Holocene history of fire,vegetation and land use from the central Pyrenees (France)

Located on a mountain pass in the west-central Pyrenees, the Col d'Ech peat bog provides a Holocene fire and vegetation record based upon nine ¹⁴C (AMS) dates. We aim to compare climate-driven versus human-driven fire regimes in terms of frequency, fire episodes distribution, and impact on vegetation. Our results show the mid-Holocene (8500–5500 cal yr BP) to be characterized by high fire frequency linked with drier and warmer conditions. However, fire occurrences appear to have been rather stochastic as underlined by a scattered chronological distribution. Wetter and colder conditions at the mid-to-late Holocene transition (4000–3000 cal yr BP) led to a decrease in fire frequency, probably driven by both climate and a subsequent reduction in human land use. On the contrary, from 3000 cal yr BP, fire frequency seems to be driven by agro-pastoral activities with a very regular distribution of events. During this period fire was used as a prominent agent of landscape management.     

Stratigraphic record of thrust propagation, Carboniferous foreland basin, Pyrenees, with emphasis on Pays-de-Sault (France/Spain)

The Carboniferous culm of the Pays-de-Sault is divided into two diachronous and synshortening series. These series are dated Late Visean (Pic d'Ourtiset series in a northern overthrust unit) and Early Namurian E2 (La Fajolle series in a southern underthrust unit) from an association of foraminifers, algae, and microproblematica identified in clasts of conglomerates. According to structural positions and facies criteria, these two series are interpreted as two turbiditic depocenters which were generated by southward thrust propagation during Late Visean and Early Namurian. At the scale of the Pyrenean Hercynian range, this evolution is consistent with a thrust and depocenter sequence propagating on the wedge-top depozone of a foreland basin system from the northeast (Mouthoumet subpyrenean massif) to the southwest (end of the High Primary Range) during Late Visean to Westphalian C time interval.     

Late Holocene sedimentary changes in floodplain and shelf environments of the Tagus River (Portugal)

Sedimentary changes during the last ∼2500 years have been reconstructed from cored sedimentary records from the deltaic floodplain of the Lower Tagus Valley and the Tagus mudbelt on the continental shelf offshore Lisbon. We used a multi-proxy approach consisting of sedimentology, grainsize, pollen data and magnetic susceptibility. In the floodplain grainsize coarsened and sedimentation rate and magnetic susceptibility increased during the late Holocene due to an increased flooding frequency and/or intensity. On the Tagus shelf the mudbelt grainsize fined, together with a higher sedimentation rate and increased magnetic susceptibility. The fining grainsize is explained by an increased suspended sediment flux towards the shelf and subdued winnowing. Floodplain and shelf records were correlated by radiocarbon dating and changes in sediment characteristics. We identified four depositional phases (∼2300/∼1600/∼1100/∼670 cal BP) on the floodplain and the shelf. These are tentatively explained by land-use changes in the Tagus catchment.     

Late Holocene fire impact and post-fire regeneration from the Bereket basin, Taurus Mountains, southwest Turkey

The 800 cm long sequence from the Bereket provides the first detailed fire history in the western Taurus Mountains for the last three millennia. The main disturbances occurred during the Bey?ehir Occupation Phase (BO Phase), a period of intensive polyculture and dated at Bereket from ca. 2230 to 1550 cal yr BP. Four phases of local and extra-local fire periods have been recorded at 2320-2240, 1985-1970, 1865-1820 cal yr BP and post-AD 1950. The fire history established for the BO Phase is complex, with fire periods alternating with periods without local fires, as is the case for the late BO Phase from 1820 until 1550 cal yr BP. It is suggested that the past agricultural practices including fires cause a higher soil erodibility than agricultural practices without fires. A climatic shift towards aridity during Roman times may have triggered the observed change in fire regime but local processes, mainly human disturbances, appear to be the proximal cause of all recorded changes. In the Bereket surroundings, fires led to a simplification of the vegetation structure, favouring soil erosion, pastures and intensive cultivation.     

