Oxygen isotope analysis of the eyes of pelagic trilobites: Testing the application of sea temperature proxies for the Ordovician

The oxygen isotope composition of well-preserved trilobite eye calcite, retaining its original optical properties, represents a possible source of information on Paleozoic sea temperatures. Species of the epipelagic telephinid genera Carolinites and Opipeuterella from strata of Early to Middle Ordovician age in Spitsbergen and Australia were analyzed, and compared with benthic asaphid species. Scanning electron microscope (SEM), cathodoluminescence (CL). electron microprobe and Electron Backscatter Diffraction (EBSD) techniques were used to assess eye preservation prior to isotope analysis. Some apparently well-preserved eyes are identified from the Valhallfonna (Spitsbergen) and Emanuel (Australia) formations. The eyes show a wide variation in delta O-18 values: -6.2%. to -9.8 parts per thousand for the Valhallfonna Formation, -3.2 parts per thousand to -10.4 parts per thousand for the Emanuel Formation, and -3.6 parts per thousand to -7.4 parts per thousand for the Horn Valley Siltstone (Australia). Intra-eye Secondary Ion Mass Spectrometry (SIMS) isotope results reveal an even larger range in delta O-18 in some specimens (delta O-18 of 2.4 parts per thousand to 10.4 parts per thousand), suggesting that the trilobite eyes have undergone cryptic recrystallization. A sub-set of trilobite cuticle from the three formations were analyzed for their carbonate clumped isotope compositions (6,47), and yielded crystallization temperatures above 50 degrees C, consistent with diagenetic alteration. The SIMS and Delta(47) results suggest that classic preservation assessment protocols for the stable isotope study of deep-time carbonate samples may be insufficient, especially for these techniques. There is a need for extensive microstructural chai-acterization of lower Paleozoic biogenic carbonates, by techniques including EBSD, SIMS and Delta(47), before their stable isotope signatures can be used with certainty in paleoclimate studies. (C) 2018 International Association for Gondwana Research. Published by Elsevier B.V. All rights reserved.