Oxygen isotope analysis of Mesozoic radiolarites using SIMS

Abstract

The oxygen isotope (δ 18 O) analysis of carbonate fossils is widely applied for palaeoceanographic analysis, whereas that of siliceous fossils is only limited partly due to technical constraints and uncertain fractionation factors. Here we used a secondary ion mass spectrometer (SIMS) for δ 18 O of radiolarian silica, precipitated inside radiolarian molds in Mesozoic radiolarites from Japan, Italy, Switzerland and Romania in order to examine its potential for palaeoceanographic proxy. 507 measurements of the isotopic oxygen signature relative to the Vienna Standard Mean Ocean Water (δ 18 OVSMOW) of 53 chert samples range between 19.8 to 35.3 ‰ overlapping with that of modern and Cenozoic radiolarian tests in the equatorial Pacific. Relatively large intra-chert variability supports that δ 18 O of the Mesozoic radiolarian tests are not perfectly homogenized within a chert bed during the diagenetic segregation. The temporal changes in the δ 18 O values of radiolarians (δ 18 Oradiolarians) show an Early-Middle Triassic slight positive excursion, a Late Triassic high plateau, an Early Jurassic negative excursion with up to 8 ‰ , a Middle Jurassic slight positive excursion, and a few light values for the Cretaceous despite of their low resolution. A comparison of δ 18 O between radiolarian molds, conodont apatite, and the low magnesium calcium shells show overall similar secular variations during the Triassic, but different trends was observed during the Early Jurassic. Because our data is low-resolution, further cross check of δ 18 Oradiolarians is necessary to use as a proxy for paleoceanography.