Evidence for Modal Metasomatism in the Orogenic Spinel Lherzolite Body from Caussou (Northeastern Pyrenees, France)

Metasomatic mineral-bearing and/or trace element-enriched ultramaficassemblages have been reported from very few Alpine-type massifs.The small ultramafic body from Caussou (Ari?ge, northeasternPyrenees) compared with other north Pyrenean ultramafic complexesshows distinctive features which are similar to those of modallymetasomatized mantle xenoliths found in alkali basalts. It ismainly composed of clinopyroxene-rich spinel lherzolites (cpx/opxratios 1), with subordinate titanian pargasite-rich peridotites,both greatly depleted in orthopyroxene. Moreover the Caussouperidotites differ from other Ari?ge peridotites in the presenceof ilmenite, the abundance of sulfide inclusions in pyroxenesand amphiboles, higher Al, Ca, Na, K, Ti, and lower Mg contents,and enrichment in incompatible trace elements (ITE). Such mineralogicaland geochemical features are interpreted as resulting from modalmetasomatism produced by influxes of silicate melt into theperidotites. At Caussou, the metasomatic assemblage comprisesTi-pargasite+Ti-bearing clinopyr oxene+ilmenite+Ti-phlogopite+sulphide+fluid,suggesting that K, Ti, Na, ITE (including S, H₂O CO₂ and possiblyFe and Ca, were introduced by the metasomatizing agent. Thismetasomatism was probably imposed on an ultramafic associationdominated by LREE-depleted peridotites similar to the northPyrenean spinel lherzolites. These features indicate that, underupper lithospheric mantle conditions, a mafic melt locally infiltratedlherzolites by a grain-boundary percolation process and reactedwith the original mineral assemblage. The infiltration of alkali-basalticliquids into spinel peridotite led to: (1) partial dissolutionof orthopyroxene and, locally, spinel; (2) crystallization ofclinopyroxene directly from introduced melts; and (3) re-crystallization/equilibrationof pre-existing clinopyroxene with these magmatic liquids. Inthe last stage of the metasomatism, segregation of more fractionatedsilicate liquids, coexisting with a (CO₂+H₂O) fluid phase, mayhave been responsible for the crystallization of titanian pargasite,possibly by means of hydro-fracturing mechanism. The pervasive modal metasomatism at Caussou was contemporaneouswith the segregation of amphibole-bearing dykes in the Lherz-Freychin?debodies (northeast Pyrenees) (101–103 Ma). They representtwo manifestations of the same magmatic event in the lithosphericmantle, probably related the Middle Cretaceous alkaline magmatisrnof the Pyrenees.     

Holocene vegetation and climate changes in Hoton-Nur basin, northwest Mongolia

Radiocarbon-dated pollen and diatom records from Hoton-Nur lake (48°40'N, 88°18'E), northwest Mongolia are used to reconstruct Holocene vegetation and climate changes. Steppe, which covered the area some time before 9000 BP (all ages are given in ¹⁴C BP), was replaced by boreal conifer forest-steppe by 9000-8500 BP. At the same time, planktonic diatoms increased in abundance from 5 to 45%. After 4000 BP there was a sharp reduction in forest and re-establishment of steppe. Changes in the pollen composition were accompanied by a decrease in the abundance of planktonic diatoms from 35-50% (between 9000 and 5500 BP) to less than 10% soon after 4000 BP. These and other published data from Mongolia suggest wetter conditions during the early and middle Holocene than those of today. This pattern probably reflects an expansion of the Pacific monsoon recorded in geological data from China and simulated in climate-model experiments, and suggests that the summer monsoon influenced a larger area than might appear if the Chinese records only are taken into consideration.     

