Ciguatoxins, the principal causative toxins of ciguatera seafood poisoning, are large ladder-like polycyclic ethers with the 13 ether rings ranging from five- to nine-membered. In this paper, we describe the total synthesis of the two most toxic members of the ciguatoxin family, ciguatoxin 1 and 51-hydroxyCTX3C 2, based on a unified synthetic strategy. The key features in our syntheses were (i) direct construction of the O,S-acetal from the corresponding left and right wing fragments (3, 4, 14); (ii) stereo- and chemoselective radical reaction of the alpha-oxyradical with pentafluorophenyl acrylate to achieve cyclization of the seven-membered G-ring; (iii) ring-closing metathesis reaction to build the nine-membered F-ring; and (iv) an efficient protective group strategy using the oxidatively removable 2-naphthylmethyl groups.

We conducted a self-potential survey at an active hydrothermal field, the Izena hole in the mid-Okinawa Trough, southern Japan. This field is known to contain Kuroko-type massive sulphide deposits. This survey measured the self-potential continuously in ambient seawater using a deep-tow array, which comprises an electrode array with a 30-m-long elastic rod and a stand-alone data acquisition unit. We observed negative self-potential signals not only above active hydrothermal vents and visible sulphide mounds but also above the flat seafloor without such structures. Some signals were detectable >50 m above the seafloor. Analysis of the acquired data revealed these signals’ source as below the seafloor, which suggests that the self-potential method can detect hydrothermal ore deposits effectively. The self-potential survey, an easily performed method for initial surveys, can identify individual sulphide deposits from a vast hydrothermal area.

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To explain the origin of a high heat flow anomaly observed within 150 km seaward of the Japan Trench, we construct a thermal model for an oceanic plate prior to subduction that includes the effect of hydrothermal circulation within a high-permeability aquifer in its uppermost part. The model includes the effects of aquifer thickening, which is expected to occur near subduction zones where plate bending prior to subduction causes fracturing and faulting within the oceanic plate. Using typical parameter values for the Japan Trench, we find that hydrothermal circulation in the thickening aquifer mines heat from the underlying basement and can account for the observed high heat flow anomaly. The ratio of heat supply below the aquifer as a result of aquifer thickening to the inverse of the thermal resistance of the sediment layer is a control parameter for the system. As long as the aquifer permeability is higher than 10 213 m 2 , a typical value for the uppermost part of the oceanic plate, variations in other details of the hydrothermal circulation such as the exact value of the aquifer permeability and the size of the convection cells do not significantly change model results. Despite its strong influence on seafloor heat flow seaward of the trench, this hydrothermal heat mining does not affect significantly the thermal structure of the subducted oceanic plate. This finding indicates that surface heat flow anomaly around the trench may not correspond to temperature anomaly within the subducted oceanic plate and the megathrust seismogenic zone.

Phase separation of seawater is an important process controlling the dynamics and chemistry of hydrothermal circulation. We numerically investigate hydrothermal circulation in porous media, including phase separation of seawater. Seawater enters the crust at the seafloor, is heated at depth, and returns to the seafloor as hydrothermal fluids. The seafloor and the bottom of the calculation region are set at depths of 2500 m and 4000 m from the sea surface, respectively. The temperature at the base of the calculation region is set at 600• C. Under these pressure and temperature ranges, supercritical phase separation is inevitable. Here we focus on steady-state conditions, as a first step to investigate the complex process of convection with phase separation. Under these conditions, we demonstrate that phase separation leads to a two-layer structure. Seawater circulates vigorously in the upper layer, and this overlies a stagnant lower layer formed by sinking of dense brine. We find that the key quantity which governs this structure is the ratio of the relative velocity between the two phases to the mean flow velocity in the transition zone between the two layers. As the relative velocity increases, the brine layer becomes thick, and the transition zone becomes thin. Under steady state conditions, the mean salinity at the seafloor should be the same as that of seawater because the total mass of salt should be conserved. Fluids which vent near the ridge axis are more saline than seawater, whereas fluids which vent more than about 100 m away from the axis are less saline than seawater.

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