Der Ozeanboden

From Surface to Seafloor: Mechanistic Models of Sinking Particle Transformation

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Project Description

What controls whether carbon produced in surface waters is sequestered for millennia at the seafloor—or transformed back to CO2 along the way? Quantifying the amount of organic carbon reaching the deep ocean or sea floor today and in future climate scenarios is a key task of global ocean models. However, we still lack a quantitative understanding of how and why particulate organic carbon (POC) fluxes differ across ocean regions, so that current models do not even agree on the direction of the projected export flux in the future.

In order to understand global patterns and derive parameterizations for the use in global models, this project “zooms in” to the particle scale: In the surface ocean, organic aggregates are formed and colonized by microorganisms. During the particles’ descent, surrounding temperature, pressure and oxygen conditions impact carbon remineralisation and flux attenuation through influencing microbial succession on particles. Using a unique, multidimensional dataset spanning the Atlantic Ocean—from the Arctic to the Weddell Sea—the aim of this project is to:

1. Quantify how nutrient availability in surface waters influences the C:N ratios and remineralization profiles of sinking particles.
2. Decode the traits and trade-offs of microbial communities colonizing these particles and how the communities evolve during descent, to develop parameterizations for trait-based models.
3. Assess the role of environmental gradients, such as oxygen and temperature, in modulating microbial activity, remineralization rates, and particle sinking speeds.

This project offers the successful candidate the opportunity to develop a broad and interdisciplinary skill set—ranging from the analysis of field and experimental data to trait-based biogeochemical modeling—while contributing to a deeper understanding of a key uncertainty in the global carbon cycle with direct relevance for improving climate projections.

Further Reading

• Arandia-Gorostidi, N., Berthelot, H., Calabrese, F., Stryhanyuk, H., Klawonn, I., Iversen, M., ... & Musat, N. (2022). Efficient carbon and nitrogen transfer from marine diatom aggregates to colonizing bacterial groups. Scientific reports, 12(1), 14949.
• Cael, B. B., & Bisson, K. (2018). Particle flux parameterizations: Quantitative and mechanistic similarities and differences. Frontiers in Marine Science, 5, 395.
• Lennartz, S. T., Keller, D. P., Oschlies, A., Blasius, B., & Dittmar, T. (2024). Mechanisms underpinning the net removal rates of dissolved organic carbon in the global ocean. Global Biogeochemical Cycles, 38(3), e2023GB007912.
• Moradi, N., Liu, B., Iversen, M., Kuypers, M. M., Ploug, H., & Khalili, A. (2018). A new mathematical model to explore microbial processes and their constraints in phytoplankton colonies and sinking marine aggregates. Science Advances, 4(10), eaat1991.
• Ploug, H., & Bergkvist, J. (2015). Oxygen diffusion limitation and ammonium production within sinking diatom aggregates under hypoxic and anoxic conditions. Marine Chemistry, 176, 142-149.

Modalities

The doctoral project will be supervised by Sinikka Lennartz (University of Oldenburg) and Morten Iversen (AWI). Depending on the personal expertise of the doctoral candidate, it will be supervised within the working group “Biogeochemical Ocean Modelling” (Oldenburg) or the working group “Polar Biological Oceanography” (Bremerhaven). The remuneration corresponds to pay group TV-L 13 with 75% of the weekly working hours if based at Oldenburg or pay group TVöD with 75% of the weekly working hours if based at the AWI. The positions is for a fixed-term period of 4 years, starting January 1st 2026 the earliest and until at most December 31, 2029 (according to § 2 WissZeitVG) with the aim of obtaining a doctorate.

The project mentoring team also includes: Nasrollah Moradi (AWI).

Main Tasks

• Scientific research in the field of:
  Biogeochemical trait-based modelling, including modification of source code, performing numerical simulations and processing model output
• Analysis of multivariate datasets including chemical and biological parameters across space and time
• Writing of scientific publications
• Participation in international conferences

Formal Requirements:

Completed scientific university degree (Master/University diploma or comparable) in natural sciences or mathematics, in particular in the context of Environmental Sciences, Environmental Modelling, Biogeochemistry or related fields. Documented programming skills in at least one of the following languages (Matlab, Fortran, Python, Julia, R) as well as language skills in written and spoken English are required.

Further questions?

Questions about the project can be addressed to Sinikka Lennartz ([Bitte aktivieren Sie Javascript]) or Morten Iversen ([Bitte aktivieren Sie Javascript]).