Moderate levels of oxygenation during the late stage of Earth’s Great Oxidation Event

The later stages of Earth’s transition to a permanently oxygenated atmosphere during the Great Oxidation Event (GOE; ∼2.43–2.06 Ga) is commonly linked with the suggestion of an “oxygen overshoot” during the ∼2.22–2.06 Ga Lomagundi Event (LE), which represents Earth’s most pronounced and longest-lived positive carbon isotope excursion. However, the magnitude and extent of atmosphere-ocean oxygenation and implications for the biosphere during this critical period in Earth’s history remain poorly constrained. Here, we present nitrogen (N), selenium (Se), and carbon (C) isotope data, as well as bio-essential element concentrations, for Paleoproterozoic marine shales deposited during the LE. The data provide evidence for a highly productive and well-oxygenated photic zone, with both inner and outer-shelf marine environments characterized by nitrate-and Se oxyanion-replete conditions. However, the redoxcline subsequently encroached back onto the inner shelf during global-scale deoxygenation of the atmosphere-ocean system at the end of the LE, leading to locally enhanced water column denitrification and quantitative reduction of selenium oxyanions. We propose that nitrate-replete conditions associated with fully oxygenated continental shelf settings were a common feature during the LE, but nitrification was not sufficiently widespread for the aerobic nitrogen cycle to impact the isotopic composition of the global ocean N inventory. Placed in the context of Earth’s broader oxygenation history, our findings indicate that O2levels in the atmosphere-ocean system were likely much lower than modern concentrations. Early Paleoproterozoic biogeochemical cycles were thus far less advanced than after Neoproterozoic oxygenation.University of TubingenGerman Research Foundation (DFG) SCHO1071/11-1 VA 1568/1-1UK Research & Innovation (UKRI)Natural Environment Research Council (NERC) NE/V004824/1University of LausanneEuropean Research Council (ERC) 636808National Research Foundation of South Africa (NRF Grant) 75892Spanish Government RYC2020-030014-INatural Sciences and Engineering Research Council of Canada (NSERC)ACS PF grant 624840ND2NERC Frontiers grant NE/V010824/1Royal Society of Londo

Mesoarchaean acidic volcanic lakes: A critical ecological niche in early land colonisation

The antiquity of life in marine environments has been demonstrated, with examples of microfossils and stromatolites extending back to at least 3.5 billion years ago (Ga). In contrast, emerged land was likely a more challenging environment during the Archaean, and only sparse evidence of life in non-marine environments has so far been identified. Here we document the abundance of isotopically light carbon (with δ13C values from −46.6 to −31.3‰), diagnostic of a biogeochemical methane cycle or acetogenesis, in shale and sandstone deposited in ∼3 billion-years-old acidic volcanic lakes on the Kaapvaal Craton of southern Africa. A distinctive Al-rich mineral assemblage with abundant pyrophyllite in lacustrine sedimentary rocks bears similarity to modern volcanic rocks affected by circulation of hot acidic fluids. This is compounded with an enrichment of Ni, Mo, W, As and Cu in whole-rock analyses of sedimentary rocks, which is also observed in geothermal areas of modern volcanic environments. Analysis of early diagenetic pyrite in these sedimentary rocks indicates high nutrient level in the lake, which might reflect hydrothermal input with leaching of volcanic material. Despite the restricted and ephemeral nature of volcanic lakes, a highly productive and complex ecosystem established itself in this environment. Volcanic lakes during the Mesoarchaean thus served as an ecological niche for the development and diversification of microbial life on emerged continental landmasses

Deflating the shale gas potential of South Africa’s Main Karoo basin

The Main Karoo basin has been identified as a potential source of shale gas (i.e. natural gas that can be extracted via the process of hydraulic stimulation or ‘fracking’). Current resource estimates of 0.4–11x109 m3 (13–390 Tcf) are speculatively based on carbonaceous shale thickness, area, depth, thermal maturity and, most of all, the total organic carbon content of specifically the Ecca Group’s Whitehill Formation with a thickness of more than 30 m. These estimates were made without any measurements on the actual available gas content of the shale. Such measurements were recently conducted on samples from two boreholes and are reported here. These measurements indicate that there is little to no desorbed and residual gas, despite high total organic carbon values. In addition, vitrinite reflectance and illite crystallinity of unweathered shale material reveal the Ecca Group to be metamorphosed and overmature. Organic carbon in the shale is largely unbound to hydrogen, and little hydrocarbon generation potential remains. These findings led to the conclusion that the lowest of the existing resource estimates, namely 0.4x109 m3 (13 Tcf), may be the most realistic. However, such low estimates still represent a large resource with developmental potential for the South African petroleum industry. To be economically viable, the resource would be required to be confined to a small, well-delineated ‘sweet spot’ area in the vast southern area of the basin. It is acknowledged that the drill cores we investigated fall outside of currently identified sweet spots and these areas should be targets for further scientific drilling projects

