Structural Agnostic Modeling: Adversarial Learning of Causal Graphs

A new causal discovery method, Structural Agnostic Modeling (SAM), is presented in this paper. Leveraging both conditional independencies and distributional asymmetries in the data, SAM aims at recovering full causal models from continuous observational data along a multivariate non-parametric setting. The approach is based on a game between $d$ players estimating each variable distribution conditionally to the others as a neural net, and an adversary aimed at discriminating the overall joint conditional distribution, and that of the original data. An original learning criterion combining distribution estimation, sparsity and acyclicity constraints is used to enforce the end-to-end optimization of the graph structure and parameters through stochastic gradient descent. Besides the theoretical analysis of the approach in the large sample limit, SAM is extensively experimentally validated on synthetic and real data

Lithofacies and depositional environment of volcano-sedimentary deposits of SE Mayo Oulo basin

The study of volcano-sedimentary deposits of Mangbai in SE Mayo Oulo basin-Cameroon leads to the identification of five major distinct lithofacies types. From the coarset to the finest lithofacies, there are : Gb-clast supported by massive coble and boulder, Gg-granule to massive gravel, Sm-massive sand, Shhorizontal layered sandstones, Fml-massive laminated claystones. Coarse facies (Gb, Gg, Sm) are cemented with volcanic matrix while the finest facies (Sh, Fml) are normal sedimentary compacted deposits. The coarsest facies distribution is identified on the top SE of the mountain while the medium to finest facies deposits are located at the base of the sequence. The formation of the volcano-sedimentary deposits of Mangbai are associated to the development and establishment of Africa major rift system while sedimentary deposits in the Mayo Oulo basin are more recent and date from Lower Cretaceous

Can the thermal stability of soil organic matter reflect disturbance and resilience in rubber tree-based agrosystems?

The capacity of soils to mitigate climate change through carbon sequestration depends on the quantitative but also the qualitative changes in soil organic carbon (SOC). In particular, the stability of SOC has to be considered to evaluate the long-term fate of the soil carbon stocks. The aim of this study was to assess the accurateness of thermal analyzes (Rock-Eval pyrolysis) to monitor the evolution of SOC quality and quantity in different land management situations. Using new I/R diagram (Sebag et al., 2016), dynamics of SOC was assessed in rubber tree-based agrosystems in Thailand. This case study allowed analyzing the changes in SOC properties after land use change (cassava to rubber) and over the ageing of the rubber plantation (up to 25 years). Soil from secondary forest plots were included in the study as a reference of an undisturbed system. While I (contribution of fresh OM) and R indices (global thermal stability) were well correlated in forest soils, our results show a higher I value in cassava field, and increasing R values with rubber plantation age. These results indicate that both SOC stocks and SOC stability increased with the age of plantations. However, R-index in the oldest rubber plantations was lower than in forest. Finally, this study highlights the potential of thermal analyses for monitoring SOC quantity and quality with promising application under the 4‰ initiative

Variabilité des précipitations au sahel Central et Recherche du forcage climatique par analyse du signal : la station de Maïne-Soroa (SE Niger) entre 1950 et 2005. Rainfall variability in the Central Sahel and climate forcing by signal analysis: Maïné-Soroa station (SE Niger) over the period 1950-2005

