Abstract
AbstractPeatland carbon may enter long‐term storage in sedimentary basins preserved as either coal or lignite. The time required to account for the carbon in 1–10 m thick coal seams must represent 105–106 years, an order of magnitude more than previously assumed. To understand the process by which this happens requires extrapolation of our understanding of peatland carbon accumulation over timescales that greatly exceed those of Holocene peat. We analyze the consequences of extrapolating peat growth to periods of 106 years. We deduce that that key to sustained peat growth are hydrologic landscapes that can maintain a saturated peat body above the level of clastic deposition. Contrary to current stratigraphic frameworks, we conclude that the generation of accommodation space at low rates of 0.1–0.2 mm/yr can adequately accommodate thick peat accumulation over periods >105 years. However, generation of accommodation space at rates >0.5 mm/yr cannot. The low rates that permit accommodation of thick peat are typical of the rates of subsidence in specific tectonic settings, particularly foreland basins, and this has implications for our understanding of the links between terrestrial carbon burial, tectonics and the carbon cycle. The long‐term stability of extensive peatland required to form coal also requires sediment bypass, modifying basin wide sediment transport and deposition. Limits to peatland growth under very low accommodation rates must exist but the relative importance of the limiting process is not understood. Finally, we discuss the consequences of these factors for predicting the future of the peatland carbon reservoir.
Highlights
Peatland carbon may enter into long-term crustal storage in sedimentary basins where it occurs as either coal or lignite
To understand the process by which this happens requires extrapolation of our understanding of peatland carbon accumulation over timescales that greatly exceed those of Holocene peat
Contrary to current stratigraphic frameworks, we conclude that the generation of accommodation space at low rates of 0.1–0.2 mm/yr can adequately accommodate thick peat accumulation over periods >105 years
Summary
Peatland carbon may enter into long-term crustal storage in sedimentary basins where it occurs as either coal or lignite. The balance between the rate at which accommodation space changes (on account of tectonic subsidence, change in sea level and sediment accumulation) and the peat production rate determines the resulting thickness, areal extent and composition of the peat This stratigraphic framework acknowledges but does not focus on other factors (e.g., climate, groundwater, vegetation, and geomorphology) that influence peat accumulation. Stratigraphic frameworks consider only the volumetric growth of peat (predominantly water) and subsequent compaction, while ignoring the processes that determine carbon accumulation They assume that peat properties, including thickness and composition, are highly responsive to the rate at which accommodation space is generated. Peat growth models (e.g., Baird et al, 2012; Frolking et al, 2010; Morris et al, 2012) are never extended over timeframes that greatly exceed those of the Holocene Their validity with respect to long-term crustal storage of peat remains untested. The result is a more informed interpretative framework that encapsulates the interaction of geological structures, geomorphology and hydrology over a wide range of timescales and enables a greater understanding of peatlands within the Earth system
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