In two of my recent posts about Earth history, I didn’t get to go very far into how scientists know what we know about the prehistoric climate or about Earth’s past in general. Paleontology is a fascinating field to me because it requires rather creative methods of finding information in fossil and geologic evidence where one might not immediately realize such information could be gleaned. And as we’ve covered before, studying the history of Earth’s climate has practical benefits for understanding our current changing climate. So let’s talk briefly about how the science of paleoclimatology is done.
Paleoclimate Proxies
To study the climate of the distant past, scientists use climate proxies; materials which are structurally affected by temperature, precipitation, and other aspects of prehistoric climate, which then preserve these measurements into the present. One of the best examples of a climate proxy is the polar and glacial ice sheets. I’ve talked before about ice cores being used in scientific research, but to recap ice sheets are formed when more snow falls in a region than melts, so snow piles up and gets crushed by its own weight into sheets of ice. This ice can remain unchanged for millions of years, locking up whatever was buried in the snow when it first fell. And since new ice forms on top of old ice, one can use the depth of where samples are taken to determine the age of the sample. These samples may include;
Air bubbles. As snow piles up, the spaces between snowflakes get crushed into bubbles and the air in these spaces gets trapped. These bubbles contain samples of the prehistoric atmosphere unchanged since the snow first fell. CO2 concentrations of this ancient air can provide information about global climate.
Isotopes. Some atoms of an element are ever-so-slightly heavier than the others. Over 99% of all oxygen atoms are Oxygen-16 (8 protons, 8 neutrons), but about 0.2% of atoms are the slightly heavier Oxygen-18 (8 protons, 10 neutrons). Because Oxygen-18 atoms are slightly heavier, water molecules made up of Oxygen-18 require slightly more energy to evaporate. In warmer climates, these heavier water molecules evaporate more readily, so rain and snow contain more Oxygen-18 atoms. By analyzing the amount of Oxygen-18 in ice cores, one can calculate the temperature when this snow fell.
Layering. Since snow only falls for part of the year, clear layers are visible in the ice demarking what year the snow fell, similar to tree rings. Not only does this make dating samples easier, but the width of these layers tell how much snow fell during a time period, which gives precipitation data.
Impurities. Material frozen in the ice can provide even more information. Large amounts of dust can denote a drier global climate since arid conditions lead to dust storms. Volcanic ash can inform about eruptions, which both impact global climate and can help date samples if the particular eruption is known. And pollen frozen in the ice can give a glimpse into what plants were present when the ice formed, which is extremely useful for determining local climate.
Aside from current ice sheets themselves, physical evidence of long-gone ice sheets can be left behind on the ground they once covered. How much of the world is covered in ice sheets tells a lot about the global climate at the time. Evidence of these former ice sheets present themselves in the following ways;
Glaciers produce very distinct erosion patterns on rock as they move over it. This includes everything from small abrasions created by glaciers moving over the rock to enormous fjords and U-shaped valleys cut by glaciers as they flow overhead. (Watch this video from Be Smart for more about how glaciers move)
As glaciers move, they carry material with them just as a river would. But because ice is denser than water, glaciers can carry far heavier material than a river. These deposits of glacial material include everything from distinctively shaped- piles of sediment to large boulders, none of which match the geology of the surrounding landscape.
Ice sheets are heavy, so the ground beneath them is slowly compressed by the weight. Once the ice sheets are gone, the ground very slowly rebounds to its original shape over the course of thousands of years. This rebounding means land once covered by ice is slowly rising, only by a centimeter or less per year which is enough to be detected by surveyors. Gravity can be slightly different in these rebounding regions due to the crust being denser and these regions can be prone to earthquakes, such as the 1811 New Madrid Earthquake.
Image 1: Glacial Striations
Image 2: Diagram of boulder being deposited by glacier
There are other places where scientists look for climate data besides current ice sheets, including;
Sand and sediment on lake beds and the ocean floor can have cores taken from them just like ice cores. These sediment cores contain isotopes, layering, and impurities similar to ice cores. Some of this sediment consists of the shells of diatoms, microorganisms that make shells. Identifying species of diatoms can give biome information and the shells themselves can give isotope data.
Tree rings contain information about a tree’s environment. Growth rings form in trees in temperate regions as their growth rate waxes and wanes with the seasonal cycle, creating one ring for each year the tree has been alive. The width of these rings and the density of wood within these rings is a proxy for temperature and precipitation during the associated year. Now, the oldest living trees are only 4,000 years old, so they can only provide data going back that far (for perspective, the last glaciation ended 12,000 years ago). But the fact that individual years are so clearly demarcated makes tree rings incredibly useful for near-term climate data and fossilized trees can provide much of the same data from far older trees.
