Sea level rise causes barrier islands to migrate landward. Coastal evolution modelling reveals a centennial-scale lag in island response time and suggests migration rates will increase by 50% within the next century, even if sea level were to stabilize.
Unlike the other terrestrial planets, Earth has a substantial silica-rich continental crust with a bulk andesitic composition. A small number of meteorites with andesitic bulk compositions have been identified that are thought to be the products of partial melting of chondritic protoliths, a mode of petrogenesis distinct from that of Earth’s continental crust. Here we show, using geochemical analyses, that unlike other known andesitic meteorites, Erg Chech 002 has strongly fractionated and low abundances of the highly siderophile elements and mineralogy consistent with origin from a melt. The meteorite’s bulk composition, which is similar to terrestrial andesites, cannot be explained by partial melting of basaltic lithologies and instead requires a metal-free chondritic source. We argue that Erg Chech 002 probably formed by ~15–25% melting of the mantle of an alkali-undepleted differentiated asteroid. Our findings suggest that extensive silicate differentiation after metal–silicate equilibration of chondritic parent bodies was already occurring within the first 2.25 million years of Solar System history and that andesitic crust formation does not necessarily require plate tectonics.
Rocket emissions and debris from spacecraft falling out of orbit are having increasingly detrimental effects on global atmospheric chemistry. Improved monitoring and regulation are urgently needed to create an environmentally sustainable space industry.
Analyses of the 2014 Iceland–Holuhraun volcanic eruption revealed the emitted aerosols induced a 10% increase in cloud coverage above the region, suggesting anthropogenic aerosols might strongly cool the Earth’s climate by increasing the cloud coverage.
Aerosol–cloud interactions have a potentially large impact on climate but are poorly quantified and thus contribute a substantial and long-standing uncertainty in climate projections. The impacts derived from climate models are poorly constrained by observations because retrieving robust large-scale signals of aerosol–cloud interactions is frequently hampered by the considerable noise associated with meteorological co-variability. The 2014 Holuhraun effusive eruption in Iceland resulted in a massive aerosol plume in an otherwise near-pristine environment and thus provided an ideal natural experiment to quantify cloud responses to aerosol perturbations. Here we disentangle significant signals from the noise of meteorological co-variability using a satellite-based machine-learning approach. Our analysis shows that aerosols from the eruption increased cloud cover by approximately 10%, and this appears to be the leading cause of climate forcing, rather than cloud brightening as previously thought. We find that volcanic aerosols do brighten clouds by reducing droplet size, but this has a notably smaller radiative impact than changes in cloud fraction. These results add substantial observational constraints on the cooling impact of aerosols. Such constraints are critical for improving climate models, which still inadequately represent the complex macro-physical and microphysical impacts of aerosol–cloud interactions.
Antarctica preserves Earth’s largest ice-sheet, which in response to climate warming, may lose ice mass and raise sea level by several metres. The ice-sheet bed exerts critical controls on dynamic mass loss through feedbacks between water and heat fluxes, topographic forcing, till deformation and basal sliding. Here we show that sedimentary basins may amplify critical feedbacks that are known to impact ice-sheet retreat dynamics. We create a high-resolution subglacial geology classification for Antarctica by applying a supervised machine-learning method to geophysical data, revealing the distribution of sedimentary basins. Hydro-mechanical numerical modelling demonstrates that during glacial retreat, where sedimentary basins exist, the groundwater discharge rate scales with the rate of ice unloading. Antarctica’s most dynamic ice streams, including Thwaites and Pine Island glaciers, possess sedimentary basins in their upper catchments. Enhanced groundwater discharge and its associated feedbacks are likely to amplify basal sliding and increase the vulnerability of these catchments to rapid ice retreat and enhanced dynamic mass loss.
Carbon and nitrogen losses from degraded wetlands and methane emissions from flooded wetlands are both important sources of greenhouse gas emissions. However, the net-exchange dependence on hydrothermal conditions and wetland integrity remains unclear. Using a global-scale in situ database on net greenhouse gas exchanges, we show diverse hydrology-influenced emission patterns in CO2, CH4 and N2O. We find that total CO2-equivalent emissions from wetlands are kept to a minimum when the water table is near the surface. By contrast, greenhouse gas exchange rates peak in flooded and drained conditions. By extrapolating the current trajectory of degradation, we estimate that between 2021 and 2100, wetlands could result in greenhouse gas emissions equivalent to around 408 gigatons of CO2. However, rewetting wetlands could reduce these emissions such that the radiative forcing caused by CH4 and N2O is fully compensated by CO2 uptake. As wetland greenhouse gas budgets are highly sensitive to changes in wetland area, the resulting impact on climate from wetlands will depend on the balance between future degradation and restoration.
