Chert in the Pacific: Biogenic silica and hydrothermal circulation

Abstract

Data on sections drilled by DSDP and ODP in the Pacific have been compiled for 187 sites, 105 of which contain chert. The spatial pattern of siliceous deposits in the Pacific generally follows the pattern of biologic productivity. There is some evidence that enhanced vertical diffusivity in the ocean during the Eocene warm climate gave rise to siliceous deposits on and near bathymetric highs and basin margins. The most common occurrence of chert is in Eocene and older sections; however, cherts do occur in sections as young as 7 Ma in the sites investigated. Over half of the youngest chert occurrences in sedimentary sections are within 150 m of basement. Nearly all the chert occurrences on crust younger than 60 Ma are within 150 m of basement. The deficit in heat flow over ocean crust younger than 60 Ma is explained as the result of heat advection by hydrothermal waters circulating in the upper part of the ocean crust. These waters appear to dissolve biogenic silica and deposit it as chert in the lower parts of sedimentary sections. Diffusion of these waters into the lower sediment section is enhanced by small-scale faulting. This faulting may be associated with intra-plate stresses that develop during ridge jumps and reorientations of plate motion. The relatively warm bottom waters of the early Cenozoic may have also enhanced the dissolution of biogenic silica during hydrothermal circulation in the upper oceanic crust and basal sediment section.