Stable oxygen isotopes of late Quaternary sediments

A simple, untuned "constant sedimentation rate" timescale developed using three radiometric age constraints and eleven d18O records longer than 0.8 Myr provides strong support for the validity of the SPECMAP timescale of the late Quaternary (Imbrie et al., 1984). In particular, the present study independently confirms the link between major deglaciations (terminations) and increases in northern hemisphere summer radiation at high latitudes and shows that this correlation is not an artifact of orbital tuning. In addition, the excess ice characteristic of late Quaternary "100-kyr" climate cycles typically accumulates when July insolation at 65°N has been unusually low for more than a full precessional cycle, or >21 kyr, and once established does not last beyond the next increase in summer insolation. Thus, the timing of the growth and decay of large 100-kyr ice sheets, as depicted in the deep sea d18O record, is strongly (and semipredictably) influenced by eccentricity through its modulation of the orbital precession component of northern hemisphere summer insolation.本研究基于3项放射性年龄约束（radiometric age constraints）与11条时长超0.8百万年的δ¹⁸O（d18O）记录，构建了一套简易且未经调谐的"恒定沉积速率"时间标尺，为晚第四纪SPECMAP时间标尺的合理性提供了强有力的支撑（Imbrie等，1984）。尤为关键的是，本研究独立验证了大型冰消作用与终止期（terminations）之间的关联，并证实该相关关系并非轨道调谐（orbital tuning）所带来的人为假象。此外，晚第四纪"100-kyr"气候周期所特有的过剩冰体，通常在北纬65°处的七月日照持续异常偏低超过一个完整岁差周期（precessional cycle，即>2.1万年）时开始积累；且这类冰体一旦形成，便不会持续至下一次夏季日照增强事件发生之后。由此可见，深海δ¹⁸O（d18O）记录所揭示的大型10万年尺度冰盖的生长与消亡时间，受到偏心率（eccentricity）的强烈且半可预测的调控——偏心率通过调制北半球夏季日照的轨道岁差分量来实现这一作用。