Foraminiferal abundances and preservation in ODP Hole 130-805C

Downcore changes in various carbonate dissolution indexes are documented for Hole 805C for the last 1.2 m.y. These indexes include degree of fragmentation of planktonic foraminifers, percent sand, abundance ratio of species of contrasting solution susceptibilities (Globigerinoides sacculifer vs. Pulleniatina, Globorotalia tumida, and Globorotalia menardii), and the difference in d18O between species of contrasting solution susceptibilities (G. sacculifer vs. Pulleniatina). These preservation indexes have been combined into a single composite dissolution index that corresponds closely to the d18O record. The rate of change of the oxygen isotope signal is also important, with glacial-to-interglacial transitions corresponding to maximum preservation events and vice versa. For information on changing productivity (which is important because an increased supply of organic matter may enhance dissolution by lowering pH upon degradation), we present the abundance of coarse-fraction benthic foraminifers per gram and the ratio between two planktonic foraminiferal species, one of which is strongly associated with equatorial upwelling (Globorotalia tumida vs. Pulleniatina). Our results suggest that productivity plays a subordinate role in determining foraminifer preservation. Furthermore, our results confirm previous observations that associate enhanced preservation events with glacial periods and with glacial-to-interglacial transitions. A correlation between preservation and sedimentation rates of these carbonate-rich sediments could not be established. Notable differences are present between the responses of individual dissolution indexes, indicating that processes other than dissolution determine proxy indexes to varying degrees.

本文记录了过去120万年间805C钻孔的多项碳酸盐溶解指标的岩芯垂向变化。这些指标涵盖浮游有孔虫（planktonic foraminifers）破碎度、砂粒百分含量、溶解敏感性迥异的类群丰度比值（包括泡抱球虫*Globigerinoides sacculifer*与圆球抱球虫*Pulleniatina*、膨圆辐球虫*Globorotalia tumida*、门氏圆辐球虫*Globorotalia menardii*的比值），以及溶解敏感性迥异的类群间的δ¹⁸O差值（泡抱球虫*G. sacculifer*与圆球抱球虫*Pulleniatina*的差值）。上述保存指标被整合为单一的综合溶解指数，该指数与δ¹⁸O记录高度吻合。氧同位素信号的变化速率同样关键：冰期向间冰期的转换对应保存作用最强的事件，反之亦然。为探究生产力变化（其重要性在于有机质输入增加会在降解过程中降低pH，从而增强溶解作用），本文提供了每克沉积物中粗组分底栖有孔虫（benthic foraminifers）的丰度，以及两种浮游有孔虫类群的丰度比值——其中一类与赤道上升流密切相关（膨圆辐球虫*Globorotalia tumida*与圆球抱球虫*Pulleniatina*）。研究结果表明，生产力对有孔虫保存的影响居于次要地位。此外，本研究验证了前人的观测结论：保存作用增强的事件与冰期以及冰期-间冰期转换阶段相关。本研究未发现这类富碳酸盐沉积物的保存作用与沉积速率之间存在相关性。各项单一溶解指标的响应特征存在显著差异，这表明除溶解作用外，其他过程在不同程度上影响着代用指标（proxy indexes）的变化。