Chemical Geology, ( ISI ), Volume (422), Year (2016-4) , Pages (94-107)

Title : ( Discerning primary versus diagenetic signals in carbonate carbon and oxygen isotope records: An example from the Permian–Triassic boundary of Iran )

Authors: M. Schobben , C. Vinzenz Ullmann , L. Leda , D. Korn , U. Struck , W. Uwe Reimold , Abbas Ghaderi , T. J. Algeo , Ch. Korte ,

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Abstract

Sedimentary successions across the Permian-Triassic boundary (PTB) are marked by a prominent negative carbon isotope excursion. This excursion, found in both fossil (e.g., brachiopod) and bulk carbonate at many sites around the world, is generally considered to be related to a global carbon cycle perturbation. Oxygen isotopes also show a negative excursion across the PTB, but because δ18O is more prone to diagenetic overprint (especially in bulk carbonate), these data are often not used in palaeoenvironmental analyses. In the present study, bulk-rock and brachiopod δ13C and δ18O, as well as conodont δ18O, were analyzed in PTB successions at Kuh-e-Ali Bashi and Zal (NW Iran) in order to evaluate diagenetic overprints on primary marine isotopic signals. The results show that the use of paired C-O isotopes and Mn-Sr concentrations is not sufficient to identify diagenetic alteration in bulk materials, because δ13C-δ18O covariation can be due to environmental factors rather than diagenesis, and Sr/Ca and Mn/Ca ratios can vary as a function of bulk-rock lithology. Comparison of δ13C profiles shows that all bulk carbonate is altered to some degree, although the general bulk-rock trend mimics that of the brachiopod data with a systematic offset of –1.2(±0.4)‰. This suggests that the first-order δ13C trend in bulk carbonate is generally robust but that the significance of small-scale carbon isotope fluctuations is uncertain, especially when such fluctuations are linked to lithologic variation. The PTB interval, which is marked by a low-carbonate ‗Boundary Clay‘ in the study sections, may be especially prone to diagenetic alteration, e.g., via late-stage dolomitization. Comparison of oxygen-isotope profiles for bulk rock and well-preserved fossils (both brachiopods and conodonts) shows that the former are offset by – 2.1(±0.4)‰. Diagenetic modeling suggests that these offsets were the product mainly of early diagenesis at burial temperatures of ~50-80°C and water/rock ratios of <10. Authigenic carbonates precipitated during early diagenesis represent a potentially major sink for isotopically light carbon - - at a global scale - that has received relatively little attention to date.

Keywords

diagenesis; trace elements; chemostratigraphy; authigenic carbonates; biogeochemical; carbon cycle
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@article{paperid:1054277,
author = {M. Schobben and C. Vinzenz Ullmann and L. Leda and D. Korn and U. Struck and W. Uwe Reimold and Ghaderi, Abbas and T. J. Algeo and Ch. Korte},
title = {Discerning primary versus diagenetic signals in carbonate carbon and oxygen isotope records: An example from the Permian–Triassic boundary of Iran},
journal = {Chemical Geology},
year = {2016},
volume = {422},
month = {April},
issn = {0009-2541},
pages = {94--107},
numpages = {13},
keywords = {diagenesis; trace elements; chemostratigraphy; authigenic carbonates; biogeochemical; carbon cycle},
}

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%0 Journal Article
%T Discerning primary versus diagenetic signals in carbonate carbon and oxygen isotope records: An example from the Permian–Triassic boundary of Iran
%A M. Schobben
%A C. Vinzenz Ullmann
%A L. Leda
%A D. Korn
%A U. Struck
%A W. Uwe Reimold
%A Ghaderi, Abbas
%A T. J. Algeo
%A Ch. Korte
%J Chemical Geology
%@ 0009-2541
%D 2016

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