Impact of subsurface methane transport on shallow marine sediment geochemistry

Abstract

Marine sediments host a vast amount of methane, a potent greenhouse gas, in the subsurface. Transport of this subsurface methane towards the seafloor creates unique biogeochemical interactions which result in important consequences for the chemical and biological composition of the oceans at present and over the Earth’s geological history. This dissertation studied the impact of subsurface methane venting to shallow marine sediment geochemistry with a goal to quantify the role of methane induced biogeochemical processes in marine carbon cycling and to recognize geochemical proxies that will enable better reconstruction of these processes from the geological record. Key results suggest the following: (i) Globally, diffusive methane charged sediments are significantly contributing to the oceanic dissolved inorganic carbon (DIC) pool (comparable to ~20% global riverine DIC flux to oceans) and sedimentary carbonate accumulation (comparable to ~15% of carbonate accumulation on continental shelves), primarily due to microbially induced carbon-sulfur (C-S) coupling. (ii) C-S coupling induced by methane seeps and crude oil seeps can be distinguished from the sediment records using a combined stable carbon (δ13C) and sulfur (δ34S) analysis of authigenic carbonate and sulfide mineral phases formed in seep settings. (iii) Molecular fossil records of methane metabolizing archaea in the sediment column involve unique isomer patterns of Isoprenoid Glycerol dialkyl glycerol tetraether (GDGT) lipids, which can serve as an important proxy to study paleo-methane flux records. These results will substantially contribute to our existing coastal and geological carbon models as well as enhance our existing inventory of geochemical proxies to characterize the methane venting systems in the geological past.