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Meandering rivers are diagnostic landforms of hydrologically active planets, and their migration regulates the continental component of biogeochemical cycles that stabilize climate and allow for life on Earth. The rise of river meanders on Earth has been linked to riverbank stabilization driven by the Palaeozoic evolution of plant life about 440 million years ago. Here we provide a fundamental test for this hypothesis using a global analysis of active meander migrations that includes previously ignored unvegetated rivers from the arid interiors of modern continents. When normalized by channel size, unvegetated meanders universally migrate an order of magnitude faster than vegetated ones. While providing irrefutable evidence that vegetation is not required for meander formation, we demonstrate how profoundly vegetation transformed the pace of change for Earth’s landscapes, and we at last offer a mechanistic explanation for the radically distinct stratigraphic records of barren and vegetated rivers. We posit that the migration slowdown driven by the rise of land plants dramatically impacted biogeochemical fluxes and rendered Earth’s landscapes even more hospitable to life. Therefore, the tenfold faster migration of unvegetated rivers may be key to deciphering the environments of barren worlds such as early Earth and Mars.

Stabilization of riverbanks by vegetation has long been considered necessary to sustain single-thread meandering rivers. However, observation of active meandering in modern barren landscapes challenges this assumption. Here, we investigate a globally distributed set of modern meandering rivers with varying riparian vegetation densities, using satellite imagery and statistical analyses of meander-form descriptors and migration rates. We show that vegetation enhances the coefficient of proportionality between channel curvature and migration rates at low curvatures, and that this effect wanes in curvier channels irrespective of vegetation density. By stabilizing low-curvature reaches and allowing meanders to gain sinuosity as channels migrate laterally, vegetation quantifiably affects river morphodynamics. Any causality between denser vegetation and higher meander sinuosity, however, cannot be inferred owing to more frequent avulsions in modern non-vegetated environments. By illustrating how vegetation affects channel mobility and floodplain reworking, our findings have implications for assessing carbon stocks and fluxes in river floodplains. Riparian vegetation densities critically mediate the morphodynamics of meandering rivers: plants slow the rate at which channels move laterally and reinforce the key, first-order control that curvature exerts on meander planform evolution.

Sand mobilized by wind forms decimeter-scale impact ripples and decameter-scale or larger dunes on Earth and Mars. In addition to those two bedform scales, orbital and in situ images revealed a third distinct class of larger meter-scale ripples on Mars. Since their discovery, two main hypotheses have been proposed to explain the formation of large martian ripples—that they originate from the growth in wavelength and height of decimeter-scale ripples or that they arise from the same hydrodynamic instability as windblown dunes or subaqueous bedforms instead. Here we provide evidence that large martian ripples form from the same hydrodynamic instability as windblown dunes and subaqueous ripples. Using an artificial neural network, we characterize the morphometrics of over a million isolated barchan dunes on Mars and analyze how their size and shape vary across Mars’ surface. We find that the size of Mars’ smallest dunes decreases with increasing atmospheric density with a power-law exponent predicted by hydrodynamic theory, similarly to meter-size ripples, tightly bounding a forbidden range in bedform sizes. Our results provide key evidence for a unifying model for the formation of subaqueous and windblown bedforms on planetary surfaces, offering a new quantitative tool to decipher Mars’ atmospheric evolution. Dust storms on Mars drive water escape to space. Here, the authors show the impact Martian dust storms have on the abundance of atmospheric hydrogen and oxygen, and how this helps to overall oxidize the Martian atmosphere.

