2. Ocean-Earth interactions

Contacts : Marcia Maia, Laurent Toffin & Clément Vic

Scientific challenges, overall objectives

Theme 2 sits at the crossroad of many disciplines: geology, oceanography, geochemistry, micro-biology, social sciences and humanities. We wish to encourage research lines at the core of these disciplines and also to foster strong interactions between them. This theme 2 is divided into four main complementary scientific challenges and two cross disciplinary topics.

1. The dynamic lithosphere: telluric processes from Earth’s interior to its surface

Understanding the processes responsible for the formation and evolution of the Earth’s outer shell, also called lithosphere, is crucial to evaluate the exchanges between the Earth and the oceans. Magmatic, tectonic, hydrothermal, sedimentary and erosional processes shaping the ocean floor and the continental margins, are key to understand and quantify the chemical and energy fluxes of our planet. Moreover, studying the Earth’s ancient times allows the reconstruction of the history of past oceanic circulation, species dispersion and interactions between land masses and the ocean that have shaped our planet until today.

Among our objectives, we can cite: (i) improving our understanding of the formation and evolution of the primitive Earth, (ii) constraining the impact of plate kinematics to oceanic dynamics and the evolution of species, (iii) quantifying the magmato-tectonic budget and its variations in space and time at spreading centers and its connections to hydrothermal fluxes, and (iv) understanding the deformation processes at major oceanic faults in depth which promote wateroceanic crust interactions. We intend to encourage the development of cooperation between different scientific domains but also support projects dealing with advances in each disciplinary field, as these form the bases for future interdisciplinary developments. We also intend to encourage the development of new sensors and methodologies, especially those reducing the anthropic impact of our research on the ecosystems and the environment.

2. Unlock the sediment’s memory: past records of climate, tectonics, and geohazards

The “memory” of sediment provides a concrete record of Earth’s dynamic past, deciphering the evolution of climate, ecosystems, and tectonics. This field of study contributes significantly to gaining a better understanding of the planet’s history and the forces that have shaped it. Marine sediments, covering millions of years with varied resolutions, may offer insights into past environmental conditions such as temperature, salinity, sea level changes, and sea ice distribution. They also provide information on changes in sediment sources, including erosion and denudation, turbidity, and anthropogenic disturbance. Moreover, within deep-water basins, sediments act as natural seismographs, capturing past earthquakes over tens of thousands of years. Unlocking the memory of sediment in terms of past mechanical stresses and deformations enables better analysis and understanding of the mechanical processes and tectonic activities that have shaped continental shelves and slopes.

Compared to the previous projects funded during the first phase of Isblue (theme 2), our goal in this second phase is to enhance the development of multidisciplinary and complementary approaches through integrated analysis for studying interactions among coupled processes. For instance, how do climate variations affect sedimentary dynamics and trigger geohazards, or how do environmental conditions contribute to the deposition of sediments sensitive to anthropogenic disturbance or tectonic stresses? This will be carried out through a combination of lab-based experimental studies with fieldwork, monitoring and observation, as well as numerical modelling that incorporates innovative numerical techniques.

3. The living deep seafloor where fluids, minerals and life interact

The deep seafloor constitutes a “critical zone” where physicochemical properties of the environment are tightly coupled to energy source, biomass and activity of the living organisms. However, deep-sea ecosystems are still some of the least explored on Earth. A variety of geological settings associated with the generation and/or release of gas fluids, mineralisation and sediments at the ocean floor and in the hydrosphere have now been recognised on the seafloor, among which hydrothermal vents, submarine volcanoes, cold seeps, cold water coral reefs, carbonate mounds and canyons are of particular interest for Theme 2.

The complex fluids generated in in these environments interact with the seafloor, including sediment, basaltic and/or ultramafic rocks and the surrounding seawater, creating unique ecosystems where microorganisms and biological fauna thrives together. Hot hydrothermal fluids rich in gases and metals generate hydrothermal chimneys, sulfide deposits or hydrothermal plumes that host significant carbon biomass. Similarly cold seeps ecosystems are usually characterized by significant chemosynthetic biomass fueled by methane emission. These deep-sea ecosystems sustain important mineral and biological resources as well as critical ecological functions.

