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

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 28Si from the ocean, the δ30Si of biosiliceous tests can thus be used for a potential proxy of productivity. Additionally, δ30Si of oceanic silica could have reflected changes in the isotopic ratio of sources and sinks. Here we show δ30Si 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 δ30Si 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 δ30Si 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 δ 30Si and biogenic silica (BSi) burial flux, which contradicts with a conventional interpretation of δ30Si as paleoproductivity proxy, despite the lowresolution and scattered our δ30Si records. Although most of the factors controlling oceanic δ30Si are difficult to be constrained, this inverse relation might be explained by changes in δ30Si of mafic/felsic rock weathering ratio, which inferred from paleogeography. Further high-resolution δ30Si records will allow a better understanding of the past silica cycle.