Mineralogy and chemistry of Cu-Fe-Ni sulfides in orogenic-type spinel peridotite bodies from Ariege (Northeastern Pyrenees,France)

Mantle-derived peridotite bodies of Ariège are composed of spinel lherzolites and harzburgites ranging from remarkably fresh (less than 5% serpentinized) samples with protogranular texture to secondary foliated samples, which are generally 10%–20% serpentinized. The foliated samples have passed through two cycles of deformation and re-crystallization, the earlier ones occurring at temperatures above 950° C for 15 kbar pressure, the later ones at temperatures between 950° and 750° C for 8–15 kbar. Microscopic investigation of 140 samples reveals an accessoy sulfide component which is more abundant in lherzolie than in harzburgite. This component occurs in two differet textural locations, either as inclusions trapped within silicates during the first stage of re-crystallization or as interstitial grains among silicates. Mineralogy and chemistry of both sulfide occurrences are quite similar, at least in samples less than 5% serpentinized. In these fresh samples, sulfides are composed of complex intergrowths between nickel-rich pentlandite and pyrite, coexisting with minor primary pyrrhotite (Fe₇S₈) and chalcopyrite. Pentlandite and pyrite are interpreted as low-temperature breakdown products of upper mantle monosulfide solid solutions. The mineralogy and chemistry of interstitial sulfides in serpentinized rocks vary in parallel with the degree of serpentinization. In samples less than 10% serpentinized, primary pyrrhotite grades into FeS. In samples more than 10% serpentinized, pyrite is replaced by secondary pyrrhotite, and then disappears totally, whereas the coexisting pentlandite is Fe-enriched and replaced by mackinawite. This sequence of alteration indicates a decrease of sulfur fugacity, resulting from serpentinization of olivine at temperatures below 300° C. This is also the case for the inclusions which have been fractured during the tectonic emplacement of the host peridotites within the crust. The presence of non-equilibrium sulfide assemblages in both cases reflects the sluggishness of solid state reactions at near-surface temperatures. It is inferred from these results that sulfides disseminated within orogenic peridotite massifs are so sensitive to serpentinization that most sulfur fugacity estimates based on fractured inclusions and intergranular sulfides are unreliable.     

Holocene history of Lake Soldatskoje (Kola Peninsula, Russia) inferred from sedimentary diatom assemblages

A sediment core from Lake Soldatskoje, a small tundra lake located in the northern coastal area of the Kola Peninsula and surrounded by numerous archaeological dwelling sites, was analysed for diatom species. The core covers the entire Holocene, i.e. 10000 radiocarbon years. The diatom record has similarities with studies made earlier from tundra lakes of northern Russia and from Arctic lakes in general. The genus Fragilaria was dominant and many other small, benthic, nordic, cold-water diatom species typical of Arctic lakes were common, including Achnanthes minutissima Kützing, A. pusilla (Grunow) De Toni, Cyclotella tripartita Håkansson and Navicula absoluta Hustedt. Multivariable ordinations were used to characterize the changes in the diatom flora. Diatom-based pH and total phosphorus inferences indicate that the lake has become progressively more acidic and poorer in nutrients. Disturbances in the diatom stratigraphy of Lake Soldatskoje around 4000–5500 ¹⁴C yr BP may be related to human activity.     

Tracking recorded fires using charcoal morphology from the sedimentary sequence of Prosser Lake, British Columbia (Canada)

Quantitative analyses of variations in morphological features of charcoal were undertaken in a ²¹⁰Pb-dated sediment core from Prosser Lake (British Columbia, Canada). Seven morphological types of charcoal were defined by particle shape, major-minor axis ratio, apparent porosity and progradation to unburned material. The distribution of morphotypes and total charcoal abundances were assessed as a proxy for fire events recorded between 1919 and 2000 and to subsequent mechanisms of transportation-sedimentation to lake sediments. Charcoal morphotypes showed distinct relationships to recorded area burned by fires. Fragile charcoal fragments with highly irregular porosity (termed Type M) displayed the strongest correlation to burned area (r² = 0.51; P = 0.0001) and did not produce any false-positive signal for fires recorded within a radius of 20 km around the lake. We infer that high porosity and low density Type M fragments are aerially transported and directly deposited on the lake, and that the fragility of Type M charcoal prevents significant quantities from being secondarily transported and incorporated into the sedimentary record. We propose that charcoal morphology is an important but underutilized technique that can yield important insights into fire type, proximity and transportation-sedimentation processes.     

