Ocean Alkalinity Enhancement (OAE) simultaneously mitigates atmospheric concentrations of CO2 and ocean acidification; however, no previous studies have investigated the response of the non-linear marine carbonate system sensitivity to alkalinity enhancement on regional scales. We hypothesise that regional implementations of OAE can sequester more atmospheric CO2 than a global implementation. To address this, we investigate physical regimes and alkalinity sensitivity as drivers of the carbon-uptake potential response to global and different regional simulations of OAE. In this idealised ocean-only set-up, total alkalinity is enhanced at a rate of 0.25 Pmol a-1 in 75-year simulations using the Max Planck Institute Ocean Model coupled to the HAMburg Ocean Carbon Cycle model with pre-industrial atmospheric forcing. Alkalinity is enhanced globally and in eight regions: the Subpolar and Subtropical Atlantic and Pacific gyres, the Indian Ocean and the Southern Ocean. This study reveals that regional alkalinity enhancement has the capacity to exceed carbon uptake by global OAE. We find that 82–175 Pg more carbon is sequestered into the ocean when alkalinity is enhanced regionally and 156 PgC when enhanced globally, compared with the background-state. The Southern Ocean application is most efficient, sequestering 12% more carbon than the Global experiment despite OAE being applied across a surface area 40 times smaller. For the first time, we find that different carbon-uptake potentials are driven by the surface pattern of total alkalinity redistributed by physical regimes across areas of different carbon-uptake efficiencies. We also show that, while the marine carbonate system becomes less sensitive to alkalinity enhancement in all experiments globally, regional responses to enhanced alkalinity vary depending upon the background concentrations of dissolved inorganic carbon and total alkalinity. Furthermore, the Subpolar North Atlantic displays a previously unexpected alkalinity sensitivity increase in response to high total alkalinity concentrations.

<p class="western" align="justify">Nitrogen (N) plays a central role in marine biogeochemistry by regulating biological productivity, influencing the cycles of carbon, oxygen, and other nutrients, and controlling oceanic emissions of the potent greenhouse gas nitrous oxide (N<sub>2</sub>O). Although the marine N cycle consists of multiple chemical species, global biogeochemical models often employ simple parameterizations of N transformations and omit key tracers and processes. Here we present an extended numerical representation of the marine N cycle that includes explicit tracers for nitrate, dinitrogen, nitrous oxide, ammonium, and nitrite. The extended model simulates heterotrophic denitrification, DNRN, DNRA, anammox, and nitrification of ammonium and nitrite and thus allows for a detailed representation of a step-wise reduction of fixed nitrogen in hypoxic zones and oxidation of reduced N-species in oxic waters. The updated biogeochemical model is included in the new global ICOsahedral Non-hydrostatic Ocean model ICON-O developed at the Max Planck Institute for Meteorology. ICON-O features a flexible, triangular grid created by recursively subdividing the original 20 triangles of the icosahedron resulting, in our configuration, in an average resolution of 40 km and 235,403 triangles. We describe the tuning and spin-up of a pre-industrial control simulation and compare model results with global and local estimates to quantify the N cycle (e.g., rates of primary production, N<sub>2</sub> fixation, nitrification, DNRN, DNRA, anammox). We further report results from a historical transient simulation focusing on N dynamics within the oxygen minimum zone of the Eastern Tropical South Pacific.</p>

<p>The largest mass extinction on Earth with an estimated 90% loss of species occurred at the Permian-Triassic Boundary (~252 Ma). The end-Permian mass extinction coincides with extreme temperature increases and changes in ocean circulation and biogeochemistry. These climate perturbations are associated with carbon emissions linked to Siberian Trap volcanism. Fully-coupled Earth System Models can be applied to investigate the feedbacks and sensitivities of the background latest Permian climate to such carbon emissions. Past studies have focussed on constraining the magnitude of these carbon emissions without examining the sensitivity of palaeo-configured Earth System models designed for modern simulations. We modified a version of the Max Planck Earth System Model v1.2, similar to that used in the 6<sup>th</sup>-phase of the Coupled Model Intercomparison Project, to simulate the latest Permian climate-carbon system and use geochemical and palaeobiological proxy data to constrain the boundary conditions of the modelled climate state.<br />We first characterise the latest Permian climate state before presenting first results on a sensitivity study of the latest Permian climate-carbon state to CO<sub>2</sub> emission pulses. A 100 year global mean 2 m surface air temperature of 17.5°C is simulated, rising up to 34.7°C in the low-latitude continental interior. The continental interior is also largely arid from ~50°N to ~50°S with a total precipitation maximum of 11.1 mm day<sup>-1</sup> at the equatorial boundary of the Tethys and Panthalassic Oceans. The prevailing hydrological regime drives woody single-stemmed evergreens and soft-stemmed plant functional groups to dominate in the dynamic vegetation model. The 100 year global mean surface ocean of the latest Permian illustrates a warm-pool across the equatorial boundary between the Tethys and Panthalassic Oceans with a maximum temperature of 30.2°C decreasing to temperatures as low as -1.9°C near the poles. Surface salinities vary broadly across the global oceans with 100 year global mean values ranging from 22.9, in well-flushed regions of strong freshwater flux, to 48.6, in low-latitude regions of restricted exchange. Large-scale seasonal mixing below 60°S in the Panthalassic Ocean dominates the global meridional overturning circulation. These model data fit within the bounds represented by the available proxy data for the Late Permian. The widespread shallow ocean mixed-layer also restricts recirculation of nutrients, driving a high gross primary production with weak seasonality. Furthermore, regions of seasonal deep mixing correlate with seasonal <em>p</em>CO<sub>2</sub> patterns at high latitudes. I will also present further analyses of the simulated ocean biogeochemical cycles in the Hamburg Ocean Carbon Cycle model with a focus on the novel extended Nitrogen-cycle processes.</p>