Anomalous fractionation of mercury isotopes in the Late Archean atmosphere

This work was funded by a Natural Environment Research Council (NERC) Fellowship NE/H016805/2 and Standard Grant NE/J023485/2 (to A.L.Z.). R.Y. was funded by the Chinese Academy of Sciences through the Hundred Talent Plan. G.J.I. recognizes continued support from R. Summons under the auspices of the Simons Collaboration on the Origin of Life. We thank J. Kirschvink, J. Grotzinger, A. Knoll, and the Agouron Institute for organizing and funding the Agouron Drilling Project, and the Council for Geoscience in South Africa, specifically those at the National Core Library in Donkerhoek, for facilitating access to the core materials.Earth’s surface underwent a dramatic transition ~2.3 billion years ago when atmospheric oxygen first accumulated during the Great Oxidation Event, but the detailed composition of the reducing early atmosphere is not well known. Here we develop mercury (Hg) stable isotopes as a proxy for paleoatmospheric chemistry and use Hg isotope data from 2.5 billion-year-old sedimentary rocks to examine changes in the Late Archean atmosphere immediately prior to the Great Oxidation Event. These sediments preserve evidence of strong photochemical transformations of mercury in the absence of molecular oxygen. In addition, these geochemical records combined with previously published multi-proxy data support a vital role for methane in Earth’s early atmosphere.Publisher PDFPeer reviewe

A copper isotope investigation of methane cycling in Late Archaean sediments

This research was supported by NERC award NE/L002590/1 to the IAPETUS DTP, and by NERC Standard Grant NE/J023485/2 to A.L.Z. The initiation of Cu isotope analysis at the University of St Andrews was aided significantly by a Carnegie Trust Research Incentive Grant awarded to P.S.S.The rise of oxygenic photosynthesis arguably represents the most important evolutionary step in Earth history. Recent studies, however, suggest that Earth’s pre-oxidative atmosphere was also heavily influenced by biological feedbacks. Most notably, recent geochemical records propose the existence of a hydrocarbon haze which periodically formed in response to enhanced biospheric methane fluxes. Copper isotopes provide a potential proxy for biological methane cycling; Cu is a bioessential trace metal and a key element in the aerobic oxidation of methane to carbon dioxide (methanotrophy). In addition, Cu isotopes are fractionated during biological uptake. Here, we present a high-resolution Cu isotope record measured in a suite of shales and carbonates from core GKF01, through the ~2.6–2.5 Ga Campbellrand-Malmani carbonate platform. Our data show a 0.85‰ range in Cu isotope composition and a negative excursion that predates the onset of a haze event. We interpret this excursion as representing a period of enhanced aerobic methane oxidation before the onset of the Great Oxidation Event. This places valuable time constraints on the evolution of this metabolism and firmly establishing Cu isotopes as a biomarker in Late Archaean rocks.PostprintPeer reviewe

Nitrogen fixation sustained productivity in the wake of the Palaeoproterozoic Great Oxygenation Event

The marine nitrogen cycle is dominated by redox-controlled biogeochemical processes and, therefore, is likely to have been revolutionised in response to Earth-surface oxygenation. The details, timing, and trajectory of nitrogen cycle evolution, however, remain elusive. Here we couple nitrogen and carbon isotope records from multiple drillcores through the Rooihoogte–Timeball Hill Formations from across the Carletonville area of the Kaapvaal Craton where the Great Oxygenation Event (GOE) and its aftermath are recorded. Our data reveal that aerobic nitrogen cycling, featuring metabolisms involving nitrogen oxyanions, was well established prior to the GOE and that ammonium may have dominated the dissolved nitrogen inventory. Pronounced signals of diazotrophy imply a stepwise evolution, with a temporary intermediate stage where both ammonium and nitrate may have been scarce. We suggest that the emergence of the modern nitrogen cycle, with metabolic processes that approximate their contemporary balance, was retarded by low environmental oxygen availability

Nitrogen fixation sustained productivity in the wake of the Palaeoproterozoic Great Oxygenation Event

The marine nitrogen cycle is dominated by redox-controlled biogeochemical processes and, therefore, is likely to have been revolutionised in response to Earth-surface oxygenation. The details, timing, and trajectory of nitrogen cycle evolution, however, remain elusive. Here we couple nitrogen and carbon isotope records from multiple drillcores through the Rooihoogte-Timeball Hill Formations from across the Carletonville area of the Kaapvaal Craton where the Great Oxygenation Event (GOE) and its aftermath are recorded. Our data reveal that aerobic nitrogen cycling, featuring metabolisms involving nitrogen oxyanions, was well established prior to the GOE and that ammonium may have dominated the dissolved nitrogen inventory. Pronounced signals of diazotrophy imply a stepwise evolution, with a temporary intermediate stage where both ammonium and nitrate may have been scarce. We suggest that the emergence of the modern nitrogen cycle, with metabolic processes that approximate their contemporary balance, was retarded by low environmental oxygen availability.National Science Foundation (U.S.) (Grant EAR-1338810)National Science Foundation (U.S.) (Grant EAR-1455258

