Évènements

Lithology and Tectonics: Building the inorganic carbon budget of a young  mountain range

Erica Erlanger, Aaron Bufe, Guillaume Paris, Ilenia D’Angeli, Luca Pisani, Preston Kemeny, Jessica Stammeier, Negar Haghipour, Niels Hovius

Résumé: Mountain building has classically been linked with CO₂ drawdown from chemical weathering of silicate minerals in the critical zone, although recent views on mountain building recognize the importance of rock-derived CO₂ emissions from the weathering of petrogenic organic carbon and inorganic carbonate. However, the focus on near-surface weathering reactions within the orogenic carbon factory does not consider the emission of metamorphic CO₂ from subduction processes in the deep crust and mantle. Such deep carbon sources could dwarf the surficial drawdown and release of carbon, in particular in actively extending mountain ranges that subduct large volumes of carbonate rock. Thus, accounting for weathering processes at depth and in the critical zone in parallel is crucial to fully assess how mountain-range uplift impacts the carbon cycle. Here, we quantify the exchange of CO₂ between rock and the atmosphere from subduction-related processes and from critical zone weathering reactions in two major river systems in the central Apennine Mountains of Italy. The catchments straddle a geodynamic gradient across the subduction zone that is expressed as changes in surface heat flow and crustal thickness, whereas climatic boundary conditions are relatively constant.  At the regional scale, we find that metamorphic CO₂ sources outpace critical zone inorganic carbon sources and sinks by 2 orders of magnitude above a window in the subducting slab that is characterized by high heat flow and low crustal thickness, and could have driven efficient degassing over the last 2 Ma. In contrast, surficial weathering processes dominate the carbon budget where crustal thickness is greater and heat flow is lower. Importantly, the difference in metamorphic degassing fluxes across the geodynamic gradient is multiple orders of magnitude larger than the difference in critical zone weathering fluxes. Thus, modulations of metamorphic decarbonation reactions are the most efficient process by which tectonics can regulate the inorganic carbon cycle in the Apennines. Both near-surface and deep sources of CO₂ must be considered when constructing the carbon budget of orogenic systems that include the subduction of carbonate rock.

à venir

Planetesimal rings as the cause of the Solar System’s planetary architecture
On Zoom

One outstanding challenge in planetary sciences is to understand how km-size planetary bodies (planetesimals) form out of mm-size dust grains in protoplanetary disks. Intuitively, one could expect that planetesimals grow by gradual accretion of smaller objects. In reality, the limited effect on meter-size objects of forces normally enhancing accretion (i.e., electrostatic forces and gravity) and their fast drift towards the central star make this scenario implausible. In this context, magnetic fields present in protoplanetary disks are increasingly regarded as an alternative force acting upon the accretion of planetesimals. However, emerging accretion models that include magnetic effects remain largely unconstrained. Astronomical observations of extrasolar disk magnetic fields are extremely limited, because current technology only allows us to probe exceptionally strong fields with a poor spatial resolution. Fortunately, paleomagnetic studies of meteorites give us access to a record of the magnetic field of a protoplanetary disk (our own solar nebula) with temporal and spatial resolution far superior to those of astronomical observations. I will give an overview of our current knowledge of the solar nebula field as recorded by meteorites. As a case study, I will also present the results of our recent paleomagnetic investigation of the Erg Chech 002 meteorite, which provides us with one of the oldest and best resolved record.

Reconstruction des déplacements méridiens des vents d’ouest au-dessus de l’océan Atlantique austral au Quaternaire :
implications sur la pression atmosphérique de CO2

Dans l’océan Atlantique austral, la position méridienne et l’intensité des vents d’ouest (“South Westerlies”) contrôlent 1) la stratification verticale de l’océan et 2) les apports de fer, micronutriment limitant la productivité biologique au nord du front polaire. Le premier facteur influence la remontée de CO2 des eaux profondes et son relargage dans l’atmosphère. Le second régule la pompe des tissus mous qui soustrait du CO2 de l’atmosphère. Lors du Quaternaire, 40 à 50% des variations glaciaires/interglaciaires de la pression atmosphérique de CO2 (pCO2) pourraient être expliquées par un changement de position et d’intensité des South Westerlies. Cependant, la temporalité et l’amplitude des déplacements méridiens de ces vents restent largement débattues.

Reconstruire l’origine des poussières est l’un des rares moyen de suivre l’évolution les circulations atmosphériques dans le passé. Dans le cadre d’une collaboration internationale, nous avons étudié deux carottes sédimentaires de l’océan Atlantique austral : ODP1090 (42°55’S, 8°54’E, 3702m) et MD07-3076Q (44°9’S, 14°14’W, 3770m) afin de reconstruire les circulations atmosphériques à l’échelle glaciaire/interglaciaire et lors d’événements abrupts, comme le Maximum Isotopique Antarctique 8 (AIM 8). Nous utilisons les isotopes radiogéniques du Pb, Sr et Nd dans la fraction détritique fine du sédiment comme proxy de la provenance des poussières. A l’échelle glaciaire/interglaciaire, nous montrons que les compositions isotopiques varient avec le flux de poussières. Cette observation indique un changement de la contribution relative des sources de poussières qui se déposent en Atlantique austral. Nous montrons que, lors des périodes froides, la Patagonie contribue jusqu’à 75% du flux de poussière. Lors des périodes chaudes (LGM, MIS 5), ce flux est constitué entre 40 et 70% de matériel issu du haut plateau du Puna (Argentine). Ces variations permettent de confirmer le déplacement vers le nord des “South Westerlies” lors des périodes froides suggéré par des approches indirectes sur la fertilisation ou la stratification des océans. Durant l’événement abrupt de l’AIM8, une troisième source contribue aux apports de poussières : les hauts plateaux de l’ouest de l’Argentine, au sud de la région du Puna. Ces résultats indiquent que les “South Westerlies” migrent vers le sud par rapport à leur position glaciaire ou froide. Ce déplacement induit une diminution des apports de fer au nord du front polaire qui réduit l’activité de la pompe des tissus mous, ainsi qu’une diminution de la stratification verticale de l’océan austral, ce qui intensifie le rejet de CO2 stocké dans l’océan profond vers l’atmosphère. Ces phénomènes peuvent expliquer l’augmentation de la pCO2 de ca. 30 ppmv durant les Maximums Isotopiques Antarctiques.

à venir

10 février 2023, 13h15 amphithéâtre du CRPG | Patrick Carr (GeoRessources)

Geochronology of tin and tungsten mineralisation in the Variscan Orogeny

The sprawl of the Peloponnese or how I learned to stop worrying and love gravity tectonics

Résumé: The Peloponnese (Southern Greece) belongs to the External Hellenides, an alpine orogeny that reached the late phase of its evolution during Miocene times. As it is the case for most of the alpine belts spread along the southern Eurasian margin, its deformation is now dominated by the gravitational collapse of its topography and the subsequent exhumation of deep metamorphic units. Southern Greece, however, is simultaneously located on the overriding plate of the Hellenic subduction, where the slab rollback already delaminated the Aegean part of the chain by a factor of two or more in the Neogene. The Peloponnese, still at a less advanced, more brittle stage of collapse, is therefore a remarkably suitable playground for studying the competition between volume and boundary forces in the early stages of orogeny dismantling.

Jeudi 23 Février 2023 à 13h30 | Emmanuelle Pucéat (Biogéosciences – Université de Bourgogne)

“Systèmes isotopiques couplés Sm-Nd et Lu-Hf dans les argiles : un nouveau regard sur les liens entre climat, tectonique, et altération.”

Origin of hydrogen isotopic variations in chondritic water and organics