Isotopic and chemical intra-shell variations in low-Mg calcite of rudist bivalves (Mollusca-Hippuritacea): disequilibrium fractionations and late Cretaceous seasonality |
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| Authors: | T Steuber |
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| Institution: | Institut für Geologie und Mineralogie, Universit?t Erlangen-Nürnberg, Schlossgarten 5, D-91054 Erlangen, Germany e-mail: steuber@geol.uni-erlangen.de Tel.: +9131-8522514 Fax: +9131-8529295, DE
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| Abstract: | Isotopic (δ13C, δ18O) and elemental (Mg, Sr, Mn, Fe) compositions were analysed in sclerochronological profiles of several shells of late Cretaceous
rudist bivalves from Greece, Turkey, Somalia and the Arabian Peninsula. The preservation of original compositions of low-Mg
calcite of outer shell layers is indicated by constant and high Sr, generally low Fe and Mn, and the preservation of fibrous-prismatic
ultrastructures. Cyclic variations in δ18O and Mg are interpreted to reflect seasonal temperature/salinity cycles and, thus, annual growth increments. In shells of
Torreites, amplitudes of correlated δ13C and δ18O cycles cannot be related to reasonable palaeotemperatures or salinity. This isotopic pattern reflects vital fractionations
of an extent which is unknown from modern bivalves. In contrast, almost identical ranges and amplitudes of δ18O cycles are observed in 13 shells of five species from Santonian-Campanian localities in central Greece and northern Turkey,
suggesting that seasonal variations in environmental conditions were recorded without significant vital fractionations. The
effect of seasonal salinity changes on δ18O of the shells is evaluated, and mean palaeotemperatures are constrained within the range of 30–32.5 °C. The annual range
of temperature was estimated to be 7 °C, assuming a constant salinity. This agrees with other isotopic proxies of Late Cretaceous
palaeotemperatures, and with global circulation models which predict higher low-latitude sea-surface temperatures than the
present ones.
Received: 12 February 1998 / Accepted: 24 May 1999 |
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| Keywords: | Rudist bivalves Cretaceous Stable isotopes Sclerochronology Palaeotemperatures |
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