Earth’s climate cooled markedly during the late Miocene from 12 to 5 million years ago, with far-reaching consequences for global ecosystems. However, the driving forces of these changes remain controversial. A major obstacle to progress is the uncertainty over the role played by greenhouse gas radiative forcing. Here we present boron isotope compositions for planktic foraminifera, which record carbon dioxide change for the interval of most rapid cooling, the late Miocene cooling event between 7 and 5 Ma. Our record suggests that CO2 declined by some 100 ppm over this two-million-year-long interval to a minimum at approximately 5.9 Ma. Having accounted for non-CO2 greenhouse gasses and slow climate feedbacks, we estimate global mean surface temperature change for a doubling of CO2—equilibrium climate sensitivity—to be 3.9 °C (1.8–6.7 °C at 95% confidence) on the basis of comparison of our record of radiative forcing from CO2 with a record of global mean surface temperature change. We conclude that changes in CO2 and climate were closely coupled during the latest Miocene and that equilibrium climate sensitivity was within range of estimates for the late Pleistocene, other intervals of the Cenozoic and the twenty-first century as presented by the Intergovernmental Panel on Climate Change. Climate sensitivity in the late Miocene was comparable to the late Pleistocene and twenty-first century, with cooling at the time coupled to declining carbon dioxide, according to a CO2 record determined from boron isotopes in planktic foraminifera
The Sahara is the largest hot desert on Earth. Yet the timing of its inception and its response to climatic forcing is debated, leading to uncertainty over the causes and consequences of regional aridity. Here we present detailed records of terrestrial inputs from Africa to North Atlantic deep-sea sediments, documenting a long and sustained history of astronomically paced oscillations between a humid and arid Sahara from over 11 million years ago. We show that intervals of strong dust emissions from the heart of the continent predate both the intensification of Northern Hemisphere glaciation and the oldest land-based evidence for a Saharan desert by millions of years. We find no simple long-term gradational transition towards an increasingly arid climate state in northern Africa, suggesting that aridity was not the primary driver of gradual Neogene expansion of African savannah C4 grasslands. Instead, insolation-driven wet–dry shifts in Saharan climate were common over the past 11 Myr, and we identify three distinct stages in the sensitivity of this relationship. Our data provide context for evolutionary outcomes on Africa; for example, we find that astronomically paced arid intervals predate the oldest fossil evidence of hominid bipedalism by at least 4 Myr. Pulses of Saharan dust have been entering the North Atlantic since at least 11 Ma, a result of astronomically paced cycles between arid and humid conditions in northern Africa, according to a terrigenous input record from an ocean core off west Africa.
Dissolved organic phosphorus (DOP) has a dual role in the surface ocean as both a product of primary production and as an organic nutrient that fuels primary production and nitrogen fixation, especially in oligotrophic gyres. Although poorly constrained, the geographic distribution and environmental controls of surface ocean DOP concentrations influence the distributions and rates of primary production and nitrogen fixation in the global ocean. Here we pair DOP concentration measurements with a metric of phosphate stress, satellite-based chlorophyll a concentrations and a satellite-based iron stress proxy to explore their relationship with upper 50 m DOP stocks. Our results suggest that phosphate and iron stress work together to control surface ocean DOP concentrations at basin scales. Specifically, upper 50 m DOP stocks decrease with increasing phosphate stress, while alleviated iron stress leads to either surface DOP accumulation or loss depending on phosphate availability. Our work extends the relationship between DOP concentrations and phosphate availability to the global ocean, suggests a linkage between marine phosphorus cycling and iron availability and establishes a predictive framework for DOP distributions and their use as an organic nutrient source that supports global ocean fertility. Production and consumption of dissolved organic phosphorus in the surface ocean is controlled by the interplay between phosphate and iron stress, according to global analyses of the distribution of marine nutrients.
The world’s largest tropical peatland complex is found in the central Congo Basin. However, there is a lack of in situ measurements to understand the peatland’s distribution and the amount of carbon stored in it. So far, peat in this region has been sampled only in largely rain-fed interfluvial basins in the north of the Republic of the Congo. Here we present the first extensive field surveys of peat in the Democratic Republic of the Congo, which covers two-thirds of the estimated peatland area, including from previously undocumented river-influenced settings. We use field data from both countries to compute the first spatial models of peat thickness (mean 1.7 ± 0.9 m; maximum 5.6 m) and peat carbon density (mean 1,712 ± 634 MgC ha⁻¹; maximum 3,970 MgC ha⁻¹) for the central Congo Basin. We show that the peatland complex covers 167,600 km², 36% of the world’s tropical peatland area, and that 29.0 PgC is stored below ground in peat across the region (95% confidence interval, 26.3–32.2 PgC). Our measurement-based constraints give high confidence of globally significant peat carbon stocks in the central Congo Basin, totalling approximately 28% of the world’s tropical peat carbon. Only 8% of this peat carbon lies within nationally protected areas, suggesting its vulnerability to future land-use change.
High biota mercury levels are persisting in the Arctic, threatening ecosystem and human health. The Arctic Ocean receives large pulsed mercury inputs from rivers and the atmosphere. Yet the fate of those inputs and possible seasonal variability of mercury in the Arctic Ocean remain uncertain. Until now, seawater observations were possible only during summer and fall. Here we report polar night mercury seawater observations on a gradient from the shelf into the Arctic Ocean. We observed lower and less variable total mercury concentrations during the polar night (winter, 0.46 ± 0.07 pmol l−1) compared with summer (0.63 ± 0.19 pmol l−1) and no substantial changes in methylmercury concentrations (summer, 0.11 ± 0.03 pmol l−1 and winter, 0.12 ± 0.04 pmol l−1). Seasonal changes were estimated by calculating the difference in the integrated mercury pools. We estimate losses of inorganic mercury of 208 ± 41 pmol m−2 d−1 on the shelf driven by seasonal particle scavenging. Persistent methylmercury concentrations (−1 ± 16 pmol m−2 d−1) are probably driven by a lower affinity for particles and the presence of gaseous species. Our results update the current understanding of Arctic mercury cycling and require budgets and models to be reevaluated with a seasonal aspect.
