11 research outputs found
Formation of carbonate chimneys in the Mediterranean Sea linked to deep-water oxygen depletion
Marine sediments at ocean margins vent substantial amounts of methane1, 2. Microbial oxidation of the methane released can trigger the precipitation of carbonate within sediments and support a broad diversity of seafloor ecosystems3, 4. The factors controlling microbial activity and carbonate precipitation associated with the seepage of submarine fluid over geological time remain poorly constrained. Here, we characterize the petrology and geochemistry of rocks sampled from metre-size build-ups of methane-derived carbonate chimneys located at the Amon mud volcano on the Nile deep-sea fan. We find that these carbonates comprise porous structures composed of aggregated spherules of aragonite, and closely resemble microbial carbonate reefs forming at present in the anoxic bottom waters of the Black Sea5. Using U-series dating, we show that the Amon carbonate build-ups formed between 12 and 7 thousand years ago, contemporaneous with the deposition of organic-rich sediments in the eastern Mediterranean, the so-called sapropel layer S1. We propose that the onset of deep-water suboxic or anoxic conditions associated with sapropel formation resulted in the development of intense anaerobic microbial activity at the sea floor, and thus the formation of carbonate chimneys
Formation of carbonate concretions in surface sediments of two mud mounds offshore Costa Rica: A stable isotope study
The surface sediments of two mud mounds
(ââMound 11ââ and ââMound 12ââ) offshore southwest Costa
Rica contain abundant authigenic carbonate concretions
dominated by high-Mg calcite (14â20 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 4â5 °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)