L5: Paleoclimate Applications of Mixing Parameterizations in an Earth-System Model

Principal investigators: Dr. Paul André (MARUM/University of Bremen), Prof. Michael Schulz (MARUM/University of Bremen)

In paleoclimatology, we are curious about past climates. Since instrumental records only span a tiny fraction of Earth’s climate history, we rely on methods to obtain proxy data from paleo-archives such as marine sediments, corals and glacial ice, to name a few.

In addition, we use the same Earth-system models to simulate past climates that are used to simulate the present and future climates. This opens up the unique opportunity to independently test Earth-system models on climate states that are greatly different from the present climate state for which they were originally developed and calibrated, and that is exactly what we aim for in our project.

More specifically, we are interested in the impact of the state-of-the-art treatment of ocean mixing in current Earth-system models on the past and future ocean circulation and marine biogeochemistry. We focus on the marine oxygen and carbon cycles, because the oxygen content of the global ocean exerts an important control on marine life, which in turn is closely linked to the carbon content of the global ocean. Oxygen availability in the ocean interior has been observed to decline during the past decades and is projected to decline further in the future. However, the future projections depend on Earth-system models that tend to underestimate the rate of the observed current oxygen decline, which is possibly related to an incomplete representation of ocean mixing.

In order to explore the role of ocean mixing on the oxygen availability in the ocean interior in the past and improve future projections, we will make use of two very different settings in Earth’s history as documented by proxy data.

The first time period is the Last Glacial Maximum (LGM; 21,000 to 19,000 years before present). During the LGM, the climate was much colder than at present, the temperature difference between the tropics and the poles was larger, sea level was lower by about 130 m and the ventilation and oxygenation of the deep ocean was reduced due to weaker and shallower spreading of the North Atlantic Deep Water.

In a highly idealized way, this figure sketches the deep-water circulation for the three time intervals to be investigated: present day (top), Last Glacial Maximum (LGM, middle) and (middle) Cretaceous (bottom). The global-mean sea level varied by approximately ±100 m between the paleo and modern states. Whereas for the modern and LGM states, deep waters form near the south and north poles (SP and NP), they probably form near the equator (EQ) during the Cretaceous. Tidal dissipation shifted towards the open ocean during the LGM and towards the shelf seas during the Cretaceous. The oxygen content varied between three scenarios as indicated by the symbol size: It was high during the present day, lower during the LGM and further reduced with a partly anoxic open ocean during the Cretaceous.

The second time period is the middle Cretaceous (more precisely, the Cenomanian-Turonian, approximately 94 million years ago) with a climate that was warmer and a sea level at least 100 m higher than at present. The widespread occurrence of the famous “black shales” (deep-ocean deposits with extraordinarily high organic-carbon content) indicate an even more reduced oxygenation of the deep ocean and deposition under partly anoxic conditions.

Hence we are presented with a riddle: As shown in the figure, very different climatic conditions during the LGM and the middle Cretaceous both lead to oxygen levels lower than today. To shed some new light on this riddle, we want to study the impact of changes in the spatial distribution of tidal energy dissipation due to changes in sea level and the continental configuration on the global marine oxygen and carbon cycles.

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