Geologic intervals of sustained warmth such as the mid-Pliocene Warm Period can inform our understanding of future climate change, including the long-term consequences of oceanic uptake of anthropogenic carbon. Here we generate carbon isotope records and synthesize existing records to reconstruct the position of water masses and determine circulation patterns in the deep Pacific Ocean. We show that the mid-depth carbon isotope gradient in the North Pacific was reversed during the mid-Pliocene compared with today, which implies water flowed from north to south and deep water probably formed in the subarctic North Pacific Deep Water. An isotopically enabled climate model that simulates this North Pacific Deep Water reproduces a similar carbon isotope pattern. Modelled levels of dissolved inorganic carbon content in the North Pacific decrease slightly, although the amount of carbon stored in the ocean actually increases by 1.6% relative to modern due to an increase in dissolved inorganic carbon in the surface ocean. Although the modelled Pliocene ocean maintains a carbon budget similar to the present, the change in deep ocean circulation configuration causes pronounced downstream changes in biogeochemistry. Marine carbon isotope patterns point to substantial deep water formation in the North Pacific during the mid-Pliocene Warm Period, according to a synthesis of carbon isotope records and isotope-enabled climate modelling.
Subduction of hydrated oceanic lithosphere can carry water deep into the Earth, with consequences for a range of tectonic and magmatic processes. Most of the fluid is released in the forearc where it plays a critical role in controlling the mechanical properties and seismic behaviour of the subduction megathrust. Here we present results from three-dimensional inversions of data from nearly 400 long-period magnetotelluric sites, including 64 offshore, to provide insights into the distribution of fluids in the forearc of the Cascadia subduction zone. We constrain the geometry of the electrically resistive Siletz terrane, a thickened section of oceanic crust accreted to North America in the Eocene, and the conductive accretionary complex underthrust along the margin. We find that fluids accumulate over timescales exceeding 1 My above the plate in metasedimentary units, while the mafic rocks of Siletzia remain dry. Fluid concentrations tend to peak at slab depths of 17.5 and 30 km, suggesting control by metamorphic processes, but also concentrate around the edges of Siletzia, suggesting that this mafic block is impermeable, with dehydration fluids escaping up-dip along the megathrust. Our results demonstrate that the lithology of the overriding crust can play a critical role in controlling fluid transport in a subduction zone. The lithology of the overriding plate plays a critical role in determining fluid transport in subduction zones, according to magnetotelluric imaging of the impact of the dry, mafic Siletzia terrane on fluids in the Cascadia subduction zone, North America.
The response of coastal barrier islands to relative sea-level rise (SLR) is a long-debated issue. Over centennial and longer periods, regional barrier retreat is generally proportional to the rate of relative SLR. However, over multi-decadal timescales, this simplification does not hold. Field observations along the USA East Coast indicate that barrier retreat rate has at most increased by ~45% in the last ~100 years, despite a concurrent ≥200% increase in SLR rate. Using a coastal evolution model, we explain this observation by considering disequilibrium dynamics—the lag in barrier behaviour with respect to SLR. Here we show that modern barrier retreat rate is not controlled by recent SLR (last decades), but rather by the baseline SLR of the past centuries. The cumulative effect of the baseline SLR is to establish a potential retreat, which is then realized by storms and tidal processes in the following centuries. When SLR accelerates, the potential for retreat is first realized through removal of geomorphic capital. After several centuries, barrier retreat accelerates proportionally to the increase in SLR. As such, we predict a committed coastal response: even if SLR remains at present rates, barrier retreat in response to SLR will accelerate by ~50% within a century. The lag dynamics identified here are probably general, and should be included in predictions of barrier-system response to climate change. Coastal evolution simulations suggest that the modern retreat of coastal barrier islands is controlled by cumulative sea-level rise over the past several centuries and will accelerate by 50% within a century, even if sea-level rise remains at present rates.
Considerable expansion of shrubs across the Arctic tundra has been observed in recent decades. These shrubs are thought to have a warming effect on permafrost by increasing snowpack thermal insulation, thereby limiting winter cooling and accelerating thaw. Here, we use ground temperature observations and heat transfer simulations to show that low shrubs can actually cool the ground in winter by providing a thermal bridge through the snowpack. Observations from unmanipulated herb tundra and shrub tundra sites on Bylot Island in the Canadian high Arctic reveal a 1.21 °C cooling effect between November and February. This is despite a snowpack that is twice as insulating in shrubs. The thermal bridging effect is reversed in spring when shrub branches absorb solar radiation and transfer heat to the ground. The overall thermal effect is likely to depend on snow and shrub characteristics and terrain aspect. The inclusion of these thermal bridging processes into climate models may have an important impact on projected greenhouse gas emissions by permafrost. Arctic shrubs cool permafrost in winter by acting as a thermal bridge through the snowpack, according to ground temperature observations and heat transfer simulations.
The bulk crustal porosity of the lunar highland may have been generated early in the Moon’s history by basin-forming impacts and then declined exponentially. A new porosity evolution model constrains the timing and sequence of basin formation.
