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Much of the current volume of Earth’s continental crust had formed by the end of the Archaean eon 1 (2.5 billion years ago), through melting of hydrated basaltic rocks at depths of approximately 25–50 kilometres, forming sodic granites of the tonalite–trondhjemite–granodiorite (TTG) suite 2 – 6 . However, the geodynamic setting and processes involved are debated, with fundamental questions arising, such as how and from where the required water was added to deep-crustal TTG source regions 7 , 8 . In addition, there have been no reports of voluminous, homogeneous, basaltic sequences in preserved Archaean crust that are enriched enough in incompatible trace elements to be viable TTG sources 5 , 9 . Here we use variations in the oxygen isotope composition of zircon, coupled with whole-rock geochemistry, to identify two distinct groups of TTG. Strongly sodic TTGs represent the most-primitive magmas and contain zircon with oxygen isotope compositions that reflect source rocks that had been hydrated by primordial mantle-derived water. These primitive TTGs do not require a source highly enriched in incompatible trace elements, as ‘average’ TTG does. By contrast, less sodic ‘evolved’ TTGs require a source that is enriched in both water derived from the hydrosphere and also incompatible trace elements, which are linked to the introduction of hydrated magmas (sanukitoids) formed by melting of metasomatized mantle lithosphere. By concentrating on data from the Palaeoarchaean crust of the Pilbara Craton, we can discount a subduction setting 6 , 10 – 13 , and instead propose that hydrated and enriched near-surface basaltic rocks were introduced into the mantle through density-driven convective overturn of the crust. These results remove many of the paradoxical impediments to understanding early continental crust formation. Our work suggests that sufficient primordial water was already present in Earth’s early mafic crust to produce the primitive nuclei of the continents, with additional hydrated sources created through dynamic processes that are unique to the early Earth. Oxygen isotopes and whole-rock geochemistry show that the water required to make Earth’s first continental crust was primordial and derived from the mantle, not surface water introduced by subduction.

Since the early 1950s, the number of international measurement standards for anchoring stable isotope delta scales has mushroomed from 3 to more than 30, expanding to more than 25 chemical elements. With the development of new instrumentation, along with new and improved measurement procedures for studying naturally occurring isotopic abundance variations in natural and technical samples, the number of internationally distributed, secondary isotopic reference materials has blossomed in the last six decades to more than 150 materials. More than half of these isotopic reference materials were produced for isotope-delta measurements of seven elements: H, Li, B, C, N, O, and S. The number of isotopic reference materials for other, heavier elements has grown considerably over the last decade. Nevertheless, even primary international measurement standards for isotope-delta measurements are still needed for some elements, including Mg, Fe, Te, Sb, Mo, and Ge. It is recommended that authors publish the delta values of internationally distributed, secondary isotopic reference materials that were used for anchoring their measurement results to the respective primary stable isotope scale.

The Palaeoarchean supracrustal belts in Greenland contain Earth’s oldest rocks and are a prime target in the search for the earliest evidence of life on Earth. However, metamorphism has largely obliterated original rock textures and compositions, posing a challenge to the preservation of biological signatures. A recent study of 3,700-million-year-old rocks of the Isua supracrustal belt in Greenland described a rare zone in which low deformation and a closed metamorphic system allowed preservation of primary sedimentary features, including putative conical and domical stromatolites 1 (laminated accretionary structures formed by microbially mediated sedimentation). The morphology, layering, mineralogy, chemistry and geological context of the structures were attributed to the formation of microbial mats in a shallow marine environment by 3,700 million years ago, at the start of Earth’s rock record. Here we report new research that shows a non-biological, post-depositional origin for the structures. Three-dimensional analysis of the morphology and orientation of the structures within the context of host rock fabrics, combined with texture-specific analyses of major and trace element chemistry, show that the ‘stromatolites’ are more plausibly interpreted as part of an assemblage of deformation structures formed in carbonate-altered metasediments long after burial. The investigation of the structures of the Isua supracrustal belt serves as a cautionary tale in the search for signs of past life on Mars, highlighting the importance of three-dimensional, integrated analysis of morphology, rock fabrics and geochemistry at appropriate scales. In contrast to a previous study of 3,700-million-year-old rocks of the Isua supracrustal belt in Greenland, which presented fossil evidence of stromatolites (macroscopic remains of layered microbial communities), this study shows that these ‘stromatolites’ are features of deformation unconnected to the processes of organic life.

