Imagine standing on the top floor of the Empire State Building. Above you, the frigid ice-capped waters of a lake in Siberia. Below you sits nearly a quarter of a mile of lake sediment resting atop impact breccia, a layer of rock formed when a meteorite slammed into Earth 3.6 million years ago.
An international team of scientists with the Lake El’gygytgyn Drilling Project is studying a sediment core extracted from the site. The remnants of plants, pollen, ash, and microscopic organisms like plankton in that core tell a story. Past climate conditions can be reconstructed by deriving information from different properties of the sediment content. These properties which help paleoclimatologists discover and quantify the details of ancient climate are called proxy indicators. By gaining data about past climates, we can better predict our climate future.
Julie Brigham-Grette is a professor of Quaternary Geology in the Geosciences department at University of Massachusetts Amherst. She served as the U.S. Lead in the Lake El’gygytgyn Drilling Project. She says that for geologists studying cores, a vital step in studying paleoclimates is to establish a chronology– to date when layers of the core were created. In less than one minute, listening to this segment* of her presentation, I realize how much I still have to learn about paleoclimatology. Show me a host of wiggles graphed on a page, I want to understand that host so that I can better understand the amazing and ongoing discoveries the Lake E team members have made.
Accordingly, here’s more paleoclimatology 101.
Insolation measures how much radiant solar energy from the Sun reaches a specific surface area on Earth at a specific time. It describes the input of solar energy into the system. Earth’s energy budget and equilibrium temperature are determined by a balance between energy being received from the Sun and energy being lost to space.
Solar radiation does not heat Earth evenly. The NASA graphic attached to the article illustrates this well. For simplicity it shows the total energy that reaches the top of the atmosphere, not the Earth’s surface. In this graphic, lighter pink colors indicate that locations along the equator receive high daily amounts of incoming energy every month of the year. Meanwhile the high latitudes north of the Arctic Circle are characterized by no incoming energy (black) during the sunless winter months and extremely high amounts of incoming energy (pink-nearly-white) during summer months when the Northern Hemisphere is tilted toward the Sun. The Southern Hemisphere experiences this in reverse; for the south, summer centers on December. Latitudes south of the Antarctic Circle receive even higher amounts of solar radiation than the far north does during its June-centered summer because Earth’s elliptical orbit brings Earth slightly closer to the Sun during December.
Thanks to qualities of the atmosphere and ocean like evaporation, rainfall, winds and ocean currents, the unevenly-received heat from the sun is redistributed throughout Earth’s regions. Complex interactions among these and other components of the Earth system are what govern the expression of our climate.
To determine the total amount of solar radiation energy received at Lake E’s coordinates over the last 3.6 million years the scientists would have had to include many factors in their calculations. We already know that insolation varies depending on latitude and time of the year. On a longer time scale, solar activity and thus radiation from the Sun vary slightly over time (these changes can prove impactful despite varying only to the extent of a tiny fraction of 1%). Earth’s orbital path and the tilt of the planet’s axis also cycle through changes over time. On shorter time frames, tiny particulates ejected into the atmosphere by i.e. volcano eruptions and wildfires can impact how much solar radiation actually reaches the ground’s surface.
Brigham-Grette references the alternating paleomagnetic normal reverse time scale.
Layers of the sediment cores have measurable magnetic properties; they were formed on Earth and Earth has a magnetic field. Currently it’s oriented toward the North Magnetic Pole (also North geographically) which we term a position of normal polarity. When the field reverses and orients toward the South Magnetic Pole it’s termed reverse polarity. A reversal event is rare, and happens unpredictably. But when we look at geologic records, push back through time, we can see the polarity alternating between normal and reversed intervals something like really disorderly piano keys.
Magnetostratigraphy is used to determine the polarity of Earth’s magnetic field at the time sediment was created. Sediments in any given layer or strata of a stratified sediment core have a remnant magnetic polarity; segments characterized by a given polarity have a great name: chrons, or magnetostratigraphic polarity units. Since periods of normal polarity and reverse polarity and their time spans are documented in other geologic records, the stratigraphic sequences’ magnetic properties help date the core segments.
Oxygen isotopes also serve as proxy indicators. One variety of oxygen atom has 8 neutrons, 8 protons and a cloud of electrons; 8 + 8 = 16 and so this oxygen isotope’s atomic weight is 16, and can be written as delta 16 oxygen: δ16O. Another heavier variety of oxygen isotope has 10 neutrons, 8 protons and a cloud of electrons; 10 + 8 = 18 or δ18O. This heavy variety occurs less frequently. There is only about one δ18O for every five hundred δ16O atoms. (I’m about to switch from superscript to normal script because superscript can be hard to see.)
