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Conditions are also judged from the concentrations of noble gases found dissolved in old groundwaters. Some such records are subject to substantial loss of information through diffusion of the components being analyzed, which limits the ability to interpret older events.

Weldemichael, Awet | Department of History

Physical indicators include the characteristics of sediments and land features. For example, the presence of sand dunes can indicate past arid conditions, and glacially polished bedrock is an indication of prior glacial conditions. Isotopic indicators are widely used in paleoclimate science. The subtle differences in behavior between chemically similar atoms having different weights isotopes prove to be sensitive indicators of paleoenvironmental conditions. One common application is paleothermometry. The physical and chemical discrimination of atoms of differing isotopic mass increases with decreasing temperature.

For example, carbonate shells grow-. For example, gas bubbles trapped in ice can be analyzed to understand the atmosphere at the time the bubbles were trapped. This table lists examples of paleoclimatic proxies, what the proxy measures, and from where the proxy data originated. Isotopic ratios also are used to estimate the concentration of a chemical. Marine photosynthesis increasingly favors the light isotope of carbon as carbon dioxide becomes more abundant, and this allows estimation of changes in carbon dioxide concentration from the isotopic composition of organic matter in oceanic sediments.

Similarly, the growth of ice sheets removes isotopically light water ordinary water from the ocean, increasing the use of isotopically heavy oxygen from water in carbonate shells, which then provide information on the size of ice sheets over time. Stable isotopic values in organic matter also provide important information on photosynthetic pathways and so can afford insight into the photosynthesizing organisms that were dominant at a given location in the past. Many chemical proxies of environmental change act like isotopic ratios in the measurement of availability of a species.

For example, if decreased rainfall increases the concentration of magnesium or strontium ions in lake water, they will become more common in calcium-carbonate shells that grow in that water. However, warming can also allow increased incorporation of substitute ions in shells. Such nonuniqueness can usually be resolved through use of multiple indicators. Other chemical indicators are allied to biological processes. For example, some species of marine diatoms incorporate stiffer molecules in their cell walls to offset the softening effects of higher temperature, and these molecules are resistant to changes after the diatoms die.

The fraction of stiffer molecules in sediments yields an estimate of past temperatures. This analytic technique, known as alkenone paleothermometry, is increasingly used to learn about paleotemperatures in the marine environment. Biological indicators of environmental conditions typically involve the presence or absence of indicator species or assemblages of species.

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For example, the existence of an old rooted tree stump shows that the climate was warm and wet enough for trees, and the type of wood indicates how warm and wet the climate was; if that tree stump is in a region where trees do not grow today, the climate change is clear. In ocean and lake sediments, the microfossil species present can indicate the temperature, salinity, and nutrient concentration of the water column when they were deposited.

Pollen and macrofossils preserved in sediments are important records of variability in the terrestrial environment see Plate 3. The presence of specific organic compounds called biomarkers in sediments can reveal what species were present, how abundant they were, and other information.


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The complicated nature of paleoclimatic interpretation can be seen when proxies are viewed in a practical example. During ice ages, the oceans were colder, but the water in them was also isotopically heavier because light water was removed and used in growing ice sheets. Shells that grew in water during ice age intervals contain heavier isotopes owing to cooling and changes in the isotopic composition of ocean waters.

The change in ocean isotopic composition can be estimated independently from the composition of pore waters in sediments, whereas the change in temperature can be estimated from both the abundance of cold- or warm-loving shells in sediment and the abundance of stiff diatom cell-wall molecules in sediments. Concentrations of non-carbonate ions substituted into calcium carbonate shells provide further information. Because there is redundancy in the available data, reliable results can be obtained.

Any paleoclimatic record requires age estimates, and many techniques are used to obtain them.

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Annual layers in trees, in sediments of some lakes and shallow marine basins, in corals, and in some ice cores allow high-resolution dating for tens of thousands of years, or longer in exceptional cases. Various radiometric techniques are also used. Dates for the last 50, years are most commonly obtained by using radiocarbon 14 C. Changes in production of radiocarbon by cosmic rays have occurred over time, but their effects are now calibrated by using annual-layer counts or other radiometric techniques, such as the use of radioactive intermediates generated during the decay of uranium and thorium and also through the potassium-argon system.

