Thierry Moutin & Guillaume Le Gland (M1S2)
Objective
This unit provides the fundamental understanding of the physical and biogeochemical processes controlling the cycles of biogenic elements (N, P, Si), their quantification and the establishment of elemental balances from regional to global scales. The cycles of the major elements are presented, as well as the couplings between elements and between the major domains (atmosphere, continent, ocean) in a comparative approach between past evolutions (latest glacial/interglacial transitions) and current climate change.
► Theoretical lectures : 40 h (2 teachears)
- The global oceanic nitrogen cycle: Different forms of reactive nitrogen and biogeochemical reactions, reservoirs and fluxes – ocean, continent, atmosphere-, anthropogenic perturbations, processes controlling the oceanic nitrogen cycle (deep nitrate reservoir and thermohaline circulation, biological control, the Dugdale & Goering model – new or regenerating production, exportable production, exported production), diazotrophy/nitrification/denitrification coupling on a global scale, coupling between the continental and oceanic cycles.
- The phosphorus cycle : Distribution and controlling factors of phosphate in marine environments (the role of phosphate in limiting oceanic production -, distribution, composition and availability of phosphate pools in the ocean, sources – rivers, atmosphere, volcanoes, hydrothermal processes – and sinks – burial of organic matter, adsorption on clays and iron oxohydroxides, phosphorite burial -, residence time), the biogeochemical cycle of phosphate (cycle in the World Ocean – Broecker & Peng’s 1st order model, cycle in the surface ocean – Thingstad model, climate change and Karl shift hypothesis -, coupling with the cycles of other biogenic elements (C, N, Si).
- The silicon cycle: The biogeochemical cycle of silicon (techniques for studying stocks and flows, dissolution of silica in the natural environment – reactivity of particulate silica and dissolution constants, effect of temperature, relationship with bacterial degradation processes, influence of aluminum content -). The global silicon cycle in the oceans – a textbook case in biogeochemical budgeting (production and dissolution of biogenic silica in the oceans – estimation of biogenic silica production and export, comparison of lower and upper limits -, biogeochemical balance of silicon in the World Ocean).
- Coupling the nitrogen and silicon cycles on a global scale: Isotopic fractionation of silicon and nitrogen (proxy for past climate change), silicon and the control of biogeochemistry on a global scale – coupling between thermocline nutrient salt contents in the Southern Ocean and low-latitude biological production, sub-Antarctic modal water (conservativity of the tracer Si* in SAMW , the conceptual model of Sarmiento et al. , 2003, the “Silicic Acid Leakage” hypothesis and the scenario of the last glacial-interglacial transition, implications in the context of global change), competition between species (optimal nutrient ratio, coexistence and dominance, competition in a variable environment, nutritional co-limitations).
► Tutorials : 20 h
TD-1. Nutrient availability, primary production and carbon export in the Mediterranean Sea
TD-2. Carbon fixation and mineral nitrogen assimilation in the equatorial Pacific Ocean
TD-3. Assimilation of mineral nitrogen and silicic acid by phytoplankton in the Southern Ocean
TD-4. Sulfur biogeochemical cycle and consequences for terrestrial climate dynamics
TD-5. The Broecker & Peng model (introducing a quantitative concept to the study of internal ocean cycles)
TD-6. The oceanic silicon cycle. Estimation of the main stocks and fluxes using the PANDORA model.
TD-7. The oceanic iron cycle, a control element for HNLC systems.
TD-8. Bibliography-1. Preparation of a synthesis paper on a current topic, presentation and discussion with students.
TD-9. Bibliography-2. Preparation of a summary paper on a current topic, presentation and discussion with students.
TD-10. Determination of “turn-over” times and direct and indirect estimates of dissolved inorganic phosphate concentrations in the surface ocean.