Abstract
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 δ13C 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.
Highlights
Like many other elements, carbon cycles through different reservoirs on Earth including the atmosphere, mantle, hydrosphere, petrogenic reservoirs, soil and vegetal cover and biosphere
This study demonstrates that the stability of organic carbon (OC) in shale is governed by their variable association with different mineral phases like carbonate, clay minerals and silicates related to how they are trapped/hosted with them
This study aims to look into the effect of oxidation with increasing temperature on the mobility of carbon locked in shales, by combusting them with incremental heating
Summary
Carbon cycles through different reservoirs on Earth including the atmosphere, mantle, hydrosphere (oceans and rivers), petrogenic reservoirs (mainly sedimentary rocks), soil and vegetal cover and biosphere (mainly as biomass). The carbon component in organic matter, referred to as the organic carbon (OC) can be incorporated in the plant biomass by photosynthesis It subsequently cycles through the soil and oceanic sediments where, a fraction of it can be subject to decomposition. For the long term cycle, a fraction of the carbon subducted to the Earth’s mantle is returned to the surface via degassing as CO2 and CH4 from mid-oceanic ridges (MORs) and hotspots within plate settings, arcs and forearc regions in subduction settings (Chiodini et al, 2010; Kelemen and Manning, 2015; Aiuppa et al, 2019; Voyer et al, 2019). Different types of OC have different degrees of resistance to oxidative degradation depending on their type (Wallmann and Aloisi, 2012) and their host sites in rocks (Zhu et al, 2016) Isolating these components in terms of their variable stability and mobility can be quite challenging. No discrimination between the different OC components present in different sites is possible
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