Publication Date:
2019-09-23
Description:
Methane (CH4) concentrations and CH4 stable
carbon isotopic composition (d13CCH4 ) were investigated in
the water column within Jaco Scar. It is one of several scars
formed by massive slides resulting from the subduction of
seamounts offshore Costa Rica, a process that can open up
structural and stratigraphical pathways for migrating CH4.
The release of large amounts of CH4 into the adjacent
water column was discovered at the outcropping lowermost
sedimentary sequence of the hanging wall in the northwest
corner of Jaco Scar, where concentrations reached up to
1,500 nmol L-1. There CH4-rich fluids seeping from the
sedimentary sequence stimulate both growth and activity of
a dense chemosynthetic community. Additional point
sources supplying CH4 at lower concentrations were
identified in density layers above and below the main
plume from light carbon isotope ratios. The injected CH4 is
most likely a mixture of microbial and thermogenic CH4 as
suggested by d13CCH4 values between -50 and -62 %
Vienna Pee Dee Belemnite. This CH4 spreads along isopycnal
surfaces throughout the whole area of the scar, and
the concentrations decrease due to mixing with ocean water
and microbial oxidation. The supply of CH4 appears to be
persistent as repeatedly high CH4 concentrations were
found within the scar over 6 years. The maximum CH4
concentration and average excess CH4 concentration at
Jaco Scar indicate that CH4 seepage from scars might be as
significant as seepage from other tectonic structures in the
marine realm. Hence, taking into account the global
abundance of scars, such structures might constitute a
substantial, hitherto unconsidered contribution to natural
CH4 sources at the seafloor.
Type:
Article
,
PeerReviewed
Format:
text
DOI:
10.1007/s00531-012-0822-z
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