Other techniques rely on measurement of accumulated damage to mineral grains, rocks, or chemicals; this permits dating on the basis of cosmogenic exposure ages, thermoluminescence, obsidian hydration, fission tracks, amino-acid racemization, and so on. Numerous techniques allow correlation of samples and assignment of ages from well-dated to initially less well-dated records.

Sedimentary records reveal numerous large, widespread abrupt climate changes over the last , years and beyond. The best known of them is the Younger Dryas cold interval. The Younger Dryas was a nearly global event that began about 12, years ago when there was an interruption in the gradual warming trend that followed the last ice age. The Younger Dryas event ended abruptly about 11, years ago Figures 2. Because the Younger Dryas can be tracked quite clearly in geologic records and has received extensive study, a rather detailed summary of the evidence is given here, followed by briefer reviews of other abrupt climate changes.

We then target Holocene 1 abrupt climate events as examples of substantial changes that have taken place when physical conditions on the earth were more similar to today.

Understanding the causes of both types of abrupt. The Younger Dryas cold reversal is especially prominent in ice-core records from Greenland, but it is also observed in ice cores from other locations. The ice-core records provide a unique perspective that demonstrates the synchronous nature of the large, widespread changes observed. Annual-layer counting in Greenland ice cores allows determination of the age, duration, and rapidity of change of the Younger Dryas event with dating errors of about one percent Alley et al. Annual-layer thicknesses corrected for the effects of ice flow give the history of snow accumulation rate in Greenland Alley et al.

Concentrations of wind-blown materials—such as dust which in central Greenland has characteristics showing its origin in central Asia [Biscaye et al. Gases trapped in bubbles reveal past atmospheric composition. Methane is of special interest because it probably records the global area of wetlands. Furthermore, differences between methane concentrations observed in Greenland ice cores and those from Antarctica allow inference of changes in the wetland areas in the tropics and high latitudes Chappellaz et al.

The combination of the isotopic record of water making up the Greenland ice see Plate 2 ; Figure 1. Ice-core records from Greenland thus provide high-resolution reconstructions of local environmental conditions in Greenland temperature and snow accumulation rate , conditions well beyond Greenland wind-blown materials including sea salt and Asian dust , and even some global conditions wetland area inferred from methane , all on a common time scale Figures 2.

A review of available Greenland ice-core data is given by Alley The data were collected by two international teams of investigators from multiple laboratories.

The duplication shows the high reliability of the. The upper-most curve is the gray-scale light or dark appearance of the Cariaco Basin core, and probably records changes in windiness and rainfall Hughen et al. The rate of snow accumulation and the temperature in central Greenland were calculated by Cuffey and Clow , using the layer-thickness data from Alley et al. The independent Severinghaus et al. Methane data are from Brook et al.

Changes in the d 15 N values as measured by Severinghaus et al.

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Highs in sea-salt sodium indicate windy conditions from beyond Greenland, and even larger changes in calcium from continental dust indicate windy and dry or low-vegetation conditions in the Asian source regions Mayewski et al. Calcium and sodium concentrations measured in the ice have been converted to concentrations in the air over Greenland, and are displayed by dividing by the estimated average atmospheric concentrations over Greenland in the millennium before the Little Ice Age, following Alley et al.

Figure is modified from Alley Briefly, the data indicate that cooling into the Younger Dryas occurred in a few prominent decade s -long steps, whereas warming at the end of it occurred primarily in one especially large step Figure 1. This matches well the change in wind-driven upwelling in the Cariaco Basin, offshore Venezuela, which occurred in 10 years or less [Hughen et al.

Ice core evidence also shows that wind-blown materials were more abundant in the atmosphere over Greenland by a factor of 3 sea-salt, submicrometer dust to 7 dust measuring several micrometers in the Younger Dryas atmosphere than after the event Alley et al. Taylor et al. Variability in at least some indicators was enhanced near this and other transitions in the ice cores Taylor et al. Beginning immediately after the main warming in Greenland by less than or equal to 30 years , methane rose by 50 percent over about a century; this increase included tropical and high-latitude sources Chappellaz et al.

Modified from Alley et al.

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Ice cores from other sites, including Baffin Island, Canada Fisher et al. The Byrd Station, Antarctica, ice core and possibly other southern cores Bender et al. The record from Taylor Dome, Antarctica, a near-coastal site, appears to show a slight cooling during the Younger Dryas, although details of the synchronization with other ice cores remain under discussion Steig et al. The Southern Hemisphere records are not comparable with those from central Greenland in time resolution; further coring is planned.