TY - JOUR

T1 - The onset of oxidative weathering traced by uranium isotopes

AU - Brüske, A.

AU - Martin, A. N.

AU - Rammensee, P.

AU - Eroglu, S.

AU - Lazarov, M.

AU - Albut, G.

AU - Schuth, S.

AU - Aulbach, S.

AU - Schoenberg, R.

AU - Beukes, N.

AU - Hofmann, A.

AU - Nägler, T.

AU - Weyer, S.

N1 - Funding Information: Ingo Horn (Leibniz University of Hannover) is thanked for help and guidance during (MC-) ICP-MS measurements of U isotopes and trace elements. Vanessa Sutterer (University of T?bingen) is thanked for assistance with X-ray fluorescence analyses of samples from core DP-16. Funding for SW, SS, ANM, ML, and AB was provided by the Deutsche Forschungsgemeinschaft (DFG) priority program SPP-1833 ?Building a habitable Earth? and is thankfully appreciated (to SW grant WE2850-15/1 and to SS grant SCHU2695/4). Part of this study was funded by the Deutsche Forschungsgemeinschaft (DFG) grant AU356/7 to SA. NJB and AH received research funding from the DST-NRF Centre of Excellence for Integrated Mineral and Energy Resource Analysis (CIMERA). Drilling of core GKP-01 was funded by the Agouron Institute in Pasadena during 2002-03 and that of BARB-3 and 5 by the ICDP in 2010. Core BH-1 (SACHA-W216) was drilled by the Council of Geoscience in Pretoria and cores DP-16 and KMF-5 by Gold Fields Limited. We thank the editor and reviewers for constructive comments that helped to improve the manuscript.

PY - 2020/3

Y1 - 2020/3

N2 - The invention of photosynthesis was a key interval in Earth's history, initiating major changes in the evolution of the oceans and atmosphere. Many studies suggest that oxygen levels were already enhanced before the Paleoproterozoic Great Oxidation Event (GOE, 2.45–2.32 Ga). However, the timing of the onset of photosynthetic oxygenation, as well as spatio-temporal fluctuations of atmospheric and oceanic oxygen levels are highly debated. Here, we present a new and substantial uranium (U) isotope dataset comprising marine sediments, e.g. black shales, carbonates and iron-rich sedimentary rocks, from the Hamersley Basin (Western Australia), Kaapvaal and Zimbabwe Cratons (both southern Africa) with depositional ages ranging from 3.4 to 2.2 Ga. Sub-recent U mobilization was monitored by analyzing 234U along with δ238U, and detrital U contribution was monitored with [Al]/[U] or [Th]/[U] ratios. Samples with significant authigenic U enrichment (EFU > 2), relative to post-Archean Australian Shale (PAAS), exhibit authigenic δ238U ranging from −0.96 to −0.02‰ (±0.05‰, 2 s.d. long-term reproducibility), which deviates markedly from typical δ238U of the continental crust (−0.40 to −0.20‰). Remarkably, we find increased δ238U variability in the upper Ghaap Group (2.50–2.47 Ga, carbonates and black shales) and the lower Pretoria Group (2.42–2.32 Ga, Duitschland Formation, shales) of the Kaapvaal Craton. The predominantly light U isotope composition of those samples is likely explained by U isotope fractionation related to the onset of partial U mobilization during slow oxidative weathering of uraninite shortly before and during the GOE. The lack of low δ238U values in the Timeball Hill Formation (overlying the Duitschland Formation) indicate the onset of quantitative U mobilization during (modern-style) oxidative weathering of uraninite and other U-bearing minerals with limited U isotope fractionation. These limited U isotope variations measured in the Paleoproterozoic Timeball Hill Formation further suggest that, in contrast to modern oceans, in the period following the GOE, U was largely quantitatively reduced and removed from the water column of the Transvaal basin which remained mainly anoxic at depth.

AB - The invention of photosynthesis was a key interval in Earth's history, initiating major changes in the evolution of the oceans and atmosphere. Many studies suggest that oxygen levels were already enhanced before the Paleoproterozoic Great Oxidation Event (GOE, 2.45–2.32 Ga). However, the timing of the onset of photosynthetic oxygenation, as well as spatio-temporal fluctuations of atmospheric and oceanic oxygen levels are highly debated. Here, we present a new and substantial uranium (U) isotope dataset comprising marine sediments, e.g. black shales, carbonates and iron-rich sedimentary rocks, from the Hamersley Basin (Western Australia), Kaapvaal and Zimbabwe Cratons (both southern Africa) with depositional ages ranging from 3.4 to 2.2 Ga. Sub-recent U mobilization was monitored by analyzing 234U along with δ238U, and detrital U contribution was monitored with [Al]/[U] or [Th]/[U] ratios. Samples with significant authigenic U enrichment (EFU > 2), relative to post-Archean Australian Shale (PAAS), exhibit authigenic δ238U ranging from −0.96 to −0.02‰ (±0.05‰, 2 s.d. long-term reproducibility), which deviates markedly from typical δ238U of the continental crust (−0.40 to −0.20‰). Remarkably, we find increased δ238U variability in the upper Ghaap Group (2.50–2.47 Ga, carbonates and black shales) and the lower Pretoria Group (2.42–2.32 Ga, Duitschland Formation, shales) of the Kaapvaal Craton. The predominantly light U isotope composition of those samples is likely explained by U isotope fractionation related to the onset of partial U mobilization during slow oxidative weathering of uraninite shortly before and during the GOE. The lack of low δ238U values in the Timeball Hill Formation (overlying the Duitschland Formation) indicate the onset of quantitative U mobilization during (modern-style) oxidative weathering of uraninite and other U-bearing minerals with limited U isotope fractionation. These limited U isotope variations measured in the Paleoproterozoic Timeball Hill Formation further suggest that, in contrast to modern oceans, in the period following the GOE, U was largely quantitatively reduced and removed from the water column of the Transvaal basin which remained mainly anoxic at depth.

KW - Archean

KW - Early Proterozoic

KW - Great Oxidation Event

KW - Modern-style weathering

KW - Oxidative weathering

KW - Uranium isotope fractionation

UR - http://www.scopus.com/inward/record.url?scp=85077239124&partnerID=8YFLogxK

U2 - 10.1016/j.precamres.2019.105583

DO - 10.1016/j.precamres.2019.105583

M3 - Article

AN - SCOPUS:85077239124

VL - 338

JO - Precambrian research

JF - Precambrian research

SN - 0301-9268

M1 - 105583

ER -