Sulphide oxidation coupled to carbonate dissolution can provide a transient source of carbon dioxide to Earth’s atmosphere and so balance the Cenozoic increase in carbon dioxide consumption by silicate weathering, reconciling this increase with the need for mass balance in the long-term carbon cycle. Balancing the Cenozoic carbon budget It is thought that mountain uplift stimulated carbon dioxide consumption by silicate mineral weathering during the Cenozoic era, roughly spanning the past 66 million years, but there are no signs of a corresponding increase in volcanic carbon dioxide emissions that could balance the carbon budget. This paper suggests that some of the missing carbon dioxide could have been produced by coupled sulphide oxidation and carbonate dissolution, a process that may also have been accelerated in response to uplift. This hypothesis is consistent with isotopic records and may help explain the interactions between the long-term carbon cycle, tectonics, and Earth's climate. The observed stability of Earth’s climate over millions of years is thought to depend on the rate of carbon dioxide (CO 2 ) release from the solid Earth being balanced by the rate of CO 2 consumption by silicate weathering 1 . During the Cenozoic era, spanning approximately the past 66 million years, the concurrent increases in the marine isotopic ratios of strontium, osmium and lithium 2 , 3 , 4 suggest that extensive uplift of mountain ranges may have stimulated CO 2 consumption by silicate weathering 5 , but reconstructions of sea-floor spreading 6 do not indicate a corresponding increase in CO 2 inputs from volcanic degassing. The resulting imbalance would have depleted the atmosphere of all CO 2 within a few million years 7 . As a result, reconciling Cenozoic isotopic records with the need for mass balance in the long-term carbon cycle has been a major and unresolved challenge in geochemistry and Earth history. Here we show that enhanced sulphide oxidation coupled to carbonate dissolution can provide a transient source of CO 2 to Earth’s atmosphere that is relevant over geological timescales. Like drawdown by means of silicate weathering, this source is probably enhanced by tectonic uplift, and so may have contributed to the relative stability of the partial pressure of atmospheric CO 2 during the Cenozoic. A variety of other hypotheses 8 , 9 , 10 have been put forward to explain the ‘Cenozoic isotope-weathering paradox’, and the evolution of the carbon cycle probably depended on multiple processes. However, an important role for sulphide oxidation coupled to carbonate dissolution is consistent with records of radiogenic isotopes 2 , 3 , atmospheric CO 2 partial pressure 11 , 12 and the evolution of the Cenozoic sulphur cycle, and could be accounted for by geologically reasonable changes in the global dioxygen cycle, suggesting that this CO 2 source should be considered a potentially important but as yet generally unrecognized component of the long-term carbon cycle.

The functioning and productivity of pre-Columbian raised fields (RFs) and their role in the development of complex societies in Amazonian savannas remain debated. RF agriculture is conducted today in the Congo Basin, offering an instructive analogue to pre-Columbian RFs in Amazonia. Our study of construction of present-day RFs documents periodic addition of organic matter (OM) during repeated field/fallow cycles. Field investigations of RF profiles supported by spectrophotometry reveal a characteristic stratigraphy. Soil geochemistry indicates that the management of Congo RFs improves soil fertility for a limited time when they are under cultivation, but nutrient availability in fallow RFs differs little from that in uncultivated reference topsoils. Furthermore, examination of soil micromorphology shows that within less than 40 years, bioturbation almost completely removes stratigraphic evidence of repeated OM amendments. If Amazonian RFs were similarly managed, their vestiges would thus be unlikely to show traces of such management centuries after abandonment. These results call into question the hypothesis that the sole purpose of constructing RFs in pre-Columbian Amazonia was drainage.