Atmospheric ozone (O3) is a pollutant produced through chemical chain reactions where volatile organic compounds (VOCs), carbon monoxide and methane are oxidized in the presence of oxides of nitrogen (NOx). For decades, the controlling chain termination step has been used to separate regions into either ‘NOx limited’ (peroxyl-radical self-reactions dominate) or ‘VOC limited’ (hydroxyl radical (OH) + nitrogen dioxide (NO2) reaction dominates). The controlling regime would then guide policies for reducing emissions and so O3 concentrations. Using a chemical transport model, we show that a third ‘aerosol inhibited’ regime exists, where reactive uptake of hydroperoxyl radicals (HO2) onto aerosol particles dominates. In 1970, 2% of the Northern Hemisphere population lived in an aerosol-inhibited regime, but by 2014 this had increased to 21%; 60% more than lived in a VOC-limited regime. Aerosol-inhibited chemistry suppressed surface O3 concentrations in North America and Europe in the 1970s and is currently suppressing surface O3 over Asia. This third photochemical O3 regime leads to potential trade-off tensions between reducing particle pollution in Asia (a key current health policy and priority) and increasing surface O3, should O3 precursors emissions not be reduced in tandem. Global chemical transport simulations reveal an ozone photochemistry regime where the uptake of hydroperoxyl radicals onto aerosol particles dominates ozone production.
Large-scale chemical depletion of ozone due to anthropogenic emissions occurs over Antarctica as well as, to a lesser degree, the Arctic. Surface climate predictability in the Northern Hemisphere might be improved due to a previously proposed, albeit uncertain, link to springtime ozone depletion in the Arctic. Here we use observations and targeted chemistry–climate experiments from two models to isolate the surface impacts of ozone depletion from complex downward dynamical influences. We find that springtime stratospheric ozone depletion is consistently followed by surface temperature and precipitation anomalies with signs consistent with a positive Arctic Oscillation, namely, warm and dry conditions over southern Europe and Eurasia and moistening over northern Europe. Notably, we show that these anomalies, affecting large portions of the Northern Hemisphere, are driven substantially by the loss of stratospheric ozone. This is due to ozone depletion leading to a reduction in short-wave radiation absorption, when in turn causing persistent negative temperature anomalies in the lower stratosphere and a delayed break-up of the polar vortex. These results indicate that the inclusion of interactive ozone chemistry in atmospheric models can considerably improve the predictability of Northern Hemisphere surface climate on seasonal timescales. Ozone depletion in the Arctic stratosphere consistently disrupts surface temperature and precipitation patterns across the Northern Hemisphere, according to atmospheric chemistry–climate modelling and observations.
For decades, ozone pollution mitigation efforts relied on two chemical regimes. A global modelling analysis has revealed a third regime involving aerosols that would help with the concurrent control of both ozone and particulate pollution.
The formation and evolution of the terrestrial planets were shaped by a bombardment of large impactors in a cluttered early Solar System. However, various surface processes degrade impact craters, and the early impact history of the Moon and the ages of its ancient impact basins remain uncertain. Here we show that the porosity of the lunar crust, generated by the cumulative crustal processing of impacts, can be used to determine the Moon’s bombardment history. We use a numerical model constrained by gravity data to simulate the generation of porosity by basin-forming impacts and the subsequent removal by smaller impacts and overburden pressure. We find that, instead of steadily increasing over the history of the Moon, lunar crustal porosity was largely generated early in lunar evolution when most basins formed and, on average, has decreased after that time. Using the Moon as a proxy for the terrestrial planets, we find that the terrestrial planets experienced periods of high crustal porosity early in their evolution. Our modelled porosities also provide an independent constraint on the chronological sequence of basin-forming impacts. Our results suggest that the inner solar system was subject to double the number of smaller impacts producing craters exceeding 20 km in diameter than has been previously estimated from traditional crater-counting analyses, whereas the bombardment record for the lunar basins (>200 km in diameter) is complete. This implies a limited late delivery of volatiles and siderophile elements to the terrestrial planets by impactors.
The Azores High is a persistent atmospheric high-pressure ridge over the North Atlantic surrounded by anticyclonic winds that steer rain-bearing weather systems and modulate the oceanic moisture transport to Europe. The areal extent of the Azores High thereby affects precipitation across western Europe, especially during winter. Here we use observations and ensemble climate model simulations to show that winters with an extremely large Azores High are significantly more common in the industrial era (since ce 1850) than in pre-industrial times, resulting in anomalously dry conditions across the western Mediterranean, including the Iberian Peninsula. Simulations of the past millennium indicate that the industrial-era expansion of the Azores High is unprecedented throughout the past millennium (since ce 850), consistent with precipitation proxy evidence from Portugal. Azores High expansion emerges after ce 1850 and strengthens into the twentieth century, consistent with anthropogenically driven warming. The Azores High over the North Atlantic has expanded due to anthropogenic climate change, disrupting precipitation patterns in western Europe, according to climate modelling and precipitation proxy records spanning the past millennium.
