Stable Isotope Signatures of Middle Palaeozoic Ahermatypic Rugose Corals - Deciphering Secondary Alteration, Vital Fractionation Effects, and Palaeoecological Implications.

Michal Jakubowicz,Matthias López Correa,Blazej Berkowski,Zdzislaw Belka,Emilia Jarochowska,Michael Joachimski,David P Gillikin

doi:10.1371/journal.pone.0136289

Michal Jakubowicz, Matthias López Correa + Show 5 more

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https://doi.org/10.1371/journal.pone.0136289

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Abstract

This study investigates stable isotope signatures of five species of Silurian and Devonian deep-water, ahermatypic rugose corals, providing new insights into isotopic fractionation effects exhibited by Palaeozoic rugosans, and possible role of diagenetic processes in modifying their original isotopic signals. To minimize the influence of intraskeletal cements on the observed signatures, the analysed specimens included unusual species either devoid of large intraskeletal open spaces ('button corals': Microcyclus, Palaeocyclus), or typified by particularly thick corallite walls (Calceola). The corals were collected at four localities in the Holy Cross Mountains (Poland), Mader Basin (Morocco) and on Gotland (Sweden), representing distinct diagenetic histories and different styles of diagenetic alteration. To evaluate the resistance of the corallites to diagenesis, we applied various microscopic and trace element preservation tests. Distinct differences between isotopic compositions of the least-altered and most-altered skeleton portions emphasise a critical role of material selection for geochemical studies of Palaeozoic corals. The least-altered parts of the specimens show marine or near-marine stable isotope signals and lack positive correlation between δ13C and δ18O. In terms of isotopic fractionation mechanisms, Palaeozoic rugosans must have differed considerably from modern deep-water scleractinians, typified by significant depletion in both 18O and 13C, and pronounced δ13C-δ18O co-variance. The fractionation effects exhibited by rugosans seem similar rather to the minor isotopic effects typical of modern non-scleractinian corals (octocorals and hydrocorals). The results of the present study add to growing evidence for significant differences between Scleractinia and Rugosa, and agree with recent studies indicating that calcification mechanisms developed independently in these two groups of cnidarians. Consequently, particular caution is needed in using scleractinians as analogues in isotopic studies of extinct coral lineages. Answering some of the pertinent palaeoecological questions, such as that of the possibility of photosymbiosis in Palaeozoic corals, may not be possible based on stable isotope data.

Highlights

Stable isotope analyses of scleractinian corals have been successfully applied to address various types of palaeoenvironmental and palaeoecological questions (e.g., [1,2,3,4,5])
This implies that mechanisms of incorporation of carbon and oxygen isotopes in Palaeozoic rugosans differed markedly from fractionation processes imposed by modern azooxanthellate scleractinians, which show strong vital fractionation effects resulting in conspicuous depletion in both 13C and 18O and a clear positive correlation between δ13C and δ18O values (e.g., [7,53,54,55,59])
The results of our study show that, having specimens with still distinguishable primary microstructural features, suitable luminescence and trace element characteristics, and embedded in sediments hampering diagenetic fluid movement and fluid-rock interactions, it is possible to obtain primary stable isotope signals from corallites of rugose corals

Summary

Introduction

Application of stable isotope signatures of extinct coral groups in palaeoceanographic and palaeoecological studies remains, problematic and poses a considerable challenge. The use of coral skeletons as palaeoenvironmental archives is very attractive, because, by analogy with Recent forms, one may be able to identify seasonal or even sub-seasonal patterns of skeleton development (e.g., [2,4]). As they typically form more rapidly than many inorganic marine precipitates, biogenic materials are less prone to significant time averaging of their geochemical signals. Due to the presence of intraskeletal organic matrix and being precipitated out of isotopic equilibrium with ambient seawater, coral carbonate may be more susceptible to diagenetic alteration than surrounding inorganic precipitates, and in the case of extinct taxa the degree of this alteration may be difficult to determine (cf., [1,12])

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