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Paleoceanography – how and why should we reconstruct the ocean’s past?

By Milena Ceccopieri

English edit by Carla Elliff


Illustration by Alexya Queiroz.


One of the topics we hear most about these days is climate change and its consequences for the future of the planet. Global warming became evident after the observation of the increase in the average global temperature of the air and ocean caused by the increase in the concentration of greenhouse gases in the atmosphere from human activities, such as the burning of fossil fuels, industrialization and deforestation. In a special report produced in 2018 at the invitation of the United Nations Framework Convention on Climate Change, the Intergovernmental Panel on Climate Change (IPCC) showed that the global temperature is already 1.0 °C above pre-industrial levels and could reach 1.5 °C between 2030 and 2052, which is already causing some impacts on terrestrial and oceanic ecosystems and could have even more drastic consequences for the Earth's climate. 


Diagram of the increase in global average temperature according to forecast reports until the year 2080. The first diagram shows milder temperatures if carbon dioxide emissions decrease. The second diagram shows much higher temperatures if carbon dioxide emissions triple by 2080.

Projection for global temperature rise by 2090 based on CO2 increase. Figure from Wikimedia. License: CC BY SA 4.0 International


But after all, if we are so concerned about future climate change, why do we need to study the past? Knowing how our planet's climate behaved in the past under certain conditions helps us understand and predict how the climate will behave in the future if we encounter similar conditions. For example, we know today that the concentration of CO2 in the atmosphere is rising rapidly, but we do not know for sure what the consequences of this increase will be. To try to predict these consequences, we need to ask some questions. Has this increase happened before? At what speed? Has the temperature increased? What about sea level? What was the climate like on Earth during the period when temperature and CO2 concentrations were similar to what we see today? This is where paleoclimatic and paleoceanographic studies come in.


Graph showing the variation in CO2 and CH4 concentrations over time estimated from ice cores from Antarctica and Greenland. The x-axis represents age, which ranges from 400,000 years ago to the present. The left y-axis shows CO2 values ​​in ppm and the right y-axis shows CH4 values ​​in ppb and temperature values. The graph shows a steep increase in CO2 and CH4 in the present. The graph also shows the 4 glaciations that occurred during this period, where CO2, CH4 and temperature values ​​were lower.

Variation of CO2 and CH4 concentrations based on ice cores from Antarctica and Greenland. Figure from Wikimedia. License: CC BY SA 4.0 International



The term “paleo” means ancient, old. In paleoceanography, the researcher acts as a detective of the past, whose investigation is based on evidence such as how the relationship between the ocean and the Earth’s climate varies on different time scales, which can be decades, hundreds, thousands, or even millions or billions of years ago. 


But what role does the ocean play in the global climate? The Earth's energy balance is modulated by four compartments: atmosphere, ocean, continent and ice. In terms of energy exchange/transport, the main compartments are the atmosphere (more dynamic) and the ocean (slower). The oceans play a fundamental role in the global climate due to their ability to store and transport large amounts of heat, being the largest heat reservoir on the planet! The heat from the sun arrives with much greater intensity at low latitudes and is redistributed to high latitudes through ocean circulation. 


To better understand this heat transport, it is important to understand the thermohaline circulation as a whole. Thermohaline circulation is driven by potential changes in temperature and salinity between different water masses, generating differences in density. The formation of ice at high latitudes results in the formation of cold water with higher salinity that is extremely dense and will sink, generating a bottom current and driving this global circulation. So if thermohaline circulation is directly related to the formation of ice, what could happen to it in a scenario of rising global temperatures and melting glaciers? What happens to the transport and distribution of global heat if the thermohaline circulation weakens? Paleoceanographers are concerned with reconstructing parameters such as paleotemperature and paleosalinity to investigate the patterns of variation in the global circulation of the past, which makes it possible to assess the consequences of variations in the present and future.


Okay, but if scientists only started collecting and recording temperature and salinity data in the 1950s, how can we reconstruct the characteristics of seawater from millions of years ago? Since the properties of the ocean’s past cannot be measured directly, we measure them indirectly using tools or what we call proxies (don’t know what a proxy is? No problem, we’ll explain in a moment). The main matrix of paleoceanography, that is, the type of sample used to measure proxies, are marine cores, which are a vertical section of the sedimentary column collected from the ocean floor. These sedimentary records are formed after many, many years of particle deposition in ocean basins. These particles are deposited in layers that accumulate one on top of the other and store information about the environmental conditions of the ocean at the time of deposition.


Schematic figure showing the steps involved in sampling a marine core. The figure shows a southern section of the ocean with the stratified marine sediment at the bottom, followed by the water column just above, and a ship operating on the surface. Below the ship is the sampler cable that descends and is finally connected to the sampler that is inserted in the sediment at the bottom. Drawings of the core sectioned after sampling are shown next to the ship.

Marine core sampling. Figure adapted from Wikimedia. License: CC BY SA 4.0 International


Images of marine core sampling with a piston corer on a Brazilian Navy research vessel. The image on the left shows the moment when the sampler is removed from the water and positioned on the ship's deck. The image on the right shows the core being carefully removed from inside the sampler.

