Pervasively anoxic surface conditions at the onset of the Great Oxidation Event: New multi-proxy constraints from the Cooper Lake paleosol

• Oceanic element inventories derived from marine sedimentary rocks place important constraints on oxidativecontinental weathering in deep time, but there remains a scarcity in complementary observations directly fromcontinental sedimentary reservoirs. This study focuses on better defining continental weathering conditions nearthe Archean-Proterozoic boundary through the multi-proxy (major and ultra-trace element, Fe and Cr stableisotopes, μ-XRF elemental mapping, and detrital zircon U-Pb geochronology) investigation of the ca. 2.45 billionyear old (giga annum, Ga) Cooper Lake paleosol (saprolith) developed on a sediment-hosted mafic dike withinthe Huronian Supergroup (Ontario, Canada).Throughout the variably altered Cooper Lake saprolith, ratios of immobile elements (Nb, Ta, Zr, Hf, Th, Al, Ti)are constant, indicating a uniform pre-alteration dike composition, lack of extreme pH weathering conditions,and no major influence from ligand-rich fluids during weathering or burial metasomatism/metamorphism. Theloss of Mg, Fe, Na, Sr, and Li, a signature of albite and ferromagnesian silicate weathering, increases towards thetop of the preserved profile (unconformity) and dike margins. Coupled bulk rock behaviour of Fe-Mg-Mn and colocalizationof Fe-Mn in clay minerals (predominantly chlorite) indicates these elements were solubilized primarilyin their divalent state without Fe/Mn-oxide formation. A lack of a Ce anomaly and immobility of Mo, V, and Cr further support pervasively anoxic weathering conditions. Subtle U enrichment, if primary, is the onlygeochemical evidence that could be consistent with oxidative element mobilization. The leaching of ferromagnesiansilicates was accompanied by variable mobility and depletion of transition metals with a relativedepletion order of Fe≈Mg≈Zn > Ni > Co > Cu (Cu being significantly influenced by secondary sulfideformation). Mild enrichment of heavy Fe isotopes (δ56/54Fe from 0.169 to 0.492‰) correlating with Fe depletionin the saprolith indicates open-system loss of isotopically light aqueous Fe(II). Minor REE+Y fractionation withincreasing alteration intensity, including a decreasing Eu anomaly and Y/Ho ratio, is attributed to albitebreakdown and preferential scavenging of HREE > Y by clay minerals, respectively. Younger metasomatismresulted in the addition of several elements (K, Rb, Cs, Be, Tl, Ba, Sn, In, W), partly or wholly obscuring theirearlier paleo-weathering trends.The behavior of Cr at Cooper Lake can help test previous hypotheses of an enhanced, low pH-driven continentalweathering flux of Cr(III) to marine reservoirs between ca. 2.48–2.32 Ga and the utility of the stable Crisotope proxy of Mn-oxide induced Cr(III) oxidation. Synchrotron μ-XRF maps and invariant Cr/Nb ratios revealcomplete immobility of Cr despite its distribution amongst both clay-rich groundmass and Fe-Ti oxides.Assuming a pH-dependent, continental source of Cr(III) to marine basins, the Cr immobility at Cooper Lakeindicates either that signatures of acidic surface waters were localized to uppermost and typically unpreservedregolith horizons or were geographically restricted to acid-generating point sources. However, given detritalpyrite preservation in overlying fluvial sequences, it is probable that the oxidative sulfide corrosion required todrive surface pH < 4 lagged behind in this region relative to other early Proterozoic sequences. The entiresaprolith exhibits a consistently light stable Cr isotope composition (δ53/52Cr: −0.321 ± 0.038‰, 2sd, n=34)that cannot be linked to Cr(III) oxidation and is instead interpreted to have a magmatic origin.

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NATURVETENSKAP  -- Geovetenskap och miljövetenskap -- Geokemi (hsv//swe)

NATURAL SCIENCES  -- Earth and Related Environmental Sciences -- Geochemistry (hsv//eng)

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