For more accurate prediction of the consequences of current global warming, it is important to disentangle the past impact on ecosystems of climate variability and human activities, in both the long and short term (Dearing and Jones, 2003). The mechanical erosion of continental surfaces mainly results from climate forcing (precipitation/runoff, vegetation changes…), but may be initiated, amplified and accelerated by anthropization (deforestation, plowing, grazing…). High altitude ecosystems are sensitive and therefore constitute relevant targets in which soil-erosion quantification can be used both to reconstruct climate changes (Simonneau et al., 2014) and to document the local consequences of human activities. In addition, such human-induced soil erosion indirectly reflects socio-ecological trajectories over time. For thousands of years, grazing has affected Pyrenean areas and is considered to be the main biotic factor creating pressure on ecosystem structure and the dynamics of mountain pastures (Galop et al., 2004; Mazier et al., 2009). Indeed, recent studies suggest that pastoral activities increase soil erosion rates (Adler and Morales, 1999; Ayala and French, 2005), especially in mountainous regions (Hall et al., 1999). Yet, though such interaction between grazing and erosion can be hypothesized, the causal relationship has been neither established nor quantified (Thormes, 2007). Such proof, however, is essential if public policy in the field of land-use management is to be credible. With this in mind, the ongoing French research program pastoralisM versus erosiOn: expLoration of molECULar biomarkers for tracking human/Environment interactions (MOLECULE, Labex DRIIHM – CNRS INEE) clearly aims to reconstruct the impact of grazing on soil erosion during the Late Holocene. In well-dated peat bog archives (Orry de Theo and Troumouse, Pyrenees), soil erosion (from organic and minerogenic markers) is studied alongside pastoralism (from coprophilous fungi and fecal molecular biomarkers: bile acids and sterols). The peat bog archive from Orry de Theo (Eastern Pyrenees, fig. 1) over the last two centuries is used to calibrate fecal tracers. Quantitative evolution of pastoralism tracers is compared to size and composition of livestock populations as described in detail in local archives (Galop et al., 2011). The markers of pastoralism can thus be quantitatively related to proportions of the livestock (i.e. ovine versus bovine). The authors then explore any quantitative relationship between the number of tracers and the size of the livestock. Finally, it is hoped that crucial information will emerge concerning any latency or time lag in the recording of molecular tracers due to varying residence times in soils or to varying transportation times from source to the archive. The Troumouse peat bog (Central Pyrenees, fig. 1) covers the last 6000 years and is located only a few kilometers from Lake Barroude (fig. 1), where climate-induced erosion processes over the Holocene have been quantified. At Troumouse, soil erosion fluxes reveal six major detrital phases dated from 3910-3855, 3445-3225, 2780-2740, 2655-2525, 1700-1510 and 735-515 cal BP. Anthropogenic indicators suggest that human activities in the vicinity of the bog date from 5000 cal BP. Moreover, Louis Ramond de Carbonnieres described intensive historical land-use management for grazing activities in the area but specified neither the type nor the number of animals involved in these practices. By applying our calibration to this new sequence and comparing local soil erosion fluxes, potentially influenced by grazing, to the local climate signal, the authors hope to demonstrate and quantify pastoralism and to establish whether or not it is a true agent of erosion.
Understanding social-ecological system dynamics is a major research priority for sustainable management of landscapes, ecosystems and resources. But the lack of multi-decadal records represents an important gap in information that hinders the development of the research agenda. Without improved information on the long-term and complex interactions between causal factors and responses, it will be difficult to answer key questions about trends, rates of change, tipping points, safe operating spaces and pre-impact conditions. Where available long-term monitored records are too short or lacking, palaeoenvironmental sciences may provide continuous multi-decadal records for an array of ecosystem states, processes and services. Combining these records with conventional sources of historical information from instrumental monitoring records, official statistics and enumerations, remote sensing, archival documents, cartography and archaeology produces an evolutionary framework for reconstructing integrated regional histories. We demonstrate the integrated approach with published case studies from Australia, China, Europe and North America.
A lake-level record for the late Holocene at Lake Ledro (Trentino, northeastern Italy) is presented. It is based on the sediment and pollen analysis of a 1.75 m high stratigraphic section observed on the southern shore (site Ledro I) and a 3.2 m long sediment core taken from a littoral mire on the southeastern shore (site Ledro II). The chronology is derived from 15 radiocarbon dates and pollen stratigraphy. The late-Holocene composite record established from these two sediment sequences gives evidence of centennial-scale fluctuations with highstands at c. 3400, 2600, 1700, 1200 and 400 cal. BP, in agreement with various palaeohydrological records established in central and northern Italy, as well as north of theAlps. In addition, high lake-level conditions at c. 2000 cal. BP may be the equivalent of stronger river discharge observed at the same time in Central Italy’s rivers. In agreement with the lake-level record of Accesa (Tuscany), the Ledro record also suggests a relatively complex palaeohydrological pattern for the period around 4000 cal. BP. On a millennial scale, sediment hiatuses observed in the lower part of the Ledro I sediment sequence indicate that, except for a highstand occurring just after 7500 cal. BP, lower lake levels generally prevailed rather before c. 4000 cal. BP than afterwards. Finally, the lake-level data obtained at Lake Ledro indicate that the relative continuity of settlements in humid areas of northern Italy during the Bronze Age (in contrast to their general abandonment north of the Alps between c. 3450 and 3150 cal. BP), does not reflect different regional patterns of climatic and palaeohydrological conditions. In contrast, the rise in lake level dated to c. 3400 cal. BP at Ledro appears to coincide with a worldwide climate reversal, observed in both the hemispheres, while palaeoenvironmental and archaeological data collected at Lake Ledro may suggest, as a working hypothesis, a relative emancipation of protohistoric societies from climatic conditions.
