Geochemical studies of the Cretaceous-Tertiary boundary in ODP Holes 113-689B and 113-690C

In a study of ODP Hole 689B no iridium (Ir) anomaly was found in Sections 1 through 6 of Core 25X or in Core 26X from the top down to section 2, 3-12 cm. The background Ir abundance averaged 11 parts per trillion (ppt) and a clay-enriched region had nearly the same average, 26 ± 12 ppt. If the Cretaceous-Tertiary (K-T) contact is in the region studied, then sedimentation was not continuous, and the K-T boundary was probably either not deposited or it was eroded away.
In a study of Cores 15X and 16X of ODP Hole 690C, an iridium peak with a maximum abundance of 1566 ± 222 ppt was found in Section 4 of Core 15X at 39-40 cm with a half-width of 6.6 cm. Background abundances were ~15 ppt and distinctly higher Ir abundances were observed from 119 cm below to 72 cm above the main peak. The Ir distribution below the main peak is attributed to bioturbation by organisms with burrows extending at least 0.4 m. The Ir distribution above the main peak may be due to the same cause but other explanations may be significant.
There are variable enrichments of clay in the mainly CaCO3 sediment of Core 15X, and the stratigraphically lowest part of the most abundant clay deposits is found (within 2 cm) in the same position as the main Ir peak. The clay deposit, which is estimated to be about 50% of the sediment, extends upward ~19 cm and then slowly decreases to a background level of 10% over 1 m. The degree of homogeneity of the clay-rich interval suggests it was not due to episodic volcanism but may have been due to a decrease of the CaCO3 deposition rate which was possibly triggered by the impact of a large asteroid or comet on the Earth.

对大洋钻探计划（Ocean Drilling Program，ODP）689B钻孔的研究显示，在25X号岩心的第1至6岩心段，以及26X号岩心自上而下至第2岩心段3~12cm处，均未发现铱（Iridium，Ir）异常。背景铱丰度平均值为11万亿分之一（parts per trillion，ppt），富黏土区域的丰度平均值与之相近，为26±12 ppt。若白垩纪-古近纪（Cretaceous-Tertiary，K-T）接触带位于本次研究区域内，则表明沉积作用并非连续进行，K-T界线要么未曾沉积，要么已被侵蚀殆尽。

对大洋钻探计划（Ocean Drilling Program，ODP）690C钻孔的15X、16X号岩心开展研究后，在15X号岩心第4岩心段39~40cm处发现一处铱丰度峰值，其最大丰度为1566±222 ppt，半高宽为6.6 cm。背景丰度约为15 ppt，主峰值下方119 cm至上方72 cm范围内的铱丰度显著偏高。主峰值下方的铱分布可归因于生物扰动：相关生物的潜穴深度至少可达0.4 m。主峰值上方的铱分布或许也由相同原因导致，但其他解释也具有一定参考价值。

15X号岩心以碳酸钙（CaCO3）沉积物为主，其中黏土含量存在不均匀富集现象。黏土富集程度最高的地层区段，其最低层位与主铱丰度峰值的位置重合（误差在2 cm以内）。据估算，该黏土沉积物占沉积物总量的50%左右，向上延伸约19 cm后，在1 m的范围内缓慢降低至10%的背景水平。富黏土段的均一程度表明，其成因并非间歇性火山活动，而可能是碳酸钙沉积速率下降所致——该沉积速率下降或许由地球遭遇大型小行星或彗星撞击引发。