Stable isotope record and age determination of ODP Site 121-758 in the northeastern Indian Ocean

Numerous studies have shown that delta18O records from benthic and planktonic foraminifera, primarily a proxy of global ice volume variations, reflect Milankovitch periodicities. To study climatic response to orbital forcing at Ocean Drilling Program site 758, we have generated continuous delta18O and delta13C records from a single benthic foraminiferal species Cibicides wuellerstorfi for the last 3.6 m.y. and extended the planktonic foraminiferal isotope records of Farrell and Janecek (1991, doi:10.2973/odp.proc.sr.121.124.1991) (0-2.5 Ma, based on Globigerinoides sacculifer) to 3.6 Ma (Chen, 1994). We then constructed an age model by matching, correlating and tuning the benthic delta18O record to a model simulation of ice volume (Imbrie and Imbrie, 1980, doi:10.1126/science.207.4434.943). The filtered 41- and 23-kyr signals based on the resultant astronomically tuned age model are highly correlated to obliquity (r=0.83) and precession (r=0.75), respectively. Although derived with methodology different from Shackleton et al. (1990) and Hilgen (1991, doi:10.1016/0012-821X(91)90206-W, 1991, doi:10.1016/0012-821X(91)90082-S), our results generally agree with their published astronomical timescales for the time interval from 0 to 3.0 Ma, providing additional support for the newly emerging chronology based on orbital tuning. Slight discrepancies exist in the time interval from 3.0 to 3.6 Ma, suggesting several possibilities, including differences in the approaches of orbital tuning and the relatively low amplitude of delta18O variations in our record. However, even if the discrepancies are due to the relatively low amplitude of the isotope signals in our record at 3.0-3.6 Ma, our resultant timescale as a whole does not adversely affect our evaluation of the paleoclimatology and paleoceanography of the Indian Ocean, such as the evolution of the 100-, 41- and 23-kyr cycles, and variation of global ice volume and deepwater temperature during the past 3.6 m.y.

大量研究表明，底栖有孔虫（benthic foraminifera）与浮游有孔虫（planktonic foraminifera）的氧同位素18（delta18O）记录，主要作为全球冰量变化的代用指标，可反映米兰科维奇周期。为探究大洋钻探计划（Ocean Drilling Program）758站位对轨道强迫的气候响应，我们针对过去3.6百万年（3.6 Ma）的单一底栖有孔虫物种伍氏卷口虫（Cibicides wuellerstorfi），构建了连续的氧同位素18与碳同位素13（delta13C）记录，并将Farrell与Janecek（1991, doi:10.2973/odp.proc.sr.121.124.1991）基于泡抱球虫（Globigerinoides sacculifer）建立的浮游有孔虫同位素记录（时间跨度0~2.5 Ma）延伸至3.6 Ma（Chen, 1994）。
随后，我们通过将底栖有孔虫的氧同位素18记录与冰量模型模拟结果（Imbrie和Imbrie, 1980, doi:10.1126/science.207.4434.943）进行匹配、关联与轨道调谐（orbital tuning），构建了年龄模型。基于最终得到的天文调谐年龄模型，经滤波处理的41千年与23千年周期信号，分别与黄赤交角（obliquity，相关系数r=0.83）和岁差（precession，相关系数r=0.75）呈现高度相关性。
尽管本研究采用的方法与Shackleton等人（1990）以及Hilgen（1991, doi:10.1016/0012-821X(91)90206-W; 1991, doi:10.1016/0012-821X(91)90082-S）存在差异，但我们的结果在0~3.0 Ma的时间区间内，与他们发布的天文时间尺度总体一致，为新兴的基于轨道调谐的年代学体系提供了额外支撑。
在3.0~3.6 Ma的时间区间内存在细微差异，这可能源于多种因素，包括轨道调谐方法的不同，以及本记录中氧同位素18变化的振幅相对偏低。即便这些差异是由3.0~3.6 Ma时段内本记录的同位素信号振幅较低所导致，整体得到的年代尺仍未对我们评估印度洋的古气候学（paleoclimatology）与古海洋学（paleoceanography）特征造成负面影响——这些特征涵盖过去3.6百万年间100千年、41千年与23千年周期的演化，以及全球冰量与深层海水温度的变化。