Abstract. On the basis of a multi-proxy approach and a strategy combining lacustrine and marine records along a north–south transect, data collected in the central Mediterranean within the framework of a collaborative project have led to reconstruction of high-resolution and well-dated palaeohydrological records and to assessment of their spatial and temporal coherency. Contrasting patterns of palaeohydrological changes have been evidenced in the central Mediterranean: south (north) of around 40° N of latitude, the middle part of the Holocene was characterised by lake-level maxima (minima), during an interval dated to ca. 10 300–4500 cal BP to the south and 9000–4500 cal BP to the north. Available data suggest that these contrasting palaeohydrological patterns operated throughout the Holocene, both on millennial and centennial scales. Regarding precipitation seasonality, maximum humidity in the central Mediterranean during the middle part of the Holocene was characterised by humid winters and dry summers north of ca. 40° N, and humid winters and summers south of ca. 40° N. This may explain an apparent conflict between palaeoclimatic records depending on the proxies used for reconstruction as well as the synchronous expansion of tree species taxa with contrasting climatic requirements. In addition, south of ca. 40° N, the first millennium of the Holocene was characterised by very dry climatic conditions not only in the eastern, but also in the central- and the western Mediterranean zones as reflected by low lake levels and delayed reforestation. These results suggest that, in addition to the influence of the Nile discharge reinforced by the African monsoon, the deposition of Sapropel 1 has been favoured (1) by an increase in winter precipitation in the northern Mediterranean borderlands, and (2) by an increase in winter and summer precipitation in the southern Mediterranean area. The climate reversal following the Holocene climate optimum appears to have been punctuated by two major climate changes around 7500 and 4500 cal BP. In the central Mediterranean, the Holocene palaeohydrological changes developed in response to a combination of orbital, ice-sheet and solar forcing factors. The maximum humidity interval in the south-central Mediterranean started ca. 10 300 cal BP, in correlation with the decline (1) of the possible blocking effects of the North Atlantic anticyclone linked to maximum insolation, and/or (2) of the influence of the remnant ice sheets and fresh water forcing in the North Atlantic Ocean. In the north-central Mediterranean, the lake-level minimum interval began only around 9000 cal BP when the Fennoscandian ice sheet disappeared and a prevailing positive NAO-(North Atlantic Oscillation) type circulation developed in the North Atlantic area. The major palaeohydrological oscillation around 4500–4000 cal BP may be a non-linear response to the gradual decrease in insolation, with additional key seasonal and interhemispheric changes. On a centennial scale, the successive climatic events which punctuated the entire Holocene in the central Mediterranean coincided with cooling events associated with deglacial outbursts in the North Atlantic area and decreases in solar activity during the interval 11 700–7000 cal BP, and to a possible combination of NAO-type circulation and solar forcing since ca. 7000 cal BP onwards. Thus, regarding the centennial-scale climatic oscillations, the Mediterranean Basin appears to have been strongly linked to the North Atlantic area and affected by solar activity over the entire Holocene. In addition to model experiments, a better understanding of forcing factors and past atmospheric circulation patterns behind the Holocene palaeohydrological changes in the Mediterranean area will require further investigation to establish additional high-resolution and well-dated records in selected locations around the Mediterranean Basin and in adjacent regions. Special attention should be paid to greater precision in the reconstruction, on millennial and centennial timescales, of changes in the latitudinal location of the limit between the northern and southern palaeohydrological Mediterranean sectors, depending on (1) the intensity and/or characteristics of climatic periods/oscillations (e.g. Holocene thermal maximum versus Neoglacial, as well as, for instance, the 8.2 ka event versus the 4 ka event or the Little Ice Age); and (2) on varying geographical conditions from the western to the eastern Mediterranean areas (longitudinal gradients). Finally, on the basis of projects using strategically located study sites, there is a need to explore possible influences of other general atmospheric circulation patterns than NAO, such as the East Atlantic–West Russian or North Sea–Caspian patterns, in explaining the apparent complexity of palaeoclimatic (palaeohydrological) Holocene records from the Mediterranean area.
