TY - JOUR
T1 - Partial diagenetic overprint of late jurassic belemnites from New Zealand
T2 - implications for the preservation potential of δ7Li values in calcite fossils
AU - Ullmann, Clemens Vinzenz
AU - Campbell, Hamish J.
AU - Frei, Robert
AU - Hesselbo, Stephen P.
AU - Pogge von Strandmann, Philip A.E.
AU - Korte, Christoph
PY - 2013/11/1
Y1 - 2013/11/1
N2 - The preservation potential and trends of alteration of many isotopic systems (e.g. Li, Mg, Ca) that are measured in fossil carbonates are little explored, yet extensive paleoenvironmental interpretations have been made on the basis of these records. Here we present a geochemical dataset for a Late Jurassic (~153 Ma) belemnite (Belemnopsis sp.) from New Zealand that has been partially overprinted by alteration. We report the physical pathways and settings of alteration, the resulting elemental and isotopic trends including δ7Li values and Li/Ca ratios, and assess whether remnants of the primary shell composition have been preserved or can be extrapolated from the measured values. The δ18O and δ13C values as well as Sr/Ca and Mn/Ca ratios were analysed along two profiles. In addition, 6 samples were analysed for 87Sr/86Sr, Sr/Ca and Mn/Ca ratios. Five samples from the same specimen and 2 from the surrounding sediment were analysed for δ7Li values, Li/Ca, Sr/Ca and Mn/Ca ratios and are compared to results for 6 other Late Jurassic belemnite rostra (Belemnopsis sp. and Hibolithes sp.) from the same region. The 87Sr/86Sr ratios are lower (less radiogenic) in the most altered part of the rostrum, whereas δ7Li values become more positive with progressive alteration. The direction and magnitude of the trends in the geochemical record indicate that one main phase of alteration that occurred in the Late Cretaceous caused most of the diagenetic signature in the calcite. Despite relatively deep burial, down to 4 km, and thus elevated temperatures, this diagenetic signature has subsequently been preserved even for the highly mobile element lithium, suggesting that primary lithium-isotope values can be maintained over geological timescales, at least in thick macrofossil shells. Our best δ7Li estimate for pristine Late Jurassic (~155-148 Ma) belemnites is +27 ± 1 ‰, which points to a Late Jurassic seawater δ7Li of ~29-32 ‰, compatible with the modern value of 31 ‰.
AB - The preservation potential and trends of alteration of many isotopic systems (e.g. Li, Mg, Ca) that are measured in fossil carbonates are little explored, yet extensive paleoenvironmental interpretations have been made on the basis of these records. Here we present a geochemical dataset for a Late Jurassic (~153 Ma) belemnite (Belemnopsis sp.) from New Zealand that has been partially overprinted by alteration. We report the physical pathways and settings of alteration, the resulting elemental and isotopic trends including δ7Li values and Li/Ca ratios, and assess whether remnants of the primary shell composition have been preserved or can be extrapolated from the measured values. The δ18O and δ13C values as well as Sr/Ca and Mn/Ca ratios were analysed along two profiles. In addition, 6 samples were analysed for 87Sr/86Sr, Sr/Ca and Mn/Ca ratios. Five samples from the same specimen and 2 from the surrounding sediment were analysed for δ7Li values, Li/Ca, Sr/Ca and Mn/Ca ratios and are compared to results for 6 other Late Jurassic belemnite rostra (Belemnopsis sp. and Hibolithes sp.) from the same region. The 87Sr/86Sr ratios are lower (less radiogenic) in the most altered part of the rostrum, whereas δ7Li values become more positive with progressive alteration. The direction and magnitude of the trends in the geochemical record indicate that one main phase of alteration that occurred in the Late Cretaceous caused most of the diagenetic signature in the calcite. Despite relatively deep burial, down to 4 km, and thus elevated temperatures, this diagenetic signature has subsequently been preserved even for the highly mobile element lithium, suggesting that primary lithium-isotope values can be maintained over geological timescales, at least in thick macrofossil shells. Our best δ7Li estimate for pristine Late Jurassic (~155-148 Ma) belemnites is +27 ± 1 ‰, which points to a Late Jurassic seawater δ7Li of ~29-32 ‰, compatible with the modern value of 31 ‰.
KW - Faculty of Science
KW - belemnites
KW - diagenesis
KW - New Zealand
KW - stable isotopes
KW - non-traditional isotopes
KW - Trace Elements
KW - Radiogenic isotopes
KW - calcite
KW - Fossils
U2 - 10.1016/j.gca.2013.06.029
DO - 10.1016/j.gca.2013.06.029
M3 - Journal article
SN - 0016-7037
VL - 120
SP - 80
EP - 96
JO - Geochimica et Cosmochimica Acta
JF - Geochimica et Cosmochimica Acta
ER -