Several celestite nodules were recovered on DSDP Leg 90 from four drilling sites on the Lord Howe Rise, southwest Pacific Ocean. The sediments at these sites are predominantly very pure calcareous nannofossil oozes and chalks. As a result of a higher-than average accumulation rate, they undergo relatively rapid burial diagenesis, which causes the expulsion of Sr from the biogenic calcite, to the interstitial waters. Another result of the high accumulation rate is the occurrence of microbial sulfate reduction in the interstitial waters. The downcore Sr increase is proportionately greater than the sulfate decrease, and celestite precipitates below about 100 m subbottom at each of these sites. The celestite contains high concentrations of many substituent cations: 4.3-7.8 mole% BaSO 4, 1.7-6.4 mole% CaSO 4, 1100-2600 ppm Al, and 400-750 ppm K. Ion activity products of Sr and sulfate at each site were calculated from the Pitzer equations and the measured concentrations of porewater constituents, and are in close agreement with the celestite solubility product corrected for in situ temperatures and pressures. Strontium concentrations of the porewaters are nearly at equilibrium with respect to celestite, and are controlled by the extent of microbial sulfate reduction. Celestite solubility increases with increasing water depth, in excellent agreement with values of the standard state partial molal volume change.