Stable oxygen isotope ratios of benthic foraminifera from sediment core RC13-110 (Table 1)

The Milankovitch theory of climate change predicts that variations of the climate system should match the dominant frequencies of the orbital forcing in the 41 and 23 kyr**-1 frequency bands. Such a linear theory would predict that the amplitude variations of the climate response in these bands should match amplitude variations in orbital forcing. Here we compare amplitude variations of the marine oxygen isotope record with orbital forcing in these bands over the last 700,000 years and find systematic changes through time. We express these amplitude mismatches as variations in the glacial response time, a measure of the climate system's sensitivity to orbitally induced insolation changes. Variations in the glacial response time occur in all frequencies bands without strong concentration of variance in any given band, and have a 'red' spectrum with larger variations at the longer periods. The response time is coherent with delta18O at periods of 100 and 41 kyr, which suggests that the variations in glacial response time in part reflect internal feedback mechanisms of the global climate system. The phase relationship between the estimated glacial response time and the delta18O (ice volume) record is very different at these two frequencies, which suggests at least two separate feedback mechanisms. The first mechanism enhances the 100,000-year climate cycle by increasing rates of change during major glacial terminations. Candidates for this feedback include lithospheric depression and rebound, enhanced ice calving from large marine based ice sheets, and possibly others. A second set of mechanisms, which is detected in the response to the 41,000-year orbital cycle of Earth's obliquity, accelerates ice growth events and slows glacial melting. Some models which include feedbacks between ice sheets, sea ice, and deep ocean temperatures predict early rapid ice growth, followed by slower growth, and this general feature is consistent with our analysis. While we can not at present identify the specific feedbacks leading to asymmetry of growth and decay rates at different frequency bands, the finding of this ice-growth acceleration mechanism in the 41,000-year frequency band suggests that high-latitude processes, where insolation varies most strongly at this rhythm, may be involved. Our finding of systematic changes in climate sensitivity has implications for orbitally tuned chronologies in Pleistocene sediments. Instead of a constant phase shift within a frequency band between orbital forcing and glacial response, as has been assumed in the past, we suggest a variable phase. The largest changes in age estimates for isotopic events are at the glacial terminations, which in our chronology are as much as 3500 years older that estimated previously.

气候变化的米兰科维奇理论（Milankovitch theory）预测，气候系统的变化应与41和23千年⁻¹频段内轨道强迫（orbital forcing）的主导频率相匹配。此类线性理论预测，上述频段内气候响应的振幅变化应与轨道强迫的振幅变化相一致。本文针对过去70万年的海洋氧同位素记录（marine oxygen isotope record）与上述频段内的轨道强迫展开振幅变化对比，发现其随时间呈现系统性变化。我们将这类振幅不匹配现象表述为冰期响应时间（glacial response time）的变化——这是衡量气候系统对轨道诱导日射量（insolation）变化敏感性的指标。冰期响应时间的变化存在于所有频段中，未在任一特定频段呈现显著的方差集中，且具有“红”谱特征，即在更长周期下存在更大的变化幅度。在100千年和41千年的周期尺度上，响应时间与δ¹⁸O（delta18O）存在相关性，这表明冰期响应时间的变化在一定程度上反映了全球气候系统的内部反馈机制。在这两个频率下，估算得到的冰期响应时间与δ¹⁸O（冰体积）记录之间的相位关系存在显著差异，这暗示至少存在两种独立的反馈机制。第一种机制通过在主要冰期终止期提升变化速率，强化了10万年的气候周期。这类反馈的候选机制包括岩石圈的沉降与回弹、大型海基冰盖的冰崩加剧等，以及其他潜在机制。第二组机制则体现在对地球倾角4.1万年轨道周期的响应中，该机制会加速冰体增长事件并减缓冰期消融过程。部分纳入冰盖、海冰与深海温度间反馈的模型预测，冰体会先快速增长，随后增速放缓，这一整体特征与本文的分析结果相符。尽管目前我们尚无法明确导致不同频段内冰体增长与消融速率不对称的具体反馈机制，但在4.1万年频段中发现的冰体增长加速机制表明，该过程可能与高纬度区域的过程有关——这里的日射量随该周期出现最显著的变化。本文发现的气候敏感性系统性变化，对更新世沉积物的轨道调谐年代学（orbitally tuned chronologies）具有启示意义。过往研究通常假设某一频段内轨道强迫与冰期响应间存在恒定相移，而我们则提出相移应是可变的。同位素事件年龄估算的最大变化出现在冰期终止期，根据本文的年代学框架，这些冰期终止期的年龄比此前估算结果最多老3500年。