Low-salinity submarine groundwater contained within continental shelves is a global phenomenon. Mechanisms for emplacing offshore groundwater include glacial processes that drove water into exposed continental shelves during sea-level low stands and active connections to onshore hydrologic systems. While low-salinity groundwater is thought to be abundant, its distribution and volume worldwide is poorly understood due to the limited number of observations. Here we image laterally continuous aquifers extending 90 km offshore New Jersey and Martha’s Vineyard, Massachusetts, on the U.S. Atlantic margin using new shallow water electromagnetic geophysical methods. Our data provide more continuous constraints on offshore groundwater than previous models and present evidence for a connection between the modern onshore hydrologic system and offshore aquifers. We identify clinoforms as a previously unknown structural control on the lateral extent of low-salinity groundwater and potentially a control on where low-salinity water rises into the seafloor. Our data suggest a continuous submarine aquifer system spans at least 350 km of the U.S. Atlantic coast and contains about 2800 km 3 of low-salinity groundwater. Our findings can be used to improve models of past glacial, eustatic, tectonic, and geomorphic processes on continental shelves and provide insight into shelf geochemistry, biogeochemical cycles, and the deep biosphere.

During the ongoing uplift and expansion of the southeastern margin of the Tibetan Plateau, the front edge of the Sichuan‐Yunnan rhombic block (SYB) has experienced intense tectonic activity and frequent seismicity. In this study, the fluid geochemistry in the primary active faults at the front edge of the SYB was investigated, with the aim of understanding the tectonic activity and intersection relationship between the Xiaojiang fault (XJF) and the Red River fault (RRF). Thermal spring water and gases exhibit a coupled spatial distribution relationship; relatively high ion concentrations and 3He/4He ratios (Rc/Ra ratios of 0.21 to 0.62Ra) are observed along the RRF, Qujiang fault (QJF), and Shiping‐Jianshui fault (SJF). Multidisciplinary research results have indicated that mantle‐derived intrusion has been detected in the crust beneath the QJF and SJF. The current tectonic activity in the front edge of the SYB remains intense, with compressive stresses shifting toward the western side of the XJF and accumulating on the QJF and SJF. This has led to the development of fractures, enhancing the water–rock interaction and deep‐derived gas degassing along the faults. The unmixing characteristics of fluids at the intersection area of these two faults suggest the absence of conduits for fluid migration between the faults. Owing to the lower gas 3He/4He ratios, lower shear strain rates, stable reservoir temperature field, and extremely low historical seismicity in the Indo‐Chinese block, it is speculated that the current movement of the XJF may not cut through the RRF and continue southward. Plain Language Summary Fluids serve as carriers of information regarding deep activities, and are known to migrate along active faults. Additionally, fluid geochemistry is highly sensitive to tectonic activity. Given the intense tectonic activity and frequent seismicity experienced at the front edge of the Sichuan‐Yunnan rhombic block (SYB), our study focuses on investigating the fluid geochemical characteristics of the primary active faults in this region. Significant spatial differences in fluid chemistry are observed, particularly with respect to relatively high ion concentrations and mantle He values along the Red River fault (RRF), Qujiang fault (QJF), and Shiping‐Jianshui fault (SJF). Furthermore, the lack of conduits facilitating fluid migration between the Xiaojiang (XJF) and RRF is evident from the distinct unmixed characteristics of the fluids. Multidisciplinary results indicate the presence of mantle‐derived intrusion into the crust beneath the QJF and SJF. The compressive stresses have shifted toward the western side of the XJF and are accumulated on the QJF and SJF, resulting in the observed spatial variations in fluid geochemistry. Ultimately, these spatial differences can be attributed to the unique intersection relationship between the XJF and RRF. Key Points Mantle‐derived intrusion has been detected in the crust beneath the Qujiang fault (QJF) and Shiping‐Jianshui fault (SJF) Tectonics remains active at the front edge of the Sichuan‐Yunnan block, and stresses accumulate on the QJF and SJF The current movement of the Xiaojiang fault may not cut through the Red River fault and continue southward

Sediment microbial fuel cells (SMFCs) generate electricity through the oxidation of reduced compounds, such as sulfide or organic carbon compounds, buried in anoxic sediments. The ability to remove sulfide suggests their use in the remediation of sediments impacted by point source organic matter loading, such as occurs beneath open pen aquaculture farms. However, for SMFCs to be a viable technology they must remove sulfide at a scale relevant to the environmental contamination and their impact on the sediment geochemistry as a whole must be evaluated. Here we address these issues through a laboratory microcosm experiment. Two SMFCs placed in high organic matter sediments were operated for 96 days and compared to open circuit and sediment only controls. The impact on sediment geochemistry was evaluated with microsensor profiling for oxygen, sulfide, and pH. The SMFCs had no discernable effect on oxygen profiles, however porewater sulfide was significantly lower in the sediment microcosms with functioning SMFCs than those without. Depth integrated sulfide inventories in the SMFCs were only 20% that of the controls. However, the SMFCs also lowered pH in the sediments and the consequences of this acidification on sediment geochemistry should be considered if developing SMFCs for remediation. The data presented here indicate that SMFCs have potential for the remediation of sulfidic sediments around aquaculture operations.

