TY - JOUR
T1 - Molybdenum isotope fractionation in glacial diamictites tracks the onset of oxidative weathering of the continental crust
AU - Greaney, Allison T.
AU - Rudnick, Roberta L.
AU - Romaniello, Stephen J.
AU - Johnson, Aleisha C.
AU - Gaschnig, Richard M.
AU - Anbar, Ariel D.
N1 - Publisher Copyright:
© 2020 Elsevier B.V.
PY - 2020/3/15
Y1 - 2020/3/15
N2 - Molybdenum isotopes in twenty-four composites of glacial diamictites spanning depositional ages of 2900 to 300 Ma show a systematic shift to lighter compositions and a decrease in Mo concentration over time. The diamictites fall into three age groups relative to the Great Oxidation Event (GOE): pre-GOE (2.43 – 2.90 Ga), syn-GOE (2.20 – 2.39 Ga), and post-GOE (0.33 – 0.75 Ga). Pre-GOE composites have an average δ98MoNIST3134 of +0.03‰ (± 0.18‰), syn-GOE composites average −0.29‰ (± 0.60‰), and post-GOE composites average −0.45‰ (± 0.51‰). These groups are statistically different at p=0.05. We use the pre-GOE data to estimate the average Archean upper continental crust (UCC) δ98Mo signature as +0.03 ± 0.18‰ (2σ), which falls within the range of previous estimates of modern igneous rocks. As the diamictites represent a mixture of igneous and weathered crust, the shift to lighter Mo values over time likely reflects Mo isotope fractionation during oxidative weathering and increased retention of light Mo isotopes in weathered regolith and soils. We hypothesize that this fractionation is due to the mobilization of oxidized Mo following the GOE, and subsequent adsorption of light Mo onto Fe-Mn oxides and/or organic matter in weathered regolith. We conclude that Mo isotopes in continental weathering products record the rise of atmospheric oxygen and onset of oxidative weathering. As the regolith formed under oxidative conditions is isotopically lighter than average continental igneous rocks, mass balance dictates that Mo isotope fractionation during oxidative weathering should result in isotopically heavy groundwater and river water, which is observed in modern systems.
AB - Molybdenum isotopes in twenty-four composites of glacial diamictites spanning depositional ages of 2900 to 300 Ma show a systematic shift to lighter compositions and a decrease in Mo concentration over time. The diamictites fall into three age groups relative to the Great Oxidation Event (GOE): pre-GOE (2.43 – 2.90 Ga), syn-GOE (2.20 – 2.39 Ga), and post-GOE (0.33 – 0.75 Ga). Pre-GOE composites have an average δ98MoNIST3134 of +0.03‰ (± 0.18‰), syn-GOE composites average −0.29‰ (± 0.60‰), and post-GOE composites average −0.45‰ (± 0.51‰). These groups are statistically different at p=0.05. We use the pre-GOE data to estimate the average Archean upper continental crust (UCC) δ98Mo signature as +0.03 ± 0.18‰ (2σ), which falls within the range of previous estimates of modern igneous rocks. As the diamictites represent a mixture of igneous and weathered crust, the shift to lighter Mo values over time likely reflects Mo isotope fractionation during oxidative weathering and increased retention of light Mo isotopes in weathered regolith and soils. We hypothesize that this fractionation is due to the mobilization of oxidized Mo following the GOE, and subsequent adsorption of light Mo onto Fe-Mn oxides and/or organic matter in weathered regolith. We conclude that Mo isotopes in continental weathering products record the rise of atmospheric oxygen and onset of oxidative weathering. As the regolith formed under oxidative conditions is isotopically lighter than average continental igneous rocks, mass balance dictates that Mo isotope fractionation during oxidative weathering should result in isotopically heavy groundwater and river water, which is observed in modern systems.
KW - continental crust
KW - great oxidation event
KW - isotope fractionation
KW - molybdenum
KW - weathering
UR - http://www.scopus.com/inward/record.url?scp=85078251907&partnerID=8YFLogxK
UR - http://www.scopus.com/inward/citedby.url?scp=85078251907&partnerID=8YFLogxK
U2 - 10.1016/j.epsl.2020.116083
DO - 10.1016/j.epsl.2020.116083
M3 - Article
AN - SCOPUS:85078251907
SN - 0012-821X
VL - 534
JO - Earth and Planetary Science Letters
JF - Earth and Planetary Science Letters
M1 - 116083
ER -