A multiple sulfur record of super-large volcanic eruptions in Archaean pyrite nodules

Archaean supracrustal rocks carry a record of mass-independently fractionated S that is interpreted to be derived from UV-induced photochemical reactions in an oxygen-deficient atmosphere. Experiments with photochemical reactions of SO2 gas have provided some insight into these processes. However, reconciling experimental results with the multiple S isotopic composition of the Archaean sedimentary record has proven difficult and represents one of the outstanding issues in understanding the Archaean surface S-cycle. We present quadruple S isotope data (32S, 33S, 34S, 36S) for pyrite from Mesoarchaean carbonaceous sediments of the Dominion Group, South Africa, deposited in an acidic volcanic lake, which help reconcile observations from the Archaean sedimentary record with the results of photochemical experiments. The data, which show low S/S ratios (mostly ≪ 1) and very negative S/S ratios (−4 and lower), contrast with the composition of most Archaean sedimentary sulfides and sulfates, having S/ (the so-called ‘Archaean reference array’), but match those of modern photochemical sulfate aerosols produced in the stratosphere, following super-large volcanic eruptions, and preserved in Antarctic ice. These data are also consistent with the results of UV-irradiation experiments of SO2 gas at variable gas pressure. The S isotope composition of the Dominion Group pyrite is here interpreted to reflect the products of photolysis in a low-oxygen-level atmosphere at high SO2 pressure during large volcanic eruptions, mixed with Archaean ‘background’ (having a composition broadly similar to the Archaean reference array) S pools. It is inferred that high sedimentation rates in a terrestrial basin resulted in an instantaneously trapped input of atmospheric S during short-lasted depositional intervals, which faithfully represents transient photochemical signals in comparison with marine sedimentary records

Moderate levels of oxygenation during the late stage of Earth's Great Oxidation Event

FOO and RS acknowledge financial support from the University of Tübingen and the German Research Foundation (DFG Grant SCHO1071/11-1). FOO and MBA are thankful for support from the Natural Environment Research Council (NERC grant NE/V004824/1). The stable isotope facilities at IDYST were funded by the University of Lausanne. SK, YA and MIV-R acknowledge European Research Council (ERC) Starting Grant 636808 (O2RIGIN). AH and FOO acknowledge support from National Research Foundation of South Africa (NRF Grant 75892). SK also acknowledges the Ramon y Cajal contract (RYC2020-030014-I). Participation by AB was supported by Discovery and Accelerator Grants from the Natural Sciences and Engineering Research Council of Canada (NSERC) and ACS PF grant (624840ND2). EES acknowledges funding from a NERC Frontiers grant (NE/V010824/1). SWP acknowledges support from a Royal Society Wolfson Research Merit Award. MIV-R additionally acknowledges funding support from the German Research Foundation (DFG Grant VA 1568/1-1).The later stages of Earth's transition to a permanently oxygenated atmosphere during the Great Oxidation Event (GOE; ∼2.43–2.06 Ga) is commonly linked with the suggestion of an “oxygen overshoot” during the ∼2.22–2.06 Ga Lomagundi Event (LE), which represents Earth's most pronounced and longest-lived positive carbon isotope excursion. However, the magnitude and extent of atmosphere-ocean oxygenation and implications for the biosphere during this critical period in Earth's history remain poorly constrained. Here, we present nitrogen (N), selenium (Se), and carbon (C) isotope data, as well as bio-essential element concentrations, for Paleoproterozoic marine shales deposited during the LE. The data provide evidence for a highly productive and well-oxygenated photic zone, with both inner and outer-shelf marine environments characterized by nitrate- and Se oxyanion-replete conditions. However, the redoxcline subsequently encroached back onto the inner shelf during global-scale deoxygenation of the atmosphere-ocean system at the end of the LE, leading to locally enhanced water column denitrification and quantitative reduction of selenium oxyanions. We propose that nitrate-replete conditions associated with fully oxygenated continental shelf settings were a common feature during the LE, but nitrification was not sufficiently widespread for the aerobic nitrogen cycle to impact the isotopic composition of the global ocean N inventory. Placed in the context of Earth's broader oxygenation history, our findings indicate that O2 levels in the atmosphere-ocean system were likely much lower than modern concentrations. Early Paleoproterozoic biogeochemical cycles were thus far less advanced than after Neoproterozoic oxygenation.Publisher PDFPeer reviewe

