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

对尤卡坦半岛希克苏鲁布陨石坑的ICDP钻探和对整个墨西哥东北部K-T露头的研究揭示了地层学、地球化学、矿物学、古地磁学和古生物学证据，表明这次撞击比K-T边界早了大约30万年，因此不可能像通常认为的那样是白垩纪末大灭绝的原因。该项目的主要目的是通过对巴西和巴西新取样和取芯材料的地球化学方法，在一个跨学科的国际研究项目中，帮助解决目前关于希克苏鲁布撞击年龄及其与K-T边界大规模灭绝的关系的争议。这场辩论不是谁对谁错的问题，而是一个巨大的影响问题，远远超出了希克苏鲁布撞击的时代，对地球历史上大规模撞击的性质和原因产生了巨大的影响。第一个项目年的初步结果与对最新文献的比较和批判性评价相结合，似乎加强了以下结论：希克苏鲁布的影响和Brazos和巴西剖面的生物地层学K-T边界在时间上并不一致。然而，一系列关键特征仍然是有争议的，必须在得出无可辩驳的结论之前回答。其中包括：（i）“事件沉积物”的性质，（ii）“事件沉积物”和生物地层K-T边界上方夹层沉积物的沉积时间，（iii）最近在Brazos事件沉积物下方发现的“黄色粘土层”是否与原始Chixculub撞击喷出物层相对应，或（iv）巴西Poty Quarry的Danian年龄沉积物中的Ir异常是否被改造或标志着一个不同的、较年轻的撞击事件。进一步的主要兴趣是清除其他应力因素的可能作用，而不是影响（如“超级火山”，缺氧）白垩纪末的大规模灭绝。

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.