Figure 5

Relationship among Cl⁻, δ¹⁸O _H2O , and δD _H2O of sources for the pore fluids. Relationship among Cl⁻, δ¹⁸O_H2O, and δD_H2O of sources for the pore fluids at cold seep sites in the Oomine Ridge. The plot of seawater (SW) is Cl⁻ = 547 mmol/kg, δ¹⁸O_H2O = −0.09‰, and δD_H2O = +1.1‰. The plot of shallow pore fluids (SPF) is Cl⁻ = 549 mmol/kg, δ¹⁸O_H2O = −2.5‰, and δD_H2O = −10.0‰. The plot of the pore fluids at cold seep sites in the Oomine Ridge (Oomine) is represented by D949 C3-3, Cl⁻ = 498 mmol/kg, δ¹⁸O_H2O = −0.40‰, and δD_H2O = −3.4‰. The plots of the clay-derived freshwater (CMD) are on the theoretical curve, drawn by the calculation of theoretical δ¹⁸O_H2O and δD_H2O values of water at 50°C to 160°C assuming equilibrium fractionation between pore water and clay minerals according to Sheppard and Gilg ([1996]) using clay minerals δ¹⁸O_clay = +21.5‰, a medium value for an example within a reported range of +17‰ to +26‰ for δ¹⁸O_clay, and that reported by Capuano ([1992]) using clay minerals δD_clay, and a medium value for an example within a reported range of −50‰ to +43‰ for δD_clay. In addition, the plot of freshwater derived from methane hydrate (MH) is Cl⁻ = 0 mmol/kg, δ¹⁸O_H2O = +0.3 to +0.7‰, and δD_H2O = +6.0 to +10.0‰. We determined the mixing ratios for each sources, as the Oomine plot is in one plane with SW, SPF, CMD, and MH, represented by the shaded quadrangle.