the Mediterranean area and the Massif Central, in part because of fewer studies on geomorphological adjustments of watercourses during the LIA (Gob et al., 2008, Cubizolle, 2009). In this regard, Jacob (2003) concluded that southeastern Massif Central rivers experienced some phases of short hydrosedimentary fluctuations, i.e., aggradation of their alluvial floor during periods of high sediment supply followed by incision rather than a bed metamorphosis like the Alpine rivers. Moreover, the manifestation of the LIA does not appear to be homogenous across the Mediterranean region (Berger et al., 2010) because low temperatures were associated with drought in the eastern part as in the Iberian Peninsula (Carozza et al., 2014); such an evolution could have decreased sediment production. Finally, Macklin et al., 1994, Macklin et al., 1995 reported a negative feedback in semiarid Mediterranean mountainous landscapes, where the wetter climate could have increased the vegetation cover and then decreased hillslope coarse material supply.
In the present work, we focus on the internal secular rhythmicity of the LIA. Notably, the LIA was not a period of continuously disturbed climatic conditions over three centuries, but characterized by quiet phases interrupting the general trend. Indeed, some authors have shown that this period was not characterized by disturbed climatic conditions but known as an alternance of crises and lulls. It has been pointed out first in the southern Alps (Douguedroit, 1979, Jorda, 1980, Neboit, 1983, Gautier, 1992, Pichard, 1995, Miramont and Guilbert, 1997) then in the Languedoc (Berger et al., 2010) and the southeastern Massif Central (Jacob, 2003, Gob, 2005, Gob et al., 2008, Astrade et al., 2011). The fluvial sensitivity to these fluctuations may depend on the size of the basin. The timing of adjustments can also vary because the downstream progradation of debris from headwater streams to lower valleys presumably requires a relatively long time (Bravard, 1989, Bravard, 1993, Gob et al., 2008, Astrade et al., 2011, Jacob-Rousseau and Astrade, 2014). According to Gob et al. (2008), the three main sedimentation phases of rivers and deltas occurred exactly at the same time as the principal torrential crises that affected the western Mediterranean during the LIA (see discussion below). Conversely, these episodes are interspersed with two multidecennial phases of severe droughts during the second half of the seventeenth and early eighteenth centuries and during the early nineteenth century (Carozza et al., 2014). Nevertheless, Wilhelm et al. (2012) observed a time delay in flood frequencies during the last four centuries between the southern Alps and the Mediterranean coast of Spain and the Cévennes area. According to these authors, these differences could be explained by two northwestern Mediterranean atmospheric circulation paths affecting the area with a frequency of 50 to 150 years.
This article presents a case study from the Yzeron basin based on optically stimulated luminescence (OSL) and radiocarbon dating of valley bottom deposits in order to understand the relative influence of climate and human occupation during the LIA. The first aim of this paper is to describe the internal secular rhythmicity of the LIA and its geomorphic implications as observed in valley bottom deposits. In light of what has been stated above, the geographical situation (in the northeastern part of the Massif Central) and the size of the studied hydrosystems (headwater basins) should argue for an early and sensitive recording of the internal LIA hydroclimatic fluctuations.
In line with our previous work (Preusser et al., 2011), the second aim is to assess the relative influence of land use evolution in the sedimentary filling of valley bottoms. Our initial hypothesis was that the valley floor in which current incision occurs has been developed during widespread agricultural activity (mainly ploughlands) and that incision could have been triggered by a later decline in sediment supply from cultivated hillslopes. However, Preusser et al. (2011) also highlighted that such small basins show high sensitivity to climate and land use variations. In order to answer the question regarding the forcing factors of sediment dynamics (hillslope soil erosion, valley bottom deposition, remobilization of valley bottom sediments), we require (i) a diachronic mapping of land use since at least the nineteenth century in the four studied subcatchments and (ii) a chronostratigraphic model of these valley bottom deposits. On the latter point, a review of different dating methods has been provided by Lang et al. (1999) showing that radiocarbon dating and dendrochronology are problematic for such sediments, as these methods rely on the dating of reworked material (i.e., wood). As a consequence, such dating approaches may overestimate the true age of sediment accumulation (e.g., Lang and Hönscheidt, 1999, Huckleberry and Rittenour, 2014). Luminescence methods are now frequently used for the dating of colluvial and alluvial sediments, as recently reviewed by Fuchs and Lang (2009).
