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What Causes Carbonates to Form “Shrubby” Morphologies? An Anthropocene Limestone Case Study

Bastianini, Laura; Rogerson, Mike; Mercedes-Martín, Ramon; Prior, Timothy J.; Cesar, Edgley A.; Mayes, William M.

Authors

Laura Bastianini

Mike Rogerson

Ramon Mercedes-Martín

Edgley A. Cesar



Abstract

The South Atlantic Aptian “Pre-Salt” shrubby carbonate successions offshore Brazil and Angola are of major interest due to their potential hydrocarbon accumulations. Although the general sedimentology of these deposits is widely recognized to be within saline, alkaline lakes in rift volcanic settings, the specific genesis of shrubby carbonate morphologies remains unclear. This study reports the first petrographically comparable shrubby carbonates amongst other carbonate microfacies from an Anthropocene limestone formed under hyperalkaline (pH 9–12) and hypersaline (conductivity 425–3200 μS) conditions at ambient temperature (12.5–13°C) (Consett, United Kingdom). This discovery allows us to capitalize on exceptional long-term hydrochemical monitoring efforts from the site, demonstrating that shrubby carbonates occur uniquely within the waters richest in calcium (∼240 mg/L) and with highest pH (∼12) and consequently with very high levels of supersaturation. However, the physical distribution of shrubs is more comparable with estimated local kinetic precipitation rate than it is to thermodynamic saturation, indicating that the fundamental control on shrub formation arises from crystal surface processes. The shrubby carbonate we report grows in the presence of significant diatomaceous and cyanobacterial biofilms, despite the highly alkaline conditions. These biofilms are lost from the deposited material early due to the high solubility of organic and silica within hyperalkaline settings, and this loss contributes to very high intercrystalline porosity. Despite the presence of these microbes, few if any of the fabrics we report would be considered as “boundstones” despite it being clear that most fabrics are being deposited in the presence of abundant extra-cellular polymeric substances. We are aware of no previous petrographic work on anthropogenic carbonates of this type, and recommend further investigation to capitalize on what can be learned from these “accidental laboratories.”.

Citation

Bastianini, L., Rogerson, M., Mercedes-Martín, R., Prior, T. J., Cesar, E. A., & Mayes, W. M. (2019). What Causes Carbonates to Form “Shrubby” Morphologies? An Anthropocene Limestone Case Study. Frontiers in Earth Science, 7, https://doi.org/10.3389/feart.2019.00236

Journal Article Type Article
Acceptance Date Aug 28, 2019
Online Publication Date Sep 11, 2019
Publication Date Sep 11, 2019
Deposit Date Dec 11, 2020
Publicly Available Date Jan 8, 2021
Journal Frontiers in Earth Science
Print ISSN 2296-6463
Electronic ISSN 2296-6463
Publisher Frontiers Media
Peer Reviewed Peer Reviewed
Volume 7
Article Number 236
DOI https://doi.org/10.3389/feart.2019.00236
Keywords Pre-Salt reservoirs; Alkaline steel slag; Consett; Shrubby carbonate; Microbial; Hydrochemistry; Crystal growth; Intercrystalline porosity
Public URL https://hull-repository.worktribe.com/output/2909416
Publisher URL https://www.frontiersin.org/articles/10.3389/feart.2019.00236/full

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Publisher Licence URL
https://creativecommons.org/licenses/by/4.0/

Copyright Statement
Copyright © 2019 Bastianini, Rogerson, Mercedes-Martín, Prior, Cesar and Mayes. This is an open-access article distributed under the terms of the Creative Commons Attribution License (CC BY). The use, distribution or reproduction in other forums is permitted, provided the original author(s) and the copyright owner(s) are credited and that the original publication in this journal is cited, in accordance with accepted academic practice. No use, distribution or reproduction is permitted which does not comply with these terms.





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