Strontium-isotope stratigraphy and oxygen- and carbon-isotope record of Middle Jurassic to Early Cretaceous belemnites in the South Atlantic

87Sr/86Sr data of belemnites are presented from a Middle Jurassic-Early Cretaceous succession from the Falkland Plateau (Deep Sea Drilling Project Sites 511 and 330) that was deposited in a periodically anoxic, semi-enclosed shallow water basin. Diagenetically screened strontium-isotope values of 0.706789 rise to 0.707044 before increasing sharply to 0.707428 in the uppermost part of the sampled succession. Comparison with published strontium calibration curves suggests that the oldest samples were Callovian to Oxfordian in age, whilst the remainder of the Jurassic part of the succession consisted of Kimmeridgian and Early Tithonian age sediments. The nannofossil, dinoflagellate and molluscan assemblages provide comparable age determinations. The strontium-isotope analysis of the youngest belemnites points to a Hauterivian-Barremian age, whilst age interpretations based upon the fauna provide a wide age range from the Barremian to early Albian. Strontium-isotope stratigraphy of this succession hence offers increased age resolution providing data regarding the timing of episodes of bottom water anoxia which have been recorded throughout the South Atlantic Basin. Well-preserved belemnite specimens display an oxygen-isotope range between +0.08 and -2.22‰ (PDB, Peedee belemnite international standard) and a carbon-isotope range from +2.35 to -1.33‰ (PDB). Delta13C values become increasingly negative through the Late Jurassic-Early Cretaceous and in concert with the 87Sr/86Sr data reveal a trend that could be accounted for by increasing levels of weathering and erosion. The oxygen-isotope data if interpreted in terms of palaeotemperature are consistent with warm palaeotemperatures in the Kimmeridgian and slightly cooler temperatures for the Tithonian and Early Cretaceous parts of the succession. The proposed relative Kimmeridgian warmth (based upon strontium-isotope age assignments) is thus in good agreement with other published palaeotemperature records.

本数据集呈现了采自福克兰高原（深海钻探计划（Deep Sea Drilling Project）511与330站位）中侏罗世至早白垩世地层的箭石（belemnites）87Sr/86Sr同位素数据，该地层沉积于周期性缺氧、半封闭的浅水环境盆地。经成岩作用筛选的锶同位素数值从0.706789升至0.707044，随后在采样地层的最上部急剧升高至0.707428。结合已发表的锶同位素校准曲线进行对比可知，最古老的样品时代为卡洛夫期至牛津期，而地层中其余侏罗系沉积物则形成于启莫里期和早提通期。钙质超微化石、沟鞭藻与软体动物组合也提供了一致的年代判定结果。对最年轻箭石样品的锶同位素分析显示其时代为豪特里维期至巴列姆期，而基于动物群的年代解释则给出了从巴列姆期至早阿尔布期的宽泛年代范围。因此，本套地层的锶同位素地层学研究提升了年代分辨率，可为南大西洋盆地全程记录的底水缺氧事件的发生时序提供数据支撑。保存完好的箭石样品的氧同位素范围为+0.08‰至-2.22‰（PDB，佩迪箭石国际标准（Peedee belemnite international standard）），碳同位素范围为+2.35‰至-1.33‰（PDB）。晚侏罗世至早白垩世期间，δ13C值逐渐偏负，结合87Sr/86Sr同位素数据可揭示出与风化侵蚀作用增强相一致的演化趋势。若以古温度视角解读氧同位素数据，则其与启莫里期温暖的古温度、提通期及早白垩世地层的略微降温特征相符。本次基于锶同位素年代标定提出的启莫里期相对温暖的结论，与其他已发表的古温度记录具有良好的一致性。