The terrestrial subsurface contains nearly all of Earth’s freshwater reserves and harbours the majority of our planet’s total prokaryotic biomass. Although genetic surveys suggest these organisms rely on in situ carbon fixation, rather than the photosynthetically derived organic carbon transported from surface environments, direct measurements of carbon fixation in the subsurface are absent. Using an ultra-low level 14C-labelling technique, we estimate in situ carbon fixation rates in a carbonate aquifer. We find these rates are similar to those measured in oligotrophic marine surface waters and up to six-fold greater than those observed in the lower euphotic zone. Our empirical carbon fixation rates agree with nitrification rate data. Metagenomic analyses reveal abundant putative chemolithoautotrophic members of an uncharacterized order of Nitrospiria that may be behind the carbon fixation. On the basis of our determined carbon fixation rates, we conservatively extrapolate global primary production in carbonate groundwaters (10% of global reserves) to be 0.11 Pg carbon per year. These rates fall within the range found for oligotrophic marine surface waters, indicating a substantial contribution of in situ primary production to subsurface ecosystem processes. We further suggest that, just as phototrophs are for marine biogeochemical cycling, such subsurface carbon fixation is potentially foundational to subsurface trophic webs. Direct measurements of carbon fixation rates in groundwater suggest a substantial contribution of in situ primary production to subsurface ecosystem processes.
Plume magmatism and continental breakup led to the opening of the northeast Atlantic Ocean during the globally warm early Cenozoic. This warmth culminated in a transient (170 thousand year, kyr) hyperthermal event associated with a large, if poorly constrained, emission of carbon called the Palaeocene–Eocene Thermal Maximum (PETM) 56 million years ago (Ma). Methane from hydrothermal vents in the coeval North Atlantic Igneous Province (NAIP) has been proposed as the trigger, though isotopic constraints from deep sea sediments have instead implicated direct volcanic carbon dioxide (CO2) emissions. Here we calculate that background levels of volcanic outgassing from mid-ocean ridges and large igneous provinces yield only one-fifth of the carbon required to trigger the hyperthermal. However, geochemical analyses of volcanic sequences spanning the rift-to-drift phase of the NAIP indicate a sudden ~220 kyr-long intensification of magmatic activity coincident with the PETM. This was likely driven by thinning and enhanced decompression melting of the sub-continental lithospheric mantle, which critically contained a high proportion of carbon-rich metasomatic carbonates. Melting models and coupled tectonic–geochemical simulations indicate that >104 gigatons of subcrustal carbon was mobilized into the ocean and atmosphere sufficiently rapidly to explain the scale and pace of the PETM. A change in the style of rifting in the North Atlantic led to carbon fluxes from subcrustal melting that helped trigger the Palaeocene–Eocene Thermal Maximum, according to geochemical analyses of volcanic sequences as well as melting and tectonic modelling.
Forecasting eruption is the ultimate challenge for volcanology. While there has been some success in forecasting eruptions hours to days beforehand, reliable forecasting on a longer timescale remains elusive. Here we show that magma inflow rate, derived from surface deformation, is an indicator of the probability of magma transfer towards the surface, and thus eruption, for basaltic calderas. Inflow rates ≥0.1 km3 yr−1 promote magma propagation and eruption within 1 year in all assessed case studies, whereas rates <0.01 km3 yr−1 do not lead to magma propagation in 89% of cases. We explain these behaviours with a viscoelastic model where the relaxation timescale controls whether the critical overpressure for dyke propagation is reached or not. Therefore, while surface deformation alone is a weak precursor of eruption, estimating magma inflow rates at basaltic calderas provides improved forecasting, substantially enhancing our capacity of forecasting weeks to months ahead of a possible eruption. Using magma inflow rate improves eruption forecasting on timescales of weeks to months for basaltic caldera systems, compared with using surface deformation alone, according to analysis of 45 unrest case studies and viscoelastic modelling.
Variation in the effective strength of the lithosphere allows for active plate tectonics and is permitted by different deformation mechanisms operating in the crust and upper mantle. The dominant mechanisms are debated, but geodynamic models often employ grain-size-independent mechanisms or evaluate a single grain size. However, observations from nature and rock deformation experiments suggest a transition to grain-size-dependent mechanisms due to a reduction in grain size can cause lithospheric weakening. Here, we employ a two-dimensional thermo-mechanical numerical model of the upper mantle to investigate the nature of deformation and grain-size evolution in a continental rift setting, on the basis of a recent growth law for polycrystalline olivine. We find that the average olivine grain size is greater in the asthenospheric mantle (centimetre-scale grains) than at the crust–mantle boundary (millimetre-scale grains). This grain-size distribution could result in dislocation creep being the dominant deformation mechanism in the upper mantle. However, we suggest that along lithospheric-scale shear zones, a reduction in grain sizes due to localized deformation causes a transition to diffusion creep as the dominant deformation mechanism, causing weakening of the lithosphere and facilitating the initiation of continental rifting.
Oceanic crust forms at mid-ocean spreading centres through a combination of magmatic and tectonic processes, with the magmatic processes creating two distinct layers: the upper and the lower crust. While the upper crust is known to form from lava flows and basaltic dykes based on geophysical and drilling results, the formation of the gabbroic lower crust is still debated. Here we perform a full waveform inversion of wide-angle seismic data from relatively young (7–12-Myr-old) crust formed at the slow-spreading Mid-Atlantic Ridge. The seismic velocity model reveals alternating, 400–500 m thick, high- and low-velocity layers with ±200 m s ⁻¹ velocity variations, below ~2 km from the oceanic basement. The uppermost low-velocity layer is consistent with hydrothermal alteration, defining the base of extensive hydrothermal circulation near the ridge axis. The underlying layering supports that the lower crust is formed through the intrusion of melt as sills at different depths, which cool and crystallize in situ. The layering extends up to 5–15 km distance along the seismic profile, covering 300,000–800,000 years, suggesting that this form of lower crustal accretion is a stable process.
