Brigitte Talon

Holocene biomass burning and global dynamics of the carbon cycle

Fire regimes have changed during the Holocene due to changes in climate, vegetation, and in human practices. Here, we hypothesise that changes in fire regime may have affected the global CO2 concentration in the atmosphere through the Holocene. Our data are based on quantitative reconstructions of biomass burning deduced from stratified charcoal records from Europe, and South-, Central- and North America, and Oceania to test the fire-carbon release hypothesis. In Europe the significant increase of fire activity is dated approximately 6000 cal. yr ago. In north-eastern North America burning activity was greatest before 7500 years ago, very low between 7500-3000 years, and has been increasing since 3000 years ago. In tropical America, the pattern is more complex and apparently latitudinally zonal. Maximum burning occurred in the southern Amazon basin and in Central America during the middle Holocene, and during the last 2000 years in the northern Amazon basin. In Oceania, biomass burning has decreased since a maximum 5000 years ago. Biomass burning has broadly increased in the Northern and Southern hemispheres throughout the second half of the Holocene associated with changes in climate and human practices. Global fire indices parallel the increase of atmospheric CO2 concentration recorded in Antarctic ice cores. Future issues on carbon dynamics relatively to biomass burning are discussed to improve the quantitative reconstructions.

Reconstruction of Holocene high-altitude vegetation cover in the French southern Alps: evidence from soil charcoal

The study presented here from the southern French Alps demonstrates the reliability of soil charcoal analysis for the study of Holocene past treeline positions. The occurrence of charcoal in mineral soils along transects from 1950 m up to 2920 m demonstrates the role of fire in the establishment of the present vegetation patterns. The abrupt decrease of charcoal concentration at about 2400—2600 m (which varies across the study sites) corresponds to the modern transition between subalpine forest and alpine tundra. Charcoal particles formed in situ provide high spatial-resolution data for the reconstruction of past forest and treeline changes. Soil charcoal analysis indicated that: (1) treeline was 300 m higher around 6800 cal. BP than today; and (2) the uppermost forest belt up to 2810 m was colonized by larch ( Larix decidua Mill.) and arolla pine (Pinus cembra L.). This pine is present today but patchily distributed: it is absent from the three areas studied. Radiocarbon dates, ranging from c. 6800 cal. BP to the modern period, along with historical and archaeological data, suggest that the present pattern of the uppermost forest belt, and the patchy distribution of arolla pine in the studied area are the results of anthropogenic fire (mainly agropastoral activities). The question of global warming consequences on treelines in this part of the French Alps is discussed.

Past uppermost tree limit in the Central European Alps (Switzerland) based on soil and soil charcoal

The uppermost limits of past treelines in the Alps are established using soil type and soil charcoal mass. In all the studied sites, a sharp decrease of soil charcoal mass is correlated with the upper altitudinal limit of podzols. On the basis of this evidence, the uppermost tree limit reached 2500±100m a.s.l. in the Valaisan Alps during the Holocene, i.e., it was 250±100 m higher than todays potential treeline. Consequently, the timberline would have reached 2400±100m a.s.l.. From the strong decline of charcoals concentration in soils above 2500m a.s.l., we infer that conifer species were rare or very rare above this altitude during the Holocene. Joint interpretation of charcoal, pollen, soil and macrofossil data suggest that alpine meadows with at most scattered conifers were present throughout the Holocene in the todays middle and upper alpine belt.

