Dynamics of marine dissolved organic matter: ocean metabolism and climate transitions

• Autores: Patricia de la Fuente Gamero
• Directores de la Tesis: José Luis Pelegrí Llopart (dir. tes.), Celia Marrasé (codir. tes.), Agustín Sánchez-Arcilla Conejo (tut. tes.)
• Lectura: En la Universitat Politècnica de Catalunya (UPC) ( España ) en 2019
• Idioma: español
• Tribunal Calificador de la Tesis: Eva Calvo Costa (presid.), Oscar Guadayol i Roig (secret.), Eva Ortega Retuerta (voc.)
• Programa de doctorado: Programa de Doctorado en Ciencias del Mar por la Universidad de Barcelona y la Universidad Politécnica de Catalunya
• Materias:
  • Ciencias de la vida
    • Microbiología
      • Procesos microbianos
  • Ciencias de la tierra y del espacio
    • Climatología
      • Paleoclimatología
    • Oceanografía
      • Oceanografía biológica
      • Oceanografía física
    • Ciencias del suelo
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• Resumen

  • GameroThe Global Ocean is the largest Earth compartment holding carbon and nutrients that reaches the upper-ocean at temporal scales ranging from months to 10 kyr. The availability of these nutrients is fundamental to sustain primary production and the concentration of dissolved inorganic carbon (DIC) in surface waters controls the glacial-interglacial changes in atmospheric CO2. One process that influences both nutrients and carbon availability is the Microbial Carbon Pump (MCP), which refers to the production of refractory dissolved organic carbon (RDOC) compounds via heterotrophic microbial activity. Variations in the RDOC pool affect long-term carbon storage in the ocean, hence influencing the carbon cycle and climate. The general objective of this thesis is to expand our understanding of the connections between RDOC production by MCP and the ocean metabolism (understood as the upper-ocean net autotrophic community production), paying special attention to the role of the marine microbial processes in the glacial-interglacial transitions of the Earth system. The RDOC production by MCP is inferred through the lineal dependence of fluorescent dissolved organic matter (FDOM) with apparent oxygen utilization (AOU) and nutrients. This relationship, however, depends on the preformed content in the water masses. In this thesis, a valuable dataset, obtained from a high-resolution spatial sampling along 7.5ºN in the equatorial Atlantic Ocean, is used to distinguish the variability of FDOM distribution associated with in situ production from that related to the water properties at origin. A simple objective nonlinear-global methodology for resolving the non-conservative fraction of biogeochemical variables distribution is presented. The approach focuses on fitting high-order polynomial models over the entire temperature-salinity space. The differences between the modelled values and the observations are identified as biogeochemical anomalies. The goodness of the method is compared, for each water stratum, with the traditional approach, which is based on the local linear mixing of a maximum of three source water masses. The new methodology has good skill at distinguishing between the conservative and non-conservative contributions to biogeochemical variables, lending information about biogeochemical processes, stoichiometric ratios and patterns of connectivity within a certain region. For the first time, a general relationship between humic-like FDOM and AOU in the dark equatorial Atlantic Ocean is formulated, irrespective of the water masses. The results endorse the idea that FDOM is mostly produced in situ in the dark ocean. In the second part of the thesis, the role of RDOC pool in quaternary climate transitions is explored. The glacial-interglacial transitions are considered as functional states of the complex Earth system, with different energetic conditions in terms of solar energy conversion through marine photosynthesis. The oceanic system capacity to capture and transform the incident solar radiation depends on the availability of DIC and nutrients to the productive upper ocean. The supply of DIC and nutrients by the Meridional Overturning Circulation (MOC) and the DOM pool are evaluated through a simple two-box and two-state relaxation-type model for the DIC and nutrients in the upper ocean. The model, inspired on physiological concepts, considers the upper ocean to switch between basal (glacial) and enhanced (interglacial) metabolic states. The model reproduces well the atmospheric CO2 time series for the last 420 kyr, providing a solution for the size and temporal dependence of the MOC and setting global constraints on primary production and remineralization in the upper ocean. The RDOC accumulates during the glacial period and its availability at the end of this cycle sets the metabolic intensity of the subsequent interglacial, in what constitutes a central component of the Earth’s pulsating homeostatic organization.