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A widespread positive δ 13 C carb anomaly at around 2.33–2.06 Ga on the Fennoscandian Shield: a paradox?
Author(s) -
Melezhik Victor A.,
Fallick Anthony E.
Publication year - 1996
Publication title -
terra nova
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.353
H-Index - 89
eISSN - 1365-3121
pISSN - 0954-4879
DOI - 10.1111/j.1365-3121.1996.tb00738.x
Subject(s) - geology , precambrian , excursion , carbonate , diagenesis , proterozoic , sedimentary rock , paleontology , isotopes of carbon , baltic shield , geologic time scale , geochemistry , isotope , chemistry , tectonics , physics , organic chemistry , quantum mechanics , political science , law
ABSTRACT Carbon isotope measurements carried out on 201 carbonate samples from the early Proterozoic of the Kola Peninsula, N. Karelia and Norway yield δ 13 C (PDB) spanning ‐ 20.5% to + 11%. A general δ 13 C secular trend shows that prior to 2.33 Ga values are typically ‘normal’ marine, averaging around ‐ 3%0. Between 2.33 and 2.06 Ga, in Jatulian time, there follows a rapid excursion to positive δ 13 C of around + 6%. Post‐Jatulian time is characterized by δ 13 C of sedimentary carbonates fluctuating between ‐ 5% and +3%; also it is remarkable for the first pronounced development of diagenetic carbonates, which have δ 13 C between ‐ 14 % and ‐ 6% . The c. 6% positive δ 13 C shift with a duration of about 270 Myr coincides with a maximum in the diversity and abundance of stromatolites, and with widespread development of ‘red beds’, but does not coincide with the maximum of buried C org mass. The Fennoscandian Shield represents the largest isotoically anomalous carbonate province yet reported, and the positive δ 13 C excursion together with a series of major global palaeoenviromental changes seems to be more intense than the Precambrian/Cambrian transition events. However, it is still not clear what kind of mechanism this phenomenon could be attributed to. An increase of the ‘Ronov ratio’, and/or ‘Broecker ratio’ and other possible models are discussed as the target for future investigations.