Not so non-marine? Revisiting the Stoer Group and the Mesoproterozoic biosphere

Funding for this project was provided by the NASA postdoctoral program (EES), the Lewis and Clark Fund (EES), an NSERC PGS-D grant (EJB), the NSF ELT (TWL, NJP) and FESD (TWL) programs, and the NASA Astrobiology Institute (TWL, NJP).The Poll a’Mhuilt Member of the Stoer Group (Torridonian Supergroup) in Scotland has been heralded as a rare window into the ecology of Mesoproterozoic terrestrial environments. Its unusually high molybdenum concentrations and large sulphur isotope fractionations have been used as evidence to suggest that lakes 1.2 billion years ago were better oxygenated and enriched in key nutrients relative to contemporaneous oceans, making them ideal habitats for the evolution of eukaryotes. Here we show with new Sr and Mo isotope data, supported by sedimentological evidence, that the depositional setting of this unit was likely connected to the ocean and that the elevated Mo and S contents can be explained by evapo-concentration of seawater. Thus, it remains unresolved if Mesoproterozoic lakes were important habitats for early eukaryotic life.Publisher PDFPeer reviewe

Micropaleontology and chemostratigraphy of the Neoproterozoic Mbuji-Mayi Supergroup, Democratic Republic of Congo.

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A new soil-based approach for empirical monitoring of enhanced rock weathering rates

Enhanced Rock Weathering (ERW) is a promising scalable and cost-effective Carbon Dioxide Removal (CDR) strategy with significant environmental and agronomic co-benefits. However, a major barrier to the widescale implementation of ERW is a robust Monitoring, Reporting, and Verification (MRV) framework. To successfully quantify the amount of carbon dioxide removed by ERW at scale, MRV must be accurate, precise, and cost-effective. Here, we outline a new method based on mass balance where metal analysis on soil samples is used to accurately track the extent of in-situ alkaline mineral weathering. We show that signal-to-noise issues of in-situ soil analysis can be mitigated by using isotope-dilution mass spectrometry to reduce analytical error. We implement a proof of concept experiment demonstrating the method in controlled mesocosms. In our experiment, basalt feedstock is added to soil columns containing the cereal crop Sorghum bicolor at a rate equivalent to 50 t ha-1. Using our approach, we calculate an average initial CDR value of 2.24 +- 1.33 tCO2eq ha-1 from our experiments after 235 days, within error of an independent estimate calculated using conventional elemental budgeting of reaction products. Our result corresponds to an initial CDR efficiency of 24.4 +- 14.5 % for the feedstock used. Our method provides a robust time-integrated estimate of initial CDR, and offers a path to track and validate large-scale carbon removal through ERW

Subglacial Meltwater Supported Aerobic Marine Habitats During Snowball Earth

The Earth’s most severe ice ages interrupted a crucial interval in eukaryotic evolution with widespread ice coverage during the Cryogenian Period (720 to 635 Ma). Aerobic eukaryotes must have survived the “Snowball Earth” glaciations, requiring the persistence of oxygenated marine habitats, yet evidence for these environments is lacking. We examine iron formations within globally distributed Cryogenian glacial successions to reconstruct the redox state of the synglacial oceans. Iron isotope ratios and cerium anomalies from a range of glaciomarine environments reveal pervasive anoxia in the ice-covered oceans but increasing oxidation with proximity to the ice shelf grounding line. We propose that the outwash of subglacial meltwater supplied oxygen to the synglacial oceans, creating glaciomarine oxygen oases. The confluence of oxygen-rich meltwater and iron-rich seawater may have provided sufficient energy to sustain chemosynthetic communities. These processes could have supplied the requisite oxygen and organic carbon source for the survival of early animals and other eukaryotic heterotrophs through these extreme glaciations

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

Identifying early Earth microfossils in unsilicified sediments

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Native Cu from the oceanic crust: Isotopic insights into native metal origin

Ocean drilling has revealed that, although a minor mineral phase, native Cu ubiquitously occurs in the oceanic crust. Cu isotope systematics for native Cu from a set of occurrences from volcanic basement and sediment cover of the oceanic crust drilled at several sites in the Pacific, Atlantic and Indian oceans constrains the sources of Cu and processes that produced Cu0. We propose that both hydrothermally-released Cu and seawater were the sources of Cu at these sites. Phase stability diagrams suggest that Cu0 precipitation is favored only under strictly anoxic, but not sulfidic conditions at circum-neutral pH even at low temperature. In the basaltic basement, dissolution of primary igneous and potentially hydrothermal Cu-sulfides leads to Cu0 precipitation along veins. The restricted Cu-isotope variations (δ65Cu = 0.02 – 0.19‰) similar to host volcanic rocks suggest that Cu0 precipitation occurred under conditions where Cu+-species were dominant, precluding Cu redox fractionation. In contrast, the Cu-isotope variations observed in the Cu0 from sedimentary layers yield larger Cu-isotope fractionation (δ65Cu = 0.41 – 0.95‰) suggesting that Cu0 precipitation involved redox processes during the diagenesis, with potentially seawater as the primary Cu source. We interpret that native Cu precipitation in the basaltic basement is a result of low temperature (20°-65 °C) hydrothermal processes under anoxic, but not H2S-rich conditions. Consistent with positive δ65Cu signatures, the sediment cover receives major Cu contribution from hydrogenous (i.e., seawater) sources, although hydrothermal contribution from plume fallout cannot be entirely discarded. In this case, disseminated hydrogenous and/or hydrothermal Cu might be diagenetically remobilized and reprecipitated as Cu0 in reducing microenvironment

Composition of native Cu, Cu isotope and mineral analysis from different samples of ODP and DSDP

Ocean drilling has revealed that, although a minor mineral phase, native Cu ubiquitously occurs in the oceanic crust. Cu isotope systematics for native Cu from a set of occurrences from volcanic basement and sediment cover of the oceanic crust drilled at several sites in the Pacific, Atlantic and Indian oceans constrains the sources of Cu and processes that produced Cu**0. We propose that both hydrothermally-released Cu and seawater were the sources of Cu at these sites. Phase stability diagrams suggest that Cu**0 precipitation is favored only under strictly anoxic, but not sulfidic conditions at circum-neutral pH even at low temperature. In the basaltic basement, dissolution of primary igneous and potentially hydrothermal Cu-sulfides leads to Cu**0 precipitation along veins. The restricted Cu-isotope variations (delta 65Cu = 0.02-0.19 per mil) similar to host volcanic rocks suggest that Cu**0 precipitation occurred under conditions where Cu+-species were dominant, precluding Cu redox fractionation. In contrast, the Cu-isotope variations observed in the Cu**0 from sedimentary layers yield larger Cu-isotope fractionation (delta 65Cu = 0.41-0.95 per mil) suggesting that Cu**0 precipitation involved redox processes during the diagenesis, with potentially seawater as the primary Cu source. We interpret that native Cu precipitation in the basaltic basement is a result of low temperature (20°-65 °C) hydrothermal processes under anoxic, but not H2S-rich conditions. Consistent with positive delta 65Cu signatures, the sediment cover receives major Cu contribution from hydrogenous (i.e., seawater) sources, although hydrothermal contribution from plume fallout cannot be entirely discarded. In this case, disseminated hydrogenous and/or hydrothermal Cu might be diagenetically remobilized and reprecipitated as Cu**0 in reducing microenvironment