GRASSLAND PRIMING OF A SILICATE PUMP IN THE MIOCENE PACIFIC OCEAN

The rise of the grassland ecosystem helped generate a silicate pump in the Miocene ocean that is recorded by the diatomites and cherts of the Monterey Formation and related facies around the northern Pacific rim. Volcanism around this active continental margin would have stimulated diatom productivity by supplying silica and nutrients. Ocean circulation favored concentration of those nutrients and silica into significant siliceous deposits (e.g. Vincent and Berger's Monterey Hypothesis).

Inputs of nutrients and silica to the Miocene ocean included volcanic delivery, dissolved river load, suspended river load, wind, and groundwater influx. Eruptive phases of Pacific-rim volcanism no doubt made voluminous amounts of fresh materials available, stimulating ocean productivity in the short term. The bulk of the material that accumulated as deposits of lava and ash on land released a slower flux of raw materials to the ocean via weathering and erosion. The relative contributions of these varied sources are evaluated herein in light of new information from a number of investigators studying diverse aspects of the biogeochemical cycle of silica.

Grasslands extract high levels of opaline silica from soils. Much grass-mobilized silica is stored as opal phytoliths in soil, but reactive phytoliths are also released. Grass fires accelerate stripping of silica and nutrients. Miocene delivery of grass-generated opaline silica to the oceans probably increased via both river and windblown pathways. New measurements from modern African rivers reveal a phytolith-rich suspended load. These opal grains join previously recognized riverine diatoms as another important flux of usable silica in rivers, particularly when they dissolve in the silica-undersaturated ocean. Germanium/silicon ratios support a strong biogenic silica component in Hawaiian rivers.

Although oceanographic factors were important in producing the unique Monterey-type facies, grasslands assisted in the delivery of usable silica and nutrients from fresh terrestrial volcanic deposits and other surficial materials. The rise of the grassland ecosystem probably magnified the effects of both Pacific-rim volcanism and changes in ocean circulation as they pertain to genesis of Monterey-type siliceous facies.