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.
Rights
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