Gas Chemistry of Pore Fluids from Oomine Ridge on the Nankai Accretionary Prism - Accretionary Prisms and Convergent Margin Tectonics in the Northwest Pacific Basin - page 256

upward had already undergone sulfate reduction by some organic carbon sources,
including sedimentary organic carbon (Eq. 5) and methane (Eq. 6):

( ) ( ) ( )

CH O NH H PO + 55SO

2 −

2 3 3 4 4

106 16

2 −

(5)

→ + 106CO 16NH + 55S + H PO + 106H O

2 3 3 4 2

− − −
+ →+ +

2

CH SO HS HCO H O (6)

4 4 3 2

However, the latter interpretation (sulfate-free fluid flowing upward) contradicts
with the interpretation for Cl profile (very slow advection rate). It is clear that in
situ sulfate reduction plays substantially important role to control SO 4 2− profile and
fluid ascent affects negligibly the SO 4 2− profile.
Sulfate reduction by sedimentary organic carbon occurs widely in sediments and
produces ammonium (NH 4 + ), as described in Eq. 5, but sulfate reduction by meth-
ane generates no NH 4 + at all (Eq. 6). NH 4 + concentrations increased even below the
depth at which sulfate was completely reduced (Fig. 3c ), which indicates that there
are processes of organic matter decomposition, such as fermentation, producing
ammonium even where sulfate is thoroughly depleted:

( ) ( ) ( ) +
→ +

CH O NH H PO 106H O

2 3 3 4 2

106 16

106CO 212H + 16NH + H PO (7)

2 2 3 3 4

Mg and Ca concentrations showed decreases below 1 m that represent consump-
tion by carbonate precipitation, as is frequently observed in seepage areas (e.g.,
Kulm et al. 1986 ). Anaerobic oxidation of methane results in an alkalinity increase,
favoring precipitation of calcium carbonate.
Such pore-water chemistry profiles have been reported in numerous seepage fields
(see references in Reeburgh 2007 ). Reductive fluids, methane-rich or sulfate-free, rise
to surface sediments of seepage fields and reduce the depth at which early diagenesis
occurs, including microbial anaerobic methane oxidation consuming sulfate.

4.3 Gas Chemistry of Reactive Components

CH 4 concentrations showed a concave-up depth profile (Fig. 4a ), and the carbon
isotopic composition of SCO 2 was considerably lighter than normal sedimentary
organic matter (Fig. 4e ), both of which are consistent with concurrent microbially
mediated oxidation of sedimentary organic matter (Eq. 5) and isotopically light
methane (Eq. 6) (Barnes and Goldberg 1976 ; Martens and Berner 1977 ; Reeburgh
1976 ; Reeburgh and Heggie 1977 ). Such isotopically light methane shows defini-
tively that the methane is derived from microbial production in sediment by carbon-
ate reduction (Schoell 1983 ):

CO 4H +→+ CH 2H O (8)

2 2 4 2