Limestones of mid-Cretaceous age containing as much as 8.6 percent organic carbon were cored at one site (463) in the Mid-Pacific Mountains and at two sites (465, 466) on southern Hess Rise, central North Pacific Ocean, during Leg 62 of the Deep Sea Drilling Project. In the Mid-Pacific Mountains, three layers containing more than 2% organic carbon are associated with volcanic ash in silicified limestone of early Aptian age (—112-113 m.y. old), a time when the site was south of the equator and considerably shallower than at present (2525 m). The beds of laminated, organiccarbon-rich limestone, implying deposition in an oxygen-deficient environment, are interbedded with normal pelagic limestones. This sequence lies on interbedded pelagic limestones and clastic limestones containing locally derived shallow-water carbonate debris, and it is overlain by interbedded green, gray, and pink limestones. The organic-carbonrich strata on southern Hess Rise are dark-olive, laminated limestones with rare interbeds of altered volcanic ash of the late Aptian to early Cenomanian ( — 98 to 103 m.y. old), a time when the site was passing under the equatorial highproductivity zone and subsiding from shallow to intermediate water depths. The organic-carbon-rich limestone sequence overlies volcanic basement (trachyte) in this part of Hess Rise. The association of volcanogenic sediments with organic-carbon-rich strata on Hess Rise is not as striking as in the Mid-Pacific Mountains, but the occurrences do suggest a coincidence of mid-plate volcanic activity and accumulation of organic matter at intermediate water depths in the tropical North Pacific Ocean during the mid-Cretaceous. Pyrolysis assays indicate that most of the organic matter in the limestone on Hess Rise is composed of lipid-rich kerogen derived from marine organisms. Limestones from the Mid-Pacific Mountains contain lower concentrations of organic matter.

Pyrolysis assays and extractable hydrocarbons indicate that the organic matter in samples from Hole 463 is also predominantly of marine origin, and that contributions of terrigenous organic matter were probably minor. Analyses of stable carbon isotopes indicate that the organic carbon in all analyzed samples is isotopically light (δ13C 24 to - 29‰) relative to most modern marine organic carbon (δ13C - 9 to - 30%), and that the lightest carbon (δ13C 28 to - 29‰) is also the most lipid-rich.

The organic-carbon-rich mid-Cretaceous limestones on Hess Rise, the Mid-Pacific Mountains, and other elevated plateaus and seamounts in the Pacific Ocean are approximately equivalent in age (mid-Cretaceous, -85-120 m.y.) to organic-carbon-rich lithofacies elsewhere in the world ocean, particularly in the Atlantic Ocean and parts of the Indian Ocean. However, strata of equivalent age deposited in the Pacific Ocean at greater depths do not contain any carbon rich horizons. This observation, together with the evidence that the plateau sites were considerably shallower and close to the equator during the mid-Cretaceous, suggests that an expanded mid-water oxygen minimum, together with local restrictions in circulation, may have resulted in the preservation of organic matter in an oxygen-deficient environment where the oxygen minimum impinged on elevated platforms such as Hess Rise and the Mid-Pacific Mountains.