Page 80 - Introduction to Paleobiology and The Fossil Record
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TAPHONOMY AND THE QUALITY OF THE FOSSIL RECORD 67
within many skeletons are chemically calcite. The commonest diagenetic process
unstable, and they break down after death is the conversion of aragonite to calcite.
while the specimen lies on the sediment After burial, pore fluids within the sediment
surface, and also for some time after burial. may be undersaturated in CaCO 3 , and the
Carbonates are liable to corrosion and dis- aragonite dissolves completely, leaving a void
solution by weakly acidic waters. The most representing the original shell shape. Later,
stable skeletal minerals are silica and pore fluids that are supersaturated in CaCO 3
phosphate. allow calcite to crystallize within the void,
thus producing a perfect replica of the origi-
nal shell. This process of replacement of ara-
Burial and modifi cation
gonite by calcite occurs commonly, and may
Animal and plant remains are typically buried be detected by the change of the crystalline
after a great deal of scavenging, decay, break- structure of the shell (see Fig. 3.6g). The
age and transport. Sediment is washed or regular layers of aragonite needles have given
blown over the remains, and the specimen way to large irregular calcite crystals (sparry
becomes more and more deeply buried. During calcite) or tiny irregular calcite crystals
and after burial, the specimen may undergo (micrite).
physical and chemical change. A common diagenetic phenomenon is the
The commonest physical change is fl atten- formation of carbonate concretions, bodies
ing by the weight of sediment deposited above that form within sediment and concentrate
the buried specimen, and this may occur soon CaCO 3 (calcite) or FeCO 3 (siderite). Carbon-
after burial. These forces fl atten the specimen ate concretions generally form early during
in the plane of the sedimentary bedding. The the burial process, and this is demonstrated
nature of flattening depends on the strength by the fact that enclosed fossils are uncrushed,
of the specimen: the first parts to collapse are having been protected from compaction by
those with the thinnest skeleton and largest the formation of the concretion. Carbonate
cavity inside. Greater forces are required to concretions form typically in black shales,
compress more rigid parts of skeletons. sediments deposited in the sea in anaerobic
Ammonites, for example, have a wide body conditions. Black shales contain abundant
chamber cavity that would fill up with sand organic carbon, and, when this is buried, bac-
or water after the soft body decayed. This terial processes of anaerobic decay begin.
part collapses first (Fig. 3.6f) and, because the These decay processes reduce oxides in the
shell is hard, it fractures. The other chambers sediment, and produce bicarbonate ions that
are smaller, fully enclosed and hence mechani- may combine with any calcium or iron ions
cally stronger: they collapse later. Plant fossils to generate carbonate and siderite concentra-
such as logs are usually roughly circular in tions. Such concentrations may grow rapidly
cross-section, and they flatten to a more ovoid to form concretions around the source of
cross-section after burial. The woody tissues calcium and iron ions, usually the remains of
are flexible and they generally do not fracture, an organism.
but simply distort. Another early diagenetic mineral that
These are examples of diagenesis, and they occurs in anaerobic marine sediments is pyrite
may occur early, very soon after burial (for (FeS 2 ). It is also produced as a by-product of
example, flattening and some chemical anaerobic processes of microbial reduction
changes), or thousands or millions of years within shallow buried sediments. Pyrite may
later, as a result of the passage of chemicals replace soft tissues such as muscle in cases of
in solution through rocks containing fossils. rapid burial, and replaces hard tissues under
Other examples of late diagenesis include appropriate chemical conditions. Wood, for
various kinds of deformation by metamorphic example, may be pyritized, and dissolved ara-
and tectonic processes, often millions of years gonite or calcite shells may be entirely replaced
after burial (Box 3.2). by pyrite. In both cases, the original skeletal
The calcium carbonate in shells occurs in structures are lost.
four forms: aragonite, calcite (in two variet- Phosphate is a primary constituent of ver-
ies: high magnesium (Mg) calcite, and low Mg tebrate bone and other skeletal elements. In
calcite), and combinations of aragonite + some cases, masses of organic phosphates are
