Silicon Isotopes Of The Mesozoic Radiolaria: Implications For Carbon-Silicon Cycle Coupling.

The global carbon and silicon cycles regulates long-term (>105 yr) changes in Earth's climate through the negative feedback mechanism that operates between atmospheric CO₂, climate and silicate weathering rate, followed by carbonate and biogenic silica (BSi) deposition. However, their temporal relation and their 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 paleoproductivity and δ³⁰Si of oceanic silica, which could have reflected changes in the isotopic ratio of sources (weathering) and sinks. Here we show δ³⁰Si_radiolaria of Triassic to Cretaceous bedded chert with bio-astrochronology, in Franciscan, Japan, Italy and Turkey. We found an overall inverse correlation between δ³⁰Si and biogenic silica (BSi) burial flux on 10-Myr timescale, which contradicts with a conventional interpretation of δ³⁰Si as paleoproductivity proxy, despite of the low-resolution and scattering of our δ³⁰Si records. Although most of 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, inferred from paleogeographic distribution of Mesozoic volcanic rocks. We will also discuss Milankovitch-type orbital-scale to Myr-scale variations in δ³⁰Si across some geologic events, such as mid-Norian Manicouagan impact event, the end-Triassic extinction, Pliensbachian-Toarcian and Bathonian-Callovian oceanic anoxic events (OAEs) will be discussed.