AGU Advances最新文献

The Agulhas Current, like other western boundary currents (WBCs), transports nutrients laterally from the tropics to the subtropics in a subsurface “nutrient stream.” These nutrients are predominantly supplied to surface waters by seasonal convective mixing, to fuel a brief period of productivity before phytoplankton become nutrient-limited. Episodic mixing events characteristic of WBC systems can temporarily alleviate nutrient scarcity by vertically entraining deep nutrients into surface waters. However, our understanding of these nutrient fluxes is lacking because they are spatio-temporally limited, and once they enter the sunlit layer, the nutrients are rapidly consumed by phytoplankton. Here, we use a novel application of nitrate Δ(15–18), the difference between the nitrogen and oxygen isotope ratios of nitrate, to characterize three (sub)mesoscale events of upward nitrate supply across the Agulhas Current in winter: (1) mixing at the edges of an anticyclonic eddy, (2) inshore upwelling associated with a submesoscale meander of the Agulhas Current, and (3) overturning at the edge of the current core driven by submesoscale instabilities. All three events manifest as upward injections of high-Δ(15–18) nitrate into the thermocline and surface where nitrate Δ(15–18) is otherwise low; these entrainment events are not always apparent in the other co-collected data. The dynamics driving the nitrate supply events are common to all WBCs, implying that nutrient entrainment facilitated by WBCs is quantitatively significant and supports productivity in otherwise oligotrophic subtropical surface waters. A future rise in energy across WBC systems may increase these nutrient fluxes, partly offsetting the predicted stratification-induced decrease in subtropical ocean fertility.

Aqueous S[IV] species ($��{�H�S�O�}_3^{-}�$, $��{�S�O�}_3^{�2�-�}�$) derived from volcanogenic atmospheric SO₂ are important to planetary habitability through their roles in proposed origins-of-life chemistry and influence on atmospheric sulfur haze formation, but the early cycling of S[IV] is poorly understood. Here, we combine new laboratory constraints on S[IV] disproportionation kinetics with a novel aqueous photochemistry model to estimate the concentrations of S[IV] in natural waters on prebiotic Earth. We show that S[IV] disproportionation is slow in pH ≥ 7 waters, with timescale T ≥ 1 year at room temperature, meaning that S[IV] was present in prebiotic natural waters. However, we also show that photolysis of S[IV] by UV light on prebiotic Earth limited [S[IV]] < 100 µM in global-mean steady-state. Because of photolysis, [S[IV]] was much lower in natural waters compared to the concentrations generally invoked in laboratory simulations of origins-of-life chemistry (≥10 mM), meaning further work is needed to confirm whether laboratory S[IV]-dependent prebiotic chemistries could have functioned in nature. [S[IV]] ≥ 1 µM in terrestrial waters for: (a) SO₂ outgassing ≥20× modern, (b) pond depths <10 cm, or (c) UV-attenuating agents present in early waters or the prebiotic atmosphere. Marine S[IV] was sub-saturated with respect to atmospheric SO₂, meaning that atmospheric SO₂ deposition was efficient and that, within the constraints of present knowledge, UV-attenuating sulfur hazes could only have persisted on prebiotic Earth if sulfur emission rates were very high (≳100× modern). Our work illustrates the synergy between planetary science, geochemistry and synthetic organic chemistry toward understanding the emergence and maintenance of life on early Earth.

The circulation in Europa's ocean determines the degree of thermal, mechanical and chemical coupling between the ice shell and the silicate mantle. Using global direct numerical simulations, we investigate the effect of heterogeneous tidal heating in the silicate mantle on rotating thermal convection in the ocean and its consequences on ice shell thickness. Under the assumption of no salinity or ocean-ice shell feedbacks, we show that convection largely transposes the latitudinal variations of tidal heating from the seafloor to the ice, leading to a higher oceanic heat flux in polar regions. Longitudinal variations are efficiently transferred when boundary-driven thermal winds develop, but are reduced in the presence of strong zonal flows and may vanish in planetary regimes. If spatially homogeneous radiogenic heating is dominant in the silicate mantle, the ocean's contribution to ice shell thickness variations is negligible compared to tidal heating within the ice. If tidal heating is instead dominant in the mantle, the situation is reversed and the ocean controls the pole-to-equator thickness contrast, as well as possible longitudinal variations.

Mid-lithosphere discontinuities are seismic interfaces likely located within the lithospheric mantle of stable cratons, which typically represent velocities decreasing with depth. The origins of these interfaces are poorly understood due to the difficulties in both characterizing them seismically and reconciling the observations with thermal-chemical models of cratons. Metasomatism of the cratonic lithosphere has been reported by numerous geochemical and petrological studies worldwide, yet its seismic signature remains elusive. Here, we identify two distinct mid-lithosphere discontinuities at ∼87 and ∼117 km depth beneath the eastern Wyoming craton and the southwestern Superior craton by analyzing seismic data recorded by two longstanding stations. Our waveform modeling shows that the shallow and deep interfaces represent isotropic velocity drops of 2%–8% and 4%–9%, respectively, depending on the contributions from changes in radial anisotropy and density. By building a thermal-chemical model including the regional xenolith thermobarometry constraints and the experimental phase-equilibrium data of mantle metasomatism, we show that the shallow interface probably represents the metasomatic front, below which hydrous minerals such as amphibole and phlogopite are present, whereas the deep interface may be caused by the onset of carbonated partial melting. The hydrous minerals and melts are products of mantle metasomatism, with CO₂-H₂O-rich siliceous melt as a probable metasomatic reagent. Our results suggest that mantle metasomatism is probably an important cause of mid-lithosphere discontinuities worldwide, especially near craton boundaries, where the mantle lithosphere may be intensely metasomatized by fluids and melts released by subducting slabs.