Section snippets
Study area
The Yzeron basin (147 km2) is a right-side tributary of the Rhône River located near the city of Lyon (Fig. 1). From west to east, in the downstream direction, the basin drains a subdued, degraded granitic and gneissic mountain (Monts du Lyonnais, a mostly rural area; altitude from 900 to 400 m), a gneissic and partly granitic plateau (Plateau Lyonnais, a mostly peri-urban area in the Lyon neighborhoods; altitude from 400 to 250 m), and some inherited, mostly alpine, fluvial and glacial deposits
Sedimentological analyses
We sampled stratigraphic sections to analyze grain size distributions (Delile et al., 2014, for details of the pretreatment) in order to understand the nature of the valley bottom filling and the hydrosedimentary processes. While the sediment fraction > 1.6 mm was sieved using several sizes of sieves, the fraction < 1.6 mm was measured by a Malvern Mastersizer 2000 laser granulometer. The interpretation of granulometric curves was based on the CM diagram (also called the Passega image), which uses
Land use history
Land use history was studied via cadastral documents (Privolt, 2009), photo interpretation of aerial photographs (data collected and analyzed by Jacqueminet et al., 2013), and analysis of old photographs and artworks (Privolt, 2010).
The document produced by the first centralized cadastration (called Napolenic in France) allows us to reconstruct land use since the beginning of the nineteenth century. It consists of two sets of data: the first map and parcel registration of the early nineteenth
Chaudanne River section
In the Chaudanne River, we investigated a 3.5-m-long sediment section (Fig. 2). The age of the sediment deposits is determined by an in situ tree trunk found 1 m away from the outcrop. Another trunk was found in a similar position about 20 m upstream. These and ten other tree trunks found along the presently incised channel are Populus, Salix, and Alnus glutinosa/incana, indicating a humid, riverine environment. Radiocarbon dating of the two trees located close to the investigated section gave
Identification of aggradation-incision phases
The OSL and radiocarbon age-depth models of the four stratigraphic sections exhibit an obvious chronological offset, as radiocarbon dates tend to be earlier than OSL ages (Fig. 4A). This first observation confirms the limits discussed in the introduction concerning radiocarbon dating, which might overestimate the age of deposits (Lang and Hönscheidt, 1999) caused by the presence of reworked materials (Lang et al., 1999). Below we will add information obtained from the radiocarbon ages frequency
Conclusions
The OSL dating has shown a high potential to explain the recent sedimentation history of valley bottoms in small watersheds of the Yzeron basin (Rhône valley, France). These ages have revealed the complex history of the LIA's internal rhythms through several phases of aggradation-incision. Indeed, while the assemblage of radiocarbon dates is influenced significantly by ancient practices of land clearing by fires, thus confirming the limits of this method for dating developmental processes in
Acknowledgments
This study was funded by the Field Observatory for Urban Water Management (Observatoire de Terrain d'Hydrologie Urbaine, OTHU), the ANR (AVuPUR) (Assessing the Vulnerability of Peri-Urban Rivers) project, which was coordinated by Isabelle Braud, and the French ARTEMIS radiocarbon dating program (CNRS). Gamma spectrometry was carried out by Sönke Szidat, Departement für Chemie und Biochemie, Universtität Bern. Sarah Ivorra (UMR 5059 CNRS, Centre de Bio-Archéologie et d'Ecologie, Montpellier,
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