Geochemistry of Paleoproterozoic Gunflint Formation carbonate: Implications for hydrosphere-atmosphere evolution

Abstract

The ~1880 Ma Gunflint Formation has played a critical role in shaping our view of the evolution of the Precambrian biosphere and the redox state of Earth's early oceans.Herein we present a study of the petrology and geochemistry of calcic grainstones and stromatolitic limestones present at the very top of the Gunflint Formation and compare them with underlying ankeritic grainstones that dominate the shore-proximal portion of the basin. Meteoric calcite cements in the upper limestones formed prior to the unit being brecciated and/or buried by debris carried by the blast cloud from the 1850 Ma Sudbury impact event. The intraclastic grains and cements in the limestone are highly enriched in V, Cr, U and REEs, with REE patterns similar to those of modern groundwater. Cr isotopes have a distinctly positive signature in most samples indicating formation of oxidized, hexavalent Cr during subaerial weathering. These redox-sensitive metals were transported in oxygenated groundwater and precipitated where the fluid encountered reducing conditions. In contrast, the underlying ankeritic marine grainstones have REE patterns similar to those of some modern-day venting hydrothermal fluids and earliest Paleoproterozoic seawater. This work suggests that significant levels of oxygen existed in the atmosphere at the time, whereas, even shallow areas of the world ocean remained very oxygen-deficient. The contrast in redox state of anoxic shallow-marine water verses oxic groundwater was probably linked to strong sub-seafloor hydrothermal circulation and extensive upwelling onto shallow shelves, which could have sustained widespread marine anoxia. More broadly, our study highlights the importance of investigating and understanding lateral geochemical gradients, and the flux rates that control them, in shallow paleomarine settings.

Cr isotopic insights into ca. 1.9 Ga oxidative weathering of the continents using the Beaverlodge Lake paleosol, Northwest Territories, Canada

“…A compilation of literature values for seawater‐derived Cr in ca . 1.9 Ga marine sediment yields an average δ 53 Cr of 0.04 ± 0.34‰ (1σ; median = –0.10‰; Frei et al., ; Fralick et al., ), which is close to the igneous inventory (–0.12 ± 0.10‰, 2σ). By contrast, modern marine carbonate and organic‐rich siliciclastic sediment yields an average δ 53 Cr of 0.52 ± 0.15‰ (1σ; median = 0.57‰; Figure a; Bonnand, James, Parkinson, Connelly, & Fairchild, ; Gueguen et al., ; Holmden et al., ; Reinhard et al., ; Wang et al., ).…”

“…Fralick et al. () proposed that the phreatic calcite cements formed in a groundwater flow system that channeled meteoric waters from oxidized surface environments into the subsurface. Reduction of Cr(VI) and other redox‐sensitive metals occurred when groundwater intersected organic‐rich sediment along the flow path, inferred on the basis that the Limestone Member contains stromatolites.…”

“…The δ 53 Cr values are much higher than those reported by Frei et al (2009) for iron formation in the marine portion of the Gunflint sedimentary succession (-0.25‰ to +0.20‰). Fralick et al (2017) proposed that the phreatic calcite cements formed in a groundwater flow system that channeled meteoric waters from oxidized surface environments into the subsurface.…”

“…Alternatively, Fralick et al (2017) considered the Limestone Member to have formed in a lake, and the stromatolites demonstrate the potential for locally high oxygen production by photosynthesis and increased local-scale production of Cr(VI) (Summer, Hawes, Mackey, Jungblut, & Doran, 2015). Redox Cr cycling in a lake could increase δ 53 Cr above the level of its weathering inputs.…”

A paleosol record of the evolution of Cr redox cycling and evidence for an increase in atmospheric oxygen during the Neoproterozoic

“…The range of paleosol δ 53 Cr data from several Paleoproterozoic paleosols are also plotted as gray bars: 2.45 Ga Cooper Lake (Babechuk et al, ), 1.9 Ga Beaverlodge Lake (Toma et al, ), and 1.85 Ga Flin Flon (Babechuk et al, ). We omit the Schreiber Beach paleosol (Frei & Polat, ) owing to the evidence for trace and radiogenic element mobility (see discussion within that paper) and questions about its geologic context (i.e., whether it is a typical paleosol; Fralick et al, ). Horizontal dashed lines indicate the crustal (BSE) δ 53 Cr array (Schoenberg et al, ; Wang et al, ) [Colour figure can be viewed at wileyonlinelibrary.com]…”