The geochemical behavior of dissolved germanium in rivers, estuaries, and the oceans is similar to that of silicon. It is chemically weathered from continental rocks and transported by rivers to the oceans, where it is taken up by siliceous organisms and deposited in biogenic opal. The ratio of germanium to silicon buried in siliceous oozes must reflect the oceanic input ratios from rivers (crustal weathering) and sea-floor vents (mid-ocean ridge hydrothermal weathering). The opal germanium/silicon ratio may thus monitor relative variations in these weathering processes and provide information about the geochemical history of seawater.