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Abstract
Journal of Sedimentary Research, Section
A: Sedimentary Petrology and Processes
Vol. 66 (1996)No.
3. (May), Pages 468-473
Possible Microbial Effects on Stable Carbon Isotopes in Hot-spring Travertines
Li Guo (1), Julian Andrews (2), Robert Riding (1), Paul
Dennis (2), Quentin Dresser (1)(*)
ABSTRACT
Recent and older (early Holocene to Pleistocene) hot-spring travertine
carbonates from central Italy have two distinctive macrofabrics, crystalline
crusts and shrubs. Crystalline crusts are laminated slope deposits that
formed abiotically following CO2 degassing from spring water.
The formation of shrub travertine--irregular, dendritic precipitates from
pool environments--is controversial and has been attributed to both abiotic
and microbial processes. Oxygen isotope variation in our travertines can
be explained by abiotic processes, mainly CO2 degassing. In
contrast, our carbon isotope data cannot be wholly explained by abiotic
CO2 degassing invoked in earlier studies. Because photosynthesis
is known to preferentially remove 12C, leaving am ient waters
enriched in 13C, this fractionation should be recorded in d13C
values of microbially influenced travertine. Our shrub carbon isotope values
are between 0.5
and 6.0
larger than values for associated abiotic precipitates, and this difference
is probably caused by microbial activity. These isotope data support previous
visual evidence that some shrub fabrics are microbially influenced.
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