2023 Author: Bryan Walter | [email protected]. Last modified: 2023-05-21 22:24
Volcano Klyuchevskaya Sopka. Volcanic activity is one of the elements of the geochemical carbon cycle
Oceanologists have built a model that fully captures the consistent dynamics of the geochemical carbon cycle and climate change over the past 50 million years. The mechanism of this relationship remained unclear for a long time, but analysis of changes in the content of carbon dioxide CO2 in the atmosphere in combination with the processes of organic carbonate sedimentation showed that marine life played an important role in the evolution of the global climate. The results of the study are reported by an article published in the journal Science Advances.
The level of carbon dioxide concentration underlies the global carbon cycle - a set of processes that ensure the carbon cycle between geochemical reservoirs - the atmosphere, biosphere, different layers of the hydrosphere and different shells of the Earth's solid body. If emissions - the flow of carbon into the atmosphere as the most mobile system - increases or decreases, the entire carbon cycle undergoes changes, and with it the climate. The same happens when a part of the carbon is removed from the cycle.
Carbon dioxide environments of the geological past can be reconstructed using indirect data. By the ratio of carbon and boron isotopes in marine carbonate sediments, by the content of the stable isotope 13C in calcites, by studying the morphology of fossil plants and other methods, paleoclimatologists have established that during most of the Cenozoic - about 60 million years - the CO2 level was steadily falling. At the same time, the emission level, apparently, did not experience a significant drop. For example, one of its main sources - volcanism - remained active. However, from the turn of the Paleocene and Oligocene to the beginning of the Miocene, over 14 million years, the CO2 content dropped from 760 to 300 parts per million v / v. This trend was accompanied by a general cooling and a decrease in the rate of chemical (carbon dioxide) weathering of limestone massifs, such as the growing Himalayas. As a result, carbon dioxide bound in limestones was not released and did not participate in the carbon cycle. In addition, the supply of calcite CaCO3 to the ocean decreased.
It is generally accepted that the latter factor determines the level of the so-called carbonate compensation depth (CCD), at which the rate of precipitation and the rate of dissolution of CaCO3 are in dynamic equilibrium (at high pressures at depth, calcite is soluble in water). Below the CCD boundary, carbonate sediments are not deposited, but if the seabed is raised above it, sedimentation occurs. The depth of carbonate compensation can shift downward with an excess of calcite and carbon dioxide in the atmosphere (after all, it is from the atmosphere that CO2 enters the ocean, where marine organisms use it to build the outer skeleton). But this situation is associated with general warming, while the climatic trend of the Cenozoic is exactly the opposite. Therefore, scientists expected that the CCD boundary in the Cenozoic would be raised to the surface of the oceans.
To figure out the position of the carbonate compensation depth level, Nemanja Komar and Richard E. Zeebe of the University of Hawaii's School of Oceanic and Terrestrial Science and Technology at Manoa conducted a model study of the evolution of the chemical composition of marine carbonate rocks. To do this, it was necessary to compare the trends in the dynamics of the CO2 content in the atmosphere and the shifts in the 13C to 12C isotope ratio from the standard signature during the Cenozoic (excluding the earliest and warmest Paleocene epoch). This deviation is higher in warm climates, when thriving life selectively assimilates the light isotope 12C, removing it from seawater. Such a model should be temperature-dependent in order to dilute and compare the competing effects of temperature - the growth of marine biomass and remineralization (decomposition of organic matter and its transformation into the simplest inorganic forms).
The study showed that with an increase in temperature, remineralization proceeds at a faster rate, and this reduces the likelihood of organic carbon burial in bottom sediments. Lowering the temperature has the opposite effect. In this case, the decomposition products of organic matter dissolve more slowly; therefore, the CCD level shifts not to the surface, as previously assumed, but to depth. This can be seen from the graph built on the basis of two models - changes in the concentration of atmospheric carbon dioxide and the content of 13C in marine carbonates. Within the framework of this model, the mechanism of carbonate compensation is not related to the dependence on the rate of weathering on land.
A. Dynamics of the fall in the concentration of atmospheric carbon dioxide (in parts per million by volume). B. Change in the shift in the ratio of carbon isotopes relative to the standard signature. C. Model of the change in depth of carbonate compensation in the Pacific region, calculated from the first two plots. The horizontal axes are time in millions of years; point 0 is the present moment. Black dots on the graphs are the values obtained by calculations, the areas of errors are indicated in orange. Blue circles - observational data
The most dramatic changes occurred about 50 million years ago. This was preceded by an event known as the Paleocene-Eocene Thermal Maximum - it ended the Paleocene warming with a CO2 peak of 2,000 vpm, or 0.2 percent (five times the current level). At the same time, the 13C / 12C ratio reached a minimum: marine life experienced a short-term crisis and then quickly recovered. Komar and Siebe associated this phenomenon with the migration of the bulk of calcifying organisms, such as coccolithophorids and foraminifera, from shallow shelf areas into the open ocean. This is evidenced by the subsequent shift in the zones of carbonate sedimentation. It is possible that the reason for the migration was the lowering of the ocean level in conditions of cold snap.
Thus, in the carbon cycle of the Cenozoic, the leading role belongs to marine organisms that deposit calcium carbonate. Despite the decrease in weathering caused by the general cooling, the activity of these organisms, which migrated to the open ocean, lowered the depth of carbonate compensation from about 3000 to almost 4500 meters. As a result, carbon removed from the atmosphere in the form of CO2 was actively bound in ocean sediments, which entailed a further decrease in its concentration in the atmosphere and an even greater cooling.
Clarification of the nature of the relationship between the geochemical carbon cycle and the history of paleoclimatic trends is necessary to understand how the Earth's climate will evolve in the future. This will enable scientists to build more realistic climate models and improve the accuracy of long-term predictions. Komar and Siebe are now working to expand the capabilities of their method and plan to include the Early Cenozoic - Paleocene and the beginning of the Eocene in the analysis of changes in the carbon cycle and climate, extending the chronological scope of the study to 66 million years.
Earlier, climatologists reported that the Earth's climate passed the point of no return in the development of a warming trend, and also explained why the tropical zone is expanding on Earth.