Fossil Diagenesis Geochemical Processing of Potential Fossils

Early diagenetic phenomena comprise the physicochemical processes that act on organism remains primarily after burial. Diagenetic features of fossils may provide information regarding the geochemistry of bottom waters and the upper "taphonomi-cally active zone" (TAZ) of the sediment column. Diagenetic features of note include evidence for early dissolution, compaction, and mineralization of fossils.

The relative timing of dissolution is commonly recorded in skeletons. Trilobite exoskeletons were impregnated with calcite and thus are relatively resistant to dissolution and are preservable; however, they still may show evidence of early dissolution. Trilobite skeletons that are dissolved prior to compaction may leave no record but, in many cases, may be preserved as plastically deformed molds (Seilacher et al. 1985). Such preservation would indicate undersaturation with respect to calcite in the upper sediments and possibly low pH conditions. Conversely, many fossil skeletons and their molds display mosaic fracture patterns on their surfaces; these are particularly prominent on large shields such as the cephala and pygidia of IsoteJus species. Such skeletons remained hard in early phases of compaction, which caused brittle fracture. (In rare instances, both plastically deformed "ghosted" specimens and brittly fractured (or unfrac-tured) well-calcified specimens occur on the same bedding planes. Such associations have been used to suggest the presence of "soft-shelled" (immediately postmolt) and intermolt individuals (Speyer 1987).)

Early diagenetic minerals such as siderite, calcite, and pyrite generally form as a result of the action of anaerobic bacteria and are partly composed of their respiratory by-products (Allison 1988a). They may provide valuable information on sediment geochemistry and rates of burial.

One of the best understood of the early diagenetic minerals is pyrite, iron disulfide (Berner 1981a; Canfield and Raiswell 1991a, 1991b). Pyrite is common in many marine mudrocks and is commonly associated with fossil trilobite remains. The ferrous iron required in pyrite formation is available in terrigenous sediments, and dissolved sulfate is abundant in marine water (but not in fresh water). Under aerobic decay of organic matter, the major respiratory by-products of organisms are water (H 0) and carbon dioxide (CO,). However, under anaerobic conditions particular types of bacteria, referred to as sulfate-reducing bacteria, use sulfate (S04~~) as an oxygen donor for metabolizing organic matter, and produce hydrogen sulfide (H,S) and bicarbonate (HC0sM) as by-products. Iron reduction, mediated by a second group of anaerobic bacteria, can generate ferrous ions, which in turn may react with the H,S generated by sulfate reduction, to produce the precursors of pyrite (Canfield and Raiswell 1991 b). The presence of pyrite shows that the sediment was anoxic but does not necessarily demonstrate that the overlying water column was anoxic. Pyrite can form either very early or relatively late in the burial history of a sediment (Hudson 1982; Brett and Baird 1986; Allison 1988a, 1988b; Canfield and Raiswell 1991a).

Under conditions of high organic-matter production, anoxic conditions may extend into the water column; in such euxinic settings H,S is in excess, and any iron that is introduced into the system is pyritized as it is deposited. Thus, pyrite tends to be distributed evenly in the sediment as disseminated tiny crystal aggregates called framboids; it is not concentrated around any organism remains that may settle into these settings. Under oxic bottom-water conditions, however, organic material is not distributed so uniformly because much of it is degraded aerobically at or near the sediment-water interface, and the sediment becomes anoxic but nonsulfidic, in Berner's (1981a) terminology. Thus, pyritization tends to occur more locally within the non-sulfidic sediment, particularly in the vicinity of anaerobically decaying organic matter. As a result of local sulfate reduction, sulfide is liberated around this decomposing organic material. Dissolved iron will react at the site of sulfate reduction so that pyrite is restricted to anoxic organic-rich microenvironments within a broadly dysoxic, low-organic setting. Well-preserved pyritized fossils, including trilobites tend to occur in bioturbated gray mudstones and thus are indicators of bottom-water oxygenation (Brett and Baird 1986; Allison and Brett 1995). The earliest phases of pyrite tend to be fine-grained fillings of cavities, such as the interiors of enrolled trilobites. Later generations of larger crystalline MoverpyriteM may nucleate on existing pyritic cores. In this way, pyritic nodules may form (Figure 3.2).

There are a number of Middle Devonian fossil beds in New York wherein large amounts of pyrite are found associated with the fossils (Dick and Brett 1986). The best known of these is the Alden Pyrite Beds in the Ledyard Shale. The Ledyard Shale is a dark-gray, generally poorly fossiliferous shale. However, a horizon in the lower Ledyard in western New York is rich in pyrite nodules and pyritized molds of fossils of all sorts. Most common are the fossils that had calcific or aragonititic shells in life, such as pelecypods, brachiopods, nautiloids, ammonoids, and trilo-bites. The pyrite nodules have fossils at their core, which indicates that the original pyrite, formed from the organic decay, acted as a nucleus for over-pyrite precipitation. Greenops grabaui is the most common trilobite in these beds, and some specimens are partially pyritized and have a solid pyritic core and some coiled specimens form the nucleus of a pyrite nodule. These beds appear to represent the rapid burial of organism bodies in an otherwise low-organic sediment. The high concentration of pyritic material around burrows and fossils indicates nucleation around local centers of anaerobic decay associated with buried organic matter.