Latest Campanian and Maastrichtian Siderolitidae (larger benthic foraminifera) from the Pyrenees (S France and NE Spain)

The siderolitids from the uppermost Campanian and Maastrichtian deposits of the Pyrenees have been re-studied. This has revealed a high diversity and rapid replacement of taxa, confirming the group as a good tool for high resolution biostratigraphy. Two genera have been found in the uppermost Campanian–Maastrichtian interval in the Pyrenean deposits: Siderolites Lamarck, and Wannierina Robles-Salcedo. Siderolites, with canaliferous spines or denticulate periphery, is represented by four species replacing each other from the latest Campanian to Maastrichtian: Siderolites praecalcitrapoides (latest Campanian), S. pyrenaicus sp. nov. (early Maastrichtian), Siderolites calcitrapoides (late Maastrichtian) and Siderolites denticulatus (late Maastrichtian). Wannierina is characterised by well-developed keels and ramified marginal canals. Two species of Wannierina have been identified and they succeeded one another from latest Campanian to early Maastrichtian: Wannierina vilavellensis sp. nov. (latest Campanian) and Wannierina cataluniensis (early Maastrichtian). The species of the genus Siderolites inhabited shallow waters of tropical to subtropical platforms with moderate-to-high water-energy conditions and those of the genus Wannierina are typical of deep–water low-energy environments but still in the eutrophic zone.     

Holocene salt‐marsh sedimentary infilling and relative sea‐level changes in West Brittany (France) using foraminifera‐based transfer functions

In order to reconstruct former sea‐levels and to better characterize the history of Holocene salt‐marsh sedimentary infillings in West Brittany (western France), local foraminifera‐based transfer functions were developed using weighted‐average‐partial‐least‐squares (WAPLS) regression, based on a modern data set of 26 and 51 surface samples obtained from salt‐marshes in the bay of Tresseny and the bay of Brest, respectively. Fifty cores were retrieved from Tresseny, Porzguen, Troaon and Arun salt‐marshes, which were litho‐ and biostratigraphically analysed in order to reconstruct palaeoenvironmental changes. A total of 26 AMS ¹⁴C age determinations were performed within the sediment successions. The Holocene evolution of salt‐marsh environments can be subdivided into four stages: (i) a development of brackish to freshwater marshes (from c. 6400 to 4500 cal. a BP); (ii) salt‐marsh formation behind gravel barriers in the bay of Brest (from 4500 to 2900 cal. a BP); (iii) salt‐marsh erosion and rapid changes of infilling dynamics due to the destruction of coastal barriers by storm events (c. 2900?2700 cal. a BP); (iv) renewed salt‐marsh deposition and small environmental changes (from 2700 cal. a BP to present). From the application of transfer functions to fossil assemblages, 14 new sea‐level index points were obtained, indicating a mean relative sea‐level rise of around 0.90±0.12 mm a^?1 since 6300 cal. a BP.     

Progressive deformation of a zone of magma transfer in a transpressional regime: The Variscan Mérens shear zone (Pyrenees, France)

The EW-striking Variscan Mérens shear zone (MSZ), located on the southern border of the Aston dome (Pyrenees), corresponds to variously mylonitized gneisses and plutonic rocks that are studied using the Anisotropy of Magnetic Susceptibility (AMS) technique. The plutonic rocks form EW-striking bands with, from south to north, gabbro-diorites, quartz diorites and granodiorites. The MSZ underwent a mylonitic deformation with an intensity progressively increasing from the mafic to the more differentiated rocks. The foliations are EW to NW–SE striking and subvertical. A first set of lineations shows a moderate WNW plunge, with a dextral reverse kinematics. More recent subvertical lineations correspond to an uplift of the northern compartment. To the east, the MSZ was cut by a N120°E-striking late shear band, separating the MSZ from the Quérigut pluton. The different stages of mylonitization relate to Late Variscan dextral transpression. This regime allowed the ascent of magmas along tension gashes in the middle crust. We interpret the MSZ as a zone of magma transfer, which fed a pluton now eroded that was similar to the Quérigut and Millas plutons located to the east. We propose a model of emplacement of these plutons by successive pulses of magmas along en-échelon transfer zones similar to the MSZ.     

