Partial diagenetic overprint of Late Jurassic belemnites from New Zealand: Implications for the preservation potential of ¦Ä⁷Li values in calcite fossils

详细信息    查看全文

• 作者：Clemens V. Ullmann ; Hamish J. Campbell ; Robert Frei ; Stephen P. Hesselbo ; Philip A.E. Pogge von Str ; mann ; Christoph Korte
• 刊名：Geochimica et Cosmochimica Acta
• 出版年：2013
• 出版时间：1 November, 2013
• 年：2013
• 卷：120
• 期：Complete
• 页码：80-96
• 全文大小：1388 K

文摘

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 ¦Ä⁷Li 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 ¦Ä¹⁸O and ¦Ä¹³C 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 ¦Ä⁷Li values, Li/Ca, Sr/Ca and Mn/Ca ratios and are compared to results for 6 other Late Jurassic belemnite rostra (Belemnopsis sp. andHibolithes sp.) from the same region. The ⁸⁷Sr/⁸⁶Sr ratios are lower (less radiogenic) in the most altered part of the rostrum, whereas ¦Ä⁷Li 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 ¦Ä⁷Li estimate for pristine Late Jurassic (¡«155-148 Ma) belemnites is +27 ¡À 1¡ë, which points to a Late Jurassic seawater ¦Ä⁷Li of ¡«29-32¡ë, compatible with the modern value of 31¡ë.