The North Atlantic Oscillation (NAO) is the principal mode of atmospheric variability over the North Atlantic, modulating the weather and climate of neighboring regions in both winter and summer. While Earth System Models generally project a more positive NAO under 21st century high-emission scenarios, uncertainties persist as to the precise response of the NAO to increased CO₂ levels, owing to large internal variability. In this study we investigate the response of the NAO to a wide range of CO₂ forcings, from two to eight times the preindustrial values. Analyzing a large sample of present-generation climate models, we find that the NAO likely becomes more positive with increasing CO₂ concentrations. Moreover, we find a reduction in NAO variability. This leads to a smaller increase in the likelihood of extremely positive NAO events than would be expected based solely on the shift in the mean. On the other hand, we also find a reduction in extremely negative NAO events, which is attributable to both the shift toward more positive values and the decrease in variance. Finally, our analysis reveals that the distribution of the NAO response at high CO₂ forcing is negatively skewed. This fact partially offsets the decrease in extremely positive NAO events associated with reduced variability. Ultimately, our results suggest a greater increase in positive NAO events compared to the decrease in extremely negative NAO events at higher CO₂ forcing.

Climate change reduces ocean oxygen levels, posing a serious threat to marine ecosystems and their benefits to society. State-of-the-art Earth System Models (ESMs) project an intensification of global oxygen loss in the future, but poorly constrain its patterns and magnitude, with contradictory oxygen gain or loss projected in tropical oceans. We introduce an oxygen water mass framework—grouping waters with similar oxygen concentrations from lowest to highest levels—and separate oxygen changes into two components: the transformation of oxygen in water masses by biological, chemical, or physical processes along their pathways in “ventilation-space,” and the redistribution of these water masses in “geographic-space.” The redistribution of water masses explains the large projection uncertainties in the tropics. ESMs with more realistic representations of water masses provide tighter constraints on future redistribution than less skilled ESMs, leading to over a third more of tropical area exhibiting consistent oxygen projections (58% vs. 22%), and a 30% reduction in model spread for tropical oxygen projections. These higher-skilled ESMs also project weaker global deoxygenation than less skilled models (median of −2.9 vs. −4.2 PmolO2 per °C of surface warming) controlled by an increase in global water residence times, and they project a stronger increase in oxygen minimum zone ventilation by ocean mixing. These tighter constraints on future oxygen changes are critical to anticipate and mitigate impacts for ecosystems and inform management and conservation strategies of marine resources.

The northern Indian Ocean is a hotspot of nitrous oxide (N2O) emission to the atmosphere. Yet, the direct link between production and emission of N2O in this region is still poorly constrained, in particular the relative contributions of denitrification, nitrification and ocean transport to the N2O efflux. Here, we implemented a mechanistically based N2O cycling module into a regional ocean model of the Indian Ocean to examine how the biological production and transport of N2O control the spatial variation of N2O emissions in the basin. The model captures the upper ocean physical and biogeochemical dynamics of the northern Indian Ocean, including vertical and horizontal N2O distribution observed in situ and regionally integrated N2O emissions of 286 ± 152 Gg N yr−1 (annual mean ± seasonal range) in the lower range of the observation-based reconstruction (391 ± 237 Gg N yr−1). N2O emissions are primarily fueled by nitrification in or right below the surface mixed layer (∼57%, including 26% in the mixed layer and 31% right below), followed by denitrification in the oxygen minimum zones (∼30%) and N2O produced elsewhere and transported into the region (∼13%). Overall, ∼74% of the emitted N2O is produced in subsurface and transported to the surface in regions of coastal upwelling, winter convection or turbulent mixing. This spatial decoupling between N2O production and emissions underscores the need to consider not only changes in environmental factors critical to N2O production (oxygen, primary productivity etc.) but also shifts in ocean circulation that control emissions when evaluating future changes in global oceanic N2O emissions. emissions.

Zooplankton diel vertical migration (DVM) is critical to ocean ecosystem dynamics and biogeochemical cycles, by supplying food and injecting carbon into the mesopelagic ocean (200–800 m). The deeper the zooplankton migrate, the longer the carbon is sequestered away from the atmosphere and the deeper the ecosystems they feed. Sparse observations show variations in migration depths over a wide range of temporal and spatial scales. A major challenge, however, is to understand the biological and physical mechanisms controlling this variability, which is critical for assessing impacts on ecosystem and carbon dynamics. Here, we introduce a migrating zooplankton model for medium and large zooplankton that explicitly resolves diel migration trajectories and biogeochemical fluxes. This model is integrated into the MOM6-COBALTv2 ocean physical–biogeochemical model and is applied in an idealized high-resolution (9.4 km) configuration of the North Atlantic. The model skillfully reproduces observed North Atlantic migrating zooplankton biomass and DVM patterns. Evaluation of the mechanisms controlling zooplankton migration depth reveals that chlorophyll shading decreases zooplankton migration depths by 60 m in the subpolar gyre compared with the subtropical gyre, with pronounced seasonal variations linked to the spring bloom. Fine-scale spatial effects (<100 km) linked to eddy and frontal dynamics can either offset or reinforce the large-scale effect by up to 100 m. This could imply that, for phytoplankton-rich regions and filaments, which represent a major source of exportable carbon for migrating zooplankton, a high chlorophyll content contributes to reducing zooplankton migration depth and carbon sequestration time.