Palaeoenvironmental changes inferred from malacofaunas in the Lateglacial and early Holocene fluvial sequence at Conty, northern France

Environmental changes are reconstructed from a Lateglacial and early Holocene sequence at Conty, northern France. The molluscan succession is put into a chronostratigraphic framework supported by numerous radiocarbon dates. Malacofaunas from the Bølling chronozone are reported for the first time in northern France and show progressive expansion of marshy communities within organic deposits. This biozone ended in a calcareous silt with the appearance of several species of arctic-alpine affinities. These sedimentological and malacological data point to colder climatic conditions after 12 220 ± 90 BP, but before 11 640 ± 80 BP, allowing allocation to the Older Dryas event. The first part of the Allerød appears to have been drier and relatively stable. After 11 400 BP, a decline in species richness and diversity in the malacofaunas suggests increasing dryness. During the Younger Dryas, two molluscan biozones are identified in a homogeneous calcareous silt, reflecting an early wet phase followed by a drier episode. At the onset of the Holocene malacofaunas show a higher diversity, suggesting climatic improvement.     

Climate variability in the Aral Sea basin (Central Asia) during the late Holocene based on vegetation changes

High-resolution pollen analyses ( 50 yr) from sediment cores retrieved at Chernyshov Bay in the NW Large Aral Sea record shifts in vegetational development from subdesertic to steppe vegetation in the Aral Sea basin during the late Holocene. Using pollen data to quantify climatic parameters, we reconstruct and date for the first time significant changes in moisture conditions in Central Asia during the past 2000 yr. Cold and arid conditions prevailed between ca. AD 0 and 400, AD 900 and 1150, and AD 1500 and 1650 with the extension of xeric vegetation dominated by steppe elements. These intervals are characterized by low winter and summer mean temperatures and low mean annual precipitation (P_mm < 250 mm/yr). Conversely, the most suitable climate conditions occurred between ca. AD 400 and 900, and AD 1150 and 1450, when steppe vegetation was enriched by plants requiring moister conditions (P_mm  250–500 mm/yr) and some trees developed. Our results are fairly consistent with other late Holocene records from the eastern Mediterranean region and the Middle East, showing that regional rainfall in Central Asia is predominantly controlled by the eastern Mediterranean cyclonic system when the North Atlantic Oscillation (NAO) is in a negative phase.     

Research on the February 18, 1996 earthquake in the caves of Saint-Paul-de-Fenouillet area, (eastern Pyrenees,France)

Abstract

Eight caves have been investigated near Saint-Paul- de-Fenouillet after the earthquake of 5.2 magnitude of February 1996 which occurred in the eastern Pyrenees (France) and caused moderate damage at the ground surface. The earthquake has been associated with the movement of an E-W fault. The caves had not been visited since the earthquake. Some damage, mainly collapses of soda straws and small rocks, could be attributed to this earthquake. The most interesting cave in the epicentral area is the Paradet cave which is situated on a recently activated fault plane. In this cave, soda straw falls could be attributed to the earthquake, but other more ancient damage was also observed. Analysis of the azimuth of fallen speleothems, which are natural pendulums, may indicate the directions, and an estimation of their mechanical properties gives the threshold of the seismic ground motion amplitude responsible for their collapse, thus supplying information to calibrate damage due to past earthquakes. A statistical study indicates that the main direction of the collapsed soda straws is E–W. Numerical simulations confirm that soda straws are relatively strong objects that may break under certain conditions during earthquakes. © Elsevier, Paris     

