User:Aeirin/Paleoecology

Paleoecological Methods
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Isotope Fractionation
One common method of determining the ecology of past organisms is through the isotope fractionation of several different possible elements within their fossils; this method is typically known as isotope paleoecology. An isotope is a specific form of an element which may contain a different number of neutron s than the standard form. For example, oxygen has three stable isotopes (and many non-stable isotopes - see Isotopes of oxygen), oxygen-16, oxygen-17, and oxygen-18, which differ purely in neutron number; oxygen-16 has 8 neutrons, oxygen-17 has 9, and oxygen-18 has 10. For all elements, the proportion of each stable element to another, or the isotope ratio, has a standard value. However, through various isotope fractionation processes, it is possible for the composition of a specific elemental reservoir in the natural world, such as a lake or a living organism, to become enriched in one isotope of an element over another. For example, due simply to the fact that heavier isotopes of elements take more energy to evaporate, water vapor (which later condenses into clouds) typically has a higher-than-standard value of the lighter oxygen and hydrogen isotopes, oxygen-16 and oxygen-17 and hydrogen-1, while liquid water typically possesses a larger-than-standard value of the heavier oxygen and hydrogen isotopes due to the evaporation of the lighter isotopes.

Within the field of isotope paleoecology, much of the work done is based around carbon and oxygen isotope composition, with the occasional application to nitrogen isotopes, as these isotopes allow researchers to learn the most about the climate in which an organism lived. The exact application depends on the type of fossil organism being researched, as carbon and oxygen isotopes are constantly used up and concentrated in the bodily tissues of organisms in different ways. (For instance, a large herbivore might have specific isotope fractionations of carbon within their body due to their consumption of different plants, while an invertebrate that relies on a carbonate shell for an exoskeleton would likely have different fractionations of carbon in the layers of their shell as they grew.) However, oxygen in particular is typically relevant for fossil organisms due to the reliance on water, or H2O, for life's existence. As such, by determining the isotope fractionation of δ18O, it is possible to determine the type of climate in which an organism lived; with a large enough fossil record, it could even potentially be possible to determine the overall climate of a region over the course of thousands of years.

Vertebrates
In vertebrates, the determination of isotope fractionation in the context of paleoecology is usually done via examination of tooth enamel composition, as such enamel often grows in layers and can thus record time periods on the order of months to years, which is highly useful for determining changes in climate over short time scales. As mentioned, the isotopes typically used in this process for vertebrates are carbon and oxygen isotopes; carbon isotopes are particularly useful for herbivorous animals, as lighter or heavier composition of such isotopes often reflects primary consumption of either C3 or C4 plants. C3 plants show higher amounts of carbon-13 depletion than C4 plants; the former also cannot survive well in dry areas. This implies that, for example, if a fossilized vertebrate has a higher concentration of carbon-13 in their tooth enamel, they likely lived in an area where droughts were uncommon.

Much of the work done in the field of isotope paleoecology in the realm of vertebrate organisms is within the concentration of megafauna, especially now-extinct North American megafauna from the Pleistocene era that ended approximately 12,000 years ago. This is due at least partially to the fact that much of the interest into megafauna is in the reason for their extinction. Many researchers have proposed that, based on the changes in oxygen and carbon isotope composition, many megafauna died out due to drought. This was likely the case for Arizonian mammoths, whose fossil records already indicate primary consumption of C4 plants. Isotope paleoecology has also been used to determine the ranges in environment and diet that some organisms were able to tolerate, such as several ground sloth species that existed in California and Nevada during the late Pleistocene. With the northern great plains bison, which, uniquely among North American megafauna, still exists in the present day, carbon and nitrogen isotope fractionations from early Holocene fossil records can be compared to those of current bison populations to determine the climate differences from that era until the present. This data, through these differences in isotope fractionation, also shows changes in bison ranges when the climate changes. (Nitrogen fractionation here is related to moisture level, and is thus a good potential indicator of drought.)

Outside of the realm of mammalian megafauna, there is also evidence through isotopic fractionation of carbon and oxygen that different, coexistent herbivorous dinosaurs preferred to eat different types of plants.

Invertebrates
For invertebrates, isotope fractionation is used heavily in paleoceanography. Of particular interest are the isotopic fractionations of oxygen, for its relation to seawater isotopic composition, and carbon, for its relationship to calcium carbonate in shells.

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