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[Administrator: this is being separated from the "Varve" thread as it is heading in a new direction.]
DAVID PLAISTED
Here are some references on fossilization. Can someone point me to
references stating that fossilization can occur without the organism
rapidly being buried? All of these references seem to imply rapid
burial.
---------
Mechanisms of fossilization of the soft-bodied and lightly armored
faunas of the Burgess Shale and of some other classical localities
Petrovich R AMERICAN JOURNAL OF SCIENCE 301 (8): 683-726 OCT 2001
Abstract: The splendid preservation of the Middle Cambrian Burgess
Shale fauna, a fauna of exceptional importance for our understanding
of the evolution of life, has not been adequately explained.
Preservation of diagenetically altered remnants of the original
organic tissues and formation of chlorite/illite coatings and cuticle
replacements, both documented in the Burgess Shale fossils though not
necessarily occurring together, can be understood as products of the
same mechanism of fossilization of soft tissues. It is argued here
that this mechanism consists of the following steps:1) adsorption on
structural biopolymers such as chitin, cellulose, and collagens of
Fe2+ ions released during the oxidation of organic matter by
iron(Ill)reducing bacteria, (2) inhibition by the adsorbed Fe2+ ions
of further bacterial decomposition of these biopolymers, which enables
them to persist and later become kerogens; (3) in some
microenvironments, nucleation of crystals of an iron(II)-rich clay
mineral, a berthierine or a ferroan saponite, on the Fe2+ ions
adsorbed on the preserved biopolymers and growth of such clay-mineral
crystals to form a coating on the organic remains and/or to replace
parts of the organism. The critical factors in the Burgess Shale-type
preservation of Early and Middle Cambrian soft-bodied and lightly
armored animals were probably: (1) rapid transport of live or freshly
killed organisms into suboxic water, (2) extensive suboxic diagenesis
in a sediment of high iron(III)/ (organic carbon) ratio, and (3)
curtailment of the supply of sulfate ions shortly after the onset of
pyritization. The proposed model of early diagenesis that results in
Burgess Shale-type fossil preservation critically depends on the
availability of steady suboxic depositional environments in open
oceanic settings at depths of the order of 100 m in which
iron(HI)-rich fine-grained sediments, rapidly deposited with the
entrained animals by turbidity currents, could accumulate without
being disturbed by storm waves and deep currents. Evidence discussed
in the present paper suggests that such conditions were common in the
Early and Middle Cambrian.
---------
Enhancement of leaf fossilization potential by bacterial biofilms Dunn
KA, McLean RJC, Upchurch GR, Folk RL GEOLOGY 25 (12): 1119-1122 DEC
1997
Abstract: Terrestrial leaf fossils often form through authigenic
preservation in which the leaf surface is coated by a variety of
minerals, especially iron oxides. The mechanism of this fossilization
is unclear, because the largely hydrophobic leaf surfaces do not
readily bind metal ions. Previously proposed mechanisms for mineral
encrustation include precipitation of minerals in sediment pore space
and precipitation of iron oxides at the surface by decay-produced
CO2. Here we demonstrate that diverse bacterial species rapidly
colonize leaf surfaces and form a biofilm within days of the leaf's
entry into a stream environment. Experimental mineralization of fresh
and biofilm-coated leaves indicates that leaves without biofilm do not
mineralize, but leaves with biofilms rapidly adsorb metal ions such as
Fe3+ onto the anionic biofilm surface where the ions form
ferrihydrite. Once these mineralized leaves are buried by the
sediment, they are more likely to be converted to fossils than
nonmineralized leaves. Examination of a fossil leaf surface by
scanning electron microscopy shows bacteria-sized structures
resembling those found in biofilms, These experimental data imply that
bacterial colonization of leaves may be an essential prerequisite for
authigenic preservation.
---------
FOSSILIZATION OF SOFT-TISSUE IN THE LABORATORY BRIGGS DEG, KEAR AJ
SCIENCE 259 (5100): 1439-1442 MAR 5 1993
Abstract: Some of the most remarkable fossils preserve cellular
details of soft tissues. In many of these, the tissues have been
replaced by calcium phosphate. This process has been assumed to
require elevated concentrations of phosphate in sediment pore
waters. In decay experiments modern shrimps became partially
mineralized in amorphous calcium phosphate, preserving cellular
details of muscle tissue, particularly in a system closed to
oxygen. The source for the formation of calcium phosphate was the
shrimp itself. Mineralization, which was accompanied by a drop in pH,
commenced within 2 weeks and increased in extent for at least 4 to 8
weeks. This mechanism halts the normal loss of detail of soft-tissue
morphology before fossilization. Similar closed conditions would
prevail where organisms are rapidly overgrown by microbial mats.
