[BCG07-08] The Carbon-Silicon cycle coupling: insight from in situ δ30Si in Mesozoic radiolarites.
キーワード：silica cycle, silicon isotopes, radiolaria
Long-term (>105 yr) changes in Earth’s climate are regulated by the global carbon and silicon cycles through the negative feedback between silica weathering followed by carbonate and BSi precipitation trapping CO2 and SiO2, respectively. The silica weathering depends on the kind and amount of silica rock exposed on continents and to climate, which is it-self induced by atmospheric CO2 and astronomical cycles (Milankovitch cycles). However, many works need still to be done to constrain the temporal relation between the carbon and silicon cycles due to poorly constrained proxy record for the past controlling factors.
Radiolarian and other biosiliceous organisms incorporate preferentially lighter 28Si from the ocean implicating that δ30Si is, at least partially, a proxy of productivity for biosiliceous organisms similar to δ13C. Other parameters influencing δ30Si are the isotopic ratio of the sources, the weathering intensity and the Si recycling in the water column. As silicon is commonly undersaturated in the water column, this proxy is very sensitive to input/output fluxes but has also fever pools that carbon at the Earth’s surface. In addition, silica is more resistant to isotopic resetting/homogenisation than carbonates.
Here, we show δ30Siradiolaria 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 δ30Si variations within chert beds.
Radiolarian and other biosiliceous organisms incorporate preferentially lighter 28Si from the ocean implicating that δ30Si is, at least partially, a proxy of productivity for biosiliceous organisms similar to δ13C. Other parameters influencing δ30Si are the isotopic ratio of the sources, the weathering intensity and the Si recycling in the water column. As silicon is commonly undersaturated in the water column, this proxy is very sensitive to input/output fluxes but has also fever pools that carbon at the Earth’s surface. In addition, silica is more resistant to isotopic resetting/homogenisation than carbonates.
Here, we show δ30Siradiolaria 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 δ30Si variations within chert beds.