Nitrous oxide (N₂O) is a potent greenhouse gas and the main stratospheric ozone-depleting agent, yet its sources are not well resolved. In this work, we experimentally show a N₂O production pathway not previously considered in greenhouse gas budgets, which we name photochemodenitrification. Sunlight induces substantial and consistent N₂O production under oxic abiotic conditions in fresh and marine waters. We measured photochemical N₂O production rates using isotope tracers and determined that nitrite is the main substrate and that nitrate can also contribute after being photoreduced to nitrite. Additionally, this N₂O production was strongly correlated to the radiation dose. Photochemodenitrification exceeded biological N₂O production in surface waters. Although previously overlooked, this process may contribute considerably to global N₂O emissions through its occurrence in fresh and marine surface waters.

We present our third and final generation joint P and S global adjoint tomography (GLAD) model, GLAD-M35, and quantify its uncertainty based on a low-rank approximation of the inverse Hessian. Starting from our second-generation model, GLAD-M25, we added 680 new earthquakes to the database for a total of 2160 events. New P-wave categories are included to compensate for the imbalance between P- and S-wave measurements, and we enhanced the window selection algorithm to include more major-arc phases, providing better constraints on the structure of the deep mantle and more than doubling the number of measurement windows to 40 million. Two stages of a Broyden–Fletcher–Goldfarb–Shanno (BFGS) quasi-Newton inversion were performed, each comprising five iterations. With this BFGS update history, we determine the model’s standard deviation and resolution length through randomized singular value decomposition.

We present a suite of major element stable isotope (δ13C, δ18O, δ44/40Ca, δ26Mg), and selected trace element (Sr/Ca and Mg/Ca) data from Pleistocene sediments from the Great Barrier Reef (IODP Expedition 325), as well as Holocene surface sediments from the Bahamas (Triple Goose Creek, Andros Island) to identify geochemical fingerprints associated with early marine and meteoric diagenesis. Sediments from both sites exhibit co-variation in δ13C and δ18O values, depletion in trace elements, and distinct geochemical trends in δ26Mg and δ44/40Ca values that reflect differences between diagenetic alteration in marine and meteoric fluids. While marine diagenesis results in lower Sr/Ca ratios, higher δ44/40Ca values, and little effect on bulk sediment δ26Mg values, meteoric diagenesis leads to lower Sr/Ca ratios, lower δ44/40Ca values, and lower δ26Mg values. Using a numerical model of diagenesis, we show how diagenetic alteration by meteoric fluids must occur after an initial period of diagenetic alteration by marine fluids, a two-stepped diagenetic history that complicates the interpretation of geochemical data in meteorically altered marine carbonate sediments. Finally, we discuss how paired metal isotopes may serve as a robust indicator of meteoric alteration in ancient shallow-water marine carbonate sediments.

Radiogenic heat production is fundamental to the energy budget of planets. Roughly half of the heat that Earth loses through its surface today comes from the three long-lived, heat-producing elements (potassium, thorium, and uranium). These three elements have long been believed to be highly lithophile and thus concentrate in the mantle of rocky planets. However, our study shows that they all become siderophile under the pressure and temperature conditions relevant to the core formation of large rocky planets dubbed super-Earths. Mantle convection in super-Earths is then primarily driven by heating from the core rather than by a mix of internal heating and cooling from above as in Earth. Partitioning these sources of radiogenic heat into the core remarkably increases the core-mantle boundary (CMB) temperature and the total heat flow across the CMB in super-Earths. Consequently, super-Earths are likely to host long-lived volcanism and strong magnetic dynamos. Entrainment of heat-producing elements in super-Earths’ cores produces intense, long-lasting volcanism and strong magnetic fields.

Crosstalk-free source-encoded elastic full-waveform inversion (FWI) using time-domain solvers demonstrates skill and efficiency at conducting seismic inversions involving multiple sources and receivers with limited computational resources. A drawback of common formulations of the procedure is that, by sweeping through the frequency domain randomly at a rate of one or a few sparsely sampled frequencies per shot, it is difficult to simultaneously incorporate time-selective data windows, as necessary for the targeting of arrivals or wave packets during the various stages of the inversion. Here, we solve this problem by using the Laplace transform of the data. Using complex-valued frequencies allows for damping the records with flexible decay rates and temporal offsets that target specific traveltimes. We present the theory of crosstalk-free source-encoded FWI in the Laplace domain, develop the details of its implementation, and illustrate the procedure with numerical examples relevant to exploration-scale scenarios. © 2024 Society of Exploration Geophysicists. All rights reserved.

We present a computational technique to model hydroacoustic waveforms from teleseismic earthquakes recorded by mid-column MERMAID floats deployed in the Pacific, taking into consideration bathymetric effects that modify seismo-Acoustic conversions at the ocean bottom and acoustic wave propagation in the ocean layer, including reverberations. Our approach couples axisymmetric spectral-element simulations performed for moment-Tensor earthquakes in a 1-D solid Earth to a 2-D Cartesian fluid solid coupled spectral-element simulation that captures the conversion from displacement to acoustic pressure at an ocean-bottom interface with accurate bathymetry. We applied our w orkflo w to 1129 seismograms for 682 earthquakes from 16 MERMAID s (short for Mobile Earthquake Recording in Marine Areas by Independent Divers) owned by Princeton University that were deployed in the Southern Pacific as part of the South Pacific Plume Imaging and Modeling (SPPIM) project. We compare the modelled synthetic waveforms to the observed records in indi viduall y selected frequency bands aimed at reducing local noise levels while maximizing earthquake-generated signal content. The modelled waveforms match the observations very well, with a median correlation coefficient of 0.72, and some as high as 0.95. We compare our correlation-based traveltime measurements to measurements made on the same data set determined by automated arri v al-Time picking and ra y-traced tra veltime predictions, with the aim of opening up the use of MERMAID records for global seismic tomography via full-waveform inversion.