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

Abstract

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