A lithium-isotope perspective on the evolution of carbon and silicon cycles

The evolution of the global carbon and silicon cycles is thought to have contributed to the long-term stability of Earth's climate. Many questions remain, however, regarding the feedback mechanisms at play, and there are limited quantitative constraints on the sources and sinks of these elements in Earth's surface environments. Here we argue that the lithium-isotope record can be used to track the processes controlling the long-term carbon and silicon cycles. By analysing more than 600 shallow-water marine carbonate samples from more than 100 stratigraphic units, we construct a new carbonate-based lithium-isotope record spanning the past 3 billion years. The data suggest an increase in the carbonate lithium-isotope values over time, which we propose was driven by long-term changes in the lithium-isotopic conditions of sea water, rather than by changes in the sedimentary alterations of older samples. Using a mass-balance modelling approach, we propose that the observed trend in lithium-isotope values reflects a transition from Precambrian carbon and silicon cycles to those characteristic of the modern. We speculate that this transition was linked to a gradual shift to a biologically controlled marine silicon cycle and the evolutionary radiation of land plants

Machine learning identifies ecological selectivity patterns across the end-Permian mass extinction

The end-Permian mass extinction occurred alongside a large swath of environmental changes that are often invoked as extinction mechanisms, even when a direct link is lacking. One way to elucidate the cause(s) of a mass extinction is to investigate extinction selectivity, as it can reveal critical information on organismic traits as key determinants of extinction and survival. Here we show that machine learning algorithms, specifically gradient boosted decision trees, can be used to identify determinants of extinction as well as to predict extinction risk. To understand which factors led to the end-Permian mass extinction during an extreme global warming event, we quantified the ecological selectivity of marine extinctions in the well-studied South China region. We find that extinction selectivity varies between different groups of organisms and that a synergy of multiple environmental stressors best explains the overall end-Permian extinction selectivity pattern. Extinction risk was greater for genera that had a low species richness, narrow bathymetric ranges limited to deep-water habitats, a stationary mode of life, a siliceous skeleton, or, less critically, calcitic skeletons. These selective losses directly link the extinctions to the environmental effects of rapid injections of carbon dioxide into the ocean-atmosphere system, specifically the combined effects of expanded oxygen minimum zones, rapid warming, and potentially ocean acidification

Marine anoxia linked to abrupt global warming during Earths penultimate icehouse.

Piecing together the history of carbon (C) perturbation events throughout Earth’s history has provided key insights into how the Earth system responds to abrupt warming. Previous studies, however, focused on short-term warming events that were superimposed on longer-term greenhouse climate states. Here, we present an integrated proxy (C and uranium [U] isotopes and paleo CO2) and multicomponent modeling approach to investigate an abrupt C perturbation and global warming event (∼304 Ma) that occurred during a paleo-glacial state. We report pronounced negative C and U isotopic excursions coincident with a doubling of atmospheric CO2 partial pressure and a biodiversity nadir. The isotopic excursions can be linked to an injection of ∼9,000 Gt of organic matter–derived C over ∼300 kyr and to near 20% of areal extent of seafloor anoxia. Earth system modeling indicates that widespread anoxic conditions can be linked to enhanced thermocline stratification and increased nutrient fluxes during this global warming within an icehouse

Response of Siliceous Marine Organisms to the Permian-Triassic Climate Crisis Based on New Findings From Central Spitsbergen, Svalbard

Siliceous marine ecosystems play a critical role in shaping the Earth's climate system by influencing rates of organic carbon burial and marine authigenic clay formation (i.e., reverse weathering). The ecological demise of silicifying organisms associated with the Permian-Triassic mass extinction is postulated to have elevated marine authigenic clay formation rates, resulting in a prolonged greenhouse climate during the Early Triassic. Yet, our understanding of the response of siliceous marine organisms during this critical interval is poor. Whilst radiolarians experienced the strongest diversity loss in their evolutionary history and perhaps also the greatest population decline of silica-secreting organisms during this event, only a small number of Griesbachian (post-extinction) localities that record siliceous organisms are known. Here, we report newly discovered latest Changhsingian to early Griesbachian (Clarkina meishanensis - Hindeodus parvus Zone) radiolarians and siliceous sponge spicules from Svalbard. This fauna documents the survival of a low-diversity radiolarian assemblage alongside stem-group hexactinellid sponges making this the first described account of post-extinction silica-secreting organisms from the Permian/Triassic boundary in a shallow marine shelf environment and a mid-northern paleolatitudinal setting. Our findings indicate that latitudinal diversity gradients for silica-secreting organisms following the mass extinction were significantly altered, and that silica productivity was restricted to high latitude and deep water thermal refugia. This result has potential to further shape our understanding of changes in marine dissolved silica levels and in turn rates of reverse weathering, with implications for our understanding of carbon cycle dynamics during this interval

Acceleration of phosphorus weathering under warm climates

The release of phosphorous (P) via chemical weathering is a vital process that regulates the global cycling of numerous key elements and shapes the size of the Earth’s biosphere. It has long been postulated that global climate should theoretically play a prominent role in governing P weathering rates. Yet, there is currently a lack of direct evidence for this relationship based on empirical data at the global scale. Here, using a compilation of temperature and P content data of global surface soils (0 to 30 cm), we demonstrate that P release does enhance at high mean annual surface temperatures. We propose that this amplification of nutrient supply with warming is a critical component of Earth’s natural thermostat, and that this relationship likely caused expanded oceanic anoxia during past climate warming events. The potential acceleration of phosphorus loss from soils due to anthropogenic climate warming may pose threats to agricultural production, terrestrial and marine ecosystems, and alter marine redox landscapes

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Traitement numérique du signal en modulation multiporteuse appliqué au modem VDSL

Les moderms VDSL (Very high rateDigital Subscriber Line) sont la dernière génération de modems à haut débit (jusqu'à 50 Mb/s) utilisant comme support physique de transmission la ligne téléphonique et succèdent aux modems ADSL (Asymetric Digital Subscriber Line). Ces types de modems utilisent tous deux une modulation multiporteuse : la modulation DMT (Discrete Multi-Tone) pour communiquer entre eux. L'objectif de ce travail est la présentation de la modulation DMT appliquée à la technologie VDSL et l'étude de nouveaux algorithmes de traitement numérique du signal en modulation multiporteuse permettant d'améliorer les performances des modems VDSL en terme de débit ou de complexité algorithmique. Après avoir introduit les dispositifs numériques d'émission et de réception d'un signal DMT utilisé en technologie VDSL, nous listerons les principaux bruits d'un système VDSL en indiquant à chaque fois les méthodes classiques permettant d'annuler ou au moins de limiter leurs impacts sur les performances du mode. On présentera alors la technique dite "Zipper" permettant une très grande flexibilité dans l'allocation des bandes de fréquences affectées aux sens montant et descendant. Lorsque plusieurs lignes téléphoniques sont regroupées dans un même câble, il se crée un bruit provenant des utilisateurs partageant le câble, appelé diaphonie. Ces perturbations étant un des principaux facteurs limitant les performances d'un modem VDSL, la suite de ce travail sera consacrée à la présentation de procédés innovants d'annulation d'écho et de diaphonies permettants d'améliorer de façon significative les performances du modem. Ce travail s'achèvera par l'étude d'une modulation multiporteuse filtrée apelée FMT (Filtered Multi-Tone) dont la particularité est de permettre une meilleure indépendance spectrale entre les porteuses. On s'intéressera à l'optimisation conjointe des filtres d'émission-réception et des coefficients d'égalisation d'un système de communication FMT.GRENOBLE1-BU Sciences (384212103) / SudocSudocFranceF