Sand ripples record interactions between planetary surfaces and environmental flows, providing paleoenvironmental archives when preserved into rocks. Two main ripple types form in sand: drag ripples, common in water, and impact ripples, exclusive to windblown surfaces. Enigmatic meter-scale aeolian ripples on Mars have been assumed to be impact ripples, though ground and orbiter-based observations suggest they may be drag ripples instead. Here, we report on low-pressure wind tunnel experiments in which large ripples formed and evolved from a flat bed. Observations demonstrate that impact and large ripples grow from distinct mechanisms. Large-ripple size aligns with predictions from drag-ripple theory, and associated sand fluxes are greater than predicted for impact ripples. These findings are inconsistent with an impact-ripple origin and instead suggest that large martian ripples are drag ripples. Windblown drag ripples constitute an untapped record of atmospheric evolution on planetary bodies with tenuous or ephemeral atmospheres across the Solar System. Low-pressure wind tunnel experiments suggest that large sand ripples on Mars are drag ripples, not impact ripples. Windblown drag ripples constitute an untapped record of atmospheric evolution under tenuous atmospheres across the Solar System.

Arctic regions are disproportionately affected by atmospheric warming, with cascading effects on multiple surface processes. Atmospheric warming is destabilizing permafrost, which could weaken riverbanks and in turn increase the lateral mobility of their channels. Here, using timelapse analysis of satellite imagery, we show that the lateral migration of large Arctic sinuous rivers has decreased by about 20% over the last half-century, at a mean rate of 3.7‰ per year. Through a comparison with rivers in non-permafrost regions, we hypothesize that the observed migration slowdown is rooted in a series of indirect effects driven by atmospheric warming, such as bank shrubification and decline in overland flow and seepage discharge along channel banks, linked in turn to permafrost thaw. As lower migration rates directly impact the residence timescales of sediment and organic matter in floodplains, these surprising results may lead to important ramifications for watershed-scale carbon budgets and climate feedbacks.Climate warming affects permafrost regions, with strong impacts on the environment such as the greening of river plains. Here the authors use satellite data to show that these changes have stabilized large Arctic sinuous rivers by slowing their lateral migration by about 20% over the past half-century.

Root chicory (Cichorium intybus var. sativum) is a cash crop cultivated for inulin production in Western Europe. This plant can be exposed to severe water stress during the last 3 months of its 6-month growing period. The aim of this study was to quantify the effect of a progressive decline in water availability on plant growth, photosynthesis, and sugar metabolism and to determine its impact on inulin production. Water stress drastically decreased fresh and dry root weight, leaf number, total leaf area, and stomatal conductance. Stressed plants, however, increased their water-use efficiency and leaf soluble sugar concentration, decreased the shoot-to-root ratio and lowered their osmotic potential. Despite a decrease in photosynthetic pigments, the photosynthesis light phase remained unaffected under water stress. Water stress increased sucrose phosphate synthase activity in the leaves but not in the roots. Water stress inhibited sucrose:sucrose 1-fructosyltransferase and fructan:fructan 1 fructosyltransferase after 19 weeks of culture and slightly increased fructan 1-exohydrolase activity. The root inulin concentration, expressed on a dry-weight basis, and the mean degree of polymerization of the inulin chain remained unaffected by water stress. Root chicory displayed resistance to water stress, but that resistance was obtained at the expense of growth, which in turn led to a significant decrease in inulin production.

Extensive dune fields nearly encircle the equatorial regions of Titan, Saturn's largest moon. Dunes evolve in response to environmental change, offering a record of recent geologic and climate history. A global analysis reveals that Titan's dunes become more narrowly spaced and increasingly more regular along a continuous eastward transport path, starting east of the Xanadu region, around the equator, and terminating abruptly at Xanadu's western margin. Xanadu is a rugged, tectonically active, water‐ice‐rich region with low topography and a thin layer of atmospherically deposited organic‐rich material. Our results demonstrate that windblown grains must withstand long transport distances. Furthermore, environmental conditions along the eastern margin of Xanadu set a template over which dunes evolve, only gradually modified as sediment supply or availability increases downwind. Together, these results highlight the oversized impact that Xanadu has on Titan's dune fields, which in turn play a critical role in regulating Titan's sedimentary and carbon cycles. Plain Language Summary Saturn's moon, Titan, is a sedimentary world like Earth and Mars. Observations from orbit reveal a geologically active surface with extensive fields of sand dunes. These dunes nearly encircle the equatorial regions, only interrupted by a mysterious terrain, Xanadu. Because dunes evolve in response to atmospheric and surface conditions, they offer a window into Titan's recent geologic and climate history. We conduct a global analysis of the patterns formed by dune crestlines. We find that dune spacing decreases continuously and their patterns become better organized along a pathway beginning at Xanadu's eastern margin, around the equator, terminating abruptly at Xanadu's western margin. These findings demonstrate that Titan's dune fields constitute a linked sedimentary system and that sand grains, previously hypothesized to be mechanically weak, must in fact withstand long travel distances. Key Points Titan's dune fields constitute a continuous sediment transport pathway rather than disconnected sediment sinks Sand grains must withstand long transport distances, even though theory and experimentation predict mechanically weak grains Xanadu exerts a primary control on the evolution of Titan's dunes as a source and sink of sediment and a physical barrier to transport

