A key aspect in interaction between upwelling mantle plumes and spreading mid-ocean ridges is the along-axis dispersion of the plume, reflecting how far the plume flows along the spreading ridge axis. Observational studies measure the dispersion distance based on the width of geophysical or geochemical anomalies, while theoretical models often define it as the distance reached by plume flow through material advection or thermal diffusion. However, variability in these measurements and the underlying causes remain unresolved. To fill this gap, we explore the dynamics of plume-ridge interactions using three-dimensional non-Newtonian geodynamic models that simulate both material and thermal flow. Unlike previous studies that suggest a steady and uniform ascent, our results show a two-stage plume upwelling process: an initially accelerated ascent from the deep mantle to the mantle dehydration zone, followed by deceleration with lateral dispersion across and along the ridge axis. During the dispersion stage, plume flux and plume-ridge separation distance significantly influence both along-axis dispersion distance and thermal topography of the plume, while ridge spreading rate primarily affects the former. Observations and models consistently show that plume thermal diffusion extends farther along the ridge axis than material advection, with the thermal dispersion distance being approximately 1.55 times greater. We further propose two practical geodynamic indicators—a 0.7 plume material isoconcentration and an isotherm at 0.1 times the excess plume temperature—that can serve as references for estimating plume properties during along-axis dispersion in future studies of more complex plume-ridge systems.

Marine carbon dioxide removal (mCDR) and geological carbon storage in the marine environment (mCS) promise to contribute to the mitigation of global climate change in combination with drastic emission reductions. However, the implementable potential of mCDR and mCS depends, apart from technology readiness, also on site-specific conditions. In this paper, we explore different options for mCDR and mCS, using the German context as a case study. We challenge each option to remove 10 Mt CO2 yr-1, which accounts for 8-22% of projected hard-to-abate and residual emissions of Germany in 2045. We focus on the environmental, resource, and infrastructure requirements of individual mCDR and mCS options at a specific site, within the German jurisdiction when possible. Furthermore, we discuss main uncertainty factors and research needs, and, where possible, cost estimates, expected environmental effects, and monitoring approaches. In total, we describe ten mCDR and mCS options; four aim at enhancing the chemical carbon uptake of the ocean through alkalinity enhancement, four aim at enhancing blue carbon ecosystems’ sink capacity, and two employ geological off-shore storage. Our results indicate that five out of ten options would potentially be implementable within German jurisdiction, and three of them could potentially rise to the challenge. This exercise provides a basis for further studies to assess the socio-economic, ethical, political, and legal aspects for such implementations.

Abrupt fluid emissions from shallow marine sediments pose a threat to seafloor installations like wind farms and offshore cables. Quantifying such fluid emissions and linking pockmarks, the seafloor manifestations of fluid escape, to flow in the sub-seafloor remains notoriously difficult due to an incomplete understanding of the underlying physical processes. Here, using a compositional multi-phase flow model, we test plausible gas sources for pockmarks in the south-eastern North Sea, which recent observations suggest have formed in response to major storms. We find that the presence of free gas in the subsurface effectively damps storm wave-induced pressure changes due to its high compressibility, so that the mobilization of pre-existing gas pockets is unlikely. Rather, our results point to spontaneous appearance of a free gas phase via storm-induced gas exsolution from pore fluids. This mechanism is primarily driven by the pressure-sensitivity of gas solubility. We show that in highly permeable sediments containing gas-rich pore fluids, wave-induced pressure changes result in the appearance of a persistent gas phase. This suggests that seafloor fluid escape structures are not always proxies for overpressured shallow gas and that periodic seafloor pressure changes can induce persistent free gas phase to spontaneously appear.