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Chemical Fossils

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Chemical fossils are chemicals found in rocks that provide an organic signature for ancient life. Molecular fossils and isotope ratios represent two types of chemical fossils.

Molecular Fossils

Molecular fossils are often referred to as biomarkers or biosignatures and represent products of cellular biosynthesis that are incorporated into sediments and eventually into rock. Many of these chemicals become altered in known ways and can be stable for billions of years.

Nucleic acids (DNA & RNA), proteins, and carbohydrates do not survive long in the geologic environment. The majority of biomarkers are hydrocarbons derived from membrane lipids, which under certain conditions can be stable over billions of years. Molecules derived from pigments, such as chlorophyll, can also act as biomarkers. In 1936 Alfred Teibs recognized that vanadyl porphyrin was a molecular fossil of chlorophyll. Teibs discovery helped support a biologic origin for petroleum (Knoll, summons, Waldbauer, & Zumberge, pp. 134-135).

Fossil fuels petroleum (crude oil), coal, and natural gas are the result of biologic activity and contain chemical fossils. Major coal deposits represent plant material that grew primarily during the Carboniferous period. Crude oil and natural gas formed primarily from prehistoric algae and zooplankton that were deposited on the ocean floor under anoxic conditions. Natural gas can also form from fossil plant material. During sedimentary rock formation the remains of algae and zooplankton are converted into a mixture of organic hydrocarbons known as kerogen. Over geologic time heat and pressure can convert kerogen into oil or natural gas. The majority of oil deposits are Mesozoic or Cenozoic in age.

It is interesting to contemplate the origins of fossil fuel energy. Ancient plants and algae converted solar energy into the chemical bond energy of carbohydrates. Converted energy from the Sun was then passed through the food chains of these prehistoric ecosystems. Organic chemicals from these organisms were incorporated into sediments and eventually rock. So, the fossil fuels we humans have come to depend upon represent ancient sunshine stored within the Earth's crust.

Fossil fuels currently provide more than 85% of all the energy consumed in the United States. Crude oil supplies 40% of our energy needs and accounts for 99% of the fuel for cars and trucks. Coal is the major source of energy for generating electricity worldwide (U.S. Department of Energy). We depend upon fossil fuel energy for our agriculture. Some studies estimate that without fossil fuels the United States could only sustain two thirds of its current population (Pfeiffer, D.A., 2006, p. 41). Our dependence upon these nonrenewable resources should be a wake up call to invest in and develop alternative energy sources. At present, it is clear that coal, oil and natural gas, chemical fossils in the form of fossil fuels, are the lifeblood of America's economy.

For paleontologists and geobiologists the information provided by molecular fossils varies greatly. Some molecular fossils can help to determine what organisms were present, while others can indicate what biosynthetic pathways were in operation, still others provide information regarding the depositional environment (Knoll, Summons, Waldbauer, and Zumberge, 2007, p. 135).

Isotope Ratios

Isotope ratios represent another type of chemical fossil and result from metabolic processes that preferentially utilize one form of an isotope over another (Cowen, R. p. 16). Molecular fossils represent biomolecules or their derivatives that were once part of a living organism. In this way, molecular fossils are similar in concept to conventional body fossils. Isotope ratios are not preserved bits of an organism, rather they result from activities during life and in this way are analogous to trace fossils.

The first chemical evidence of photosynthesis, in the form of C-12 to C-13 ratios, can be found in Archean rock of 3.8 Ga from Isua, Greenland (Kenrick & Davis, 2004, pp 10-11; Johnson & Stucky, 1995, p 22). The process of photosynthesis preferentially utilizes C-12 over C-13 when removing CO2 from the air to synthesize carbohydrates, creating ratios of these carbon isotopes that differ from normal background ratios. Thus, carbon compounds processed by photosynthetic organisms are enriched with C-12 (Rich & Fenton, 1996, p 91). The enrichment of C-12 in rocks is a test for the presence of life. Carbon isotope ratios consistent with presence of cyanobacteria are widespread in rock dated at 3.5 Ga (Johnson & Stucky, 1995, p. 22). One problem with the evidence above is the fact that chemical pathways in non-photosynthetic autotrophs and nonautotrophs can produce C-12 enrichment (Blankenship, Sadekar, & Raymond, 2007, pp 22-23).

Evolution of Primary Producers in the Sea

Molecular fossils have been a key to understanding the evolution of primary producers in Earth's oceans. Microfossils and molecular fossils have helped to establish that Earth's oceans have experienced two major shifts in the composition of primary producers. Initially, cyanobacteria along with other photosynthetic bacteria were the primary producers during the Proterozoic eon. The first shift occurred during the early Paleozoic era when eukaryotic green algae joined cyanobacteria in being major primary producers. The second shift would occur during the Mesozoic era when dinoflagellates and coccolithofores would be joined by diatoms in the Jurassic. Diatoms, dinoflagellates, and coccolithophores would assume their dominant role as the base of many modern marine ecosystems by Cretaceous times. (Knoll, Summons, Waldbauer, and Zumberge, 2007, p. 155).


Blankenship, Sadekar, & Raymond
. (2007). The Evolutionary Transition from Anoxygenic to Oxygenic Photosynthesis. In Falkowski, P.G. Knoll, A.H. [Eds] Evolution of Primary Producers in the Sea. (pp. 21-35). China: Elsevier Academic Press.

Cownen, R. (2005). History of Life [4th Edition]. Malden, Main: Blackwell Publishing.

Johnson, K.R. & Stucky R.K. (1995). Prehistoric Journey: A History of Life on Earth. Boulder, Colorado: Roberts Rinehart Publishers.

Kenrick, P. & Davis, P. (2004). Fossil Plants. Washington: Smithsonian Books.

Knoll, Summons, Waldbauer, and Zumberge. (2007). The Geological Succession of Primary Producers in the Oceans. In Falkowski, P.G. Knoll, A.H. [Eds] Evolution of Primary Producers in the Sea. (pp. 133-163). China: Elsevier Academic Press.

Pfeiffer, D.A. (2006), Eating Fossil Fuels: Oil, Food and the Coming Crises in Agriculture. Canada: New Society Publishers.

Rich P.V., Rich T. H., Fenton, M.A., & Fenton, C.L. (1996). The Fossil Book: A Record of Prehistoric Life. Mineola, NY: Dover Publications, Inc.

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