|MadSci Network: Chemistry|
Peter: You have asked quite a few questions, some of which have highly technical answers. I will do my best to get you started with some answers, but geochemistry is a complex and rapidly advancing field. Some of your questions can only be adequately answered by reference to the current scientific literature on geochemistry. You probably can get some help from a paleontologist or geochemist at the nearest university or geological survey. Your first question is: what happens when organic materials like wood or bone are replaced, and secondarily, do partially transformed fossils exist. The answer to the second part is yes, partially transformed fossils do exist. However, many fossilization processes occur pretty fast compared to the age of fossils. We can look at fossilization processes operating today, and see the effects in very young fossils (thousands of years old). For example, organic material in hot springs can become completely encased in calcium carbonate crystals in only hours. Therefore, any fossil that is millions of years old has had time, at least, to become fully mineralized. The answer to the question of how mineralization takes place is more complicated, because there are many ways. The most common method of preservation for both wood and bone is probably mineralization in which minerals are deposited within the open spaces of the wood or bone. Wood and bone are very porous (at least when they have dried and the protoplasmic components are gone) -- this means a large amount of the inside of a piece of wood, say, is open space where cells used to be. Minerals are precipitated from water in these spaces, filling them up. This makes the wood heavy, hard, and changes its color, but it preserves the detailed structure down to the level where cells appear as units. The soft cells themselves are not preserved, only the spaces where they once existed. Therefore, the internal components of cells are not preserved by this method. Why do the different parts of fossil wood and fossil bone look different? The two main reasons are different minerals and different holes. Where holes are of different sizes and different spacing, the resulting mineral patterns will look different macroscopically. Also, over time, more than one mineral may be formed within a fossilizing piece of bone or wood, and different minerals have different colors. Sometimes, mineralization works differently. For instance, the oldest fossils in the world are microorganisms preserved in chert. Chert is silica (SiO2), and has the same chemical composition as quartz. However, the crystals of chert are very very tiny, far too small to see with a low-powered microscope. Chert can form as a gel (like jello) within sediment, growing crystal by crystal, and often completely enveloping tiny objects that were in the sediment. The microfossils in chert can preserve the shapes and sizes of individual cells, but they do not usually preserve any information about what was inside the cells. This is because the toughest part of a microorganism's cell is the outside (not just a membrane, but an extra protective organic coating), and this is the only part that can usually survive being entombed in silica. Of course, sometimes organic material IS preserved in fossils. We know this because that is what coal is, that is what most plant fossils are, and there is organic material in some animal fossils too, such as some insects preserved in amber. However, most organic molecules are fragile (physically and chemically). The elements composing organic material (chiefly carbon, hydrogen, oxygen, and nitrogen) are not fragile. But the pressure and heat and chemical environments that affect buried organic matter usually change it drastically. This is where oil comes from. The oil is organic, it used to be parts of living creatures, and we can even tell by looking at the kinds of organic molecules in a particular sample of oil whether higher plants, algae, or other kinds of organisms sourced that particular oil. However, most people do not consider oil to be fossils. When we speak of fossilized organic matter, we mean protein such as collagen, DNA, or some other material that will really tell us something about the organism from which it came. Even plant fossils are mostly carbon and hydrogen when we find them, and the thin black films don't tell us a whole lot about the organisms that we can't learn simply by examining the fossils visually. The most famous example of a fossil that contains well-preserved organic matter has to be the frozen mammoths that have been found in northern eurasia. These fossils are 10,000 years old or more, and most paleontologists consider them to be true fossils, even though they have been preserved by only two processes: (1) freezing, (2) isolation from water and oxygen. Even if you don't want to consider these to be fossils, there is preserved structural protein in some fossils from a variety of places; some of these fossils are millions of years old. There is organic material with fragments of DNA and other biologic chemicals in amber, much of which is many millions of years old. These are clearly fossils with important organic parts preserved. The organic chemicals are still recognizable because they have not been heated too much, and because chemicals (especially oxygen) have not been able to degrade them. Also, very importantly, bacteria were not able to use them for food. Most organic material decays after death, and decay is mostly the process of being devoured by bacteria. Dinosaur bone. Let's see. First, dinosaur bone is often NOT harder than the surrounding sediment. However, dinosaur bone is certainly different from the surrounding sediment and therefore can be harder. Only some of the minerals in the sediment are mobile (can be readily dissolved in water) and these are the ones that end up inside the bone. Also, minerals precipitated in bone are in the form of tiny crystals cemented together and to the bone itself, which makes the resulting structure rigid and hard. Dinosaur bone theoretically can contain organic material, sealed in by the enclosing sediment. Why doesn't it? Well, sometimes it probably does. However, most of the fossils of animals that contain organic matter are younger than 65 million years. That is a great span of time, long enough for remnant organic matter to slowly decay even if it is almost perfectly sealed away from water and oxygen. I do think it is possible that interpretable organic remains of dinosaurs will be found, if they have not already been found, but preservation of organic matter is much less common in fossils of that great age. Isotope geology is complicated, but certainly it is true that some dinosaur bones will preserve the original ratios of some isotopes. Many fossil shells much older than dinosaurs do the same. Again, the key is to isolate the fossil from agents of change. If the bone is not affected by moving water that can bring in isotopes of carbon, say, then the original carbon is what you will find. Because bone is so porous it is easy to alter the isotope ratios compared to those of a shell, which is typically much less porous. Well, I hope I have begun to answer your questions. You are interested in the science of taphonomy, which is the study of the changes that organisms undergo while they become fossils, and after that. It is very tricky sometimes to understand everything that is going on, but it can be very rewarding too. When we understand the changes that have taken place, then it is easier to understand the organism as it was before it underwent those changes. Good luck, and feel free to ask more questions. David Kopaska-Merkel Geological Survey of Alabama PO Box 869999 Tuscaloosa AL 35486-6999 (205) 349-2852 FAX (205) 349-2861 web site: www.gsa.state.al.us
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