Abstract This work focuses on the impact of Maya agriculture on soil degradation. In site and out site studies in the area of the city of La Joyanca (NW Petén) show that “Maya clays” do not constitute a homogeneous unit, but represent a complex sedimentary record. A high resolution analysis leads us to document changes in rates and practices evolving in time in relation with major socio-political and economic changes. It is possible to highlight extensive agricultural practices between Early Pre-classical to Late Pre-classical times. Intensification occurs in relation with reduction of the fallow duration during Pre-classic to Classic periods. The consequences of these changes on soil erosion are discussed. However, it does not seem that the agronomic potential of the soils was significantly degraded before the end of the Classic period. Keywords: Maya claysoil erosionslash and burn cultivationPeténMaya lowlands
Dans le cadre de l’Observatoire Homme Milieu Haut Vicdessos (versant Nord-Est des Pyrenees), des remplissages lacustres de la vallee de Bassies (Lacs Majeur et Sigriou, 42°N-1°E) sont etudies dans le but de distinguer les empreintes anthropiques et climatiques au cours de l’Holocene. Calibree par l’echantillonnage des sols et des roches presents sur les versants, la caracterisation des constituants organiques et mineraux des sediments lacustres preleves dans le Lac Majeur (3 carottes sur un transect Nord-Sud) et le Lac Sigriou (carottage central) permettent de documenter l’enregistrement des periodes climatiques humides et seches de l’Holocene superieur. La sedimentation holocene de ces lacs apparait tres riche en matiere organique. La petrographie organique quantitative realisee sur ces facies demontre qu’ils sont de type dy (60% de materiel allochtone), et qu’ils resultent de l’erosion des tourbes et des sols presents en amont. En plus des descriptions visuelles et de l’imagerie X, la microfluorescence X (XRF) et les analyses elementaires (LA ICP-MS) demontrent que ces facies organiques sont interrompus par des passees riches en rubidium (de 96 a 148 ppm), souvent sableuses, interpretees comme liees a des phases d’activation de canyons dont le materiel est issu du dome granitique, lui-meme riche en rubidium (190 ppm). Ces canyons, presents sur chacun des systemes lacustres, drainent les zones d’altitude soumises preferentiellement au forcage climatique. Leur activation, liee a d’intenses orages et/ou d’importantes fontes nivales, temoigne alors de periodes plus humides datees de AD1710, AD1360, AD1080, AD940, AD570, et 1690, 1830, 2640, 4210 et 4760 cal BP. Au cours du dernier millenaire, ces phases humides apparaissent synchrones entre les deux lacs, confirmant leur caractere climatique, et semblent correlees, au moins pour cette periode, aux minima solaires. Apres AD1907, le niveau d’eau du Lac Majeur est regule par la centrale hydroelectrique de Bassies et a pour consequence un marnage de 10 metres affectant 37% du lac. Ceci entraine une forte remobilisation du materiel glaciaire issu des berges, se traduisant (1) par un changement du niveau trophique du lac vers un facies de type gyttja (75% de materiel algaire), et (2) par un doublement du taux de sedimentation durant le dernier siecle. Les faibles teneurs en rubidium enregistrees sur le facies gyttja, jusqu’ici interpretees comme marquant des phases plus seches, temoignent d’un marnage moins frequent, caracteristique de periodes plus humides. Cette interpretation est validee par la presence de passees sableuses dans le Lac Sigriou lors de ces episodes.
High-resolution seismic profiling has been combined with geochemical analyses of both watershed samples and five lacustrine cores retrieved from two natural lacustrine basins of glacial origin: Lake Majeur and Lake Sigriou (1630 m a.s.l. and 1995 m a.s.l., respectively, Eastern French Pyrenees). Identifying specific minerogenic and organic markers of autochthonous and allochthonous supply, data allow documenting past climatic and anthropogenic pressures. Over the past century, the lacustrine sediment of Lake Majeur has been essentially composed of algae, drastically contrasting with the natural sedimentary infill of the basin, mainly resulting from soil erosion from the mid–late Holocene. Since ad 1907, the Lake Majeur has been used for hydroelectricity production. Human-induced lake-level regulations, affecting up to 37% of the lacustrine surface, have increased by fourfold the accumulation rate of the lake and favoured water enrichment. Rubidium abundance within the lacustrine sediments of the two lakes reflects the mid–late Holocene palaeohydrology. After dam construction in ad 1907, greater quantities of rubidium found in Lake Majeur sedimentary infills indicate drier climatic periods, such as from ad 1975 to ad 1982, during which water reservoirs were particularly in demand. Inversely, before the dam was built, rubidium fluctuations were correlated with wetter conditions and hydrological events were recorded as sandy layers deposited by canyon reactivation, synchronous with European climatic deterioration phases. We notably document that the Mediaeval Climate Anomaly was interrupted by some humid periods dated c. ad 940, ad 1080, ad 1100 and ad 1250. We also date the onset of the ‘Little Ice Age’ c. ad 1360 and identify that this period was wetter after c. ad 1500.