Anaerobic decay processes, including sulfate reduction, also generate HCO," (bicarbonate), which may initiate the precipitation of calcite or siderite concretions around decaying organic matter. Where trilobites or other fossils are enclosed within carbonate concretions, they are nearly always three-dimensional. This proves that the concretions formed early and before organism remains could be compacted by overburden pressure.

It is less common for phosphatic nodules to form because phosphorus is only present in very small quantities in seawater. However, the anaerobic decay of organic matter does liberate phosphate-bearing compounds to solution. Also, phosphate can become adsorbed to ferrous hydroxides in the sediment. As these ferrous hydroxides are buried and pass through the anoxic-oxic boundary, they are reduced. This process also liberates phosphates to pore-water. Dissolved phosphate may be released back to the water column if anoxia persists to the sediment-water interface. However, if a micro-oxidized zone exists in the upper sediment, then the phosphates may be repre-cipitated, especially around phosphatic skeletal nuclei, such as the chitinophosphatic material forming arthropod skeletons (Swirydczuk et al. 1981; Berner 1981a; Allison 1988b). Trilobites are not uncommonly phosphatized. Phosphatization of trilobites occurs primarily under conditions of slow sedimentation. Thus, for example, in the Middle Devonian Hamilton Group, phos-phatized internal molds of enrolled trilobites may occur at minor disconformities.

If the sedimentation rate is high, then the time spent by any particular sediment layer at this micro-oxidized interface will be low, and the phosphorus concentration in pore-water will be increased only slightly. Conversely if the sedimentation rate is low, then the time spent at the interface will be high. Thus, a large proportion of the adsorbed phosphorus compounds will be concentrated at one layer in the sediment. Such concentration can increase pore-water levels of phosphorus so that phosphate minerals can precipitate. These minerals may replace organic remains or form concretions. Thus, the occurrence of phosphatic fossil molds or concretions is nearly always an indicator of low rates of sedimentation.

Other forms of diagenetic modification of trilobite material are uncommon to absent in New York. There are a few references to silica replacement of exoskeletal material in trilobite protaspids but no observations of phosphate replacement or carbonized specimens.

FIGURE 3.2. Conditions for the formation of pyritized, well-preserved fossils. A. Fauna on a bottom with poor oxygenation. There is low diversity, and the bottom is anoxic a short distance beneath the surface. The plus signs (+) indicate anoxic conditions. B. Rapid burial by sediment and the reduced oxygen levels of the water raise the anoxic level to the newly buried organisms. Anoxic bacteria in metabolizing organic matter of the trilobites also reduce the sulfate in the pore water, resulting in sulfide ions that then react with reduced (Fe2+) iron in the sediment. In a sediment rich in terrigenous material, the iron level is high enough to cause iron sulfide precipitation at or close to the decaying organic material. C. Pyrite accumulates at the source of the organic decay, and the fossil is covered with pyrite at this nucleation site. A-C from Speyer and Brett (1991). Reproduced with permission. D, A trilobite buried in a rapid burial event but in an oxygenated sediment so that only a minor amount of pyrite formed. E. A coiled trilobite buried under anoxic, low-organic (except at the site of trilobite decay), and iron-rich conditions, resulting in a total covering of pyrite, PRI 49664.

FIGURE 3.2. Conditions for the formation of pyritized, well-preserved fossils. A. Fauna on a bottom with poor oxygenation. There is low diversity, and the bottom is anoxic a short distance beneath the surface. The plus signs (+) indicate anoxic conditions. B. Rapid burial by sediment and the reduced oxygen levels of the water raise the anoxic level to the newly buried organisms. Anoxic bacteria in metabolizing organic matter of the trilobites also reduce the sulfate in the pore water, resulting in sulfide ions that then react with reduced (Fe2+) iron in the sediment. In a sediment rich in terrigenous material, the iron level is high enough to cause iron sulfide precipitation at or close to the decaying organic material. C. Pyrite accumulates at the source of the organic decay, and the fossil is covered with pyrite at this nucleation site. A-C from Speyer and Brett (1991). Reproduced with permission. D, A trilobite buried in a rapid burial event but in an oxygenated sediment so that only a minor amount of pyrite formed. E. A coiled trilobite buried under anoxic, low-organic (except at the site of trilobite decay), and iron-rich conditions, resulting in a total covering of pyrite, PRI 49664.

More complete information on the processes of fossil preservation can be found in collective volumes edited by Briggs and Crowther (1990); Allison and Briggs (1991a, 1991b); Donovan (1991); Einsele, Ricken, and Seilacher (1991); and Martin (1999).

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