|Exploring for Oil and Gas Traps|
|Series||Treatise in Petroleum Geology|
|Part||Predicting the occurrence of oil and gas traps|
|Author||Robert L. Fleisher, H. Richard Lane|
Paleoclimate can be operationally defined as those factors of climate, weather, and atmospheric and oceanic circulation that influence biotic distribution and the rate and locus of clastic, evaporite, and carbonate deposition.
Utility of fossils in paleoclimatology
Fossils have been used in an ad hoc manner to interpret climate since the beginnings of geology as a science, but a systematic methodology for using fossils to assess climate is still lacking. The following elements can contribute to such a methodology:
- Biostratigraphic correlation
- Adaptive morphology of animals and plants
- Isotopic “paleothermometers”
- Quantitative paleoclimate models
Because climate is a global phenomenon, the various lithofacies and biofacies that reflect climate in the time interval of interest must be correlated accurately. Climates may change abruptly, and biostratigraphy provides the only practical means to demonstrate the contemporaneity, or lack of it, of contrasting climates.
Living organisms adapted to distinctive climates or environments may have distinctive characteristics, or morphologies. Recognizing these features in fossils can provide information about ancient climates.
For example, many plants of the tropical rain forest have drip points on their leaves to facilitate rainwater drainage; similar features on fossil leaves can be taken to suggest a similar paleoenvironment.
This approach can be used even when the significance of the adaptive form is unclear; Wolfe for example, suggests that quantification of angiosperm leaf characteristics (i.e., the percentage of entire-margined leaves) helps us determine Tertiary temperatures to within 1°C274.15 K
An analysis of the ratios of oxygen isotopes in fossil shell material yields estimates of ancient temperatures. Oceanic paleotemperatures based on analysis of fossil calcareous foraminiferal tests are widely used to interpret Cenozoic paleoclimate and paleoceanography (see Stable isotope stratigraphy).
Quantitative paleoclimate models
Quantitative paleoclimate models often produce unique solutions to the paleoclimatic interpretation of geologic and paleontologic data.
A quantitative community climate model (CCM1) at the National Center for Atmospheric Research was used by Amoco Production Research to investigate possible causes of Late Devonian extinctions. The results suggest two things:
- There was no perennial snow cover and, hence, no glaciation in the austral regions.
- Simulated sea-surface temperatures in the tropics ranged from 27°C300.15 K
540.27 °R to 34°C307.15 K
552.87 °R—high enough to kill reefs.
These results support a climatic cause for the extinctions.
- Stratigraphic and geographic distribution of fossils
- Temporal and environmental distribution of microfossils
- Taphonomy and provenance
- Wolfe, J. A., 1979, Temperature parameters of humid to mesic forests of eastern Asia and relation to forests of other regions of the northern hemisphere and Australia: U.S. Geological Survey Professional Paper 1106-K, 37 p.
- Ormiston, A. R., and G. Klapper, 1992, Paleoclimate, controls on Upper Devonian source rock sequences and stacked extinctions (abs.), in S. Lidgard, and P. R. Crane, eds., Fifth North American Paleontological Convention Abstracts and Programs: Paleontological Society Special Publication 6, p. 227.
- Thompson, J., and C. Newton, 1989, Late Devonian mass extinction: episodic cooling or warming?, in N. McMillan, A. Embry, and D. Glass, eds., Devonian of the World: Canadian Petroleum Society Memoir 14, vol. 3, p. 29–34.