TEX86 and stable δ18O paleothermometry of early Cretaceous sediments: Implications for belemnite ecology and paleotemperature proxy application
TEX86 and stable δ18O paleothermometry of early Cretaceous sediments: Implications for belemnite ecology and paleotemperature proxy application
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DOI：
10.1016/j.epsl.2010.07.043
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发表时间：
2010-10
影响因子：
5.3
通讯作者：
J. Mutterlose;Matthias Malkoč;Stefan Schouten;J. Damsté;A. Forster
中科院分区：
文献类型：
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作者：
J. Mutterlose;Matthias Malkoč;Stefan Schouten;J. Damsté;A. Forster
最近的研究对未经调整的侏罗纪和白垩纪的δ 18 O古温度数据的使用提出了质疑，因为后者的数据往往反映了比预期更低的古温度估计。在这项研究中，我们解决了这个问题，分析岩石的Barremian早期Aptian年龄从两个露头在北方德国使用TEX 86古温度计，沿着与142个belemnite警卫研究其稳定同位素（δ 13 C，δ 18 O）和微量元素组成（镁，锶，铁，锰）。TEX86和δ 18OBel都表明一个独特的早巴列姆纪晚期（“Hauptblätterton”）黑色页岩序列的水温非常温暖，温度分别高达29°C和23°C。我们观察到TEX86温度相对于4至5°C冷却器δ18O箭石信号的恒定偏移。巴列姆阶晚期的δ18OBelvalues从−1‰增加到0‰，反映温度在16到12°C之间，而同期的TEX86温度在26到32°C之间变化。然而，在巴列姆纪早期的缺氧沉积物中普遍出现的箭石显然意味着一种类似于现代条虫的游泳生活方式，而不是像乌贼那样的游泳底栖生活方式。这反过来又表明，这里调查的箭石（属Praeoxyteuthis，Aulacoteuthis，Oxyteuthis和Neohibolites）是活跃的游泳者，它们居住在温跃层以下的更深的栖息地，在水深约100至250米的陆表海。因此，TEX86和δ 18OBelt数据的偏移反映了两种不同深度生境的温度信号，即TEX86选择性地来自温暖的海面沃茨，而在巴列姆纪晚期，由于盐度相对增加，箭石可能占据了更深更冷的沃茨。在对δ 18OBel信号进行进一步解释之前，必须考虑用于古温度重建的箭石分类群（属、种）的分类学、古生物学和生态学的重要性。22个样品的δ18O变化幅度为1.1‰，表明在恢复长期古温度变化趋势时应谨慎使用单个样品的δ18O数据。
Recent studies have cast doubt on the unadjusted usage of Jurassic and Cretaceous δ18O paleotemperature data derived from belemnites, since the latter data often reflect cooler paleotemperature estimates than would be expected. In this study we address this problem by analysing rocks of Barremian to early Aptian age from two outcrops in northern Germany using TEX86paleothermometry, along with 142 belemnite guards studied for their stable isotope (δ13C, δ18O) and trace element composition (magnesium, strontium, iron, and manganese). Both TEX86and δ18OBelindicate very warm water temperatures for a distinctive black shale sequence of late early Barremian age (“Hauptblätterton”) with temperatures of up to 29°C and 23°C, respectively. We observe a constant offset of TEX86temperatures versus the 4 to 5°C cooler δ18O belemnite signal for this interval. The late Barremian sequence shows an increase of the δ18OBelvalues from −1‰ to 0‰ reflecting temperatures around 16 to 12°C, while the contemporaneous TEX86temperatures vary between 26 and 32°C. The common occurrence of belemnites in the anoxic sediments of the early Barremian implies, however, clearly a nektonic way of life similar to that of recent teuthids, rather than a nektobenthic one like Sepia. This in turn suggests that the belemnites investigated here (genera Praeoxyteuthis, Aulacoteuthis, Oxyteuthis, and Neohibolites) were active swimmers, which inhabited a deeper habitat below the thermocline in an epicontinental sea of perhaps 100 to 250m water depth. The offset of the TEX86and δ18OBeldata is therefore interpreted to reflect temperature signals from two different depth habitats, i.e. the TEX86is selectively derived from warm sea-surface waters, and the belemnites likely occupied deeper and cooler waters with relative increasing salinities in the late Barremian. This study stresses the importance that the taxonomy, paleobiology and ecology of the belemnite taxa (genera, species) used for paleotemperature reconstructions must be considered before the δ18OBelsignal can be further interpreted. The variation of the δ18O signature gained from one belemnite population of 22 specimens by 1.1‰ suggests that the δ18O data of individual belemnites should be used with caution for reconstructing long termed paleotemperature trends.