Paleoclimates

paleoclimates are, literally, past climates. The term "paleo" is, however, normally reserved for those time periods that are prehistorical, although usage of the term to describe climate in the first millennium c.e. and earlier is not uncommon. Like reading a historical text, paleoclimates give glimpses into the var ied past of Earth's climate system, and understanding of paleoclimates may well be critical to understanding how Earth's climate will change in the future.

Unfortunately, understanding of this critical climate history does not come easily. Unlike present climate that is carefully recorded and catalogued by various observational and physical measurements, paleoclimate records must be extracted from Earth's geological history; in particular, the archives of sedimentary rocks and large ice sheets. These geological archives do not contain precise measurements of climatic variables such as temperature and precipitation, but rather record environmental responses to ambient climate conditions. As such, the climate indicators extracted from the geological record are known as proxy data.

proxy data

An example of the use of proxy data would be conducting a North American winter snow survey by searching North American garages for snow shovels in the summer. In regions where snow shovels are common in garages, a researcher might reasonably interpret cold winter temperatures and the presence of snow. The shovels, though not actually snow, are a proxy for the occurrence of winter snow. In regions without snow shovels, winter temperatures are either warmer with no snow, or snow is removed in a different way. Because proxy data may be definitive in one sense, but ambiguous in another, using them to decipher the nature of

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The Petrified Forest, in northeastern Arizona, has one of the best fossilized records of the Late Triassic period in the world.

The Petrified Forest, in northeastern Arizona, has one of the best fossilized records of the Late Triassic period in the world.

paleoclimates requires a suite of data types, or a multiproxy approach. Fortunately, paleoclimatologists have proven extremely innovative in their development of climate proxies, which range from preserved vegetation in ancient pack rat middens (nests) to the relative abundances of different stable oxygen isotopes (notably 16O and 18O) in the shells of small, ocean-dwelling plankton known as foraminifera.

Some of the oldest proxy data are those related to preserved vegetation. That vegetation may be actual fossils (permineralized plant material or traces of plant material) or simply desiccated (mummified) plant remains, some of which can be tens of millions of years old in cold climates. Vegetation is used as a climate indicator in two primary ways. The first is based on the observation that certain types of plants inhabit certain types of climates. Alders, for example, prefer wetter climates, while Ponderosa Pines thrive in drier conditions. As such, if a plant fossil is found in a given location and there is reasonable certainty that the plant actually grew there, climate in that location can be interpreted based on the known preferences of the identified vegetation.

This is relatively straightforward when the preserved plant material is from a known, living species such as an Alder or Ponderosa Pine. It becomes increasingly difficult further back in geological time when many plants are unknown. In these instances, the climate tolerances of the extinct plant's nearest living relative are used to interpret climate. The other common use of preserved vegetation as a climate indicator is through a statistical process known as leaf margin analysis. Statistical studies of large, modern data sets have indicated that the shape of a leaf's margin (smooth or toothed, elongated or rounded) has some correlation to the climatic conditions that the plant prefers. By performing the same analyses on ancient leaves, scientists can compare ancient leaf samples to the modern climate interpretation and make some assessment of paleoclimatic conditions.

While analyses of preserved vegetation continue to provide significant insight into paleoclimates, more recently, scientists have developed a suite of chemically based climate proxies that make use of the lengthy climate records stored in the sediments of the ocean floor. The most common of these chemical proxies is based on the stable isotopes of oxygen (in particular 16O and 18O, which are the most abundant).

Because different isotopes of a given element have different masses, they are relatively easier or harder to evaporate and, inversely, relatively harder or easier to precipitate. This means that water vapor will always have more 16O than the liquid it evaporated from and is, therefore, lighter. Liquid precipitation will always have more 18O than the vapor it condensed from and is, therefore, heavier. These processes can be used to decipher past climate.

For icehouse climates with large, continent-spanning ice sheets, the relative volume of water stored in large glaciers largely influences the oxygen isotopic concentration of global seawater. Because lighter isotopes evaporate preferentially, precipitation is almost always lighter than the global ocean. If precipitation falls as snow and is then trapped as glacial ice, the overall ocean isotopic composition will get heavier as more and more of the light isotope is locked up in growing ice sheets. If climate warms and ice sheets begin to melt, the lighter isotopes are returned to the ocean and the ocean isoto-pic composition becomes lighter. Thus, changes in the oxygen isotopic composition of ocean water can be tied to growing and shrinking ice sheets and, by extension, cooling and warming climate.

The primary source of information on past oceanic oxygen isotopic composition is the shells of ocean dwelling microorganisms, in particular, foraminifera. Foraminifera create their shells out of calcium carbonate (CaCO3). The oxygen incorporated into the foraminifera shells comes from the ocean water and, therefore, reflects the ocean's oxygen isotopic composition. The exact isotopic ratio of the shell, however, is also influenced by the ambient temperature and, to a lesser and largely negligible extent, salinity, as well as the organic processes of the foraminifera. Empirically derived equations relate the oxygen iso-topic composition of calcium carbonate shells to the ocean temperature and oxygen isotopic composition. If any two of those values are known, the third can be calculated. While the isotopic composition of ancient foraminifera shells can be measured in the laboratory, the overall oceanic oxygen isotopic composition and temperature in the past must either be estimated or derived from other proxies.

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