The mid-Oxfordian (Late Jurassic) positive carbon-isotope excursion recognised from fossil wood in the British Isles
The mid-Oxfordian (Late Jurassic) positive carbon-isotope excursion recognised from fossil wood in the British Isles
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从不列颠群岛的化石木材中发现了牛津中期（侏罗纪晚期）的正碳同位素偏移
DOI：
10.1016/j.palaeo.2005.03.004
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发表时间：
2005
期刊：
影响因子：
--
通讯作者：
A. Coe
中科院分区：
文献类型：
--
作者：
C. Pearce;S. Hesselbo;A. Coe
化石木材的碳同位素比值最近已被证实为古大气同位素组成变化的替代指标。从苏格兰斯凯岛侏罗纪(牛津期)斯塔芬页岩地层(北方/亚北方菊石分带)采集的化石木材样品的碳同位素数据显示，长期的正碳同位素漂移至少为3‰。这一漂移在牛津纪中期达到最大值，与之前从同一剖面收集的陨石中报告的碳同位素比率以及欧洲其他剖面中碳酸盐的碳同位素数据非常吻合。这证实了牛津纪中期正碳同位素漂移影响了总的可交换碳库。在较高的地层分辨率下采集的木材化石样品，但时间间隔较短，来自英国多塞特郡的珊瑚群(Antecedens、Parandieri和Ctisnigrae亚带；欧洲西北部菊石分带)，其碳同位素比率相当分散，没有明显的趋势。斯凯岛和多塞特岛的联合数据集表明，牛津市长期的正碳同位素趋势与长期的相对海平面变化相吻合，并且最正的碳同位素比率发生在褶皱-横槽生物分带(亚地中海菊石分带)上。这意味着碳同位素漂移不是由记录充分的横槽区海平面上升引起的。尽管在多塞特的节状碎石段(Parandieri亚带)中记录到了来自化石木材样品的非常低的碳同位素比值，但没有足够连贯的信号将这些值归因于先前根据特提斯海相碳酸盐的碳同位素比值推测的天然气-水合物解离事件。对斯塔芬页岩地层的炭化碎屑进行的显微镜分析表明，木质柏木属植物普遍存在，这种木质植物来自于一种雪松属针叶树。
The carbon-isotope ratios of fossil wood have recently been confirmed as a proxy for changes in the isotopic composition of palaeoatmospheres. Carbon-isotope data from fossil wood samples collected from the Jurassic (Oxfordian) Staffin Shale Formation on the Isle-of-Skye, Scotland (Boreal/Sub-Boreal ammonite zonation) reveal a long-term positive carbon-isotope excursion of at least 3‰. This excursion reaches a maximum in the mid-Oxfordian, and closely matches the carbon-isotope ratios previously reported from belemnites collected from the same section and carbon-isotope data from carbonates in other European sections. This confirms that the mid-Oxfordian positive carbon-isotope excursion affected the total exchangeable carbon reservoir. Fossilised wood samples collected at a higher stratigraphic resolution, but over a shorter interval from the Corallian Group in Dorset, England (antecedens, parandieri and cautisnigrae subzones; NW European ammonite zonation) show considerable scatter in their carbon-isotope ratios, and no trends are discernable. The combined Isle-of-Skye and Dorset dataset shows that the long-term Oxfordian positive carbon-isotope trend coincides with a long-term relative sea-level change, and that the most positive carbon-isotope ratios occur across the plicatilis–transversarium biozonal boundary (Sub-Mediterranean ammonite zonation). This implies that the carbon-isotope excursion was not caused by the well-documented rise in sea-level in the transversarium Zone. Although very low carbon-isotope ratios from fossil wood samples are recorded from the Nodular Rubble Member (parandieri Subzone) of Dorset, there is not a sufficiently coherent signal to ascribe these values to the gas–hydrate dissociation event previously hypothesized from the carbon-isotope ratios of Tethyan marine carbonates. A microscopal analysis of the charcoalified debris from the Staffin Shale Formation indicates a prevalence of the wood genus Cupressinoxylon, derived from a cheirolepidiaceaen conifer.