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The global silica cycle is an important component of the long-term climate system, yet its controlling factors are largely uncertain due to poorly constrained proxy records. Because radiolarians and other organisms preferentially extract lighter Si from the ocean, the δSi of biosiliceous tests can thus be used for a potential proxy of productivity. Additionally, δSi of oceanic silica could have reflected changes in the isotopic ratio of sources and sinks. Here we show δSi records measured by secondary ion mass spectrometer (SIMS) in radiolarian silica, precipitated inside radiolarian molds in early Mesozoic bedded chert of the Inuyama section, central Japan. Range of measured δSi between -0.3 and 2 ‰ is consistent with that of modern and Cenozoic radiolarian tests. Relatively large intra-chert bed variability up to ~ 0.8 ‰ (1SD) support that δSi of the Mesozoic radiolarian molds are not perfectly homogenized in a chert bed during diagenesis. We found an overall inverse correlation between 10-Myr scale δSi and biogenic silica (BSi) burial flux, which contradicts with a conventional interpretation of δSi as paleoproductivity proxy, despite the lowresolution and scattered our δSi records. Although most of the factors controlling oceanic δSi are difficult to be constrained, this inverse relation might be explained by changes in δSi of mafic/felsic rock weathering ratio, which inferred from paleogeography. Further high-resolution δSi records will allow a better understanding of the past silica cycle.

SIMS analysis of Si isotope for radiolarian test in Mesozoic bedded chert, Inuyama, central Japan