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

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posted on 04.04.2018, 10:20 by Carys. E Bennett, Mark Williams, M Leng, M Lee, M Bonifacie, D Calmels, R Fortey, J Laurie, A Owen, Alex. A Page, A Munnecke, T Vandenbroucke
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 δ18O values: −6.2‰ to −9.8‰ for the Valhallfonna Formation, −3.2‰ to −10.4‰ for the Emanuel Formation, and −3.6‰ to −7.4‰ for the Horn Valley Siltstone (Australia). Intra-eye Secondary Ion Mass Spectrometry (SIMS) isotope results reveal an even larger range in δ18O in some specimens (δ18O of −2.4‰ to −10.4‰), 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 (Δ47), and yielded crystallization temperatures above 50 °C, consistent with diagenetic alteration. The SIMS and Δ47 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 characterization of lower Paleozoic biogenic carbonates, by techniques including EBSD, SIMS and Δ47, before their stable isotope signatures can be used with certainty in paleoclimate studies.

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Citation

Gondwana Research, 2018, volume 57, pp 157-169

Author affiliation

/Organisation/COLLEGE OF SCIENCE AND ENGINEERING/School of Geography, Geology and the Environment

Version

AM (Accepted Manuscript)

Published in

Gondwana Research

Publisher

Elsevier

issn

1342-937X

Acceptance date

16/01/2018

Copyright date

2018

Publisher version

https://www.sciencedirect.com/science/article/pii/S1342937X18300339

Notes

The file associated with this record is under embargo until 24 months after publication, in accordance with the publisher's self-archiving policy. The full text may be available through the publisher links provided above.

Language

en

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