Radiolaria, marine zooplankton, have lived in the Earth's oceans for more than 500 my and have substantially determined the global silica cycle by precipitating the ocean's dissolved silica that results form chemical weathering of the Earth's crust. Only recently (the last 15 my) marine diatoms have taken the lead in controlling the marine silica cycle. This project seeks to quantitatively understand the changeover from the past radiolarian - to the present diatom-dominated marine silica cycle in connection with major climate changes over the past 200 million years (from greenhouse to icehouse). The timing and the causes of these major changes since the Mesozoic will be assessed by means of quantitative radiolarian biochronology and biodiversity through time, O and Si-stable isotope geochemistry of biosiliceous rocks, radiolarians and diatoms, and quantitative modelling of silica budgets through time. In the past and the present the silica and carbon cycles are closely linked via continental weathering rates and nutrient availability in the oceans. Understanding the silica cycle is a key to the processes linking the geosphere and the biosphere in long and short term climate change.