DAVID PLAISTED
Here are some references on fossilization. Can someone point me to
references stating that fossilization can occur without the organism
rapidly being buried? All of these references seem to imply rapid
burial.
---------
Mechanisms of fossilization of the soft-bodied and lightly armored
faunas of the Burgess Shale and of some other classical localities
Petrovich R AMERICAN JOURNAL OF SCIENCE 301 (8): 683-726 OCT 2001
Abstract: The splendid preservation of the Middle Cambrian Burgess
Shale fauna, a fauna of exceptional importance for our understanding
of the evolution of life, has not been adequately explained.
Preservation of diagenetically altered remnants of the original
organic tissues and formation of chlorite/illite coatings and cuticle
replacements, both documented in the Burgess Shale fossils though not
necessarily occurring together, can be understood as products of the
same mechanism of fossilization of soft tissues. It is argued here
that this mechanism consists of the following steps:1) adsorption on
structural biopolymers such as chitin, cellulose, and collagens of
Fe2+ ions released during the oxidation of organic matter by
iron(Ill)reducing bacteria, (2) inhibition by the adsorbed Fe2+ ions
of further bacterial decomposition of these biopolymers, which enables
them to persist and later become kerogens; (3) in some
microenvironments, nucleation of crystals of an iron(II)-rich clay
mineral, a berthierine or a ferroan saponite, on the Fe2+ ions
adsorbed on the preserved biopolymers and growth of such clay-mineral
crystals to form a coating on the organic remains and/or to replace
parts of the organism. The critical factors in the Burgess Shale-type
preservation of Early and Middle Cambrian soft-bodied and lightly
armored animals were probably: (1) rapid transport of live or freshly
killed organisms into suboxic water, (2) extensive suboxic diagenesis
in a sediment of high iron(III)/ (organic carbon) ratio, and (3)
curtailment of the supply of sulfate ions shortly after the onset of
pyritization. The proposed model of early diagenesis that results in
Burgess Shale-type fossil preservation critically depends on the
availability of steady suboxic depositional environments in open
oceanic settings at depths of the order of 100 m in which
iron(HI)-rich fine-grained sediments, rapidly deposited with the
entrained animals by turbidity currents, could accumulate without
being disturbed by storm waves and deep currents. Evidence discussed
in the present paper suggests that such conditions were common in the
Early and Middle Cambrian.
---------
Enhancement of leaf fossilization potential by bacterial biofilms Dunn
KA, McLean RJC, Upchurch GR, Folk RL GEOLOGY 25 (12): 1119-1122 DEC
1997
Abstract: Terrestrial leaf fossils often form through authigenic
preservation in which the leaf surface is coated by a variety of
minerals, especially iron oxides. The mechanism of this fossilization
is unclear, because the largely hydrophobic leaf surfaces do not
readily bind metal ions. Previously proposed mechanisms for mineral
encrustation include precipitation of minerals in sediment pore space
and precipitation of iron oxides at the surface by decay-produced
CO2. Here we demonstrate that diverse bacterial species rapidly
colonize leaf surfaces and form a biofilm within days of the leaf's
entry into a stream environment. Experimental mineralization of fresh
and biofilm-coated leaves indicates that leaves without biofilm do not
mineralize, but leaves with biofilms rapidly adsorb metal ions such as
Fe3+ onto the anionic biofilm surface where the ions form
ferrihydrite. Once these mineralized leaves are buried by the
sediment, they are more likely to be converted to fossils than
nonmineralized leaves. Examination of a fossil leaf surface by
scanning electron microscopy shows bacteria-sized structures
resembling those found in biofilms, These experimental data imply that
bacterial colonization of leaves may be an essential prerequisite for
authigenic preservation.
---------
FOSSILIZATION OF SOFT-TISSUE IN THE LABORATORY BRIGGS DEG, KEAR AJ
SCIENCE 259 (5100): 1439-1442 MAR 5 1993
Abstract: Some of the most remarkable fossils preserve cellular
details of soft tissues. In many of these, the tissues have been
replaced by calcium phosphate. This process has been assumed to
require elevated concentrations of phosphate in sediment pore
waters. In decay experiments modern shrimps became partially
mineralized in amorphous calcium phosphate, preserving cellular
details of muscle tissue, particularly in a system closed to
oxygen. The source for the formation of calcium phosphate was the
shrimp itself. Mineralization, which was accompanied by a drop in pH,
commenced within 2 weeks and increased in extent for at least 4 to 8
weeks. This mechanism halts the normal loss of detail of soft-tissue
morphology before fossilization. Similar closed conditions would
prevail where organisms are rapidly overgrown by microbial mats.