Investigating a cold case:
Tracking ancient climate change with modern data
To understand past human behavior, archaeologists need information about the environment the first people to move into Ice Age Cook County would have encountered.
• What were plant and animal communities like?
• How cold was it?
• What was there to eat?
• How fast was climate changing as glaciers retreated to the north?
Many plant and animal species are very sensitive to environmental change. Tiny, preserved bits of evidence like pollen grains (top left) and ostracods (small, freshwater crustaceans; bottom right) recovered in archaeological sites or soil cores allow us to reconstruct the Ice Age climate of Cook County and surrounding areas. The bogs and wetlands which developed around glacial Chicago are a particularly good source of this paleo-environmental data.
The remains of smaller critters in archaeological and paleontological sites also tell us about past climate change. For example, lemmings (left), arctic ground ground squirrel (center), and certain species of ground beetles (right) only lived in northeast Illinois during cold, fully glacial times. But as climates warmed and glaciers retreated around the time people first entered the region, these species moved further north and are no longer seen in Illinois sites.
Much of the evidence archaeologists use to reconstruct past climates comes from soil cores. Archaeologists trained in soil science (geoarchaeologists) can use soil cores to tell how a landscape formed over time. Knowing how a location evolved over time – whether landscapes were building up or eroding away and how they were being affected by wind, water, and other processes like seasonal freeze/thaw – can often tell us quite a bit about how climates changed over time as well.
Less obvious clues also tell us about past climate change. For example, the ratio of different oxygen isotopes found in ice cores, soil samples, or paleontological remains changes depending on how warm or cold temperatures were at a given time. As water cycles between the oceans, atmosphere, and terrestrial environments, it carries oxygen isotopes with it (H2O). Oxygen comes in two main flavors – “light” 16O (8 protons and 8 neutrons) and “heavy” 18O (8 protons and 10 neutrons). In cold, glacial times “heavy” oxygen returns to the ocean sooner as precipitation, and more “light” oxygen is trapped in glacial ice or gets incorporated into the bones of living animals that drink water and breath air. The reverse is true in warmer periods. Scientists can measure the ratio of these different oxygen isotopes in samples to get at past climate. After all, you are what you eat (and drink and breath)!