δ16O’s weight is lighter than δ18O’s and, accordingly, the two behave differently. The lighter isotope evaporates more easily, dominating water vapor composition while leaving isotopes with more neutrons behind in the world’s oceans. Heavier isotopes need more energy (and therefore warmer temperatures) to change state from a liquid to a vapor. Even once the heavier δ18O has evaporated, its greater weight prompts it to condense and fall as rain sooner than its lighter brother. Since water vapor generally leaves the world’s oceans close to the warm equator before blowing north or south, the closer you get to the poles the less heavy δ18O still lingers in clouds waiting to precipitate.
The proportion of δ18O and δ16O in ice cores and sediment cores can be analyzed to determine temperature changes in past climates.
Warmer temperatures allow for more mobility of heavy δ18O isotopes away from the equator in measurable amounts. Warmer temperatures cause higher δ18O ratios in rainfall– a difference of one part per million for every 2.7°F [1.5°C] air temperature change at evaporation can be measured in ice cores extracted from glaciers and ice sheets. The glaciers and ice sheets– found near Earth’s poles– incorporate water from rainfall. Warmer temperatures provide enough energy for more δ18O to make it to high latitudes and fall as rain. On the other side of the spectrum, envision how cooling temperatures (i.e. at the start of an ice age) cause glaciers and ice sheets to form. They incorporate and trap rainfall; therefore, a large amount of δ16O becomes trapped deep in the ice masses. In prevailing cold the δ16O can’t melt and return to the oceans, which skews the ratios. Ocean water becomes more dominated by δ18O in the cold.
Aquatic sediments incorporate the shells of dead microscopic organisms. Warmer temperatures cause lower δ18O ratios in sediment cores– a difference of 0.2 parts per million for every 1.8°F [1°C] air temperature change at evaporation. The remains of organisms like foraminifera with shells made of calcium carbonate (CaCO3) record oxygen isotope ratios in sediment because of the O in CaCO3. Warmer temperatures which provide enough energy to mobilize heavy δ18O atoms remove more δ18O from ocean waters than colder temperatures do. Cold temperatures leave the heavy δ18O behind. The microscopic aquatic organisms have to use the oxygen near them, so cold temperatures result in sediment enriched by a higher ratio of δ18O.
Of course there’s more to it than that. With creatures involved I think it always gets more complicated. When making their shells, these organisms’ processes preferentially incorporate more δ18O than δ16O, but that trend can be noted and corrected for when determining oxygen isotope ratios in the body of water the shell grew in. Other chemical analysis and other isotope ratios can be used to help calculate and fine-tune temperatures. The scientists do not have a simple task.
By analyzing oxygen isotopes measured in cores, taking into account latitude and core type, scientists use the isotope ratio to calculate past temperatures. Averaging measurements from cores drawn from locations around the world provides a proxy record of past global climate, like the one Brigham-Grette referenced as the Lisiecki – Raymo stack (published by scientists named Lisiecki and Raymo in 2005).
Lake E published papers
Work is ongoing. As of now, find two major Lake El’gygytgyn papers in Science.
2.8 Million Years of Arctic Climate Change from Lake El’gygytgyn, NE Russia
Martin Melles, Julie Brigham-Grette, Pavel S. Minyuk, Norbert R. Nowaczyk, Volker Wennrich, Robert M. DeConto, Patricia M. Anderson, Andrei A. Andreev, Anthony Coletti, Timothy L. Cook, Eeva Haltia-Hovi, Maaret Kukkonen, Anatoli V. Lozhkin, Peter Rosén, Pavel Tarasov, Hendrik Vogel, and Bernd Wagner
Science 20 July 2012: 337 (6092), 315-320.Published online 21 June 2012 [DOI:10.1126/science.1222135] (abstract link)
Pliocene Warmth, Polar Amplification, and Stepped Pleistocene Cooling Recorded in NE Arctic Russia
Julie Brigham-Grette, Martin Melles, Pavel Minyuk, Andrei Andreev, Pavel Tarasov, Robert DeConto, Sebastian Koenig, Norbert Nowaczyk, Volker Wennrich, Peter Rosén, Eeva Haltia, Tim Cook, Catalina Gebhardt, Carsten Meyer-Jacob, Jeff Snyder, and Ulrike Herzschuh
Science 21 June 2013: 340 (6139), 1421-1427.Published online 9 May 2013 [DOI:10.1126/science.1233137] (abstract link)
Frontier Scientists: presenting scientific discovery in the Arctic and beyond
Where is Lake El’gygytgyn? project