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Published on: August 2, 2018
Mechanism for Burgess Shale-type preservation
Robert R Gaines1, Emma U Hammarlund, Xianguang Hou
1Geology Department, Pomona College, Claremont, CA 91711, USA. robert.gaines@pomona.edu
Summary
Early animal diversification is revealed by exceptionally preserved Cambrian fossils. Unique ocean chemistry and rapid sediment burial inhibited microbial decay, preserving soft tissues through Burgess Shale-type fossilization.
Area of Science:
- Paleontology and Geochemistry
- Early animal evolution and fossilization processes
Background:
- Exceptionally preserved fossil biotas from Cambrian deposits like the Burgess Shale offer crucial insights into early animal diversification.
- The mechanism behind the extraordinary preservation of soft tissues in these fossil assemblages has remained largely unexplained since their discovery.
Purpose of the Study:
- To elucidate the preservation mechanism of Burgess Shale-type fossilization.
- To investigate the role of sedimentology and geochemistry in preserving labile tissues in Cambrian fossil deposits.
Main Methods:
- Analysis of sedimentologic and geochemical data from multiple Burgess Shale-type deposits (Chengjiang, Burgess Shale, etc.).
- Examination of sulfur isotope evidence from sedimentary pyrites.
- Assessment of early Paleozoic seawater chemistry and its impact on sediment diagenesis.
Main Results:
- Fossilization as carbonaceous compressions resulted from early inhibition of microbial activity due to oxidant deprivation.
- Low oceanic sulfate and low-oxygen bottom waters reduced oxidant availability.
- Rapid burial in fine sediments and early carbonate cementation created a permeability barrier, restricting oxidant flux.
Conclusions:
- Burgess Shale-type preservation was facilitated by unique early Paleozoic seawater chemistry, specifically high alkalinity.
- This seawater chemistry influenced sediment diagenesis, leading to the formation of a permeability barrier.
- These conditions provide a unique record of the period immediately following the Cambrian explosion.
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