Observations suggest that the tropical Pacific Ocean has lost oxygen since the 1960s leading to the expansion of its oxygen minimum zone (OMZ). Attribution to anthropogenic forcing is, however, difficult because of limited data availability and the large natural variability introduced by the Pacific Decadal Oscillation (PDO). Here, we evaluate the PDO influence on oxygen dynamics and OMZ extent using observations and hindcast simulations from two global ocean circulation models (NEMO‐PISCES, MOM6‐COBALT). In both models, the tropical Pacific oxygen content decreases by about 30 Tmol.decade−1 and the OMZ volume expands by 1.3 × 105 km3.decade−1 during PDO positive phases, while variations of similar magnitude but opposite sign are simulated during negative phases. Changes in equatorial advective oxygen supply, partially offset by biological demand, control the oxygen response to PDO. Observations which cover 39% of the tropical Pacific volume only partially capture spatio‐temporal variability, hindering the separation of anthropogenic trend from natural variations. Plain Language Summary Human activities cause oxygen loss in the ocean, which leads to the expansion of areas with very low oxygen concentrations located in the tropics called oxygen minimum zones (OMZ). Understanding the dynamics of OMZs is crucial because they produce greenhouse gasses and are unsuitable for the life of most large marine organisms. Quantifying the response of OMZs is however complicated by natural variability that superimposes on human‐induced changes. In the Pacific Ocean, one of the strongest natural variability phenomena is the Pacific Decadal Oscillation. We used data and numerical models to assess the magnitude of oxygen changes caused by this natural phenomena in the tropical Pacific Ocean, and show that they are comparable to that of human‐induced oxygen changes. We highlight that more oxygen data is needed to accurately separate natural variations from human‐induced changes, and that a fraction of the oxygen loss attributed to human activities in prior work could in fact be due to natural variability. Key Points Pacific Decadal Oscillation (PDO) modulates tropical Pacific oxygen content and oxygen minimum zone volume on decadal time scales The PDO‐induced variations are of the same order of magnitude as the anthropogenic deoxygenation signal Currently available data are too sparse to resolve and isolate the PDO‐induced and anthropogenic signals

Human activities, such as research, innovation and industry, concentrate disproportionately in large cities. The ten most innovative cities in the United States account for 23% of the national population, but for 48% of its patents and 33% of its gross domestic product. But why has human activity become increasingly concentrated? Here we use data on scientific papers, patents, employment and gross domestic product, for 353 metropolitan areas in the United States, to show that the spatial concentration of productive activities increases with their complexity. Complex economic activities, such as biotechnology, neurobiology and semiconductors, concentrate disproportionately in a few large cities compared to less--complex activities, such as apparel or paper manufacturing. We use multiple proxies to measure the complexity of activities, finding that complexity explains from 40% to 80% of the variance in urban concentration of occupations, industries, scientific fields and technologies. Using historical patent data, we show that the spatial concentration of cutting-edge technologies has increased since 1850, suggesting a reinforcing cycle between the increase in the complexity of activities and urbanization. These findings suggest that the growth of spatial inequality may be connected to the increasing complexity of the economy. Balland et al. use data on scientific papers, patents, employment and GDP for 353 metropolitan areas in the United States to show that economic complexity drives the spatial concentration of productive activities in large cities.