Core formation and magma ocean outgassing set planetary N-S-C ratios

Earth’s volatile elements cannot be accounted for as mixtures of different chondrites, despite their clear chondritic heritage. Early-acting, but as yet unidentified, processes apparently fractionated volatile elements now contained by planets. Here we test the hypothesis that planetary-scale differentiation, namely core formation and primordial atmosphere degassing, set Earth’s distribution of N, S, and C. To this end, we report new metal-silicate partitioning experiments on N up to 26 GPa and 3400 K; the highest pressure and temperatures conditions yet explored. Our results highlight a strong, positive effect of pressure on nitrogen partitioning into cores. We apply our new experiments with literature data for S and C partitioning to a model that couples core formation with degassing into the primordial atmosphere, to demonstrate that volatile elements ratios for Earth, and potentially Mars and Venus, can be set by primordial differentiation under conditions that also satisfy their moderately siderophile element budgets.

Element budgets in an Arctic mesocosm CO₂ perturbation study

Recent studies on the impacts of ocean acidification on pelagic communities have identified changes in carbon to nutrient dynamics with related shifts in elemental stoichiometry. In principle, mesocosm experiments provide the opportunity of determining the temporal dynamics of all relevant carbon and nutrient pools and, thus, calculating elemental budgets. In practice, attempts to budget mesocosm enclosures are often hampered by uncertainties in some of the measured pools and fluxes, in particular due to uncertainties in constraining air/sea gas exchange, particle sinking, and wall growth. In an Arctic mesocosm study on ocean acidification using KOSMOS (Kiel Off-Shore Mesocosms for future Ocean Simulation) all relevant element pools and fluxes of carbon, nitrogen and phosphorus were measured, using an improved experimental design intended to narrow down some of the mentioned uncertainties. Water column concentrations of particulate and dissolved organic and inorganic constituents were determined daily. New approaches for quantitative estimates of material sinking to the bottom of the mesocosms and gas exchange in 48 h temporal resolution, as well as estimates of wall growth were developed to close the gaps in element budgets. Future elevated pCO2 was found to enhance net autotrophic community carbon uptake in 2 of the 3 experimental phases but did not significantly affect particle elemental composition. Enhanced carbon consumption appears to result in accumulation of dissolved organic compounds under nutrient recycling summer conditions. This carbon over-consumption effect becomes evident from budget calculations, but was too small to be resolved by direct measurements of dissolved organics. The out-competing of large diatoms by comparatively small algae in nutrient uptake caused reduced production rates under future ocean CO2 conditions in the end of the experiment. This CO2 induced shift away from diatoms towards smaller phytoplankton and enhanced cycling of dissolved organics was pushing the system towards a retention type food chain with overall negative effects on export potential.