The rapidly retreating Thwaites and Pine Island glaciers together dominate present-day ice loss from the West Antarctic Ice Sheet and are implicated in runaway deglaciation scenarios. Knowledge of whether these glaciers were substantially smaller in the mid-Holocene and subsequently recovered to their present extents is important for assessing whether current ice recession is irreversible. Here we reconstruct relative sea-level change from radiocarbon-dated raised beaches at sites immediately seawards of these glaciers, allowing us to examine the response of the earth to loading and unloading of ice in the Amundsen Sea region. We find that relative sea level fell steadily over the past 5.5 kyr without rate changes that would characterize large-scale ice re-expansion. Moreover, current bedrock uplift rates are an order of magnitude greater than the rate of long-term relative sea-level fall, suggesting a change in regional crustal unloading and implying that the present deglaciation may be unprecedented in the past ~5.5 kyr. While we cannot preclude minor grounding-line fluctuations, our data are explained most easily by early Holocene deglaciation followed by relatively stable ice positions until recent times and imply that Thwaites and Pine Island glaciers have not been substantially smaller than present during the past 5.5 kyr.
Day-to-day changes in CO2 emissions from human activities, in particular fossil-fuel combustion and cement production, reflect a complex balance of influences from seasonality, working days, weather and, most recently, the COVID-19 pandemic. Here, we provide a daily CO2 emissions dataset for the whole year of 2020, calculated from inventory and near-real-time activity data. We find a global reduction of 6.3% (2,232 MtCO2) in CO2 emissions compared with 2019. The drop in daily emissions during the first part of the year resulted from reduced global economic activity due to the pandemic lockdowns, including a large decrease in emissions from the transportation sector. However, daily CO2 emissions gradually recovered towards 2019 levels from late April with the partial reopening of economic activity. Subsequent waves of lockdowns in late 2020 continued to cause smaller CO2 reductions, primarily in western countries. The extraordinary fall in emissions during 2020 is similar in magnitude to the sustained annual emissions reductions necessary to limit global warming at 1.5 °C. This underscores the magnitude and speed at which the energy transition needs to advance. Observed daily changes in CO2 emissions from across the globe reveal the sectors and countries where pandemic-related emissions declines were most pronounced in 2020.
The evolution of land plants during the Palaeozoic Era transformed Earth’s biosphere 1. Because the Earth's surface and interior are linked by tectonic processes, the linked evolution of the biosphere and sedimentary rocks should be recorded as a near-contemporary shift in the composition of the continental crust. To test this hypothesis, we assessed the isotopic signatures of zircon formed at subduction zones where marine sediments are transported into the mantle 2,3, thereby recording interactions between surface environments and the deep Earth. Using oxygen and lutetium-hafnium isotopes of magmatic zircon that respectively track surface weathering (time-independent) 4 and radiogenic decay (time-dependent) 5, we find a correlation in the composition of continental crust after 430 Myr ago, which is coeval with the onset of enhanced complexity and stability in sedimentary systems related to the evolution of vascular plants. The expansion of terrestrial vegetation brought channelled sand-bed and meandering rivers, muddy floodplains, and thicker soils, lengthening the duration of weathering before final marine deposition 6,7. Collectively, our results suggest that the evolution of vascular plants coupled the degree of weathering and timescales of sediment routing to depositional basins where they were subsequently subducted and melted. The late Palaeozoic isotopic shift of zircon indicates that the greening of the continents was recorded in the deep Earth.
Unrest episodes observed in basaltic systems indicate magma influx rates may be key to generating long-term eruption forecasts. The findings predict that, if a critical flow rate is surpassed, a volcano will erupt within a year.
Global warming-induced melting and thawing of the cryosphere are severely altering the volume and timing of water supplied from High Mountain Asia, adversely affecting downstream food and energy systems that are relied on by billions of people. The construction of more reservoirs designed to regulate streamflow and produce hydropower is a critical part of strategies for adapting to these changes. However, these projects are vulnerable to a complex set of interacting processes that are destabilizing landscapes throughout the region. Ranging in severity and the pace of change, these processes include glacial retreat and detachments, permafrost thaw and associated landslides, rock–ice avalanches, debris flows and outburst floods from glacial lakes and landslide-dammed lakes. The result is large amounts of sediment being mobilized that can fill up reservoirs, cause dam failure and degrade power turbines. Here we recommend forward-looking design and maintenance measures and sustainable sediment management solutions that can help transition towards climate change-resilient dams and reservoirs in High Mountain Asia, in large part based on improved monitoring and prediction of compound and cascading hazards.
Megathrust earthquakes release and transfer stress that has accumulated over hundreds of years, leading to large aftershocks that can be highly destructive. Understanding the spatiotemporal pattern of megathrust aftershocks is key to mitigating the seismic hazard. However, conflicting observations show aftershocks concentrated either along the rupture surface itself, along its periphery or well beyond it, and they can persist for a few years to decades. Here we present aftershock data following the four largest megathrust earthquakes since 1960, focusing on the change in seismicity rate following the best-recorded 2011 Tohoku earthquake, which shows an initially high aftershock rate on the rupture surface that quickly shuts down, while a zone up to ten times larger forms a ring of enhanced seismicity around it. We find that the aftershock pattern of Tohoku and the three other megathrusts can be explained by rate and state Coulomb stress transfer. We suggest that the shutdown in seismicity in the rupture zone may persist for centuries, leaving seismicity gaps that can be used to identify prehistoric megathrust events. In contrast, the seismicity of the surrounding area decays over 4–6 decades, increasing the seismic hazard after a megathrust earthquake.