Marine core sampling. Source: Milena Ceccopieri, license: CC BY-SA 4.0.


Photograph taken inside the marine core repository at the Alfred Wegener Institute for Polar and Marine Research in Germany. The photo shows a corridor with floor-to-ceiling shelves on both sides, filled with hundreds of properly stored and labeled marine cores. In the background, at the end of the corridor, a man is seen handling one of these cores.

Marine core repository of the Alfred Wegener Institute for Polar and Marine Research, Germany, from Wikimedia. License: CC By SA 2.5 Generic



There are other environmental matrices that also store sequential information about the Earth's paleoclimate, such as corals and ice cores. It is even possible to obtain information about the atmosphere from hundreds of thousands of years ago from air bubbles found in these ice cores! On the continents, we also have tree rings, speleothems and lake sedimentary records. The great advantage of marine cores in relation to other records is that they cover a longer period, which can go back up to 100 million years!


The age of a marine core must be clearly defined before anything else, as it is what limits the period and temporal resolution of this sedimentary record, which will be the basis for all interpretation. This chronology is developed by dating certain points of the core and constructing an age model. There are different types of dating suitable for each period and type of material to be dated, such as dating using the radioactive isotopes carbon-14 and lead-210. In the case of marine cores, the most suitable material for dating is the shells of foraminifera, very small single-celled organisms that produce a structure composed of calcium carbonate. Dating is commonly done using 14C, which covers a period of up to 45-50 thousand years, and can extend to hundreds of thousands of years when associated with other tools.


Schematic figure showing the steps of sectioning the core and sorting foraminifera. On the upper left side, a drawing of a sectioned core is shown, which would be the first step. A little further to the right, the step of sub-sampling one of the core sections and its washing inside an Erlenmeyer flask is shown. Then this Erlenmeyer flask is poured over a sieve and a beaker to separate the thickest fraction of the sediment containing the foraminifera. The last step is shown below and represents a woman sorting the foraminifera using a magnifying glass or microscope.

Core sectioning and foraminifera screening. Figure adapted from Wikimedia. License: CC BY SA 4.0 International


But what exactly is a proxy? A proxy consists of a clue or piece of information preserved over time that can be quantified and bears some relation to a parameter of interest that we would otherwise not be able to measure directly. For example, the ratio between component X and component Y in a sample of marine sediment may be related to the temperature of seawater at the time these components were formed in the water column. The colder the water, the greater the formation of X, and the warmer the water, the greater the formation of Y. If components X and Y are deposited on the seafloor and remain preserved in the sediment over time, by analyzing them today we can reconstruct the temperature from when they were formed. The X/Y ratio would then be a proxy that allows us to reconstruct the paleotemperature of the seawater.


The components preserved in marine sediment samples can be organic and inorganic compounds, shells of organisms, pollen, pieces of vegetation or volcanic ash. The proxies used in paleoceanography can provide us with information on parameters such as temperature, salinity, water masses, marine productivity, CO2 concentration, supply and type of terrestrial vegetation. When analyzed together, these proxies help us reconstruct the most varied environmental, climatic and oceanographic processes, such as variations in the mixed layer, current intensity, continental precipitation, sea level and ice volume. The combination of sedimentary records collected in various parts of the world provides us with an overview of changes in the thermohaline circulation, global heat transport and the Earth's climate. 


Most paleoceanographic work focuses on reconstructing seawater temperature. Temperature proxies can be divided into two groups: inorganic and organic. Inorganic temperature proxies include the ratio of oxygen-18 to oxygen-16 isotopes (represented by δ18O) and the elemental ratio of magnesium to calcium (Mg/Ca), which are present in the calcite of foraminiferal shells. The δ18O proxy is based on the principle that the ratio of oxygen-16 (the lighter isotope) to its heavier isotope oxygen-18 during foraminiferal calcification varies with temperature, so that increasing temperature results in the precipitation of calcite depleted in oxygen-18. δ18O is also a salinity indicator and an excellent ice volume indicator, as ice stores more oxygen-16 (lighter), leaving more oxygen-18 (heavier) in the ocean and consequently in the shells of foraminifera present on the ocean floor. In the case of the Mg/Ca ratio, the increase in temperature is responsible for increasing the incorporation of Mg during the calcification of foraminifera. Both δ18O and the Mg/Ca ratio can also be applied to reconstruct temperature from coral records, as these are also formed by calcium carbonate.


Organic temperature proxies are based on the ability of various microorganisms to adjust the stability of their cell membranes to local temperature variations through changes in the structures of certain compounds during their formation. These structural changes involve variations in the number of unsaturations or rings in their molecules, which can be easily identified and quantified through specific analytical techniques. Among the organic compounds preserved in marine sediments used to reconstruct past seawater temperatures are alkenones (long-chain ketones with 2 to 4 unsaturations) and long-chain diols produced by microalgae, as well as glycerol dialkyl glycerol tetraethers (GDGTs) produced by archaea and bacteria. 


