Benthic foraminiferal abundances at the Cretaceous/Paleogene boundary of ODP Hole 117-1049C at Blake Nose, Northwest Atlantic (Appendix A)

Sediments recovered at lower bathyal ODP Site 1049 on Blake Nose (Northwestern Atlantic) offer an opportunity to study environmental changes at the Cretaceous/Paleogene (K/P) boundary relatively close to the Chicxulub impact structure on the Yucatan peninsula, Mexico. In Hole 1049C, the boundary is located at the base of a 9-cm-thick layer with abundant spherules, considered to be impact ejecta. Uppermost Maastrichtian oozes below, and lowermost Danian pelagic oozes above the spherulebed contain well-preserved bathyal benthic foraminifera. The spherule-bed itself, in contrast, contains a mixture of shallow (neritic) and deeper (bathyal) species, and specimens vary strongly in preservation. This assemblage was probably formed by reworking and down-slope transport triggered by the K/P impact. Across the spherule-bed (i.e., the K/P boundary) only ~7% of benthic foraminiferal species became extinct, similar to the low extinction rates of benthic foraminifera worldwide. Quantitative analysis of benthic foraminiferal assemblages and morphogroups in the >63-µm size fraction indicates a relatively eutrophic, stable environment during the latest Maastrichtian, interrupted by a sudden decrease in the food supply to the benthos at the K/P boundary and a decrease in diversity of the faunas, followed by a stepped recovery during the earliest Danian. The recovery was probably linked to the gradual recovery of surface-dwelling primary producers.

采集于西北大西洋布雷克海岭大洋钻探计划（Ocean Drilling Program, ODP）1049站位半深海下带的沉积物，为研究相对靠近墨西哥尤卡坦半岛希克苏鲁伯撞击坑（Chicxulub impact structure）的白垩纪/古近纪（K/P）界线处的环境变化提供了契机。在1049C钻孔中，该K/P界线赋存于一层厚9厘米、富含球粒（spherules）的地层底部，该层被认定为撞击喷出物（impact ejecta）。球粒层之下的马斯特里赫特阶顶部软泥，以及球粒层之上的丹尼阶最底部远洋软泥中，保存有完好的半深海底栖有孔虫（benthic foraminifera）。与之相反，球粒层本身则混合了浅海（neritic）与半深海（bathyal）物种，且标本的保存状态差异悬殊。该有孔虫组合大概率是由K/P撞击触发的再沉积作用与斜坡搬运所形成的。跨越球粒层（即K/P界线）时，仅约7%的底栖有孔虫物种发生灭绝，这与全球范围内底栖有孔虫的低灭绝率特征一致。对粒径大于63微米的粒级组分中的底栖有孔虫组合与形态类群（morphogroups）开展定量分析，结果表明：马斯特里赫特阶末期的半深海环境整体处于富营养且稳定的状态，该环境在K/P界线处因底栖生物的食物供给骤减而被打断，同时伴随动物群多样性下降；随后在丹尼阶早期，生态系统经历了阶段性的恢复过程。这一恢复过程大概率与表层栖息的初级生产者的逐步复苏相关。