The Arctic Ocean - Ventilation Timescales, Anthropogenic Carbon and Variability in a Changing Environment

• Applicants: Professor Dr. Werner Aeschbach; Dr. Toste Tanhua
• Subject Area: Oceanography ; Hydrogeology, Hydrology, Limnology, Urban Water Management, Water Chemistry, Integrated Water Resources Management ; Optics, Quantum Optics and Physics of Atoms, Molecules and Plasmas
• Term: since 2020
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 456675218

In the Arctic, climate change is most apparent. This shows for example in the strong decrease of the Arctic Ocean’s sea ice cover, which in turn influences the region’s heat balance and the circulation in the ocean and the atmosphere. The formation of deep water encompasses the transport of dissolved gases into the ocean’s interior, known as ventilation. The corresponding uptake of anthropogenic carbon, which is disproportionally large in the Arctic Ocean, acts as an important buffer for greenhouse gas emissions. Its knowledge is crucial for reliable climate scenarios.The ventilation timescales can be determined by measurements of certain tracers, which carry time information by their time dependent input function or radioactive decay. However, classical tracers such as CFC-12 and sulfurhexafluoride (SF6) as well as a series of modern, so called “Medusa Tracers” are not detectable in the deepest parts of the Arctic Ocean. The new Atom Trap Trace Analysis (ATTA) method now allows for measurements of the radioisotope 39Ar in seawater and thus for coverage of exactly the age range that could not be precisely resolved in the past. In combination with the mentioned tracers as well as radiocarbon (14C), age distribution functions and ventilation timescales can thus be determined for the entire water column. This approach is complemented by measurements of noble gases to determine saturation anomalies at the surface as well as the long-lived anthropogenic radioisotopes 236U und 129I, which act as markers of Atlantic water and enable the study of the exchange between the North Atlantic and the Arctic Ocean.In this project we will take and measure samples for all mentioned tracers during an expedition on the icebreaker ODEN in the Central Arctic Ocean in 2021. The data serve to determine model parameters of transit time distributions, which in turn provide the basis for the calculation of the water column inventory of anthropogenic carbon. The results will be compared to biogeochemical approaches and used to estimate ocean acidification rates. The complementing tracer data provide insights about the circulation in the North Atlantic realm and about processes at the ocean surface. To analyze the decadal changes expected to result from climatic effects, we will additionally analyse historic tracer data from the Arctic.Furthermore, from the combination of different innovative methods we expect important methodical insights as well as data-based boundary conditions for ocean models. The results of this project will thus make extensive contributions to an improved understanding of the circulation and ventilation of the Arctic Ocean, the carbon uptake capacity of the oceans and the consequences of the changing Arctic and global climate.

• DFG Programme: Research Grants
• International Connection: China, Sweden, Switzerland
• Co-Investigators: Professor Dr. Norbert Frank; Professor Dr. Markus Kurt Oberthaler
• Cooperation Partners: Professor Dr. Leif Anderson; Professor Dr. Minggang Cai; Dr. Nuria Casacuberta Arola; Dr. Céline Heuzé; Dr. Adam Ulfsbo