Continental weathering acts as a nexus of global biogeochemical cycles yet its long‐term evolution remains unclear. Here we show that continental weathering may not have operated on a massive scale until the early Paleozoic based on a large‐number data compilation of mudstone geochemistry. We attribute the long‐term evolution of mudstone K/Al to the competing weathering inputs between continents and seafloor. Much of pre‐Phanerozoic mudstone is characterized by high K/Al relative to continents, indicating low weathering input of K and Al from continents relative to the hydrothermal input of K from seafloor alteration to the global sedimentary mud reservoir. A quantitative assessment indicates over one order of magnitude increase of weathering partition into continents during the Phanerozoic. The Phanerozoic onset of massive continental weathering reflects a fundamental evolution on the Earth's surface linked to an interactive feedback network consists of mountain building, atmospheric oxygenation, and land plant colonization.

Continental interiors were flooded by epeiric seas during many intervals of the geologic past. Few modern analogs exist for these environments, however, and basic variables such as redox, salinity, and restriction are difficult to reconstruct in deep time. Despite these challenges, constraining epeiric watermass properties is critical because much of our preserved and accessible sedimentary record was deposited in such settings. Here, we present a four‐dimensional reconstruction of watermass evolution in the Late Devonian Appalachian Seaway of North America. We use combined proxies for sediment supply, paleosalinity, paleoredox, and basin hydrography in six cores through the Upper Devonian Cleveland Shale deposited across a paleo‐depth transect. Cyclic, coupled changes in sedimentation, redox, and salinity are recorded in environments near the Catskill Delta. Additionally, a pronounced salinity gradient was present from low‐brackish conditions near the delta to fully marine conditions in the basin interior, with a lower‐salinity mixing zone recorded across the Cumberland Sill. We also identified two broad sequences—the lower and upper Cleveland Shale—each of which shows distinct watermass signatures. The lower Cleveland Shale records a redox gradient with euxinia only present along the Cumberland Sill, whereas the upper Cleveland Shale records intensification of euxinia (potentially in the photic zone) at all six sites, which may be coincident with the Hangenberg extinction event. Ultimately, this study identifies pronounced epeiric watermass gradients over short timescales (millennia) and distances (hundreds of km or less), highlighting the need for interpreting the geochemistry of epicontinental deposits in the context of basin hydrography and paleosalinity. Plain Language Summary The interiors of continents were flooded by shallow seas during many intervals of Earth history; however, little is known about the basic watermass properties of ancient inland seas and how they differed from the open ocean. Here, we provide an example of watermass reconstruction in an ancient inland sea by investigating the Cleveland Shale, which was deposited in a flooded region west of the Appalachian Mountains during the Late Devonian (∼383–359 million years ago). We use geochemical proxies for oxygen concentrations (redox) and salinity in six rock cores located across a gradient of ancient water depth. Our data reveal a pronounced salinity gradient and two discrete stages in the sea's chemical evolution, with the first stage recording a strong redox gradient across the sea and the second stage characterized by oxygen deficiency and toxic hydrogen sulfide at all sites investigated. This expansion of shallow‐water hydrogen sulfide occurred at the same time as the Hangenberg mass extinction event that killed many inhabitants of the shallow oceans. Ultimately, this study is among the first to identify pronounced chemical gradients in an ancient inland sea and link spatiotemporal watermass properties to ocean habitability and extinction during a critical time in Earth history. Key Points Four‐dimensional paleoredox, paleosalinity, and hydrographic reconstruction of the Late Devonian Appalachian Seaway Pronounced watermass heterogeneity over short timescales and distances, including large vertical and lateral salinity gradient Basin‐wide intensification of water column (and potentially photic zone) euxinia during the Hangenberg extinction event