All proxies used in paleoceanography carry uncertainties that can affect the accuracy of estimates and lead to misinterpretation of environmental variations. Therefore, it is important to recognize the limitations of each proxy and to keep in mind that there is no perfect proxy. It is essential to try to combine independent proxies to reconstruct the same parameter and observe whether they show the same trend. The potential impact of human activities on present and future climate has increased interest in understanding past climate. A reliable and well-resolved reconstruction of past climate variations is essential for better investigation and prediction of what awaits us in the future.


 

References or suggested reading:


Erez, J.; Luz, B. 1983. Experimental paleotemperature equation for planktonic foraminifera. Geochimica et Cosmochimica Acta, 47:1025–1031. https://doi.org/10.1016/0016-7037(83)90232-6


IPCC. 2018. Summary for Policymakers. In: Global Warming of 1.5°C. An IPCC Special Report on the impacts of global warming of 1.5°C above pre-industrial levels and related global greenhouse gas emission pathways, in the context of strengthening the global response to the threat of climate change, sustainable development, and efforts to eradicate poverty [Masson-Delmotte, V., P. Zhai, H.-O. Pörtner, D. Roberts, J. Skea, P.R. Shukla, A. Pirani, W. Moufouma-Okia, C. Péan, R. Pidcock, S. Connors, J.B.R. Matthews, Y. Chen, X. Zhou, M.I. Gomis, E. Lonnoy, T. Maycock, M. Tignor, and T. Waterfield (eds.)]. World Meteorological Organization, Geneva, Switzerland, 32 pp.


Kim, J.H., van der Meer, J., Schouten, S., Helmke, P., Willmott, V., Sangiorgi, F., Koç, N., Hopmans, E.C., Sinninghe Damsté, J.S. 2010. New indices and calibrations derived from the distribution of crenarchaeal isoprenoid tetraether lipids: Implications for past sea surface temperature reconstructions. Geochimica et Cosmochimica Acta, 74:4639–4654. https://doi.org/10.1016/j.gca.2010.05.027


Locarnini, R.A., Mishonov, A.V., Antonov, J.I., Boyer, T.P., Garcia, H.E., Baranova, O.K., Zweng, M.M., Paver, C.R., Reagan, J.R., Johnson, D.R., Hamilton, M., Seidov, D., 2013. World Ocean Atlas 2013, Volume 1: Temperature. NOAA Atlas NESDIS. http://www.nodc.noaa.gov


Mann, M. E.; Bradley, R. S.; Hughes, M. K. 1998. Global-scale temperature patterns and climate forcing over the past six centuries. Nature, 392:779. https://doi.org/10.1038/33859


Müller, P. J.; Fischer, G. 2004. Global core-top calibration of U37K (update). PANGAEA. https://doi.org/10.1594/PANGAEA.126662


Nürnberg, D.; Bijma, J.; Hemleben, C. 1996. Assessing the reliability of magnesium in foraminiferal calcite as a proxy for water mass temperatures. Geochimica et Cosmochimica Acta, 60:803–814. https://doi.org/10.1016/0016-7037(95)00446-7


Prahl, F.G., Wakeham, S.G. 1987. Calibration of unsaturation patterns in long-chain ketone compositions for palaeotemperature assessment. Nature 330:367–369. https://doi.org/10.1038/330367a0


Rahmstorf, S. 2002. Ocean circulation and climate during the past 120,000 years. Nature, 419:207–214. https://doi.org/10.1038/nature01090


Rampen, S. W.; Willmott, V.; Kim, J. H.; Uliana, E.; Mollenhauer, G.; Schefuß, E.; Sinninghe Damsté, J. S.; Schouten, S. 2012. Long chain 1,13- and 1,15-diols as a potential proxy for palaeotemperature reconstruction. Geochimica et Cosmochimica Acta, 84:204–216. https://doi.org/10.1016/j.gca.2012.01.024


Schouten, S., Hopmans, E.C., Schefuß, E., Sinninghe Damsté, J.S. 2002. Distributional variations in marine crenarchaeol membrane lipids: a new tool for reconstructing ancient sea water temperatures? Earth and Planetary Science Letters, 204:265–274. https://doi.org/10.1016/S0012-821X(02)00979-2


Urey, H. C. 1947. The thermodynamic properties of isotopic substances. Journal of the Chemical Society (Resumed), 582:562. https://doi.org/10.1039/JR9470000562


 

About the author:



Oceanographer from UERJ, with a master's and doctorate in chemistry from PUC-Rio, and a doctorate in Germany at the University of Bremen and the Alfred Wegener Institute for Polar and Marine Research. She is currently a postdoctoral fellow at the Laboratory of Marine and Environmental Studies at PUC-Rio, specializing in marine organic geochemistry. She is always immersed in the world of lipids and stable isotopes to understand the sources, transport and destination of organic matter, as well as its relationship with recent or past processes. She also has experience and loves to get involved with marine pollution, with a focus on petroleum hydrocarbons. She is passionate about the possibilities of collaboration and knowledge exchange between all areas of oceanography, since everything is connected and no area works alone. Her leisure time is divided between the beach, movies, books, music and, as a true Carioca, she would like Carnival to last the whole year.



 


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