Depths of the ocean told about climate changes during last half a million years

Scientists from the Immanuel Kant Baltic Federal University and Shirshov Institute of Oceanology of Russian Academy of Sciences described changes in conditions of bottom waters of the Atlantic during last 500 thousand years. As oceans plays an important role in formation of global climate, this information can help to understand contemporary changes and predict future variations in temperature and risks connected with them. The results of the research, supported by Russian Scientific Foundation’s grant, are published in the magazine Palaeogeography, Palaeoclimatology, Palaeoecology.

There is a constant interchange of heat and gases, including CO_2, which belongs to greenhouse ones, between oceans and the atmosphere. The circulation of waters – sea currents and mutual drifting of deep and surface layers of different temperatures and salinity – is closely connected with Earth’s global climate. The currents of carbone dioxide on the border “ocean-atmosphere” are controlled by such characteristics as solar energy level, heat exchange, photosynthesis, respiration, formation of carbonate shells and corals and other. That’s why it’s important to understand the mechanisms of change of ocean waters’ condition.

Equatorial Atlantic is one of the most important areas of the ocean, because it participates in redistribution of heat from equatorial to polar latitudes. Nowadays the northern part of equatorial area demonstrates the highest level of temperatures of surface waters in the Atlantic Ocean, that is connected with position of so-called intertropical convergence zone, the area, that influences the heat equation in more polar latitudes of the Nothern Atlantic and climate in general, and also the productivity in the surface layer of the ocean. The productivity of the ocean is a feature, closely connected with the income of organic substance, mineral elements, and also with amount and diversity of its inhabitants. The change in water circulation in equatorial and tropical zones of the Atlantic, that took place at the time of interchange of glacial (cold) and interglacial (warm) epochs, enables to understand the relationship between ocean dynamics and global climate change.

Scientists from the Atlantic Branch of Shirshov Institute of Oceanology of Russian Academy of Sciences (Kaliningrad), the Immanuel Kant Baltic Federal University jointly with colleagues  from Keele University (Germany) recorded latitudinal migrations of intertropical convergence zone, and also studied the connection between changes of features of the bottom layer of the ocean with surface productivity and global climate changes.

To achieve this aim the researches studied bottom sea sediments, taken from the depth of about 4000 meters in the southern part of Cape Verde basin. Such sediments, apart from mineral elements, contain shells of foraminifers – sea protista. They form carbonate shells from chemical elements containing in water that surrounds them, in particular from “heavy” and “light” isotopes of carbon and oxygen. These isotopes reflect characteristics accordingly of water masses and global ice. The more “heavy”moleculars the water contained, the larger were the icecaps of the planet and visa versa. Due to this feature it is possible to compare changes in chemical constitution of foraminifers with climatic period in which these shells formed. Moreover, state of shells’ preservation tells about aggressiveness of bottom waters to calcium carbonate. Species richness of foraminifers can tell us about the conditions on the surface and in the depth of the ocean; temperature, water ventilation, existence of biogenic matters.

These researches showed that shift of intertropical convergence zone to the north from the exploration area was followed by warming of surface layer of the ocean and increasing of biological productivity of waters. It is supposed that this shift to the north was preceded by the periods when deep sea layers raise closer to surface. As far as the composition of bottom waters is concerned it emerged that in the exploration area there was an alternative domination of old “ancestor” of cold Antarctic deep water, which was aggressive to calcium carbonate (in the periods of cold climatic intervals) and more “loyal” North-Atlantic deep water (in the periods of warm climatic intervals). Nevertheless, the connection between features of deep waters and climate is nonlinear.

“We singled out three climatic regimes, that existed in exploration area in the period of warm climatic stages: “classic model”, “effect of local productivity” and “intense influence of aggressive Antarctic bottom water’. The first two regimes present the intense income of North-Atlantic deep water to the exploration area. The singled out regime “effect of local productivity” corresponds to the intervals of migration of intertropical convergence zone to the north. It is supposed that in the period of the third regime the exploration area was under the influence of waters that were mainly of Antarctic origin”, –  reports Lyubov Kuleshova,  junior research scientist of Atlantic Branch of Shirshov Institute of Oceanology of Russian Academy of Sciences and the author of this article.

“The results we gained enable to understand how global climate changes are reflected in the local processes in the ocean. Reconstructing the changes of chemical constitution of bottom waters and especially their circulation in the past, we can understand the degree of anomality of today’s processes and forecast future climatic changes”, – tells Leila Bashirova, candidate of geological and mineralogical sciences, head of the laboratory of Geology of Atlantic of Shirshov Institute of Oceanology of Russian Academy of Sciences, director of Scientific Educational Centre“Geoecology and Sea Nature Management” of  the Immanuel Kant Baltic Federal University.