Passive coastal margin deep sedimentary carbon and methane cycles

Biogeosciences Discussions This discussion paper is/has been under review for the journal Biogeosciences (BG). Please refer to the corresponding final paper in BG if available. Abstract We present a new geologic-time and basin-spatial scale model of the continental margin methane cycle. The model, SpongeBOB, is used to simulate evolution of the carbon cycle in a passive sedimentary continental margin in response to changing oceanographic and geologic forcing over a time scale of 140 million years. The model is some-5 what less sensitive to temperature than our previous results with a one-dimensional model, but is more sensitive to reasonable changes in POC than it is to reasonable changes in temperature. This behavior could lead to higher inventories of hydrate during hothouse climate conditions, rather than lower as generally assumed, due to the enrichment of the sediments in organic carbon. The hydrate inventory in the model is 10 extremely sensitive to the ability of methane bubbles to rise within the sediment column, and how far gas-phase methane can get through the sediment column before it redis-solves when it reaches undersaturated conditions. Hydrate formation is also sensitive to deep respiration of migrating petroleum in the model. The geochemistry of the sediment column is altered by the addition of vertical high-permeability chimneys intended 15 to mimic the effects of heterogeneity in the real sediment column due to faults and chimneys , and produces results consistent with measured pore-water tracers SO 2− 4 and 129 I. Pore water DIC concentrations are consistent with chemical weathering at depth within the sediment column. The carbon isotopic composition of the DIC is consistent with a methane production efficiency from POC of 50 %, which is somewhat lower than redox 20 balance with the H/C of organic matter in the model. Other phenomena which we simulated had only small impact on the hydrate inventory, including thermogenic methane, dissolved organic carbon, and sediment transport characteristics.