Geological evidence of active tropical glaciers reaching sea level during the Neoproterozoic (1,000–541 Ma), suggesting a global ocean completely covered in ice, was the key observation in the development of the hard Snowball Earth hypothesis. These conditions are hard to reconcile with the survival of complex marine life through Snowball Earth glaciations, which led to alternative waterbelt scenarios where a large-scale refugium was present in the form of a narrow ice-free strip in the tropical ocean. Here we assess whether a waterbelt scenario maintained by snow-free dark sea ice at low latitudes is plausible using simulations from two climate models run with a variety of cloud treatments in combination with an energy-balance model. Our simulations show that waterbelt states are not a robust and naturally emerging feature of Neoproterozoic climate. Intense shortwave reflection by mixed-phase clouds, in addition to a low albedo of bare sea ice, is needed for geologically relevant waterbelt states. Given the large uncertainty in mixed-phase clouds and their interaction with radiation, our results strongly question the idea that waterbelt scenarios can explain the Neoproterozoic geology. Hence, Neoproterozoic life has probably faced the harsh conditions of a hard Snowball Earth.
The last glacial cycle began around 116,000 years before present during a period with low incoming solar radiation in Northern Hemisphere summer. Following the glacial inception in North America, the marine sediment record depicts a weakening of the high-latitude ocean overturning circulation and a multi-millennial eastward progression of glaciation across the North Atlantic basin. Modelling studies have shown that reduced solar radiation can initiate inception in North America and Siberia; however, the proximity to the temperate North Atlantic typically precludes ice growth in Scandinavia. Using a coupled Earth-system–ice-sheet model, we show that ice forming in North America may help facilitate glacial expansion in Scandinavia. As large coherent ice masses form and start filling the ocean gateways in the Canadian Archipelago, the transport of comparatively fresh North Pacific and Arctic water through the archipelago is diverted east of Greenland, resulting in a freshening of North Atlantic deep convection regions, sea-ice expansion and a substantial cooling that is sufficient to trigger glacial inception in Scandinavia. This mechanism may also help explain the Younger Dryas cold reversal and the rapid regrowth of the Scandinavian Ice Sheet following several warm events in the last glacial period.
In the relatively unproductive waters of the tropical ocean, islands can enhance phytoplankton biomass and create hotspots of productivity and biodiversity that sustain upper trophic levels, including fish that are crucial to the survival of islands’ inhabitants. This phenomenon, termed the island mass effect 66 years ago, has been widely described. However, most studies focused on individual islands, and very few documented phytoplankton community composition. Consequently, basin-scale impacts on phytoplankton biomass, primary production and biodiversity remain largely unknown. Here we systematically identify enriched waters near islands from satellite chlorophyll concentrations (a proxy for phytoplankton biomass) to analyse the island mass effect for all tropical Pacific islands on a climatological basis. We find enrichments near 99% of islands, impacting 3% of the tropical Pacific Ocean. We quantify local and basin-scale increases in chlorophyll and primary production by contrasting island-enriched waters with nearby waters. We also reveal a significant impact on phytoplankton community structure and biodiversity that is identifiable in anomalies in the ocean colour signal. Our results suggest that, in addition to strong local biogeochemical impacts, islands may have even stronger and farther-reaching ecological impacts.
Talik formation has long been acknowledged as an important mechanism of permafrost degradation. Currently, a lack of in situ observations has left a critical gap in our understanding of how ongoing climate change may influence future sub-aerial talik formation in areas unaffected by water bodies or wildfire. Here we present in situ ground temperature measurements from undisturbed sub-aerial sites across the discontinuous permafrost zone of Alaska between 1999 and 2020. We find that novel taliks formed at 24 sites across the region, with widespread initiation occurring during the winter of 2018 due to higher air temperatures and above-average snowfall insulating the soil. Future projections under a high emissions scenario show that by 2030, talik formation will initiate across up to 70% of the discontinuous permafrost zone, regardless of snow conditions. By 2090, talik in areas of black spruce forest, and warmer ecosystems, may reach a thickness of 12 m. The establishment of widespread sub-aerial taliks has major implications for permafrost thaw, thermokarst development, carbon cycling, hydrological connectivity and engineering. Temperature observations from across Alaska show widespread talik formation in the discontinuous permafrost zone due to higher air temperatures and above-average snowfall in recent years.
Imbalanced anthropogenic inputs of nitrogen (N) and phosphorus (P) have significantly increased the ratio between N and P globally, degrading ecosystem productivity and environmental quality. Lakes represent a large global nutrient sink, modifying the flow of N and P in the environment. It remains unknown, however, the relative retention of these two nutrients in global lakes and their role in the imbalance of the nutrient cycles. Here we compare the ratio between P and N in inflows and outflows of more than 5,000 lakes globally using a combination of nutrient budget model and generalized linear model. We show that over 80% of global lakes positively retain both N and P, and almost 90% of the lakes show preferential retention of P. The greater retention of P over N leads to a strong elevation in the ratios between N and P in the lake outflow, exacerbating the imbalance of N and P cycles unexpectedly and potentially leading to biodiversity losses within lakes and algal blooms in downstream N-limited coastal zones. The management of N or P in controlling lake eutrophication has long been debated. Our results suggest that eutrophication management that prioritizes the reduction of P in lakes—which causes a further decrease in P in outflows—may unintentionally aggravate N/P imbalances in global ecosystems. Our results also highlight the importance of nutrient retention stoichiometry in global lake management to benefit watershed and regional biogeochemical cycles. Lakes preferentially retain phosphorous over nitrogen, amplifying the imbalance of nutrient cycles caused by anthropogenic inputs, according to analyses of more than 5,000 lakes globally.