Soil Carbon Sequestration by Holocene Fires Inferred from Soil Charcoal in the Dry French Alps

The current global carbon budget has a missing sink, which is believed to be in terrestrial ecosystems. At least one carbon sink, wood charcoal sequestrated in soil, remains poorly detailed. We estimate the wood charcoal-carbon content in soils located in dry valleys within the French Alps. Soils were sampled at five sites along altitudinal transects, from the conifer-dominated subalpine forests to the alpine grasslands. The five sites were distributed along a bioclimatic and biogeographic gradient from the southern Mediterranean to the northern continental Alps. The altitudinal distribution of charcoal exhibits the same pattern in the five sites, despite stand fire history, and regional bioclimatic and biogeographic differences. Charcoal concentrations are low (0.01 to 10 gchar m−2) in soils from the current treeless belt, while soils at lower elevation show high concentrations (10 to 2000 gchar m−2). The results suggest that the landscape structure determine the charcoal accumulation throughout variability of vegetation type and fire frequency. Charcoal concentrations recorded in the subalpine belt in the Alps are similar to those of Swedish boreal forests, but are 10 to 100 times lower than values from Mediterranean ecosystems. Dry to subhumid ecosystems contain subfossil carbon in the form of charcoal, which should be explicitly taken into account in the global carbon budget.

Soil charcoal analysis: a reliable tool for spatially precise studies of past forest dynamics: a case study in the French southern Alps.

This paper presents and discusses the use of soil charcoal analysis (pedoanthracology) to reconstruct past forest dynamics in a larch forest of the upper Guil valley (French Alps, Queyras). We also discuss the role of anthropogenic fire in forest dynamics. The radiocarbon dates from this site demonstrate that arolla pine (Pinus cembra) and larch were present in the area since 7566—7673 cal. BP and 5934—6123 cal. BP, respectively. The identification of a piece of charcoal to fir (Abies alba) dated to 5734—5908 cal. BP suggests that this species was present at 1980 m a.s.l. since c. 6000 cal. BP and had a higher ecological tolerance than previously assumed. The cover of larch forests has increased since the second half of the Holocene because of anthropogenic practices (eg, clearing with fire, cattle grazing). However, today, owing to the abandonment of pastoral practices, the arolla pine has become dominant, which greatly modifies the forest structure and the landscape.

The age and history of the french Mediterranean steppe revisited by soil wood charcoal analysis.

In the Mediterranean basin, steppe vegetation is considered to be the result either of land clearing in the Neolithic or the result of harsh environmental conditions (shallow soils and/or Mediterranean drought). For the first time, a study was carried out in the French Mediterranean steppe area of the Crau in Southeastern France using soil wood charcoal analysis, a technique in which charcoals found in the soil are identified and dated. According to some authors, an original mixed oak and beech forest may have covered the plain and have been cleared in the Neolithic, or, in the view of other authors, the steppe vegetation may have existed in its present form since the Lateglacial period. A total of 14 soil samples and 23 C14 datings were performed. Our results show that no species of ancient forests dominated by Quercus pubescens and Fagus sylvatica or Q. ilex were present at the end of the Neolithic or during the Bronze Age. Only earlier successional species such as Pinus pinea/halepensis, Phillyrea angustifolia and Prunus sp. were burnt during these periods. The other charcoal identified from the Roman Empire, Middle Ages and Modern periods seem to be better correlated with pastoral settlements than natural secondary succession processes. These results obtained at a local scale confirm those already available from the western Mediterranean, which have shown that human impacts were the determining factors of the evolution of the Mediterranean vegetation during the Neolithic and Bronze age.

Spatial Reliability of Soil Charcoal Analysis: The Case of Subalpine Forest Soils

Abstract: Pedoanthracological studies are generally able to provide accurate spatial resolution on plant distribution. But what is the true spatial variability of the pedoanthracological data? This study uses high-resolution spatial sampling in two 0.5-ha sites (5 pits per 0.5 ha) to assess the heterogeneity of the results obtained by soil charcoal analysis at a small scale. The results show heterogeneous patterns of soil charcoal distribution in terms of concentrations and botanical assemblages. Soil charcoal concentration ranged between 128 mg·kg-1 and 3369 mg·kg-1. Plant identifications and anthracomass values were analyzed by principal component analysis (PCA). PCA showed that between 32% and 48% of variance can be explained by inter-pit variability. The high-resolution data obtained through a multisampling strategy is more reliable for the investigation of heterogeneity in soil anthracological assemblages. It is obvious that care is needed in the interpretation of data derived from the analysis of single samples. Nomenclature: Tutin et al., 1964–1980.

