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| Paleoecology |
Paleoecology is the study of ancient environments. As a field of science, paleoecology ismost closely related to paleontology, the study of fossils. It is also related to paleobotany and a number of other areas of study dealing with the distant past.
Because paleoecology and its related disciplines (paleontology, paleobotany, paleoclimatology, paleogeography, and others) deal with the past, scientists are unable to apply the usual scientific criteria of direct observation and measurement of phenomena.
In order to make any conclusions about the past, scientists must assume at least one statement to be true without direct observation: The processes that exist in the modern universe and on the modern earth existed in the past.
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Uniformitarianism and Catastrophism
Paleontologists must assume that ancient plants and animals had tolerances to temperature, moisture, and other environmental parameters similar to those of modern organisms.
The belief that the present is the key to the past is called uniformitarianism. It has been a key concept of the biological and earth sciences since the early nineteenth century. Uniformitarianism does not include the belief that the ancient earth was like the modern earth in its life-forms or geography.
During the early part of the nineteenth century, another worldview dominated: catastrophism, the belief that the earth is relatively young and was formed by violent upheavals, floods, and other catastrophes at an intensity unlike those of modern earth.
Many catastrophists explained the presence of fossils at high elevations by the biblical flood of Noah. The uniformitarian viewpoint prevailed and, although admitting that local catastrophes may be important, their long-term, earthwide importance was denied.
Catastrophism was revived in the 1980’s to explain certain important events. The rapid extinction of the large dinosaurs at the close of the Mesozoic era has been attributed to the climatic changes associated with an alleged encounter between the earth and a comet—certainly a catastrophic event.
Climatic Cycles
One of the most intensively investigated paleoecological problems has been the changing environments associated with the ice ages of the past million years.
Analysis of pollen from bogs in many parts of the world indicates that there have been at least four advances and retreats of glaciers during that period. Evidence for this is the changing proportions of pollen from tree species found at the various depths of bogs.
In North America, for example, spruces (indicators of cool climate) formerly lived much farther south than they do now. They were largely replaced almost eight thousand years ago by other tree species, such as oaks, which are indicative of warmer climates. This warming trend was a result of the latest glacial retreat.
Dendrochronology (tree-ring analysis) not only enables paleoecologists to date past events such as forest fires and droughts but also allows them to study longer-term cycles of weather and climate, especially those of precipitation and temperature.
In addition, trees serve as accumulators of past mineral levels in the atmosphere and soil. Lead levels of tree wood showed a sharp increase as the automobile became common in the first half of the twentieth century because of lead additives in gasoline.
Tree rings formed since the 1970’s have shown a decrease in lead because of the decline in use of leaded fuels. Tree-ring analysis has also been a valuable tool for archaeologists’ study of climatic changes responsible for shifting patterns of population and agriculture among American Indians of the southwestern United States.
Traces of Ancient Environments
Fossil evidence is the chief source of paleoecological information. A fossil bed of intact clam shells with both valves (halves) present inmost individuals usually indicates that the clams were preserved in the site in which they lived (autochthonous deposition). Had they been transported by currents or tides to another site of deposition (allochthonous deposition), the valves would have been separated, broken, and worn.
Similarly, many coal beds have yielded plant fossils that indicate that their ancient environments were low-lying swamp forests with sluggish drainage periodically flooded by water carrying a heavy load of sand.
The resulting fossils may include buried tree stumps and trunks with roots still embedded in their original substrate and numerous fragments of twigs, leaves, and bark within the sediment.
Certain dome- or mushroom-shaped structures called stromatolites are found in some of the most ancient of earth’s sedimentary rocks. These structures may be several meters in diameter and consist of layers of material trapped by blue-green algae (cyanobacteria).
Such structures are currently being formed in shallow, warm waters. Uniformitarian interpretation of the three-billion-year-old stromatolites is that they were formed under similar conditions.
Their frequent association with mud cracks and other shallow- and above-water features leads to the interpretation that they were formed in shallow in shore environments subject to frequent exposure to the air.
Relative oceanic temperature can be estimated by observing the direction in which the shells of certain planktonic organisms coil. The shell of Globigerina pachyderma coils to the left in cool water and to the right in warmer water. Globigerina menardii shells coil in an opposite fashion—to the right in cool water and to the left in warmer water.
Uniformitarian theory leads one to believe that ancient Globigerina populations responded towater temperature in a similar manner. Sea-bottom core samples showing fossils with left- or right-coiling shells may be used to determine the relative water temperature at certain periods.
Eighteen-thousand-year-old sediments taken from the Atlantic Ocean show a high frequency of left-handed pachyderma and right-handed menardii shells. Such observations indicate that colder water was much farther south about eighteen thousand years ago, a date that corresponds to the maximum development of the last ice age.
Interpreting Clues
Fossil arrangement and position can be clues to the environments in which the organisms lived or in which they were preserved. Sea-floor currents can align objects such as small fish and shells.
Not only can the existence of the current be inferred, but also its direction and velocity can be determined. Currents and tides can create other features in sediments which are sometimes indicators of environment.
If a mixture of gravel, sand, silt, and clay is being transported by a moving body of water such as a stream, tide, or current, the sediments will often become sorted by the current and be deposited as conglomerates—sandstones, silt stones, and shales. Such graded bedding can be used to determine the direction and velocity of currents.
Larger particles, such as gravel, would tend to be deposited nearer the sediment source than smaller particles such as clay. Similarly, preserved ripple marks indicate current direction. Mud cracks in a rock layer indicate that the original muddy sediment was exposed to the atmosphere at least for a time after its deposition.
Certain minerals within fossil beds or within the fossil remains themselves can sometimes be used to interpret the paleoenvironment. The presence of pyrite in a sediment almost always indicates that the sedimentary environment was deficient in oxygen, and this, in turn, often indicates deep, still water. Such conditions exist today in the Black Sea and even in some deep lakes, with great accumulations of dead organic matter.
The method of preservation of the remains of the fossilized organism can be an indication of the environment in which the creature lived (or died). Amber, a fossilized resin, frequently contains the embedded bodies of ancient insects trapped in the resin like flies on flypaper.
This ancient environment probably contained resin-bearing plants (mostly conifers) and broken limbs and stumps that oozed resin to trap these insects. Mummified remains in desert areas and frozen carcasses in the northern tundra indicate the environments in which the remains were preserved thousands of years ago.
Fossils and Fuels
One of themost immediately important applications of paleoecological data is in the search for increasingly scarce fossil fuels: petroleum, natural gas, and coal. Reconstruction of ancient environments and their paleogeographical distributions is probably the most accurate way to predict the presence of reservoirs of these natural resources.
Because these substances are formed in environments that are biologically highly productive, with abundant plant and animal life, they are commonly found associated with reefs (petroleum, gas) andswamps (coal).
Index fossils and fossil assemblages can indicate such environments with a high degree of accuracy. Petroleum and gas reservoirs must be porous and permeable in order for the material to accumulate in high concentrations. Reef material and porous sandstones formed from ancient sandbars meet such criteria.
Most of the historically famous oil-producing region of Pennsylvania lies within an area 40 to 100 kilometers west of a former shoreline near present-day Pittsburgh. Within this area, most oil pools are within the ancient offshore sandbar belt. Sediments forming this “Pocono formation” came from the east, from an area near present Atlantic City, New Jersey.
These sediments, all deposited within the same time frame, grade from coarse, nonmarine conglomerates and sandstones in the east near their source to fine-grained sandstones and shales to the west in the oil-producing region.
