22 May 2022

Scientists from Shirshov Institute of Oceanology of the Russian Academy of Sciences together with colleagues from Immanuel Kant Baltic Federal University (IKBFU) have described changes in the Atlantic Ocean’s bottom water which happened over the past 500,000 years, the IKBFU press service said. The findings will help clarify the causes of today's climate changes and predict future temperature fluctuations.

"The results we obtained allow us to understand how global climate changes are reflected in local processes occurring in the ocean. By reconstructing changes in the chemical composition of bottom waters and peculiarities of their circulation in the past, one can understand how anomalous today's processes are and predict future climatic changes", IKBFU cites Leyla Bashirova, Deputy Director of the Atlantic Branch of the Shirshov Institute of Oceanology for Scientific Work, Director of Scientific-Educational Centre Geology and Marine Nature Management, as saying.

There is a constant exchange of heat and gases between the oceans and the atmosphere, including CO2, which is one of the greenhouse gases. And water circulation - sea currents and mutual movements of deep and surface layers of different temperature and salinity - is closely connected to the Earth's global climate. The equatorial Atlantic is one of the most important areas of the ocean, as it participates in the redistribution of heat from low latitudes to high latitudes. Changes in water circulation in the equatorial and tropical Atlantic, which occurred against the background of changing glacial (cold) and interglacial (warm) epochs, will allow us to understand the relationship between ocean dynamics and global climate change.

Russian scientists together with colleagues from Kiel University (Germany) have recorded latitudinal migrations of the Intertropical Convergence Zone and studied the relationship between changes in the properties of the bottom layer of the ocean with surface productivity and global climate changes. For this purpose, the authors studied bottom marine sediments raised from a depth of about 4 km in the southern part of the Cape Verde Basin.

 

These sediments, in addition to mineral particles, include shells of foraminifera - marine unicellular organisms. They form carbonate shells out of the chemical elements in the surrounding water, including "heavy" and "light" isotopes of carbon and oxygen. These isotopes reflect the properties of water masses and global ice levels, respectively. The more "heavy" molecules there were in the surrounding water, the more extensive were the ice "caps" of the planet and vice versa. Thanks to this fact, it is possible to compare changes in the chemical composition of foraminifer shells with the climatic period during which these shells were formed. In addition, the degree of preservation of the shells indicates the aggressiveness of bottom waters to calcium carbonate. The species diversity of foraminifera can indicate the conditions at the surface and at the depth of the ocean: temperature, ventilation of the waters, and the presence of nutrients.

Climatic regimes

The studies showed that the northward shift of the Intertropical Convergence Zone relative to the study area was accompanied by a warming of the surface layer of the ocean and an increase in the bioproductivity of the waters. It is assumed that the northward shift of this zone was preceded by periods in which the deep ocean layers rose closer to the surface.

As for the composition of bottom waters, it turned out that the area of the study was dominated alternately by the ancient "ancestral" cold Antarctic Bottom Water, aggressive to calcium carbonate (during cold climatic intervals), and the more "loyal" North Atlantic Deep Water (during warm climatic intervals). However, the relationship between deep water properties and climate is not linear. 

"We identified three climate regimes that existed in the study area during warm climatic phases: "classical pattern," "local productivity effect," and "enhanced influence of aggressive Antarctic Bottom Water." The first two regimes represent the intensified inflow of the North Atlantic Deep Water to the study area. The highlighted "local productivity effect" regime coincides with the intervals of northward migration of the Intertropical Convergence Zone. It is assumed that during the third regime, the study area was under the influence of waters of predominantly Antarctic origin," said Junior Research Fellow of the Shirshov Institute Liubov Kuleshova, author of the article with the results of the research, which was published in the journal Palaeogeography, Palaeoclimatology, Palaeoecology.

The research was funded by a grant from the Russian Science Foundation.