What are proxy data?
Proxy data are used to reconstruct the climate and ocean states from thousands to millions of years ago. They can be thought of as nature’s record-keepers, in that proxy data come from pollen, trees, coral, ice cores, stalagtites, and ocean and lake sediments and are preserved physical characteristics of the environment that can stand in for direct measurements. Because we can’t go back in time to test the air and measure how much CO2 was around, and how hot or how cold it was, paleoclimatologists heavily rely on these records to tell them something about the Earth. Below are some common proxy records that are used in paleoclimate reconstructions, or to interpret the climate and ocean conditions of the past.
Some people hate pollen, as it can cause horrible allergic reactions and in the Spring, chances are your car may become covered in it if you park too close to a flowering tree! But paleoclimatologists love pollen, especially pollen that is fossilized. Certain types of pollen occur in very specific areas; for example, the Black Spruce, an evergreen tree, grows in taiga forests (‘boreal’ or ‘snow forest’) in high latitudes. These trees produce a certain type of pollen that is only found in cold, high latitude regions of the world today. So, if we find Black Spruce pollen in old rocks or ocean sediments at lower latitudes (closer to the equator), we can interpret that the Earth may have been colder in the geologic past. By knowing what kinds of pollen occur in each climate belt today, we can then interpret what the climate belts of the past looked like by mapping out where ancient pollen is found in old rocks and sediments.
Ice cores are one of the most useful proxies for reconstructing gas levels in the Earth’s atmosphere at times in the past. As snow falls at high latitudes (such as Antarctica and in the Arctic), it packs down over time. Eventually, snow packing leads to land glaciers, and within the ice, there are tiny air bubbles that became trapped. Paleoclimatologists are very interested in obtaining ice cores because they can measure the gas content within the air bubble, which are a direct measurement of the atmosphere thousands of years ago! Currently, the longest ice core paleoclimatologists have to work with is 3,910 meters (10,465 ft!!) and goes back 800,000 years!! The ice core was recovered by the European Project for Ice Coring in Antarctica (EPICA) at Dome C, about 550 km from Vostok. When the gas bubbles in the EPICA Dome C core were analyzed, the data indicated that the Earth had been through 8 previous glaciations within the past 800,000 years.
There are several ice core storage facilities right here in the United States. To see additional images of ice cores and read more about how they are obtained, sampled, and analyzed, visit the National Ice Core Laboratory (located in Lakewood, Colorado) website, as well as the Byrd Polar Research Center (located in Columbus, Ohio).
Trees are often not the type of proxy record you would expect to think about when talking about reconstructing climate from thousands of years ago. Trees can grow to be hundreds of thousands of years old, and their growth is influenced by climate. If you’ve ever chopped wood, or perhaps have any polished wood surface in your home, you might have noticed that there are natural rings within the wood. Typically, a tree will add a ring, or layer, every year. Thus, paleocliamtologists can take slices of old trees and count the number of rings to determine how old the tree may be.
In addition, the width of the rings as well as the colors tell us information about what the climate was like at the time the ring was being added (the tree was growing). Thicker rings may indicate favorable growing conditions (increased precipitation), whereas thin rings may indicate drought conditions or that the weather was too warm or too cold. We can even tell if there was a forest fire during a certain year, as some rings may be burned. Because tree rings provide annual resolution of climate, they are often used in archaeological studies as well. The study of using tree rings in age dating is called dendrochronology.
Corals are similar proxies to tree rings in that they grow (but they do not add rings every year) and record the chemistry of the seawater in which they precipitate their skeletons from. Corals are made of the mineral calcite (CaCO3), so we can use samples of their skeletons for isotope analyses (both carbon and oxygen). As reviewed on the Isotopes page, carbon can tell us something about productivity in the water column, and oxygen isotopes tell us about precipitation, evaporation, ice volume, and temperature of the surface waters. Today, corals occupy very specific temperature and water conditions. They are usually found in shallow waters warmer than 64.4° F, which today translates to tropical regions. Because corals and their reefs are found in restricted climates, we can even use the distribution, or occurrence, of ancient corals to aid in our understanding of Earth’s past climates. For example, in the Cincinnati, Ohio region and throughout the southern parts of Quebec, Canada, rocks of Ordovician age (~450 million years old) contain corals. Thus, we can infer that 450 million years ago, these areas (Ohio and southern Canada) had tropical climates due to the presence of corals!
To learn more about corals and how they are used in paleoclimate reconstructions, visit the NOAA Coral page here.
Have you ever toured a cave, or maybe even gone spelunking? If so, chances are you’ve seen stalactites (hanging from the cave ceiling) and stalacmites (jutting up from the cave floor), which are calcite (CaCO3 formations that form over hundreds of thousands of years. Paleoclimatologists commonly refer to the family of cave formations such as stalactites and stalacmites as ‘speleothems’. Similar to corals, speleothems record past climate conditions over the last hundreds of thousands of years through isotopes of water and hydrogen.
Speleothems that recently formed during the Earth’s most recent epoch, the Holocene, we can reconstruct the climate on a yearly resolution! In order to study speleothems, scientists trek into caves (most of which aren’t disturbed by humans, as the oils on our skin damage the speleothems) and collect the speleothems by cutting them from the cave walls.
Back in the lab, the speleothems are sliced in half and polished so that the rings formed within the speleothem are clearly visible. Using a microdrill, small samples of calcite are removed and powdered. This powder is then put into a machine (called a mass spectrometer) that measures the isotopes of hydrogen and oxygen, which are then used to reconstruct climate.
Ocean & Lake Sediments
Ocean sediments are perhaps one of the most common proxy records paleoclimatologists use to reconstruct climate on hundred, thousand, and million year timescales. Each year, billions of tons of sediment collect on the ocean floor as well as within lakes. This sediment consists of dust, minerals, organic matter (essentially dead organisms and their poop), and tiny microfossils. Within the realm of paleoclimatology, there are different scientists that study different types of sediment. For example, organic geochemists are interested in the organic material that has collected on the bottom of lakes and the ocean, as some of this material is preserved and can be analyzed to tell how how warm or cold the Earth was. On the other hand, micropaleontologists are interested in the shells of tiny marine organisms and plankton. Adriane will elaborate more on the use of ocean sediments, specifically the use of plankton known as foraminifera, in her studies to reconstruct past ocean conditions and climate over the past 15 million years.