Fossilization

[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.
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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.

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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.

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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.

 

EARL DETRA

From: 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.

Actually, David, these article suggest just the opposite. They show early diagenetic changes that forever produce a chemical gradient in the rock that shows up as fossils whether the remains were covered immediately or over a long period of time.

Note the following phrases:

"Preservation of diagenetically altered remnants of the original organic tissues and formation of chlorite/illite coatings and cuticle replacements, …"

Note that the remnants are chemically altered. They are not of the same composition as the surrounding sediments. They are distinct from those sediments and are, therefore, visible as fossils.

"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, "

Note here that the decomposition of the remains does not remove the chemical difference between them and the surrounding sediments.

"(2) inhibition by the adsorbed Fe2+ ions of further bacterial decomposition of these biopolymers, which enables them to persist and later become kerogens; ..."

So decomposition results in the formation of kerogens, insoluble gunk that will leave a permanent signature in the rock.

"(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. "

Note the early formation of ferrous minerals. What do you think happens to these whether occurring at the sediment water interface or below that interface?

"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, ..."

Here, note that the transport of the remains to a suboxic environment is deemed important. This may occur by sinking below a compositional boundary in water, as we have discussed earlier.

"(2) extensive suboxic diagenesis in a sediment of high iron(III)/ (organic carbon) ratio, ..."

Here we have suboxic diagenesis in an environment where many scavengers do not exists. The chemical changes will leave a permanent chemical gradient in the rock that we will see 500 Ma later as fossils.

"…

The proposed model of early diagenesis that results in Burgess Shale-type fossil preservation critically depends on the availability of steady …"

Note the term "steady" here. I interpret that to mean stable. Not what one would envision for a massive global flood that is spreading sediments all over the globe.

"…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, …."

Here we have the first use of the term "rapidly." However, it refers more to the deposition of thin layers and transport of remains by dilute turbidity currents rather than the accumulation of thick sequences of shale.

"… 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. "

So, as we have discussed, the remains are not highly disturbed by the currents, in relatively quiet waters. I really see nothing here to indicate the sudden deposition of massive strata to bury an organism. In fact the theory suggests that whether the sediments are deposited rapidly or not, the chemical changes differentiate the remains of the organism from the surrounding sediments.

Moving along:

"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. "

Nothing about burial yet. Notice that the leaf is in 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. "

So, the biofilms enhance rapid growth of ferric ions. No need for burial yet.

"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."

So, rapid burial does not seem to be necessary. In fact the biofilms seem to form early and attract ferric ions early. Then when the leaf or the biofilms are buried, they will form fossils, according to this theory.

"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. "

Once again, one of the elements that we have discussed: anoxic conditions. Perhaps like those encountered in enclosed basins?

"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."

Microbial mats, eh? Doesn't sound like burial by flood sediments, does it? I see little in any of these studies requiring rapid burial by flood sediments. Most of them seem to discuss the chemical environment around organic remains that produce permanent changes in the sediments by way of insoluble mineralization. The point being that rapid burial is a vague idea that may occur but is not necessary. What we really need is the formation of insolubles that will give the rock a local variation that we see later as a fossil.