P200 Introduction to Prehistoric Archaeology
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Reconstructing Ice Age climate
Global climate has fluctuated through time, driven in part by the changing energy budget of the earth. As originally hypothesized by Milankovitch, earth's orbital parameters influence the distribution of energy falling on the earth's surface through time. Some of this energy is reflected back into space off of clouds, and highly reflective surfaces, like ice or snow (land surfaces with high "albedo"). The rest is absorbed by the oceans and landsurfaces or trapped in the atmosphere (e.g. by "greenhouse gases"). The differential distribution of energy (heat) across the globe drives ocean currents and atmospheric winds, and determines short-term (weather) and long-term climatic cycles. (Note that over the very long-term, the distribution of continental landmasses, mountain chains, etc, also influences global climate.)
During the last million years, earth's climate has experienced wide swings in climate, from relatively cold ("glacial) conditions to warm ("interglacial") ones. Each cycle has lasted approximately 100,000 years, and these cycles are called Ice Ages. Within each major glacial cycle a number of short-term, small-scale fluctuations have also occurred.
The effect of these climatic changes has been relatively minor in the tropics, but has had a major impact on terrestrial environments in the middle latitude, temperate regions, like Europe and North America.
Glacial conditions: cooler summers allow winter snows to accumulate, and cause polar ice caps to grow and high altitude glaciers to spread. Cold, dry conditions in many areas cause temperate woodlands to shrink, cold-adapted evergreen tree species to spread, and treeless steppe environments and grasslands to spread. The last glacial maximum in North America occurred between 20,000-18,000 years ago. In Indiana, ice sheets extended as far south as Martinsville. New York City, Chicago, and Seattle were buried under a thick cap of ice.
Inter-Glacial conditions: hotter summers, and sometimes colder winters, and often more variable weather patterns and high-energy storms cause ice sheets and glaciers to shrink, sometimes extremely rapidly. Warm, wetter conditions in many areas encourage the expansion of woodlands and temperate forests. We are currently living in inter-glacial times, exacerbated by additional global warming caused by the greenhouse gases we are adding to the atmosphere, which help trap solar radiation and make the earth's climate even hotter than would naturally be expected during our time in the normal interglacial cycle.
Evidence of Climate Change:
There are many types of evidence that can be used to reconstruct the changing record of earth's past climate. Most of these are called "proxy indicators" of climate, because they follow the effects of ancient temperature changes, rather than documenting the temperature directly.
For example, in terrestrial environments, archaeologists can often relate their sites to:
- geological evidence of glacial environments:
- lower sea levels (strand lines and beaches)
- morraines indicating the maximum extent of ice sheets
- fine, air-blown sediments (dust) ground up by ice sheets that blow around and form extensive deposits during cold, dry conditions
- soils that freeze and crack during very cold times
- cave roofs and walls will freeze and crack and cause rock falls during very cold times
- evidence of changing frequencies of plants or animals that prefer different environments
- frequencies of pollen preserved in sites or lake sediments can show changes in regional vegetation
- micro-mammals, in particular, reflect very local conditions and can document changing habitats
- large mammal communities can also reflect habitat shifts -- large herds of grazing bison and horse roamed across open steppes in Ice Age Europe ... later replaced by smaller, territorial species adapted to the woodlands typical of inter-glacial times
For evidence of world-wide sequences of climate change, climatologists can sample sediments drilled from the deep ocean or from cores of ice drilled from ice caps (e.g. in Greenland and Antarctica). These cores have tiny strata (layers) that preserve chemical and biological indicators of environmental conditions at the time they were deposited. The strata can be dated directly using radio-metric techniques.
- Micro-organisms that live in the ocean (like plankton) die, and their exo-skeletons drift to the bottom, being incorporated in sediments. Sometimes these organisms prefer specific habits (e.g. love warm water) and so their presence in an ocean at a particular latitude can help climatologists reconstruct ocean temperature, or salinity at that spot. However, they also grow their exoskeletons (e.g. Calcium carbonate) using the elements available in the water. so the chemistry of their skeletons (called "tests") will reflect the chemical compostion of the water at the time they were alive.
- Oxygen isotopes: O-18 and O-16 are two stable isotopes of oxygen that occur in the air and water in known ratios. Because they have different atomic weights (O-18 is heavier) they behave slightly differently during chemical processes. For example, when water evaporates, molecules of water with the lighter isotope (O-16) evaporate more quickly, leaving more of the heavy isotope behind in the ocean or lake, enriching the remaining water with O-18. During glacial times, when more water has evaporated from the world's oceans, the remaining glacial waters are enriched in O-18. (The snow that accumulates in the ice caps is formed from the "light" water vapor, and thus polar ice is enriched in O-16.) Thus, plankton that lived during glacial times will have carbonate tests that are enriched in O-18, compared to inter-glacial plankton.
Links to good Paleo-climate sites and Ice Age sites on the web:
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Last updated: 15 February, 1999
Copyright Jeanne Sept 1998 : do not cite without permission