Palynostratigraphy of John's Nose, a New Cretaceous—Paleogene Boundary Section in Southwestern North Dakota, USA
Palynostratigraphy of John's Nose, a New Cretaceous—Paleogene Boundary Section in Southwestern North Dakota, USA
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DOI：
10.1080/01916122.2012.678695
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发表时间：
2012-03
期刊：
影响因子：
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通讯作者：
A. Bercovici;V. Vajda;D. Pearson;U. Villanueva-Amadoz;Douglas Kline
中科院分区：
文献类型：
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作者：
A. Bercovici;V. Vajda;D. Pearson;U. Villanueva-Amadoz;Douglas Kline
本研究记录了美国北达科他州新发现的古近纪-古近纪界线--约翰鼻剖面的陆生孢粉记录。除了泥丘和金字塔丘之外，约翰的鼻子代表了北达科他州西南部的第三个K-Pg剖面，保存了希克苏鲁布小行星撞击的直接证据，可以直接比较这一事件的孢粉学记录的时间和趋势。通过对John's Nose剖面的孢粉学分析，发现有68个花粉和孢子分类群。在边界粘土的正上方，记录了大量的杯状孢子和Laevigatosporites属的蕨类孢子（其中59%的组合由杯状孢子代表）。这一非常独特的K-Pg“蕨类尖峰”事件与小行星撞击后立即发生的陆地植物毁灭有关，并与北达科他州西南部其他地点通常报告的成分相匹配。孢粉地层学表明，根据马斯特里赫特阶典型分类群（K-分类群）的最后出现基准面（LAD）确定K-Pg界线的位置可能会产生误导。与以前的报告相比，在约翰鼻子的边界粘土上方几米处偶尔出现K-类群是一个重要的差异。根据这一观察，应谨慎使用LAD作为识别边界的主要标准;一些K-类群可能在最早的古近系中短期存在，或被改造。在John's Nose剖面，孢粉学记录中的主要变化和灭绝发生在地球化学K-Pg边界，表明在短时间内发生了灾难性的更替。
This study documents the terrestrial palynological record at the John's Nose section, a new Cretaceous—Paleogene (K—Pg) boundary site in North Dakota, USA. In addition to Mud Buttes and Pyramid Butte, John's Nose represents the third K—Pg section in southwestern North Dakota that preserves direct evidence of the Chicxulub asteroid impact, allowing for direct comparison over the timing and trends of the palynological record in respect to this event. The palynological analysis of John's Nose section reveals the presence of 68 pollen and spore taxa. Immediately above the boundary clay, a high abundance of fern spores of the genera Cyathidites and Laevigatosporites is recorded (with 59% of the assemblage being represented by Cyathidites). This very distinctive K—Pg ‘fern spike’ event is correlated with the devastation of land plants immediately following the asteroid impact and matches the composition generally reported from other sites in southwestern North Dakota. Palynostratigraphy demonstrates that the placement of the K—Pg boundary based upon the identification of the Last Appearance Datum (LAD) of typical Maastrichtian taxa (K-taxa) may be misleading. The presence of occasional K-taxa up to a few meters above the boundary clay at John's Nose represents an important difference when compared to previous reports. In light of this observation, LADs should be used cautiously as the primary criteria to identify the boundary; some K-taxa may have a short-term presence in the earliest Paleogene, or be reworked. In the John's Nose section, major changes and extinction in the palynological record occur at the geochemical K—Pg boundary, indicating that a catastrophic turnover took place over a short time.