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| Titel |
Evolution of Microbial Carbonates During Early Burial: A Multi-Proxy
Approach |
| VerfasserIn |
Chelsea Pederson, James Klaus, Peter Swart, Donald McNeill |
| Konferenz |
EGU General Assembly 2017
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| Medientyp |
Artikel
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| Sprache |
en
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 19 (2017) |
| Datensatznummer |
250138581
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| Publikation (Nr.) |
 EGU/EGU2017-1638.pdf |
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| Zusammenfassung |
| This study provides a geochemical and textural characterization of modern, Holocene, and
Pleistocene palustrine microbial muds and associated organic matter along the low-energy
shoreline of the Florida Everglades, USA.
Dense organic-rich, light brown carbonate muds are deposited above karstified
Pleistocene limestone. Deposited in shallow freshwater, the palustrine mud is a result of
precipitation induced by cyanobacterial photosynthesis, and reflects local water chemistry
and hydrologic conditions. The microbial community forms a dense mat, and excretes
exopolymeric substances (EPS) and other labile organic constituents, in which texturally
complex, fine-grained low-Mg calcite (LMC) crystals precipitate. Geochemically, this deposit
is characterized by high amounts of TOC (up to 12%), a depleted organic δ13C signal
indicative of microbial activity, and a low range of inorganic δ18O and δ13C values reflecting
freshwater deposition.
Within the Holocene microbial mud sequence, depositional and burial environments
(freshwater, brackish, and full marine) can be inferred through variation in geochemical
signatures, which arise from differences in depositional fluids, vegetation type, and metabolic
processes of organic degradation. Stable isotope values indicate that an increased marine
influence causes more positive δ18O values, whereas a freshwater source is characterized by
more negative values. Inorganic δ13C of the Holocene sequence likely represents biological
influence, and shows a fairly large range of up to 5‰ whereas sediments within the marine
burial environment have a much lower range (∼1.5). Depending on the method
of organic decomposition (aerobic, anaerobic, etc.), different δ13C signatures are
recorded in both the organic and inorganic fraction. Holocene cores indicate that the
freshwater environment degrades organic material via denitrification during early burial,
whereas the mangrove transition zone (greater marine influence) likely breaks down
organic material through sulfate reduction, leading to a more negative organic δ13C
signature. Early diagenesis of Holocene (<4.5 kyr) muds has altered both the texture, as
well as geochemical signatures of the microbial carbonates and their associated
organic material. The TOC generally decreases with depth, correlating with more
negative organic δ13C values as a result of the degradation of labile material, including
EPS.
This study implies that microbial signatures of freshwater carbonates are dependent upon
the environment in which they were formed and buried. Variation in salinity, as well as
vegetation type within the environment alter the original depositional signature, which is also
not well preserved. Deviation from modern geochemical signals implies that meteoric and
marine diagenesis affects even these dense LMC mudstones and their associated
organic matter, and has implications for the use of palustrine carbonates proxies. |
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