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| Titel |
Recrystallization of biogenic carbonates in soils: consequences for palaeological studies |
| VerfasserIn |
Kazem Zamanian, Konstantin Pustovoytov, Yakov Kuzyakov |
| Konferenz |
EGU General Assembly 2015
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| Medientyp |
Artikel
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| Sprache |
Englisch
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 17 (2015) |
| Datensatznummer |
250101220
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| Publikation (Nr.) |
 EGU/EGU2015-326.pdf |
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| Zusammenfassung |
| The isotopic signatures of biogenic carbonate (BC) in fossils are commonly used to assess
environmental conditions during the life time of organisms, their diets and extinction periods.
As a proxy, BC represents in many cases the only alternative to organic matter. However, BC
in fossils may dissolve in embedded matrix and recrystallize with CO2 respired by roots and
microorganisms. Consequently, isotopic composition of BC can be re-equilibrated and the
original paleoenvironmental signal may be lost. The dynamics of these processes still
remains poorly understood. Here the results of BC recrystallization under controlled
conditions have been presented. We aimed 1) To determine the recrystallization amounts
of BC as a function of time, 2) To investigate the effects of geogenic carbonates
(GC) availability in embedded matrix on recrystallization rate of BC and 3) To
evaluate the effects of organic matter (OM) presence in the BC structure on its
recrystallization.
Loess and a loamy soil were selected as carbonate containing and carbonate free matrixes,
respectively. Shells of “Pacific little-neck clams (Protothaca staminea)” were selected as BC.
To evaluate the role of OM presence in the BC structure, heated (550?C) and not heated shells
were used. The shells were washed by means of ultrasonic and crashed to a size of 2-2.5 mm.
The 14C labeled CO2 (pCO2= 2%) was injected into the airtight bottles. The samples
were incubated at room temperature and water content of 60% of water holding
capacity of matrixes for 1, 3, 10, 21 and 56 days. At each time the 14C activity was
measured in bottle air, dissolved organic and inorganic carbon, matrixes and the
shells.
The recrystallization of shells started even after one day of incubation. However, the
amounts of recrystallization were increased by the time. The recrystallization of CaCO3 was
higher in shells without OM. Elimination of OM probably increases the porosity of shell
structure and led to better water penetration into the shells. Recrystallization was higher in
BC surrounded by loess compared to that in carbonate free loamy soil. GC in loess can
undergo dissolution as well. This leads to release of Ca2+ ions which may partially be
recrystallized on shells. Based on the recrystallization rate of shell carbonate, we extrapolated
the time for their full recrystallization. Our results showed that a full recrystallization of
shells needs ca. 90 years for shells without OM in loess to ca. 300 years for shells
containing OM in carbonate free loamy soil. Consequently, the original isotopic
signature will be vanished completely in the shell structure after this period. Our results
suggest that the recrystallization of BC may proceed relatively rapidly: between
decades and centuries. The study further shows the high potential of 14C labelling for
understanding the processes of diagenetic alteration of BC in soils and sediments. |
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