Experimental data are presented for methane hydrate stability conditions in seawater (S --33.5 %0). For the pressure range of 2.75-I0.0 M.Pa, at any given pressure, the dissociation temperature of m,ethane hydrate is depressed by approximately-1.1 øC relative to the pure methane-pure water system. These experimental results are consistent with previously reported thermody.namic predictions and experimental results obtained with artificial seawater.Collectively these results provide a minimum constraint concerning depth ranges over which methane hydrate is stable in the oceanic environment.

Abstract. Geophysicists have recently expressed an interest in understanding how pore water composition affects CH4 hydrate stability conditions in the marine environment. It has previously been shown in the chemical engineering literature that CH4 hydrate stability conditions in electrolyte solutions are related to the activity of water (aw). Here we present additional experimental data in support of this relationship and then use the relationship to address issues relevant to geophysicists. Pressure and temperature conditions of CH4 hydrate dissociation were determined for 10 solutions containing variable concentrations of CI-, SO4 2-, Br-, Na +, K +, Mg 2+, NH4 +, and Cu 2+. The reciprocal temperature offset of CH4 hydrate dissociation between the CH4-pure water system and each of these solutions (and for other electrolyte solutions in literature) is directly related to the logarithm of the activity of water (lnaw). Stability conditions for CH4 hydrate in any pore water system therefore can be predicted simply and accurately by calculating lnaw. The effect of salinity variation and chemical diagenesis on CH4 hydrate stability conditions in the marine environment can be evaluated by determining how these processes affect lnaw of pore water.

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This paper compares results of rigorous calculations of light scattering by a distribution of coated spheres with the measured light scattering of marine Chlorella. The elastic scattering properties of any organism are described completely by the 16-element scattering matrix that gives all the intensity and polarization information as a function of angle. Fitting the angular dependence of several matrix elements affords a much more stringent test of scattering calculations than does fitting only one, such as the phase function or linear polarization. The added requirements of this test significantly narrow the range of acceptable optical models and thereby permit better characterization of the scattering medium. Measurements of several elements of the scattering matrix of laboratory cultures of Chlorella were obtained with a scanning polarization-modulation nephelometer. The results for these elements were best fit by calculations based on a model of a Gaussian distribution of spheres with a relative complex refractive index of l.OS(t-0.005)-0.05i(~0.005) and a 60-nm coating of index1.13(+0.005)-0.04i(f0.005) to approximate the cell membrane. Good agreement was obtained for only a very narrow range of particle parameters. Experimental results were broken into spherical and nonspherical contributions to evaluate the effects of nonsphericity.Particles in the oceanic water column scatter and absorb light to alter its intensity, spectrum, and polarization state. The polarization of light in the ocean has long been known to affect the behavior of marine organisms (Waterman 1954). Measurement of the intensity and polarization of light scattered by particles in the water as a function of angle can be used to investigate the nature of the particles and to predict the propagation of light in the ocean. To better understand light scattering in the ocean, we AcknowledgmentsWe thank Richard Spinrad, Eric Hartwig, and Pat Wilde for discussions and support.