Recent, deep-sourced methane/mud discharge at the most active mud volcano in the western Mediterranean

Abstract

Active mud volcanism in the West Alboran Basin (WAB) is closely associated with tectonically mobilized, overpressurized shales and shale-diapirism. This appears to control mud expulsion at Carmen mud volcano, a cone-shaped structure 65 m high and 1 km in basal diameter. The presence of gas-rich mud breccia, living chemosynthetic fauna, the absence of hemipelagic draping and the abrupt transition that occurs between high dissolved sulfate in the uppermost interval and low sulfate together with high methane concentrations in the lowermost sediment interval all point to a recent expulsion of mud breccia at the summit of Carmen MV. For the lowermost interval, the depletion of major elements (i.e., Ca2+ and Mg2+) and the enrichment of trace species (i.e., Li and B) in the pore water all indicate a deep fluid source. The δ18Opw (5.7‰ VSMOW) and δDpw (−10‰ VSMOW) of pore water in the lowermost interval correspond with smectite dehydration as the main pore-water freshening mechanism. Water-formation temperatures calculated with empirical geo-thermometers (K-Na, K-Mg; δ18Opw, δDpw, and dissolved B) reveal that fluids were generated at temperatures of ~140 ± 20 °C. Taking a regional geothermal gradient for the WAB of 25–27 °C/km, this points to a fluid source from ~5 ± 1 km sediment depth. This is not only consistent with the depth of overpressurized shales and megabreccia of Lower to Middle Miocene age, but it also fits nicely with the Upper/Middle-Miocene seawater value for the porewater 87Sr/86Sr derived from dissolving carbonates. The stable carbon and hydrogen isotopic composition of methane (δ13Cmethane ~ −59.4‰ VPDB and δDmethane −184‰ VSMOW) for the deepest samples of summit-core GP05PC is consistent with the mentioned deep origin. Mud breccia expulsion of overpressurized deep sedimentary units would be accompanied by rigorous degassing, leading to rapid, ‘instantaneous’ replacement of pore fluid by bottom water in the upper sediments. The absence of oxidized sediment draping, the seawater-like pore-water composition in the uppermost part of the mud breccia interval, and the abrupt methane to sulfate transition all provide evidence for a very recent mud expulsion. The distinctively kink-shaped pore-water Cl− profile in core GP05PC has been used in a numerical transport-reaction model to derive the timing for this event. This eruptive event appears to have taken place very recently, namely 12 ± 5 yrs prior to the 2012 coring, thus in the year 2000 CE.