High-resolution chemostratigraphy of Maastrichtian/Paleocene transition in Texas, USA, and Brazil

对尤卡坦地区希克苏鲁布陨石坑的ICDP钻探和整个墨西哥东北部的K-T露头研究揭示了地层、地球化学、矿物学、古地磁和古生物证据，表明这种影响早于K-T界线约30万年，因此不可能像人们通常认为的那样，导致白垩纪末期的大灭绝。该项目的主要目的是通过对布拉索斯和巴西新采集的样品和岩心的地球化学方法，为解决目前在一个跨学科的国际研究项目中关于希克苏鲁布撞击的年龄及其与K-T边界大灭绝的关系的争议做出贡献。这场争论不是谁对谁错的问题，而更多的是一个巨大影响的问题，远远超出了奇克苏鲁撞击的年龄，与地球S历史上大规模灭绝的性质和原因(S)有着巨大的关系。第一个项目年的初步结果，加上对最新文献的比较和批判性评价，似乎强化了这样的结论，即希克苏鲁撞击和巴西和巴西剖面的生物地层K-T边界在时间上并不重合。然而，在得出无可辩驳的结论之前，一系列关键特征仍有争议，必须得到回答。其中包括(I)“事件沉积”的性质，(Ii)“事件沉积”和生物地层K-T界线上方的沉积物的沉积时间，(Iii)最近在布拉索斯事件沉积之下发现的“黄色粘土层”是否对应于原始的Chixculub撞击喷射层，或(Iv)巴西Poty Quarry大年沉积物中的Ir异常是否被改造或标志着一次不同的、更年轻的撞击事件。更重要的是明确除影响(如“超级火山作用”、缺氧)以外的其他压力因素对白垩纪末期大灭绝的可能作用。

The ICDP drilling of the Chicxulub crater on Yucatan and K-T outcrop studies throughout NE Mexico have revealed stratigraphic, geochemical, mineralogical, paleomagnetic and paleontological evidences that this impact predates the K-T boundary by about 300,000 years and hence could not have been responsible for the end-Cretaceous mass extinction as commonly believed. The major purpose of this project is to contribute by means of geochemical methods on newly sampled and cored material in Brazos and Brasil to the solution of the current controversy over the age of the Chicxulub impact and its relationship to the K-T boundary mass extinction within an interdisciplinary, international research project. The debate is not a matter of who is right or wrong it is more a matter of tremendous implications that go way beyond the mere age of the Chicxulub impact, with tremendous bearing on the nature and cause(s) of mass extinctions during the Earth´s history. Preliminary results of the first project year combined with a comparative, critical evaluation of the latest literature, seems to reinforce the conclusion that the Chicxulub impact and the biostratigraphic K-T boundary of the sections of Brazos and Brazil are not coincident in time. However, a series of key features are still debatable and must be answered before reached to an irrefutable conclusion. Among others, these include (i) the nature of the “event deposits”, (ii) the deposition time of the sediments intercalated above the “event deposits” and the biostratigraphic K-T boundary, (iii) whether the recently discovered “yellow clay layer” below the event deposits at Brazos corresponds or not to the original Chixculub impact ejecta layer, or (iv) whether the Ir anomaly in sediments of Danian age at Poty Quarry, Brazil is reworked or marks a different, younger impact event. Of further major interest is to clear the possible role of other stress factors than impact (e.g. “super-volcanism”, anoxia) to the end-Cretaceous mass extinction.