We still need to identify and characterise the geodiversity of fluids, mineralogical composition and nature of the deposits that shape and support biodiversity of the deep-sea. The comprehensive understanding of the geochemical and microbiological processes at stake is essential to predict the health of the ocean seafloor and the effect of global changes on the deep-sea ecosystem functioning. Indeed, such scientific framework is critical to develop robust management and conservation guidelinesfor the sustainable use of deep-sea resources. The variability of activity, in space and time, of the aforementioned seafloor geological features; from extinct to diffuse to active habitats, must be defined in order to identify relevant biomarkers, assess potential connectivity among sites (through chemical and genetic dispersion) and finally preserve the deep-sea environments. These deep-sea ecosystems have been known for several decades but as they face multiple threats in the context of future seafloor mining, global climate warming, even episodic or catastrophic events several questions remain regarding their functioning.

This theme 2 axis aims (i) to understand and quantify the controlling factors and how they impact the distribution, the structure and the functioning of deep seafloor ecosystems, (ii) to identify the sources and pathways of fluid flow and energy fluxes through the trophic relationships of deep benthic and planktonic communities, (iii) to identify the biological and micro-biological communities providing sources and sink of carbon and the physio-chemical characteristics of their habitat, (iv) to assess their biodiversity, (v) to understand their energy budget and ecosystem structure in relation to geochemical interfaces or gradients and mineral characteristics. Newly acquired data on geochemistry, mineralogy, and micro-biologically will improve databases and information sources to be used in carbon cycleecosystem modeling to test scenario of deep-sea ecosystems management.

4. Deep-sea circulation, current-topography interactions and consequences for the transport and mixing of tracers and sediments

The ocean is mainly put into motion (forced) at the surface by air-sea fluxes of heat, freshwater and momentum. As oceanic surface layers are in contact with the atmosphere and are easier to observe (in situ and satellite), they have historically received more attention than deeper layers (>1000 m). The latter are difficult to sample and, due to their relative isolation from the atmosphere, have been assumed to host laminar and sluggish currents, with no dynamical or climatic interest. The deepest layers have remained poorly sampled and understood, and they suffer from large biases in climate models. Yet, a large fraction of energy dissipation is catalysed by the interaction of currents with the seafloor topography. This route to dissipation entails a rich phenomenology of processes, from the generation of submesoscale (0.1-10 km) coherent vortices to high-frequency and non-linear internal waves, which impact the transport and mixing of biogeochemical tracers (carbon, metals, …), heat, sediments, etc.

We wish to develop research projects that aim at better understanding currenttopography interactions and the energy cycle on the one hand; and at investigating the impact of such interactions in the transport and mixing of matter, e.g., chemical species and sediments, on the other hand. The methodologies would benefit from a combination of numerical models designed to study current-topography interactions and technical developments to sample in situ the variability of the deep
layers.

Cross-disciplinary topics

• Social sciences and humanities :
  Under the Law of the sea, the ocean floor beyond the limit of the continental shelf enjoys the status of common heritage of mankind. The growing need for mineral resources and the prospects offered by genetic resources for biotechnologies raise questions about the ability of international regulatory regimes to balance the trade-offs between conservation and exploitation. In recent years, the question of international governance dedicated to the protection of the high seas, in the sense of the water column, has been at the heart of UN negotiations for the BBNJ treaty (Biodiversity Beyond National Jurisdiction).
  
  The conditions for effective protection and potential exploitation of these ecosystems are the subject of multi-disciplinary research combining law, social, and natural sciences, in direct contact with political negotiation forums and stakeholders. This work can focus on the legal, political, economic and social dimensions of managing deep-sea and offshore ecosystems under anthropogenic pressure.
• Technological developments :
  The development or adaptation of innovative and integrative methods, materials and equipment to the constraints of the deep marine ecosystems is essential to address main research questions of the theme 2. The observations, sampling, measurements and experiments, whether in situ, in vivo, in mesocosms will provide valuable information on ecosystem functioning and biotic, abiotic and mineral interactions.