**Role of physical and biogeochemical fluxes of organic matter on the dynamics of deoxygenation in the Humboldt system**:- Réf- **ABG-115070**- Sujet de Thèse- 07/06/2023- Contrat doctoral- CNRS UMR7621 LOMIC- Lieu de travail- Concepcion - Chili- Intitulé du sujet- Role of physical and biogeochemical fluxes of organic matter on the dynamics of deoxygenation in the Humboldt system- Champs scientifiques- Ecologie, environnement- Terre, univers, espace- Mots clés- OMZ, coupled model, biogeochemistry, fine scale transport, Humboldt system, oxygen, nitrogen**Description du sujet**:ContextGlobal changes caused by anthropogenic forcing will severely impact all components of the biosphere both at regional and local scales, as well as their related ecosystem services and dependent socio- economic systems. It has become critical to develop a stronger capacity to understand and predict these impacts, in order to identify robust strategies for socio-economic adaptation and climate change mitigation.Despite improvements brought to global Earth System Models in recent years (Seferian et al., 2020), some areas of the world oceans remain stubbornly biased, which has precluded a realistic simulation of the evolution of essential variables such as oxygen and primary production in a warmer climate. Among such areas is the Oxygen Minimum Zone (OMZ) off Chile, spreading from the coastal to the open-ocean, where many uncertainties remain amongst state-of-the-art models in terms of its long-term trend (Kwiatkowski et al., 2020). Recently, important effort were made to detail the different physical fluxes on the OMZ formation and variability (Montes et al., 2014; Vergara et al., 2016; Pizarro-Koch et al., 2019, 2023), but the analysis of the biogeochemical fluxes and of the coupling of biogeochemical processes, as well as the small scale physical transport on those terms remain to be investigated. Further, the Humboldt OMZ system is under the combined influence of offshore and coastal, natural and anthropogenic pressures, which result in a spatio-temporal variability of the quality of the organic matter and of observed phytoplanktonic populations. And beyond the pure academic interest, this system presents critical ecosystem services such as the provision of fish stocks and landing, aquaculture production, tourism and recreation.The present PhD project contributes to the COPAS-Coastal1 mission and takes place within the CLAP2 project, whose objectives are:Specific Objective 1: Monitoring and analyzing past and current trends. Generate and analyze customized regional observational datasets, to deliver an empirical understanding of the state, variability, and trends of the ocean and coastal service pressures.Specific Objective 2: Modeling coupled ocean-biogeochemical processes. Deliver downscaled simulations of oceanic response to climate change at regional to coastal scales.Objectives of the PhD projectThe goal of the PhD project is to gain a clear understanding of the biogeochemical dynamics in the deoxygenated (Paulmier et al., 2006; Fuenzalida et al., 2008) and productive (Daneri et al., 2000, Montero et al., 2007, Testa et al., 2018) Humboldt system. This entails a better description of the drivers of oxygen (O2) fluxes with, in particular, a better comprehension of sources and sink terms thatare coupled to the nitrogen (N) cycle and to the dynamics of organic matter. The low and rather stable O2 stocks commonly observed in the OMZ core may falsely reflect an apparent homogeneity of the system, which may as well result from a strong ventilation (strong physical O2 inputs) coupled with high respiration rates, or from weak O2 inputs and low O2 consumption. This project will particularly focus on the sources of organic matter as the main biogeochemical forcing mechanism. A challenge therefore lies in the adequate representation of biogeochemical fluxes, and in particular O2. The extreme physical-biogeochemical gradients, high temporal variability and advection (Farías et al., 2021) will require fine simulation scales to be properly accounted for.Specific question: How do organic matter and its fine scale transport affect the OMZ dynamics (stability/ intensity, and variability)?Working Hypothesis: The origin, nature and spatial distribution of the organic matter affect the biogeochemistry specific to the OMZ, and O2 sinks in particularThis project is highly original, with very moderate risks as the project team possesses all the required expertise and access to adequate data to successfully mentor the PhD fellow.OutcomesModel simulations will provide an original description of the OMZ dynamics at an unprecedented resolution, including the characterization of fine scale transport within and at the limits of the Chilean OMZ. Outcomes of the project will also provide an identification of the sources of particulate and dissolved organic matter that feed the OMZ, discriminating between phyto- and
