Autogenic Timescales Set Spatial Scales of Subsurface Carbon Reservoirs in Marginal Marine Environments

Autogenic dynamics of terrestrial channels in marginal marine settings are a principal driver of shoreline movement over time. However, how autogenic channel behavior influences organic matter preservation in lowland coastal river deltas is unclear. We use observations from a physical experiment that couples a clastic delta and coastal wetland environment to develop a framework that links the autogenic movement of shorelines via channel relocation to the spatial extent of preserved organic-rich strata over geologic timescales. In the physical experiment, the input rate of water and clastic sediment were kept constant, as was the long-term creation of accommodation. Under these boundary conditions, deltaic channels self-organized to approach threshold states.

Organic sediment proxy (kaolinite clay) was deposited on the delta-top separate from the clastic input following ecological rules that link organic matter accumulation rates to elevation relative to sea level. We quantify a conservative value for the bulk fraction of preserved proxy material in the resulting deposit and characterize spatial arrangements of organic-rich strata. On average, the active wetland accretion zone was 0.20 m wide, but the subsurface extent defined by preservation of at least 25% of the maximum fraction present for all cross-sections was 2.20 m wide. We show channel mobility and compensation timescales exert a first-order control on shoreline movement and thus preservation of organic sediment proxy. Given autogenic processes operate in a range of clastic environments that accumulate appreciable quantities of organic matter, our approach shows these processes can aid in predicting the spatial extent of subsurface carbon deposits.