Abstract In the middle–late Paleocene, a marine, organic-rich sedimentary unit (Waipawa Formation [Fm]) in which the organic matter was derived mainly from terrestrial plants was deposited in many of New Zealand's sedimentary basins. The unique organofacies of this formation has not been identified in any other time interval within the geological history of the Southwest Pacific, indicating that unusual climatic and oceanographic conditions likely prevailed during this time. It has, therefore, attracted wide scientific interest due to its significance for regional and global reconstruction of the early Paleogene transitional climate as well as potential for oil and gas production. Scarcity of age-diagnostic fossils, presence of unconformities and lack of volcanic interbeds have, however, hindered precise dating and correlations of all the known occurrences of the formation. Here, rhenium‑osmium (Re-Os) geochronology has yielded the first radiometric age for the formation (57.5 ± 3.5 Ma), which is consistent with available biostratigraphic age determinations (59.4–58.7 Ma). Further, a comparison of Re-Os, bulk pyrolysis, sulfur and palynofacies data for the Waipawa Fm with those of more typical marine sediments such as the underlying Whangai Fm supports the interpretation that the chelating precursors or fundamental binding sites responsible for uptake of Re and Os are present in all types of organic matter, and that these elements have a greater affinity for organic chelating sites than for sulfides. The results also indicate that sedimentation rate may not play a dominant role in enhanced uptake of Re and Os by organic-rich sedimentary rocks. The initial 187Os/188Os values for the Waipawa (~0.28) and Whangai (~0.36) formations are broadly similar to those reported for coeval pelagic sediments from the central Pacific Ocean, further constraining the low-resolution marine 187Os/188Os record of the Paleocene. We present a compilation of 187Os/188Os values from organic-rich sedimentary rocks spanning the period between 70 and 50 Ma which shows that seawater Os gradually became less radiogenic from the latest Cretaceous, reaching a minimum value in the earliest late Paleocene (~59 Ma) during the deposition of Waipawa Fm, and then increased through the later Paleocene and into the early Eocene. The composite Os isotope record broadly correlates with global temperature (δ18O and TEX86) and carbon isotope (δ13C) records from the middle Paleocene to early Eocene, which is inferred to reflect climate-modulated changes in continental weathering patterns.

中文翻译：

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新西兰中晚期古新世 Waipawa 组的 Re-Os 年代学和同位素系统学以及有机和硫地球化学：对早古近纪海水 Os 同位素组成的洞察

摘要 在古新世中晚期，新西兰许多沉积盆地沉积了一个富含有机质的海相沉积单元（Waipawa组[Fm]），其中有机质主要来自陆生植物。该地层的独特有机相在西南太平洋地质历史的任何其他时间段内都没有被发现，这表明这段时间可能存在不寻常的气候和海洋条件。因此，由于其对早期古近纪过渡气候的区域和全球重建以及石油和天然气生产潜力的重要意义，它引起了广泛的科学兴趣。然而，年龄诊断化石的稀缺、不整合面的存在和火山夹层的缺乏，阻碍了所有已知地层发生的精确年代测定和相关性。在这里，铼 - 锇 (Re-Os) 地质年代学已经产生了地层的第一个辐射年龄（57.5 ± 3.5 Ma），这与可用的生物地层年龄测定（59.4-58.7 Ma）一致。此外，将 Waipawa Fm 的 Re-Os、大量热解、硫和孢粉相数据与更典型的海洋沉积物（例如下伏 Whangai Fm）的数据进行比较，支持了螯合前体或基本结合位点负责 Re 吸收和Os 存在于所有类型的有机物质中，并且这些元素对有机螯合位点的亲和力比对硫化物的亲和力更大。结果还表明，沉积速率可能不会在富含有机物的沉积岩对 Re 和 Os 的吸收增强中起主导作用。Waipawa (~0.28) 和 Whangai (~0.36) 地层的初始 187Os/188Os 值与中太平洋同期远洋沉积物报告的值大致相似，进一步限制了古新世的低分辨率海洋 187Os/188Os 记录. 我们提供了 70 到 50 Ma 期间富含有机质沉积岩的 187Os/188Os 值的汇编，这表明海水 Os 从白垩纪晚期开始逐渐降低放射成因，在最早的晚古新世（~59 Ma ) 在外帕瓦组沉积期间，然后增加到晚古新世并进入早始新世。