Mg isotope response to dolomitization in hinterland-attached carbonate platforms: Outlook of δ26Mg as a tracer of basin restriction and seawater Mg/Ca ratio

Or M. Bialik, Xiaomin Wang, Shugao Zhao, Nicolas D. Waldmann, Ran Frank, Weiqiang Li

Research output: Contribution to journalArticlepeer-review

Abstract

Magnesium isotopes in early diagenetic dolomite have been proposed as a potential tracer for seawater chemistry and global Mg cycles. However, the applicability of Mg isotopes of early diagenetic dolomite in studies of ancient seawater requires a detailed understanding of the behavior of Mg isotopes during dolomitization in a variety of geological settings. Hinterland attached carbonate platform is an important sink of seawater Mg through dolomitization, and basin restriction is a common feature in hinterland attached carbonate platforms, yet its effects on Mg isotope systematics in carbonates have not been well documented. The upper Albian Hevyon Formation in southern Israel was deposited in a typical hinterland attached carbonate platform setting and provides an ideal case for investigating the Mg isotope behavior during dolomitization in environments of frequent basin restrictions. The abundance of dolomite increases up-section in the Hevyon Formation and correlates with the appearance of microbial deposits and disappearance of metazoans, reflecting water level fluctuations in multiple exposure and recharging events. In conjunction with sedimentary facies analysis, multiple geochemical proxies (Ni/Co, V/Cr, Ce/Ce, and δ13C) indicate the development of anoxic conditions. These two lines of evidence, together with textural indicators and 87Sr/86Sr ratios, suggest dolomitization was microbial and penecontemporaneous in a shallow low circulation water body. δ26Mg of dolomite increases from −1.99 ± 0.12‰ in the lower part of the sequence to a value of −1.52 ± 0.02‰ in the middle interval, then decreases back to −2.06 ± 0.18‰ in the upper unit. Overall, Mg isotope compositions co-vary with δ13C, Mg/Ca ratios, as well as crystal size of dolomite, which implies that variations in δ26Mg value reflect syn-depositional changes in water chemistry as a result of dolomitization in episodically restricted basins. δ26Mg tracks the fluctuating marine connection and provides a powerful tracer for the level of restriction. Furthermore, by pairing Mg isotope variability of the dolomite with a Rayleigh distillation model and elemental mass balance in aqueous solution, it is possible to constrain the Mg/Ca ratio of seawater. Based on the records from the Hevyon Formation, Mg/Ca ratio of seawater during the Albian is constrained between 1.5 and 1, which is consistent with results from other records.

Original languageEnglish
Pages (from-to)189-207
Number of pages19
JournalGeochimica et Cosmochimica Acta
Volume235
DOIs
StatePublished - 15 Aug 2018

Bibliographical note

Funding Information:
This study was jointly supported by ISF grant No. 2208/15 to NDW and NSFC grant No. 41561144002 to WL, based on research agreements between the Israeli Science Foundation and the National Science Foundation of China. WL also acknowledges supports from NSFC with grants Nos. 41473002 and 41622301 . Special thanks are given to Gerald Auer for his input regarding the redox proxies. Nimer Taha provided assistance for analytical work at the Basin Analysis and Petrophysical laboratory (PetroLab), University of Haifa. NDW also thanks the Wolfson Family Charitable Trust for supporting the purchase of some of the analytical equipment used in the context of this study. The authors thank Adrian Immenhauser, Matthew Fantle, an anonymous reviewer and editor Fangzhen Teng for constructive comments.

Funding Information:
This study was jointly supported by ISF grant No. 2208/15 to NDW and NSFC grant No. 41561144002 to WL, based on research agreements between the Israeli Science Foundation and the National Science Foundation of China. WL also acknowledges supports from NSFC with grants Nos. 41473002 and 41622301. Special thanks are given to Gerald Auer for his input regarding the redox proxies. Nimer Taha provided assistance for analytical work at the Basin Analysis and Petrophysical laboratory (PetroLab), University of Haifa. NDW also thanks the Wolfson Family Charitable Trust for supporting the purchase of some of the analytical equipment used in the context of this study. The authors thank Adrian Immenhauser, Matthew Fantle, an anonymous reviewer and editor Fangzhen Teng for constructive comments.

Publisher Copyright:
© 2018 Elsevier Ltd

Keywords

  • Basin restriction
  • Carbonate platforms
  • Dolomite
  • Levant
  • Mg isotopes
  • Seawater Mg/Ca ratio

ASJC Scopus subject areas

  • Geochemistry and Petrology

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