Abstract. Magmatic sill intrusions into organic-rich sediments cause the release of
thermogenic CH4 and CO2. Pore fluids from the Guaymas Basin (Gulf
of California), a sedimentary basin with recent magmatic activity, were
investigated to constrain the link between sill intrusions and fluid seepage
as well as the timing of sill-induced hydrothermal activity. Sampling sites
were close to a hydrothermal vent field at the northern rift axis and at cold
seeps located up to 30 km away from the rift. Pore fluids close to the active
hydrothermal vent field showed a slight imprint by hydrothermal fluids and
indicated a shallow circulation system transporting seawater to the
hydrothermal catchment area. Geochemical data of pore fluids at cold seeps
showed a mainly ambient diagenetic fluid composition without any imprint
related to high temperature processes at greater depth. Seep communities at
the seafloor were mainly sustained by microbial methane, which rose along
pathways formed earlier by hydrothermal activity, driving the anaerobic oxidation
of methane (AOM) and the formation of authigenic carbonates. Overall, our data from the cold seep sites suggest that at present, sill-induced hydrothermalism is not active away from the ridge axis, and the vigorous venting
of hydrothermal fluids is restricted to the ridge axis.
Using the sediment thickness above extinct conduits and carbonate dating, we
calculated that deep fluid and thermogenic gas flow ceased 28 to 7 kyr ago.
These findings imply a short lifetime of hydrothermal systems, limiting the
time of unhindered carbon release as suggested in previous modeling studies.
Consequently, activation and deactivation mechanisms of these systems need to
be better constrained for the use in climate modeling approaches.
Diffuse hydrothermal fluids from Lucky Strike hydrothermal vent field: Evidence for a shallow conductively heated system