Revisiting the data reduction of seafloor heat-flow measurement: The example of mapping hydrothermal venting site around Yonaguni Knoll IV in the South Okinawa Trough

Yonaguni Knoll IV is an active hydrothermal venting site located on the southern tip of a cross back-arc volcanic trail in the South Okinawa trough. We have compiled 22 heat-flow measurements on the seafloor around the Yonaguni Knoll IV site that reveal drastic heat-flow spatial variation. To obtain the appropriate heat-flow measurements from our recently rebuilt Lister-type marine heat probe, we have updated the processing algorithm in this study to avoid utilizing the sometimes-unstable temperature evolving data from the frictional heating stage. We have carefully calibrated and processed the observed data to yield robust estimates of the seafloor heat-flow emphasizing in particular that the previous implementation of the data reduction scheme by downsizing model parameters into two subsets to be pursued in two separate steps is not theoretically and practically essential. We also examine the trade-offs of sensitivity on model parameters constrained by the thermal decay data and reveal previously unclear knowledge concerning the intertwined mutual dependence that is critical to the performance the data reduction. A contrast of more than 3000 mW/m(2) within short distances implies active local hydrothermal circulation. The observed low heat-flow area could be a cold seawater-charging site whereas Yonaguni Knoll IV might stand for a close by discharging output. Although it demands for detailed mapping to establish the 3D geometry of the local hydrothermal circulation, preliminary evidence suggests that the lateral dimension of the circulation cell is around a few kilometres.