Interaction between tectonic and erosion processes on the morphogenesis of an Alpine valley: geological and geophysical investigations in the lower Romanche valley (Belledonne massif, western Alps)

In the Belledonne massif, the steep Paleozoic Belledonne Middle Fault (BMF) separates micaschists, displaying numerous landslides, from amphibolites. The massif is incised by the lower Romanche river valley. When crossing the BMF, the valley widens into a lozenge-shaped basin recently interpreted as an active pull-apart type structure associated with a major N110 striking Quaternary fault. Multidisciplinary investigations were carried out in the basin to check if this model has implications on the seismic and landslide hazard assessment. This study demonstrated the existence of a N80 sinistral strike slip Séchilienne Fault Zone (SFZ). This fault zone is suspected to offset the BMF by 375 m across the basin. Geophysical experiments revealed that the bedrock depth increases strongly in the basin, up to 350 m. Our study invalidates the active pull-apart origin of the basin and suggests it results from Quaternary glacial and fluvial erosion processes, magnified by the intersection of two inherited structures, the BMF and the SFZ.     

Spline models of contemporary, 2030, 2060 and 2090 climates for Mexico and their use in understanding climate-change impacts on the vegetation

Spatial climate models were developed for México and its periphery (southern USA, Cuba, Belize and Guatemala) for monthly normals (1961–1990) of average, maximum and minimum temperature and precipitation using thin plate smoothing splines of ANUSPLIN software on ca. 3,800 observations. The fit of the model was generally good: the signal was considerably less than one-half of the number of observations, and reasonable standard errors for the surfaces would be less than 1°C for temperature and 10–15% for precipitation. Monthly normals were updated for three time periods according to three General Circulation Models and three emission scenarios. On average, mean annual temperature would increase 1.5°C by year 2030, 2.3°C by year 2060 and 3.7°C by year 2090; annual precipitation would decrease ?6.7% by year 2030, ?9.0% by year 2060 and ?18.2% by year 2090. By converting monthly means into a series of variables relevant to biology (e. g., degree-days > 5°C, aridity index), the models are directly suited for inferring plant–climate relationships and, therefore, in assessing impact of and developing programs for accommodating global warming. Programs are outlined for (a) assisting migration of four commercially important species of pine distributed in altitudinal sequence in Michoacán State (b) developing conservation programs in the floristically diverse Tehuacán Valley, and (c) perpetuating Pinus chiapensis, a threatened endemic. Climate surfaces, point or gridded climatic estimates and maps are available at http://forest.moscowfsl.wsu.edu/climate/.     

Modern Sphagnumδ13C signatures follow a surface moisture gradient in two boreal peat bogs,James Bay lowlands,Québec

Carbon isotopic composition of Sphagnum macrofossils can potentially be used as a palaeohydrological tool for peat‐based climatic studies since a relationship between Sphagnum δ¹³C values and peatland surface moisture has been presented in previous studies. In order to verify this hypothesis, modern Sphagnum δ¹³C values were measured along a moisture (microtopographic) gradient in two boreal peat bogs. Isotopic measurements were performed on bulk material of S. fuscum, S. magellanicum, S. capillifolium and S. pulchrum. Isotopic variations found within and between Sphagnum species along the microtopographic gradient were compared using analysis of variance. A significant positive correlation (P < 0.0001) was found between Sphagnum δ¹³C values and their position along the surface moisture gradient. Results show that ¹³C‐depleted values are related to low water table depths (WTD), while ¹³C‐enriched values correspond to a water table that is close to the peat surface. Although the mechanisms underlying carbon fractionation processes in mosses are not well understood, we demonstrate that water resistance to CO₂ diffusion is an important fractionation process that is observed in bulk Sphagnum δ¹³C measurements, since drier and wetter samples exhibit consistent and very different isotopic signatures. Copyright © 2008 John Wiley & Sons, Ltd.     

The impact and rotational light curves of Comet 9P/Tempel 1

UVES and HIRES high-resolution spectra of Comet 9P/Tempel 1 are used to investigate the impact and rotational light curves of various species with a view toward building a simple model of the distribution and activity of the sources. The emission by OH, NH, CN, C₃, CH, C₂, NH₂, and OI, are analyzed, as well as the light scattered by the dust. It is found that a simple model reproduces fairly well the impact light curves of all species combining the production of the observed molecules and the expansion of the material throughout the slit. The impact light curves are consistent with velocities of 400-600 m/s. Their modeling requires a three-step dissociation sequence “Grand-Parent → Parent → Daughter” to produce the observed molecules. The rotational light curve for each species is explained in terms of a single model with three sources. The dust component can however not easily be explained that way.     

Changing hillslope and fluvial Holocene sediment dynamics in a Belgian loess catchment

Floodplain deposition is an essential part of the Holocene sediment dynamics of many catchments and a thorough dating control of these floodplain deposits is therefore essential to understand the driving forces of these sediment dynamics. In this paper we date floodplain and colluvial deposition in the Belgian Dijle catchment using accelerator mass spectrometric radiocarbon and optical stimulated luminescence dating. Relative mass accumulation curves for the Holocene were constructed for three colluvial sites and 12 alluvial sites. A database was constructed of all available radiocarbon ages of the catchment and this database was analysed using relative sediment mass accumulation rates and cumulative probability functions of ages and site‐specific sedimentation curves. Cumulative probability functions of ages were split into different depositional environments representing stable phases and phases of accelerated clastic deposition. The results indicate that there is an important variation between the different dated sites. After an initial stable early and middle Holocene phase with mainly peat growth in the floodplains, clastic sedimentation rates increased from 4000 BC on. This first phase was more pronounced and started somewhat earlier for colluvial deposits then for alluvial deposits. The main part of the Holocene deposits, both in colluvial and alluvial valleys, was deposited during the last 1 ka. The sedimentation pattern of the individual dated sites and the catchment‐wide pattern indicate that land use changes are responsible for the main variations in the Holocene sediment dynamics of this catchment, while the field data do not provide indications for a climatological influence on the sediment dynamics. Copyright © 2011 John Wiley & Sons, Ltd.     

