Authigenic sediments, particularly the oxidized, iron-rich, volcanic sediments from the Pacific Ocean, have interstitial waters with a pH close to 7.4, whereas in less oxidized hemipelagic sediments they show a large scatter in pH. In addition to reactions between silicates and sea water, the pH of interstitial and deep ocean waters is also controlled by the production of ferric hydroxides and the inorganic precipitation of carbonates during deep-sea volcanism.