Magmatic volatiles (for example, water) are abundant in arc melts and exert fundamental controls on magma evolution, eruption dynamics and the formation of economic ore deposits. To constrain the H2O content of arc magmas, most studies have relied on measuring extrusive products and mineral-hosted melt inclusions. However, these methods have inherent limitations that obfuscate the full range of H2O in arc magmas. Here, we report secondary-ion mass spectrometry measurements of volatile (H2O, F, P, S, Cl) abundances in lower-crustal cumulate minerals from the Kohistan palaeo-arc (northwestern Pakistan) and determine H2O abundances of melts from which the cumulates crystallized. Pyroxenes retained magmatic H2O abundances and record damp (less than 1 wt% H2O) to hydrous (up to 10 wt% H2O) primitive melts. Subsequent crystal fractionation led to formation of super-hydrous melts with approximately 12–20 wt% H2O, predicted petrologically yet virtually absent from the melt-inclusion record. Porphyry copper deposits are probably a natural eventuality of fluid exsolution from super-hydrous melts, corroborating a growing body of evidence. The water content of arc magmas in the lower crust can reach up to 20 wt% during crystallization, according to geochemical analyses of minerals from the Kohistan palaeo-arc, Pakistan, underscoring the role of water in porphyry deposits formation.
The surface strength of small rubble-pile asteroids, which are aggregates of unconsolidated material under microgravity, is poorly constrained but critical to understanding surface evolution and geologic history of the asteroid. Here we use images of an impact ejecta deposit and downslope avalanche adjacent to a 70-m-diameter impact crater on the rubble-pile asteroid (101955) Bennu to constrain the asteroid’s surface properties. We infer that the ejecta deposited near the crater must have been mobilized with velocities less than Bennu’s escape velocity (20 cm s–1); such low velocities can be explained only if the effective strength of the local surface is exceedingly low, nominally ≤2 Pa. This value is four orders of magnitude below strength values commonly used for asteroid surfaces, but it is consistent with recent estimates of internal strength of rubble-pile asteroids and with the surface strength of another rubble-pile asteroid, Ryugu. We find a downslope avalanche indicating a surface composed of material readily mobilized by impacts and that has probably been renewed multiple times since Bennu’s initial assembly. Compared with stronger surfaces, very weak surfaces imply (1) more retention of material because of the low ejecta velocities and (2) lower crater-based age estimates—although the heterogeneous structure of rubble piles complicates interpretation.
On asteroids, fractures develop due to stresses driven by diurnal temperature variations at spatial scales ranging from sub-millimetres to metres. However, the timescales of such rock fracturing by thermal fatigue are poorly constrained by observations. Here we analyse images of the asteroid (101955) Bennu obtained by the Origins, Spectral Interpretation, Resource Identification and Security-Regolith Explorer (OSIRIS-REx) mission and show that metre-scale fractures on the boulders exposed at the surface have a preferential meridional orientation, consistent with cracking induced by diurnal temperature variations. Using an analytical model of fracture propagation, we suggest that fractures the length of those on Bennu’s boulders can be produced in 104–105 years. This is a comparable or shorter timescale than mass movement processes that act to expose fresh surfaces and reorient boulders and any preferential direction signature. We propose that boulder surface fracturing happens rapidly compared with the lifetime in near-Earth space of Bennu and other carbonaceous asteroids. The damage due to this space-weathering process has consequences for the material properties of these asteroids, with implications for the preservation of the primordial signature acquired during the accretional phases in the protoplanetary disk of our solar system. Fractures on the asteroid Bennu imaged by the OSIRIS-REx spacecraft are consistent with cracking induced by diurnal temperature variations over geologically rapid timescales.
Biological soil crusts (biocrusts) cover ~12% of the global land surface. They are formed by an intimate association between soil particles, photoautotrophic and heterotrophic organisms, and they effectively stabilize the soil surface of drylands. Quantitative information on the impact of biocrusts on the global cycling and climate effects of aeolian dust, however, is not available. Here, we combine the currently limited experimental data with a global climate model to investigate the effects of biocrusts on regional and global dust cycling under current and future conditions. We estimate that biocrusts reduce the global atmospheric dust emissions by ~60%, preventing the release of ~0.7 Pg dust per year. Until 2070, biocrust coverage is expected to be severely reduced by climate change and land-use intensification. The biocrust loss will cause an increased dust burden, leading to a reduction of the global radiation budget of around 0.12 to 0.22 W m⁻², corresponding to about 50% of the total direct forcing of anthropogenic aerosols. This biocrust control on dust cycling and its climate impacts have important implications for human health, biogeochemical cycling and the functioning of the ecosystems, and thus should be considered in the modelling, mitigation and management of global change.