Deciphering signals of late Holocene fluvial and aeolian supply from a shelf sediment depocentre off Senegal (north‐west Africa)

High‐resolution climatic records of the late Holocene along the north‐west African continental margin are scarce. Here we combine sediment grain size, elemental distribution and mineral assemblage data to trace dust and riverine sources at a shallow‐marine sediment depocentre in the vicinity of the Senegal River mouth. The aim is to understand how these terrigenous components reflect climate variability during the late Holocene. Major element contents were measured and mineral identification was performed on three sub‐fractions of our sediment core: (i) fluvial material < 2 µm, (ii) aeolian material of 18–63 µm and (iii) a sub‐fraction of dual‐origin material of 2–18 µm. Results show that more than 80% of the total Al and Fe terrigenous bulk content is present in the fluviogenic fraction. In contrast, Ti, K and Si cannot be considered as proxies for one specific source off Senegal. The Al/Ca ratio, recording the continental river runoff, reveals two dry periods from 3010 to 2750 cal a BP and from 1900 to 1000 cal a BP, and two main humid periods from 2750 to 1900 cal a BP and from 1000 to 700 cal a BP. The match between (i) intervals of low river runoff inferred by low Al/Ca values, (ii) reduced river discharge inferred by integrated palynological data from offshore Senegal and (iii) periods of enhanced dune reactivation in Mali confirms this interpretation. Copyright © 2011 John Wiley & Sons, Ltd.     

Experimental analysis of kaolinite particle orientation during triaxial path

The goal of this study was to analyze the relation between the behaviour of a clayey material at the macroscopic scale and its microfabric evolution. This may lead to a better understanding of macroscopic strain mechanisms especially the contractancy and dilatancy phenomena. The approach proposed in this paper is based on the study of clay particles orientation by SEM picture analysis after different phases of triaxial loading. In the initial state of the samples (one‐dimensional compression), the SEM observations highlight a microstructural anisotropy with a preferential orientation of the particles normal to the loading direction. During isotropic loading, densification of the clayey matrix occurs related to a random orientation of particles indicated by the term ‘depolarization’. In the earlier stages of constant σ₃ drained triaxial path on slightly overconsolidated specimens, the microstructural depolarization seems to persist inside a macroscopic domain, in which only the volumetric strains due to the isotropic part of the stress tensor evolve. Then, a rotation mechanism of the particles towards preferred directions seems to be activated. The phenomenon appears directly linked to the evolution of the deviatoric part of the stress tensor. Copyright © 2010 John Wiley & Sons, Ltd.     

Nuclear spin temperature of ammonia in Comet 9P/Tempel 1 before and after the Deep Impact event

The Deep Impact mission succeeded in excavating inner materials from the nucleus of Comet 9P/Tempel 1 on 2005 July 04 (at 05:52 UT). Comet 9P/Tempel 1 is one of Jupiter family short period comets, which might originate in the Kuiper belt region in the solar nebula. In order to characterize the comet and to support the mission from the ground-based observatory, optical high-dispersion spectroscopic observations were carried out with the echelle spectrograph (UVES) mounted on the 8-m telescope VLT (UT2) before and after the Deep Impact event. Ortho-to-para abundance ratios (OPRs) of cometary ammonia were determined from the NH₂ emission spectra. The OPRs of ammonia on July 3.996 UT and 4.997 UT were derived to be 1.28±0.07 (nuclear spin temperature: Tspin=24±2 K) and 1.26±0.08 (Tspin=25±2 K), respectively. There is no significant change between before and after the impact. Actually, most materials ejected from the impact site could have moved away from the nucleus on July 4.997 UT, about 17 h after the impact. However, a small fraction of the ejected materials might remain in the slit of UVES instrument at that time because an excess of about 20% in the NH₂ emission flux is observed above the normal activity level was found [Manfroid, J., Hutsemékers, D., Jehin, E., Cochran, A.L., Arpigny, C., Jackson, W.M., Meech, K.J., Schulz, R., Zucconi, J.-M., 2007. Icarus. This issue]. If the excess of NH₂ on July 04.997 UT was produced from icy materials excavated by the Deep Impact, then an upper-limit of the ammonia OPR would be 1.75 (Tspin>17 K) for those materials. On the other hand, the OPR of ammonia produced from the quiescent sources was similar to that of the Oort cloud comets observed so far. This fact may imply that physical conditions where cometary ices formed were similar between Comet 9P/Tempel 1 and the Oort cloud comets.     

High elevation of low‐relief surfaces in mountain belts: does it equate to post‐orogenic surface uplift?

We present experiments of upraising and relaxing topographies showing that peneplanation can occur above the ultimate base level (sea level). After active uplift, the erosion of a topography bounded by a piedmont generates a final smooth and highly elevated topography. Smoothing at high elevation is even possible during active uplift if the evolution of topography is disrupted by the deposition of the products of erosion on its piedmont which is the case at the transition from underfilled to overfilled conditions in foreland basins.