Research on the February 18, 1996 earthquake in the caves of Saint-Paul-de-Fenouillet area, (eastern Pyrenees,France)

Eight caves have been investigated near Saint-Paul-de-Fenouillet after the earthquake of 5.2 magnitude of February 1996 which occurred in the eastern Pyrenees (France) and caused moderate damage at the ground surface. The earthquake has been associated with the movement of an E-W fault. The caves had not been visited since the earthquake. Some damage, mainly collapses of soda straws and small rocks, could be attributed to this earthquake. The most interesting cave in the epicentral area is the Paradet cave which is situated on a recently activated fault plane. In this cave, soda straw falls could be attributed to the earthquake, but other more ancient damage was also observed. Analysis of the azimuth of fallen speleothems, which are natural pendulums, may indicate the directions, and an estimation of their mechanical properties gives the threshold of the seismic ground motion amplitude responsible for their collapse, thus supplying information to calibrate damage due to past earthquakes. A statistical study indicates that the main direction of the collapsed soda straws is E-W. Numerical simulations confirm that soda straws are relatively strong objects that may break under certain conditions during earthquakes.     

Vendian of the Fore-Yenisei sedimentary basin (southeastern West Siberia)

Fossiliferous Upper Vendian strata are discovered in the Upper Proterozoic to Lower Paleozoic Fore-Yenisei sedimentary basin under a thick Mesozoic-Cenozoic cover in southeastern West Siberia. Two depositional systems are recognized based on sedimentological features: (1) wave- and current-agitated shoreface-forereef-biohermal reef system (Vostok-3 Borehole section) and (2) tidal flat-evaporite basin (Averinskaya-150 Borehole section). The forereef facies yielded fossilized tubular calcareous skeletons of reef-building metazoans Cloudina riemkeae, Cloudina hartmannae and Cloudina carinata, phosphatized Namacalathus-like fossils, and a diversity of tubular phosphatized and agglutinated tubular fossils. The fossil assemblage can be interpreted as the evidence of ecological complexity of the reef system. Paleontological characteristics suggest correlation of the Vendian strata with the lowermost Purella antiqua Assemblage Zone and the boundary interval with the underlying Anabarites trisulcatus Assemblage Zone of the Siberian Platform. Therefore, at least in the late Proterozoic, the Fore-Yenisei sedimentary basin was part of a larger pericratonic depositional system on the western margin of the Siberian Craton.     

Hummocky cross-stratification-like structures in deep-sea turbidites: Upper Cretaceous Basque basins (Western Pyrenees, France)

Hummocky cross-stratification is a sedimentary structure which is widely interpreted as the sedimentary record of an oscillatory current generated by energetic storm waves remobilizing surface sediment on the continental shelf. Sedimentary structures named hummocky cross-stratification-like structures, similar to true hummocky cross-stratification, have been observed in the Turonian–Senonian Basque Flysch Basin (south-west France). The bathymetry (1000 to 1500 m) suggests that the observed sedimentary structures do not result from a hydrodynamic process similar to those acting on a continental shelf. The morphology of these three-dimensional structures shares similarities with the morphology of hummocky cross-stratification despite a smaller size. The lateral extent of these structures ranges from a few decimetres to many decimetres; they consist of convex-up domes (hummock) and concave-up swales with a non-erosive base. Four types of hummocky cross-stratification-like geometries are described; they occur in association with structures such as climbing current ripple lamination and synsedimentary deformations. In the Basque Flysch, hummocky cross-stratification-like structures are only found in the Tc interval of the Bouma sequence. Hummocky cross-stratification-like structures are sporadic in the stratigraphic series and observed only in few turbidite beds or bed packages. This observation suggests that hummocky cross-stratification-like structures are linked genetically to the turbidity current but form under a very restricted range of parameters. These structures sometimes show an up-current (upslope) migration trend (antidunes). In the described examples, they could result from standing waves forming at the upper flow interface because of Kelvin–Helmholtz instability.