TY - JOUR

T1 - No biological effect on magnesium isotope fractionation during stromatolite growth

AU - Hu, Zhongya

AU - Hohl, Simon V.

AU - Viehmann, Sebastian

AU - Meister, Patrick

AU - Tepe, Nathalie

N1 - Funding Information: We thank chief editor Prof. Jeffrey G. Catalano and associated editor Noah Planavsky for the editorial handling and constructive comments. This manuscript benefits from constructive reviews from Sara R. Kimmig and Tomaso Bontognali. We thank Carlos E. De Rezende and his team at the Universidade do Norte Fluminense (UENF, Brazil) for guidance on the sampling location and for providing technical support. The National Science Foundation of China supported this study (grants 42230410 and 42103004 to Z. Hu, 42150610481 to S.V. Hohl) and the China Postdoctoral Science Foundation (grant 2020M681379 to Z. Hu). We thank Zhiguang Xia at Nanjing University for assistance during XRD analyses. The Marie-Curie Research Training Network supported the sampling during the “Greenhouse-Gas Removal Apprenticeship and Student Program” (GRASP), project MRTN-CT-2006-035868 and the Swiss National Science Foundation (SNF) project PA00P2-126221.

PY - 2023/10/1

Y1 - 2023/10/1

N2 - The growth and morphology of stromatolites have been previously linked to diverse microbial activities. However, the role of microbial metabolisms on carbonate formation during stromatolite growth remains controversial. Magnesium isotopes have been proposed to serve as a tracer of microbial carbonate formation, implying a potential biological isotope fractionation. To further elucidate whether Mg isotope fractionation is modified during microbial carbonate formation, this study reports Mg isotope compositions of Holocene stromatolites and pore waters from Lagoa Salgada, a coastal ephemeral lake in Brazil. The stromatolitic carbonates are composed of high-Mg calcites characterized by extremely positive inorganic δ 13C values, up to +20‰, and variable δ 26Mg values, ranging from −2.98‰ to −0.68‰. Multiple pieces of evidence consistently demonstrate changes in microbial metabolism resulting from ecosystem fluid chemistry evolution. However, the direction of Mg isotope fractionation associated with carbonate precipitation remained invariable despite the changes in microbial activity. Mineralogical features indicate that the stromatolitic carbonates formed via 2-D nucleation. An isotopic mass balance calculation based on the observed variations in δ 26Mg values and Mg concentrations of associated pore waters argues for an Mg isotopic equilibrium fractionation factor of −2.60‰ associating with the carbonate formation at Lagoa Salgada, well-matching experimental values for abiotic calcite precipitation. Thus, the observed δ 26Mg variability of stromatolitic carbonates is primarily controlled by the changes of physicochemical conditions in the ambient fluid. We infer that during the formation of stromatolites, a consortium of different microbial communities produces extracellular polymeric substances, which serve as a substrate for carbonate nucleation from ambient fluid without significantly affecting the fractionation of Mg-isotopes. Our findings shed light on the effects of microbial processes on carbonate formation during stromatolite growth and improve the current understanding of the Mg isotope record in microbial carbonates.

AB - The growth and morphology of stromatolites have been previously linked to diverse microbial activities. However, the role of microbial metabolisms on carbonate formation during stromatolite growth remains controversial. Magnesium isotopes have been proposed to serve as a tracer of microbial carbonate formation, implying a potential biological isotope fractionation. To further elucidate whether Mg isotope fractionation is modified during microbial carbonate formation, this study reports Mg isotope compositions of Holocene stromatolites and pore waters from Lagoa Salgada, a coastal ephemeral lake in Brazil. The stromatolitic carbonates are composed of high-Mg calcites characterized by extremely positive inorganic δ 13C values, up to +20‰, and variable δ 26Mg values, ranging from −2.98‰ to −0.68‰. Multiple pieces of evidence consistently demonstrate changes in microbial metabolism resulting from ecosystem fluid chemistry evolution. However, the direction of Mg isotope fractionation associated with carbonate precipitation remained invariable despite the changes in microbial activity. Mineralogical features indicate that the stromatolitic carbonates formed via 2-D nucleation. An isotopic mass balance calculation based on the observed variations in δ 26Mg values and Mg concentrations of associated pore waters argues for an Mg isotopic equilibrium fractionation factor of −2.60‰ associating with the carbonate formation at Lagoa Salgada, well-matching experimental values for abiotic calcite precipitation. Thus, the observed δ 26Mg variability of stromatolitic carbonates is primarily controlled by the changes of physicochemical conditions in the ambient fluid. We infer that during the formation of stromatolites, a consortium of different microbial communities produces extracellular polymeric substances, which serve as a substrate for carbonate nucleation from ambient fluid without significantly affecting the fractionation of Mg-isotopes. Our findings shed light on the effects of microbial processes on carbonate formation during stromatolite growth and improve the current understanding of the Mg isotope record in microbial carbonates.

KW - Carbonate formation

KW - Lagoa Salgada

KW - Mg isotopes

KW - Microbial metabolism

KW - Stromatolite

UR - http://www.scopus.com/inward/record.url?scp=85168355376&partnerID=8YFLogxK

U2 - 10.1016/j.gca.2023.07.022

DO - 10.1016/j.gca.2023.07.022

M3 - Article

VL - 358

SP - 1

EP - 11

JO - Geochimica et Cosmochimica Acta

JF - Geochimica et Cosmochimica Acta

SN - 0016-7037

ER -