The Longriqu fault zone, eastern Tibetan Plateau: Segmentation and Holocene behavior

The dextral Longriba fault system (LFS), ~300 km long and constituting of two fault zones, has recently been recognized as an important structure of the eastern Tibetan plateau (Sichuan province), as it accommodates a significant amount of the deformation induced by the ongoing Indo-Asian collision. Although previous paleoseismological investigations highlighted its high seismogenic potential, no systematic quantification of the dextral displacements along the fault system has been undertaken so far. As such information is essential to appraise fault behavior, we propose here a first detailed analysis of the segmentation of the Longriqu fault, the northern fault zone of the LFS, and an offset inventory of morphological features along the fault, using high-resolution Pleiades satellite images. We identify six major segments forming a mature fault zone. Offsets inventory suggests a characteristic coseismic displacement of ~4 m. Two alluvial fans, with minimum ages of 6.7 and 13.2 ka, respectively displaced by 23 ± 7 m and 40 ± 5 m, give an estimate of the maximal horizontal slip rate on the Longriqu fault of 3.2 ± 1.1 mm yr A1. As a result, a minimum ~1340 year time interval between earthquakes is expected.

Landscape evolution and agro-sylvo-pastoral activities on the Gorgan Plain (NE Iran) in the last 6000 years

The Gorgan Plain (NE Iran) is characterized by fertile soils formed on a loess plateau and is at present primarily exploited for intensive agriculture. However, the timing and intensity of the human impact on the landscape in the past are still unclear. A sediment core, taken from the centre of the eastern Gorgan Plain in the Kongor Lake covering the major part of the Holocene from 6.1 to 0.8 ka (all ages are calibrated before present), has been studied for pollen, non-pollen palynomorphs, botanical macroremains, insects, charcoal, geochemistry, biomarkers and magnetism in order to provide new insights into the evolution of the landscape and to estimate the intensity of human activities. The data obtained suggest a dry period between 5.9 and 3.9 ka and an increase in regional humidity afterwards with a maximum between 2.7 and 0.7 ka, during the period of the Persian empires (Achaemenid through Sasanian) and the Islamic era. The eastern part of the Gorgan Plain was characterized by open steppe landscapes during the last 6 ka, which most likely were used for pasture and at least since 2.7 ka for agriculture including arboriculture. The strongest anthropogenic impact on the environment around the Kongor site is documented during the Parthian and Sasanian Empires (200 BC–651 AD) and the Islamic era up to the eve of the Mongol invasion.

Environmental imprints of landscape evolution and human activities during the Holocene in a small catchment of the Calanques Massif (Cassis, southern France)

This paper is based on a multidisciplinary study using both paleoenvironmental methods (biological, sedimentological, magnetic and geochemical) and archaeological data in order to characterize landscape and hydrological network changes as well as human impact on a littoral catchment in the Western Mediterranean area. Sedimentary records obtained around a coastal alluvial plain (Cassis, Southeastern France) reveal local environmental changes and human activities since the Neolithic (ca. 6000 cal. BP). Anthropogenic impact is already noticeable in the earliest record, and we follow its evolution through time. According to biological markers, the Holocene landscape of the Calanques is dominated by a mosaic of open herbaceous formations partly generated by human activity. The prevalence of Cernuella virgata in malacofauna successions and of coprophilous fungal spores in the pollen record testifies to the spread of agro-pastoral activities. Two increases of anthropogenic impact are highlighted during the Roman (ca. 2000 cal. BP) and Modern (from ca. 1450 to today) periods. Lead isotope analyses indicate an anthropogenic release of lead, possibly originating from the Greek Cyclades (ca. 2000 cal. BP). Hydrographic and agricultural work is also enhanced during the Roman period. Magnetic parameters and lead isotopes indicate that the Modern anthropogenic impact may be associated with high temperature activities (e.g. lime kiln, fossil fuel).