[SGL34-P03] Preliminary Results of the RV SONNE cruise SO251b in the Kumano Basin (Nankai Trough subduction zone, Japan)
Keywords:Mud Volcano, Kumano Basin, Long-term observation
Two scientific cruises with the RV SONNE were held on the Kumano Basin in 2012 (SO222) and 2016 (SO251b). The main purposes of the SO251b cruise were (1) to recover the long-term monitoring devices installed in 2012 on 3 mud volcanoes (MV#2, MV#3, MV#4) and (2) to monitor heat flow, pore water geochemistry, bathymetric and subbottom variations on the Kumano Basin seafloor in order to compare the newly acquired data to the SO222 ones.
Four long-term observatories, measuring pressure and temperature in MeBo boreholes, were recovered during multiple dives with the ROV PHOCA from Geomar and are currently under analysis. The long-term data series, together with records from the DONET network, will allow to define the link between mud volcanoes and earthquakes. At the same time, the multibeam echosounder survey (Kongsberg EM122, 12 kHz) was expanded from 2012 to cover most of the Kumano Basin, from 136º10’ to 137º30’ East. The recently acquired data unraveled a new mud volcano (MV#14, which was also groundtruthed with the recovery of mud breccia) and interesting bathymetric changes in correspondence of some of the most active mud volcanoes (MV#2, MV#13). The differential bathymetry provides constraints on mud flows occurrence between 2012 and 2016. Several loci of possible gas emission (>40) were discovered on the basin seafloor, often situated on top of mud volcanoes or near suspected ones, characterized by strong acoustic signals within the water column. Geochemistry results from sediment cores scattered through the research area showed, according to preliminary analyses conducted onboard, slight freshening with increasing depth, pointing towards a possible deep-seated fluid source. Heat flow measurements with a violin-bow probe through the basin and the active mud volcanoes (identified in 2012) revealed marked differences from the SO222 measurements, supporting ongoing fluid/mud emissions especially on MV#2 and MV#13.