Publication Date:
2019-09-23
Description:
The surface sediments of two mud mounds
(‘‘Mound 11’’ and ‘‘Mound 12’’) offshore southwest Costa
Rica contain abundant authigenic carbonate concretions
dominated by high-Mg calcite (14–20 mol-% MgCO3). Pore
fluid geochemical profiles (sulfate, sulfide, methane, alkalinity,
Ca and Mg) indicate recent carbonate precipitation
within the zone of anaerobic oxidation of methane (AOM) at
variable depths. The current location of the authigenic carbonate
concretions is, however, not related to the present
location of the AOM zone, suggesting mineral precipitation
under past geochemical conditions as well as changes in the
flow rates of upward migrating fluids. Stable oxygen and
carbon isotope analysis of authigenic carbonate concretions
yielded d18Ocarbonate values ranging between 34.0 and
37.7 % Vienna standard mean ocean water (VSMOW) and
d13Ccarbonate values from -52.2 to -14.2 % Vienna Pee
Dee belemnite (VPDB). Assuming that no temperature
changes occurred during mineral formation, the authigenic
carbonate concretions have been formed at in situ temperature
of 4–5 °C. The d18Ocarbonate values suggest mineral
formation from seawater-derived pore fluid (d18Oporefluid
= 0 % VSMOW) for Mound 12 carbonate concretions but
also the presence of an emanating diagenetic fluid
(d18Oporefluid &5 %) in Mound 11. A positive correlation
between d13Ccarbonate and d18Ocarbonate is observed, indicating
the admixing of two different sources of dissolved carbon
and oxygen in the sediments of the two mounds. The
carbon of these sources are (1) marine bicarbonate
(d13Cporefluid &0 %) and (2) bicarbonate which formed
during the AOM (d13Cporefluid &-70 %). Furthermore, the
d18Oporefluid composition, with values up to ?4.7 % Vienna
standard mean ocean water (VSMOW), is interpreted to be
affected by the presence of emanating, freshened and boronenriched
fluids. Earlier, it has been shown that the origin of
18O-enriched fluids are deep diagenetic processes as it was
indicated by the presence of methane with thermogenic
signature (d13CCH4 = -38 %). A combination of present
geochemical data with geophysical observations indicates
that Mounds 11 and 12 represent a single fluid system
interconnected by deep-seated fault(s).
Type:
Article
,
PeerReviewed
Format:
text
DOI:
10.1007/s00531-012-0843-7
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