A marine Mo-isotope record across OAE1a

Although the oceans have remained well oxygenated for most of the Phanerozoic, the stratigraphic record is punctuated by transient events reflecting an apparent worldwide increase in marine anoxia. The Cretaceous oceans appear to have been particularly prone to the development of marine anoxia, and feature a number of discrete oceanic anoxic events (OAEs; [1]). Research has shown that OAEs were broadly contemporaneous with the emplacement of large igneous provinces (LIPs), abrupt global warming and significant marine biotic changes. OAEs were also associated with substantial changes in the global carbon cycle, intensification of the hydrological cycle and ocean acidification. Whilst it is clear from direct observation that unusually high levels of marine organic carbon accumulated globally during OAEs, it is much harder to quantify the lateral increase in marine anoxia. Most redox proxies reflect local conditions either within the water column or beneath the sediment water interface. In contrast, the Mo-isotope system has, under certain circumstances, the unique potential to reflect the areal extent of marine anoxia globally [2, 3]. Here we present new Mo-isotope results and a comprehensive suite of other geochemical data derived from a section across Cretaceous OAE 1a (the Selli event) at Gorgo a Cerbera, in the Umbria-Marche region of Italy. Integration of our new data with an existing Os-isotope stratigraphy [4] enables us to track the course and development of marine redox fluctuations during the OAE in relation to the emplacement of the coeval Ontong-Java LIP. We observe significant shifts in the Mo-isotopic composition of seawater to light isotopic ratios. Taken together, the isotopic and trace element data provide unique insights into how the chemical composition and redox state of the Cretaceous oceans, and the contemporaneous marine biota, were affected by the emplacement of the Ontong-Java volcanics

The distribution and accumulation of mercury and methylmercury in surface sediments beneath the East China Sea

Abstract China is a massive mercury emitter, responsible for a quarter of the world’s mercury emissions, which transit the atmosphere and accumulate throughout its watercourses. The Changjiang (Yangtze) River is the third largest river in the world, integrating mercury emissions over its 1.8 × 106 km2 catchment and channelling them to the East China Sea where they can be buried. Despite its potential global significance, the importance of the East China Sea as a terminal mercury sink remains poorly known. To address this knowledge gap, total mercury and methylmercury concentrations were determined from 51 surface sediment samples revealing their spatial distribution, whilst demonstrating the overall pollution status of the East China Sea. Sedimentary mercury distributions beneath the East China Sea are spatially heterogeneous, with high mercury concentrations (> 25 ng g−1) corresponding to areas of fine-grained sediment accumulation. In contrast, some sites of fine-grained sediment deposition have significantly lower values of methylmercury (< 15 ng g−1), such as the Changjiang estuary and some isolated offshore areas. Fine-grained particles and organic matter availability appear to exert the dominant control over sedimentary mercury distribution in the East China Sea, whereas in situ methylation serves as an additional control governing methylmercury accumulation. Estimated annual sedimentary fluxes of mercury in the East China Sea are 51 × 106 g, which accounts for 9% of China’s annual mercury emissions

Reconciling discrepant minor sulfur isotope records of the Great Oxidation Event

Emerging sulfur isotope data divides opinion surrounding the Great Oxidation Event. Utilising computational approaches and additional data, Uveges et al. reconcile these disparities, offering a more refined framework of atmospheric oxygenation

Slice to Volume Registration

The molybdenum (Mo) stable isotope system has been applied to a variety of geochemical and environmental problems. In the absence of a universally accepted zero-delta reference material, different groups report their data relative to their adopted in-house standards. Rigorous comparison of results generated in different laboratories using different analytical approaches is only possible if the in-house standards are of identical Mo isotope composition. To determine potential isotopic differences among various standards, the ?98Mo (98Mo/95Mo) values of ten Mo standard solutions were measured as part of this study. For six of these solutions, four laboratories carried out an intercalibration. In contrast to previous results, ?98Mo of various in-house standards were found to differ by up to 0.37‰. Renormalisation of our new and published Mo-isotope data available for seawater taken from various sites and the USGS rock reference material SDO-1 relative to NIST-SRM-3134, provides a much better agreement among reported ?98Mo values for these samples. Relative to NIST-SRM-3134, the ?98Mo of SDO-1 is 0.80 ± 0.14‰ (2s), while oxic, open-ocean seawater is characterised by an average ?98Mo of 2.09 ± 0.10‰ (2s). This intercalibration provides a solid platform for comparing and amending existing ?98Mo values. In addition, we recommend that future Mo isotope studies adopt NIST-SRM-3134 as a universal zero-delta reference material