Corals also have growth rings. They’re growth rate can provide information about sea temperature and salinity going back several centuries. These rings also contain isotopes that serve as proxies for temperature.
Fossilized plants and plant parts give a glimpse to what biome a location was in the past, provided one can identify the species. Fossilized pollen is relatively plentiful and can be isolated from sediment cores and sedimentary rock. Leaves have adaptations to their environment that can remain visible when fossilized, so information about an area’s climate can be gleaned even if species can’t be identified. The size of fossilized leaves can inform the plant’s health which can inform precipitation levels. Isotopes of carbon in these fossils can inform atmospheric CO2 levels.
Dating
Once one has found sources of data that correlate to prehistoric climate, one must then determine what point in time this data correlates to. For tree and coral rings from living organisms this is relatively easy, just count the number of rings and subtract from today’s date. But if one is working with preserved or fossilized wood or coral from dead organisms, this doesn’t work as well. For ice core data, being buried further down correlates to older samples as snow builds up on top of samples and is slowly crushed into sheets of ice. Fossils are buried in the same way (replace snow with sediment and ice with sedimentary rock) so they too can be dated by how deeply they are buried. But for this technique to work, one must know exactly how much snow/sediment is added over a given time and how long it takes to be crushed into ice/rock in order to gauge the exact correlation between depth and time. Figuring this out requires a lot of its own data. And the most reliable way to collect this data is radiometric dating.
You’ve probably heard of radiometric or radioactive dating in some form, specifically carbon dating. I’ve talked about radioactivity before, but just to recap, not all isotopes are stable. The most common isotope of carbon is carbon-12, which accounts for 99% of all carbon atoms. Carbon-13 is far less common, but it is still a stable isotope; a carbon-13 atom will remain a carbon-13 atom indefinitely. Carbon-14 however is not stable, it has too many neutrons to balance out the magnetic forces within it so it will eventually decay into another atom in order to balance itself. The rate at which this decay happens is incredibly precise, meaning that if one knows how many radioactive atoms were in a sample at an earlier date and how many are in the sample now, one can calculate how much time has elapsed. For example, roughly one in every trillion carbon atoms are carbon-14. When an organism is alive, it will contain roughly the same amount of C-14 in its body as carbon atoms are being continuously replaced before they can decay (Don’t worry, the radiation released is so small it doesn’t pose a threat to the organism, including yourself).But once an organism dies, it stops replacing its carbon and will contain the exact same atoms indefinitely. Without continuous replacement, the number of C-14 atoms in the dead organism will slowly decline as atoms decay. The fewer C-14 atoms there are in a sample, the older it is, with the exact age being easy to calculate when you know how many atoms are in a living organism. Radiocarbon dating can be used to date any organic substance, including preserved tree rings and material frozen in ice sheets.
Radiocarbon dating only works for organic samples (things made from once-living organisms) less than 50,000 years old (any older and there won’t be enough C-14 left to detect). But radiometric dating or carbon-14 or other radioactive isotopes are a useful tool for determining how fast certain processes take. Samples taken from the top of an ice sheet will have more C-14 then samples taken a few meters below that. With that, one can determine how fast new ice is created and thus what depth correlates to what age. With this information, one can use depth to approximate the age of samples taken further down. None of these techniques are perfect; there are lots of natural processes that can vary how fast ice sheets form, how much C-14 an organism started with, or otherwise add caveats to any proxy. This is why scientists use so many proxies; multiple data sources lets one eliminate errors in any one source. If one wanted to know the average global temperature 200,000 years ago, and ice cores, sediment cores, and fossilized leaves give similar numbers for this period, one can be relatively sure of that number’s accuracy. If one proxy gave a significantly different value, that could reveal something interesting in-and-of itself. And I’ve only described a few of the most common proxies and dating techniques here, there are many other places one can find climate data.
I worry this will be one of my less interesting posts due to its technicality, but I feel it’s important for other reasons. I sometimes worry that science, like other esoteric fields of study, can come across as a black box. The layman can only see the facts it spits out and may be expected to accept its authority on blind faith. I have talked before about why I believe the scientific process and the institutions that participate in the process have more than earned that authority, but I would never tell anyone to take anything on faith. That is why I like to share how the proverbial sausage is made, because it’s easier to trust something that’s open about how it works. So I hope I’ve given you some insights into how we know what we know about both climate science and paleontology and that these insights make both fields more approachable.
I will be starting a new job, so I will be taking a hiatus as I figure out how I will incorporate Scientific Rhodes into my new routine. I hope to make it brief and to return here soon.
Comments