University of Bremen: Study of oxygen content in deep ocean water
Analyzes of sediment samples show that the breakdown of organic carbon in the water masses of the deep sea consumed the oxygen available there.
Scientists from Oklahoma State University (USA), the GEOMAR Helmholtz Center for Ocean Research Kiel and the MARUM – Center for Marine Environmental Sciences at the University of Bremen are involved.
As a natural sink for carbon, the ocean is a central component of the earth’s climate system. How much carbon is withdrawn from the system for a long time depends on how much carbon-containing particles are retained in the sea floor. The availability of dissolved oxygen is of central importance here, as this is consumed in the microbial degradation of previously formed biomass. The distribution of oxygen in the water column is primarily determined by the vertical circulation. To answer the question of whether the corresponding conditions in the deep ocean have changed in recent geological history, the authors of the new study examined sediment samples. Chemical elements were analyzed,
Sediment cores from biologically highly productive areas analyzed
The sediment cores available to the team come from the Cape Basin off the west coast of southern Africa, from water depths between 1,000 and 2,500 meters. Due to the current conditions, one of the most biologically productive areas is located here: Cold, nutrient-rich water from the depths increases the productivity of plant plankton. If organic material dies, these particles are broken down by microorganisms as they sink through the water column and on the sea floor. Oxygen is preferably consumed in this process. If large amounts of organic material sink, this can mean that more oxygen is required than can be supplied by currents. The water column becomes “anoxic”, i.e. oxygen-free.
Oxygen deficiency detected in the deep sea during the Ice Age
Using geochemical signatures in the sediments, researchers have now been able to demonstrate that much less oxygen must have been available in the deep ocean during the last glacial period than in the warmer phases. So far it was known that during cold periods the stronger temperature gradient between the poles and the equator is directly related to an increase in wind circulation, thus a stronger buoyancy of nutrient-rich water and thus, in turn, more intensive biological production. It was also known that due to the formation of polar ice caps and the associated lower sea level in cold periods, the upwelling close to the coast shifted towards the continental slope, i.e. over deeper areas of the ocean. “What is new about the current study is that the oxygen depletion is not limited to water depths of a few hundred to a thousand meters, but has now also been demonstrated at the bottom of the ocean, “says co-author Dr. Matthias Zabel from MARUM. This can essentially be traced back to two causes.
More organic carbon is stored in the deep
Intensive decomposition processes of the increased biomass produced during cold periods have consumed a great deal of oxygen. The increased content of organic carbon in the sediments examined can be seen as a clear indication that the availability of oxygen must have been severely restricted at the same time. “Today you can find oxygen-free zones on the shelf a few hundred meters flat, ie the transition from the continental shelf to the sea. In the Ice Age, on the other hand, the water of the open ocean was anoxic at greater depths, ”emphasizes Dr. Florian Scholz. The GEOMAR biogeochemist is co-author of the study and head of the Emmy Noether research group ICONOX – Iron cycling in continental margin sediments and the nutrient and oxygen balance of the ocean (Influence of the iron cycle in continental sediments on the nutrient and oxygen balance of the ocean).
Impact on the global carbon cycle
“From the sediment samples we can see that during cold periods in the deep ocean organic material was broken down less effectively and, as a result, more organic carbon was buried in the seabed sink,” said Dr. Scholz. “By analyzing these processes from the history of the earth more precisely, we can better estimate whether a slower circulation in the future could lead to an increased storage of the carbon released by humans in deep-sea sediments,” summarizes Dr. Zabel summarized the importance of the new study for research. “Against the background of the man-made increase in carbon dioxide concentrations in the atmosphere and advancing climate change, it is of crucial importance to determine and evaluate processes and mechanisms.