A framework for understanding Mo isotope records of Archean and Paleoproterozoic Fe- and Mn-rich sedimentary rocks: Insights from modern marine hydrothermal Fe-Mn oxides. (1st July 2020)

• Record Type: Journal Article
• Authors: Goto, Kosuke T.; Sekine, Yasuhito; Shimoda, Gen; Hein, James R.; Aoki, Shogo; Ishikawa, Akira; Suzuki, Katsuhiko; Gordon, Gwyneth W.; Anbar, Ariel D.
• Is Part Of: Geochimica et cosmochimica acta. Volume 280(2020)
• Page Start: 221
• Page End: 236
• Publication Date: 2020-07-01
• Subjects: Molybdenum isotope -- Modern hydrothermal iron and manganese oxides -- Ocean paleoredox -- Paleoproterozoic -- Archean; Geochemistry -- Periodicals; Meteorites -- Periodicals; Géochimie -- Périodiques; Météorites -- Périodiques; Geochemie; Astrochemie; Electronic journals; 551.905
• DOI: 10.1016/j.gca.2020.04.017
• Languages: English
• ISSNs: 0016-7037

Abstract

Abstract: Molybdenum isotopic compositions (δ 98/95 Mo) of Archean and Paleoproterozoic Fe- and Mn-rich sedimentary rocks have been used to investigate local accumulations of O2 in an O2 -lean ocean. Previous studies interpret that these δ 98/95 Mo values would be a representation of the global minimum for δ 98/95 Mo of contemporaneous seawater and would, therefore, link to global paleoredox conditions. Here, we present new δ 98/95 Mo data on modern marine hydrothermal Fe-Mn oxides, for a wide range of Fe/Mn ratios, from five hydrothermal systems. Samples composed mainly of Fe oxides (Fe/Mn > 10) show positive values (δ 98/95 Mo ∼ +0.7‰), whereas those containing substantial amounts of Mn oxides (Fe/Mn < 10 −1 ) generally exhibit negative values (δ 98/95 Mo ∼ −0.8 ‰). These δ 98/95 Mo values are consistent with isotopic fractionations due to the adsorption of seawater Mo onto Fe and Mn oxides, respectively. The aforementioned positive and negative values are connected by a positive correlation between δ 98/95 Mo values and the Fe/Mn ratios of samples with Fe/Mn ∼ 10 −1 –10 1 . The positive correlation can be explained by the mixing of δ 98/95 Mo in Fe and Mn oxides. Based on these data, we propose that the measured δ 98/95 Mo trends for the modern hydrothermal Fe-Mn oxides can be reproduced using a simple mass-balance calculation with both modern seawater δ 98/95 Mo and Mo isotopic fractionations due to the adsorption of Mo onto Fe and Mn oxides. By applying this mass-balance calculation to published Fe/Mn and δ 98/95 Mo data on ancient Fe- and Mn-rich sedimentary rocks, we estimate Archean and Paleoproterozoic seawater δ 98/95 Mo and their deviations (Δ 98/95 Mo) from modern seawater δ 98/95 Mo. The long-term evolution of seawater δ 98/95 Mo inferred as a result suggests extensive deposition of Fe and Mn oxides at ∼2.3–2.2 Ga and an expansion of euxinic conditions at ∼1.9 Ga, which are broadly consistent with the proposed redox evolution of the ocean–atmosphere system during the Archean and Paleoproterozoic.