International audienceUne régression polynomiale non-paramétrique (méthode LOESS) appliquée aux précipitations annuelles de Maïné-Soroa (SE Niger) montre trois périodes : humide (1950-1967), aride (1968-1993), semi-aride (1994-2005). L'utilisation de la transformée en ondelettes continues permet de séparer la variabilité interne (haute fréquence) et la variabilité forcée (basse fréquence). Nous avons pu mettre en évidence sept modes de fréquence localisés dans le temps : deux modes intra-saisonniers (4-9jrs, 16-20jrs), deux modes saisonniers (6 mois, 1an), deux modes pluriannuels (2-4ans et 5-8ans) et un mode quasi-décennal (12-18ans). Lors du régime humide, on retrouve une forte variance des modes saisonniers liés à la ZCIT et des modes pluriannuels se succédant dans le temps (5-8ans puis 2-4ans) en liaison avec les températures de l'Atlantique Tropical Nord (TNA) puis avec l'Anticyclone des Açores. Ces modes basses fréquences sont synchrones avec une activité convective mature (mode 4-9jrs) et l'apparition d'une perturbation (mode 16-20jrs) lors de la transition entre la phase océanique et la phase continentale de la mousson. Au contraire, lors des années arides on retrouve le mode quasi-décennal, en lien avec les températures de l'Atlantique Tropical Sud (TSA), forçant une activité convective peu développée et perturbée au coeur de la phase continentale. A non-parametric polynomial regression applied to the Maine-Soroa annual rainfall timeseries reveals three periods: wet (1950-1967), arid (1968-1993) and semi-arid (1994-2005). Continuous Wavelet Transform allows crossing internal variability (high frequency) and forcing variability (low frequency). We could reveal seven frequency modes highly localized in time: two intraseasonal modes (4-9 days, 16-20 days), two seasonal modes (6mo, 1yr), two interannuals modes (2- 4yrs, 5-8yrs) and one quasidecadal mode (12-18yrs). During wet period, we notice a strong variance across seasonal modes in relation to ITCZ and the two interannual modes succeeding in time (5-8yrs then 2-4yrs) in connection with the Tropical North Atlantic temperatures (TNA) then Azores high. At the same time, we notice great convective activity (4-9days) and the oceanic/continental transition phase disturbation (16-20days). During the arid period, we notice the quasidecadal mode in relation with the Tropical South Atlantic temperatures (TSA), forcing a weak convective activity and disturbed in the medium of the continental phase

Evolution des paysages Sahélines au cours des six dernières décennies dans la région de Niamey : de la disparition de la brousse tigrée à l'encroutement de surface des sols

International audienceIn the Sahel, the rapid increase of the population during the last decades and the climate variation lead to an important environmental degradation. This work aims to measure the impacts of the human pressure on ecosystem during the six last decades. A diachronic cartography of a 100 km² area close to Niamey was done with aerial photographs (1950 and 1975) and GPS measurements (2009). Results showed that the tiger bush vegetation was completely cleared between 1950 and 2009 while the fallow decreases from 7 % to 1 %. In the sandy valley, the increase of cultivated fields from 20,7 % (1950) to 69,4 % (1975) favoured wind and water erosions which allowed surface soil crusting. Between 1975 and 2009, the bare crusted soil dramatically developed at the expense of the cultivated area which represents only 54,4 % of the studied area. The valleys are going to be overloaded because of high sedimentation rate (> 4 cm per year).Au Sahel, l'explosion démographique de ces dernières décennies et les variations climatiques ont provoqué d'importants changements environnementaux. L'objectif de ce travail est de mesurer les impacts de la pression anthropique sur les écosystèmes dans la région de Niamey au cours des six dernières décennies. L'étude est fondée sur une cartographie diachronique d'une aire de 100 km² située près de Niamey au moyen de photographies aériennes (1950 et 1975) et relevés au GPS (2009). Il est apparu ainsi qu'entre 1950 et 2009, la végétation de la brousse tigrée a été complètement déboisée. Dans les vallées sableuses, les surfaces cultivées ont connu une extension passant de 20,7 % à 69,4 % entre 1950 et 1975. Ceci a favorisé l'emprise des érosions hydrique et éolienne qui ont abouti à une dégradation des terres par encroûtement des sols. Celui-ci est à l'origine de la baisse des surfaces cultivées entre 1975 et 2009 (de 69,4 % à 54,4 %). Dans les bas- fonds, la tendance est au comblement du fait d'un taux de sédimentation de plus de 4 cm par an