Deflating the shale gas potential of South Africa’s Main Karoo Basin

The Main Karoo basin has been identified as a potential source of shale gas (i.e. natural gas that can be extracted via the process of hydraulic stimulation or ‘fracking’). Current resource estimates of 0.4–11x109 m3 (13–390 Tcf) are speculatively based on carbonaceous shale thickness, area, depth, thermal maturity and, most of all, the total organic carbon content of specifically the Ecca Group’s Whitehill Formation with a thickness of more than 30 m. These estimates were made without any measurements on the actual available gas content of the shale. Such measurements were recently conducted on samples from two boreholes and are reported here. These measurements indicate that there is little to no desorbed and residual gas, despite high total organic carbon values. In addition, vitrinite reflectance and illite crystallinity of unweathered shale material reveal the Ecca Group to be metamorphosed and overmature. Organic carbon in the shale is largely unbound to hydrogen, and little hydrocarbon generation potential remains. These findings led to the conclusion that the lowest of the existing resource estimates, namely 0.4x109 m3 (13 Tcf), may be the most realistic. However, such low estimates still represent a large resource with developmental potential for the South African petroleum industry. To be economically viable, the resource would be required to be confined to a small, well-delineated ‘sweet spot’ area in the vast southern area of the basin. It is acknowledged that the drill cores we investigated fall outside of currently identified sweet spots and these areas should be targets for further scientific drilling projects. Significance:  • This is the first report of direct measurements of the actual gas contents of southern Karoo basin shales. • The findings reveal carbon content of shales to be dominated by overmature organic matter. • The results demonstrate a much reduced potential shale gas resource presented by the Whitehill Formation

Zinc enrichment and isotopic fractionation in a marine habitat of the c. 2.1 Ga Francevillian Group: A signature of zinc utilization by eukaryotes?

Constraining the timing of eukaryogenesis and the divergence of eukaryotic clades is a major challenge in evolutionary biology. Here, we present trace metal concentration and zinc isotope data for c. 2.1 billion-year-old Francevillian Group pyritized structures, previously described as putative remnants of the first colonial multicellular organisms, and their host black shales. Relative to the host rocks, pyritized structures are strongly enriched in zinc, cobalt and nickel, by at least one order of magnitude, with markedly lighter zinc isotope compositions. A metabolic demand for high concentrations of aqueous zinc, cobalt, and nickel combined with preferential uptake of lighter zinc isotopes may indicate metalloenzyme utilization by eukaryotes in marine habitats c. 2.1 billion years ago. Once confirmed, this would provide a critical calibration point for eukaryogenesis, suggesting that this major evolutionary innovation may have happened contemporaneously with elevated atmospheric oxygen levels during the latter part of the Great Oxidation Event, some 400 million years earlier than is currently widely accepted

Evidence for oxygenic photosynthesis half a billion years before the Great Oxidation Event

The early Earth was characterized by the absence of oxygen in the ocean–atmosphere system, in contrast to the well-oxygenated conditions that prevail today. Atmospheric concentrations first rose to appreciable levels during the Great Oxidation Event, roughly 2.5–2.3 Gyr ago. The evolution of oxygenic photosynthesis is generally accepted to have been the ultimate cause of this rise, but it has proved difficult to constrain the timing of this evolutionary innovation. The oxidation of manganese in the water column requires substantial free oxygen concentrations, and thus any indication that Mn oxides were present in ancient environments would imply that oxygenic photosynthesis was ongoing. Mn oxides are not commonly preserved in ancient rocks, but there is a large fractionation of molybdenum isotopes associated with the sorption of Mo onto the Mn oxides that would be retained. Here we report Mo isotopes from rocks of the Sinqeni Formation, Pongola Supergroup, South Africa. These rocks formed no less than 2.95 Gyr ago in a nearshore setting. The Mo isotopic signature is consistent with interaction with Mn oxides. We therefore infer that oxygen produced through oxygenic photosynthesis began to accumulate in shallow marine settings at least half a billion years before the accumulation of significant levels of atmospheric oxygen