Research Themes REACTOR

The dynamic ocean floor is a biogeochemical reactor that influences the global-scale distribution of elements that are vital for life, modulates global climate on geological timescales, and fuels rich biological communities at and below the seabed. The impacts of this reactor on the global carbon cycle, on global climate, on long-term evolution of ocean chemistry, and on the diversity of life are poorly constrained, let alone properly quantified.

Focusing on three Themes, the Research Unit REACTOR will examine reactions in the oceanic crust and at the ocean floor, quantify their regional- to global-scale impacts, explore and quantify biotic and abiotic processes and their mechanisms, and examine the associated geosphere-biosphere interactions and key components of the related ecosystems. Many factors control the direction and magnitude of exchange between the reactive ocean floor and the ocean, including the composition of subsurface rocks, sediments and hydrocarbon reservoirs in different geotectonic settings.

Main objectives / expected achievements in the REACTOR:

To examine the relationships between plate tectonics and the chemical composition of the ocean we will quantify and budget chemical fluxes in a variety of marine geological settings.
To determine habitability within the ocean floor and quantify the role of the deep biosphere
in element cycles we will reveal the chemical and physical factors that define the lower
boundary of the biosphere below the ocean floor and identify its impact on element cycles.
To disclose the impact of gas seeps and hot vents on life at the ocean floor and in the deep
ocean we will determine the relationships between geological processes, the chemical
compositions of emitted fluids, and the resulting biological activity and diversity.

An organizational overview of the program is provided in the following table.

REACTOR	Theme 1	Theme 2	Theme 3
Title	Seawater-crust interactions	Deep biosphere and element cycling in sediments	Processes and dynamics of vents and seeps
Research focus, spatial	Below ocean floor, primarily non-sedimented	Below ocean floor, primarily sedimented	At and above ocean floor, primarily sedimented
Contributions to overarching objectives (Obj.) of the Cluster	Element fluxes and budgets (Obj. 2); ecosystems and biogeochemical cycles (Obj. 3)		Element fluxes and budgets (Obj. 2); ecosystems and biogeochemical cycles (Obj. 3); warm-world scenarios (Obj. 4)
Most important links within the Cluster (T = Theme)	Climate-ecosystembiogeochemistry feedbacks (RECORDER T2); impact on ocean chemistry and climate (Enabler MODELING)	Transformation of biogenic particles (RECEIVER T1); detection of life and its activity (molecular isotopic approaches; Enabler TRACERS)	Recording the reactor (RECORDER T1 and Enabler TECHNOLOGY)
Key study sites	Eastern Equatorial Pacific ridge flanks, North Atlantic rifted margins and spreading ridges	Nankai Trough, Guaymas Basin, Helgoland mudflat, Argentine Basin	Submarine volcanic arcs, Cascadia margin, Guaymas Basin, mid-Atlantic ridge
Joint study sites	— North Atlantic, Mediterranean Sea —
Disciplines	Geophysics, geodynamics, geochemistry, biogeochemistry, geobiology	Biogeochemistry, microbial ecology, organic eochemistry, geobiology	Biogeochemistry, microbial ecology and physiology, marine geology, chemical oceanography

REACTOR Themes — Schematic overview of the research topics within the Research Unit REACTOR. Research activities will take place in oceanic rifts, ridge flanks, and subduction zones as well as in reactive sediment systems in the deep ocean and shelf seas. We will focus on examining the interrelationships between geological processes, geochemical reactions, and biology in these settings. Constraining diffusive and venting / seepage-related fluxes of matter between the ocean floor and the ocean and determining the influences of these fluxes on marine life is another major goal.