Abstract. Radium-228 (228Ra), an almost conservative trace isotope in the ocean, supplied from the continental shelves and removed by a known radioactive decay (T1∕2 = 5. 75 years), can be used as a proxy to constrain shelf fluxes of other trace elements, such as nutrients, iron, or rare earth elements. In this study, we perform inverse modeling of a global 228Ra dataset (including GEOSECS, TTO and GEOTRACES programs, and, for the first time, data from the Arctic and around the Kerguelen Islands) to compute the total 228Ra fluxes toward the ocean, using the ocean circulation obtained from the NEMO 3.6 model with a 2° resolution. We optimized the inverse calculation (source regions, cost function) and find a global estimate of the 228Ra fluxes of 8.01–8. 49 × 1023 atoms yr−1, more precise and around 20 % lower than previous estimates. The largest fluxes are in the western North Atlantic, the western Pacific and the Indian Ocean, with roughly two-thirds in the Indo-Pacific Basin. An estimate in the Arctic Ocean is provided for the first time (0.43–0.50  ×  1023 atoms yr−1). Local misfits between model and data in the Arctic, the Gulf Stream and the Kuroshio regions could result from flaws of the ocean circulation in these regions (resolution, atmospheric forcing). As radium is enriched in groundwater, a large part of the 228Ra shelf sources comes from submarine groundwater discharge (SGD), a major but poorly known pathway for terrestrial mineral elements, including nutrients, to the ocean. In contrast to the 228Ra budget, the global estimate of SGD is rather unconstrained, between 1.3 and 14. 7 × 1013 m3 yr−1, due to high uncertainties on the other sources of 228Ra, especially diffusion from continental shelf sediments. Better precision on SGD cannot be reached by inverse modeling until a proper way to separate the contributions of SGD and diffusive release from sediments at a global scale is found.

The fossil record of marine invertebrates has long fuelled the debate as to whether or not there are limits to global diversity in the sea1–5. Ecological theory states that, as diversity grows and ecological niches are filled, the strengthening of biological interactions imposes limits on diversity6,7. However, the extent to which biological interactions have constrained the growth of diversity over evolutionary time remains an open question1–5,8–11. Here we present a regional diversification model that reproduces the main Phanerozoic eon trends in the global diversity of marine invertebrates after imposing mass extinctions. We find that the dynamics of global diversity are best described by a diversification model that operates widely within the exponential growth regime of a logistic function. A spatially resolved analysis of the ratio of diversity to carrying capacity reveals that less than 2% of the global flooded continental area throughout the Phanerozoic exhibits diversity levels approaching ecological saturation. We attribute the overall increase in global diversity during the Late Mesozoic and Cenozoic eras to the development of diversity hotspots under prolonged conditions of Earth system stability and maximum continental fragmentation. We call this the ‘diversity hotspots hypothesis’, which we propose as a non-mutually exclusive alternative to the hypothesis that the Mesozoic marine revolution led this macroevolutionary trend12,13.

Understanding the phenology of phytoplankton species is a challenge and despite a lot of theoretical work on competition for resources, this process is under-represented in deterministic models. To study the main driver of the species selection, we used a trait-based model that keeps phenotypic variability through physiological trait parameterization. Next, we validated the results by using the toxic dinoflagellate Alexandrium minutum which is a toxic species. Due to their monitoring, we show that harmful algae are ideal models for studying ecological niches and for contributing to this more global challenge. As a first step, a dimensionless model of an estuary (France) was built with water temperature and water exchanges deduced from a hydro-dynamic model. The biological parametrization takes into account the size (from pico-to microphytoplankton) and the type of assimilation. The results show that temperature, competition for nutrients and dilution are important factors regulating the community structure and Alexandrium minutum dynamics (more especially the bloom initiation and magnitude). These drivers contribute to the determination of the ecological niche of A. minutum, influence the shape of its blooms and provide potential explanations of its interannual variability. This approach makes the community structure more flexible in order to study how environmental forcings could drive its evolution.