Biogenic opal content in bulk sediments of ODP Leg 181 sites, southwest Pacific

Biogenic opal concentrations were measured on bulk sediments recovered at Ocean Drilling Program Sites 1123, 1124, and 1125 off North Island of New Zealand in the southwest Pacific. Site 1124 showed opal contents ranging from approximately 2 to 8 wt%, which is relatively high compared to other sites. The subbottom maximum in biogenic opal content located between 1.0 and 1.5 m composite depth can be recognized at each site. Patterns of biogenic opal content in the uppermost parts of the cores appear to reflect the surface ocean settings relating to the migration of the Subtropical Convergence Zone.

(Table DR1) Biogenic silica and chert in the Pacific Ocean, DSDP and ODP data

Evidence for the dissolution of biogenic silica at the base of pelagic sections supports the hypothesis that much of the chert formed in the Pacific derives from the dissolution and reprecipitation of this silica by hydrothermal waters. As ocean bottom waters flow into and through the crust, they become warmer. Initially they remain less saturated with respect to dissolved silica than pore water in the overlying sediments. With the diffusion of heat, dissolved ions, and to some extent the advection of water itself, biogenic silica in the basal part of the sedimentary section is dissolved. Upon conductively cooling, these pore waters precipitate chert layers. The most common thickness for the basal silica-free zone (20 m) lies below the most common height of the top of the chert interval above basement (50 m). This mode of chert formation explains the frequent occurrence of chert layers at very shallow subbottom depths in pelagic sections of the Pacific. It is also consistent with the common occurrence of cherts