The maximum supersaturation (S_x) in clouds is a key parameter affecting the cloud's microphysical and radiative properties. We investigate the S_x of the marine boundary layer clouds by combining airborne and surface observations in the Eastern North Atlantic. The cloud droplet number concentration (N_c) in the least diluted cloud cores agrees well with the number concentration of particles larger than the Hoppel Minimum (HM) (N_>HM) below clouds, indicating that the HM represents the average size threshold above which particles are activated to form cloud droplets. The S_x values derived from surface observations vary from 0.10% to 0.50% from June 2017 to June 2018, with a clear seasonal variation exhibiting higher values during winter. Most of the S_x variance (∼60%) can be explained by the cloud condensation nuclei (CCN) concentration and updraft velocity (w), with the CCN concentration playing a more important role than w in explaining the variation of S_x. The influence of CCN concentration on S_x leads to a buffered response of N_c to aerosol perturbations. The response of N_c to low aerosol concentration during winter is further buffered by the high w. The global Community Earth System Model (CESM) simulated S_x values in the Azores have a positive bias compared to measured S_x, likely due to overestimated w and underestimated CCN concentration. The CESM simulated S_x exhibits higher values further north over the North Atlantic Ocean, which is attributed to stronger w. The suppression of S_x by aerosol is also evident in regions with high CCN concentrations.

Projections of future carbon sinks and stocks are important because they show how the world's ecosystems will respond to elevated CO₂ and changes in climate. Moreover, they are crucial to inform policy decisions around emissions reductions to stay within the global warming levels identified by the Paris Agreement. However, Earth System Models from the 6th Coupled Model Intercomparison Project (CMIP6) show substantial spread in future projections—especially of the terrestrial carbon cycle, leading to a large uncertainty in our knowledge of any remaining carbon budget (RCB). Here we evaluate the global terrestrial carbon cycle projections on a region-by-region basis and compare the global models with regional assessments made by the REgional Carbon Cycle Assessment and Processes, Phase 2 activity. Results show that for each region, the CMIP6 multi-model mean is generally consistent with the regional assessment, but substantial cross-model spread exists. Nonetheless, all models perform well in some regions and no region is without some well performing models. This gives confidence that the CMIP6 models can be used to look at future changes in carbon stocks on a regional basis with appropriate model assessment and benchmarking. We find that most regions of the world remain cumulative net sources of CO₂ between now and 2100 when considering the balance of fossil-fuels and natural sinks, even under aggressive mitigation scenarios. This paper identifies strengths and weaknesses for each model in terms of its performance over a particular region including how process representation might impact those results and sets the agenda for applying stricter constraints at regional scales to reduce the uncertainty in global projections.

The interdependent crises of climate change and biodiversity losses require strategic policies to protect, manage, and restore essential ecosystems. Here, we evaluate the relative importance of US national forests (NFs) for protection and conservation as natural climate and biodiversity solutions. We compared landscape integrity (degree of modification by humans), habitat for three keystone species, forest carbon density, accumulation, and total biomass carbon stocks across 154 NFs in the United States. Southern Alaska's Tongass and Chugach NFs hold disproportionally large amounts of high landscape integrity area among all NFs with 25.3% and 5.6% (total 30.9%) of all high (≥9.6) landscape integrity found on NF lands. The Tongass and Chugach store approximately 33% and 3% of all biomass carbon stocks that occur in NFs with high landscape integrity. These two NFs together account for about 49%, 37%, and 18% of all bald eagle, brown bear, and gray wolf habitat found on NF lands. Gray wolf habitat extent was 4% of the total or less on remaining NFs. The Tongass and Chugach were historically wetter and cooler among NFs, and are projected to experience much larger increases in precipitation and much lower increases in maximum temperatures over the coming century. Combined with relatively low recent occurrence of wildfire, this makes permanence more likely. The Tongass and Chugach forests, along with the Pacific Northwest's high carbon density forests should be a high priority for protection and conservation to meet climate and biodiversity goals given their landscape-scale scarcity and high value.

As the planet approaches local and global exceedance of the 1.5°C stabilization target, damages from climate change, mostly due to extremes, are growing far faster than projected. While assessment models have largely estimated high costs of mitigation, the cost of green energy is dropping faster than projected. Climate policy has assumed that damage costs are manageable while decarbonization is expensive. Both these assumptions are wrong, potentially leading to a tipping point in human behavior: scientists need to explore options aligned with this emerging reality.

Jupiter's atmosphere comprises several dynamical regimes: the equatorial eastward flows and surrounding retrograde jets; the midlatitudes, with the eddy-driven, alternating jet-streams and meridional circulation cells; and the jet-free turbulent polar region. Despite intensive research conducted on each of these dynamical regimes over the past decades, they remain only partially understood. Saturn's atmosphere also encompasses similar distinguishable regimes, but observational evidence for midlatitude deep meridional cells is lacking. Models offer a variety of explanations for each of these regions, but only a few are capable of simulating more than one of the regimes at once. This study presents new numerical simulations using a 3D deep anelastic model that can reproduce the equatorial flows as well as the midlatitudinal pattern of the mostly barotropic, alternating eddy-driven jets and the meridional circulation cells accompanying them. These simulations are consistent with recent Juno mission gravity and microwave data. We find that the vertical eddy momentum fluxes are as important as the meridional eddy momentum fluxes, which drive the midlatitudinal circulation on Earth. In addition, we discuss the parameters controlling the number of midlatitudinal jets/cells, their extent, strength, and location. We identify the strong relationship between meridional circulation and the zonal jets in a deep convection setup, and analyze the mechanism responsible for their generation and maintenance. The analysis presented here provides another step in the ongoing pursuit of understanding the deep atmospheres of gas giants.