Marine sediments contribute significantly to global element cycles on multiple time scales. This is due in large part to microbial activity in the shallower layers and abiotic reactions resulting from increasing temperatures and pressures at greater depths. Quantifying the rates of these diagenetic changes requires a three-dimensional description of the physiochemical properties of marine sediments. In a step toward reaching this goal, we have combined global data sets describing bathymetry, heat conduction, bottom-water temperatures, and sediment thickness to quantify the three-dimensional distribution of temperature in marine sediments. This model has revealed that ∼35% of sediments are above 60 °C, conditions that are suitable for petroleum generation. Furthermore, significant microbial activity could be inhibited in ∼25% of marine sediments, if 80 °C is taken as a major thermal barrier for subsurface life. In addition to a temperature model, we have calculated new values for the total volume (3.01 × 108 km3) and average thickness (721 m) of marine sediments, and provide the only known determination of the volume of marine-sediment pore water (8.46 × 107 km3), equivalent to ∼6.3% of the volume of the ocean. The results presented here can be used to help quantify the rates of mineral transformations, lithification, catagenesis, and the extent of life in the subsurface on a global scale.