10 research outputs found
Stability of Organic Carbon Components in Shale: Implications for Carbon Cycle
Stability and mobility of organic matter in shale is significant from the perspective of
carbon cycle. Shale can only be an effective sink provided that the organic carbon
present is stable and immobile from the host sites and, not released easily during
geological processes such as low pressure-temperature burial diagenesis and higher
pressure-temperature subduction. To examine this, three Jurassic shale samples of
known mineralogy and total organic carbon content, with dominantly continental source
of organic matter, belonging to the Haynesville-Bossier Formation were combusted by
incremental heating from temperature of 200 to 1400◦C. The samples were analyzed for
their carbon and nitrogen release profiles, bulk δ
13C composition and C/N atomic ratio,
based on which, at least four organic carbon components are identified associated
with different minerals such as clay, carbonate, and silicate. They have different stability
depending on their host sites and occurrences relative to the mineral phases and
consequently, released at different temperature during combustion. The components
identified are denoted as, C-1 (organic carbon occurring as free accumulates at the
edge or mouth of pore spaces), C-2 (associated with clay minerals, adsorbed or
as organomineral nanocomposites; with carbonate minerals, biomineralized and/or
occluded), C-3(a) (occurring with silicate minerals, biomineralized and/or occluded) and
C-3(b) (graphitized carbon). They show an increasing stability and decreasing mobility
from C-1 to C-3(b). Based on the stability of the different OC components, shale is
clearly an efficient sink for the long term C cycle as, except for C-1 which forms a
very small fraction of the total and is released at temperature of ∼200◦C, OC can be
efficiently locked in shale surviving conditions of burial diagenesis and, subduction at
fore arc regions in absence of infiltrating fluids. Under low fluid flux, C-3(b) can be
efficiently retained as a refractory phase in the mantle when subducted. It is evident
that the association and interaction of the organic matter with the different minerals play
an important role in its retention in the shale
Characterisation of carbon components and their isotopic composition in gas shales
Shales are an important reservoir of both organic and inorganic carbon that can be decoupled by incremental stepwise combustion without acid treatment. The study of shales from various depths of three cores of Haynesville-Bossier formation of Jurassic period indicates the presence of organic and inorganic carbon with comparable δ13C and nitrogen/carbon (N/C) ratios in all three. Both the isotopic and elemental ratios predominantly indicate a continental source but a minor N/C fraction corresponding to a marine/lacustrine source is also seen. This indicates either mixing from different sources and/or fractionation, perhaps due to preferential loss of nitrogen during organic matter degradation. Because some of the organic component is released at a high temperature of 1000-1200°C although ungraphitised, it may be locked up in minerals like sulphides and K-feldspars, incorporated during diagenesis and hydrothermal alteration. Carbon locked in these minerals can be isolated from the carbon cycle for a longer period of time
Ion implantation in nanodiamonds: Size effect and energy dependence
Nanoparticles are ubiquitous in nature and are increasingly important for technology. They are subject to bombardment by ionizing radiation in a diverse range of environments. In particular, nanodiamonds represent a variety of nanoparticles of significant fundamental and applied interest. Here we present a combined experimental and computational study of the behaviour of nanodiamonds under irradiation by xenon ions. Unexpectedly, we observed a pronounced size effect on the radiation resistance of the nanodiamonds: particles larger than 8 nm behave similarly to macroscopic diamond (i.e. characterized by high radiation resistance) whereas smaller particles can be completely destroyed by a single impact from an ion in a defined energy range. This latter observation is explained by extreme heating of the nanodiamonds by the penetrating ion. The obtained results are not limited to nanodiamonds, making them of interest for several fields, putting constraints on processes for the controlled modification of nanodiamonds, on the survival of dust in astrophysical environments, and on the behaviour of actinides released from nuclear waste into the environment
The Winchcombe meteorite, a unique and pristine witness from the outer solar system.
Direct links between carbonaceous chondrites and their parent bodies in the solar system are rare. The Winchcombe meteorite is the most accurately recorded carbonaceous chondrite fall. Its pre-atmospheric orbit and cosmic-ray exposure age confirm that it arrived on Earth shortly after ejection from a primitive asteroid. Recovered only hours after falling, the composition of the Winchcombe meteorite is largely unmodified by the terrestrial environment. It contains abundant hydrated silicates formed during fluid-rock reactions, and carbon- and nitrogen-bearing organic matter including soluble protein amino acids. The near-pristine hydrogen isotopic composition of the Winchcombe meteorite is comparable to the terrestrial hydrosphere, providing further evidence that volatile-rich carbonaceous asteroids played an important role in the origin of Earth's water