Centennial-scale climate cooling with a sudden cold event around 8,200 years ago

The extent of climate variability during the current interglacial period, the Holocene, is still debated. Temperature records derived from central Greenland ice cores show one significant temperature anomaly between 8,200 and 8,100 years ago, which is often attributed to a meltwater outflow into the North Atlantic Ocean and a slowdown of North Atlantic Deep Water formation--this anomaly provides an opportunity to study such processes with relevance to present-day freshening of the North Atlantic. Anomalies in climate proxy records from locations around the globe are often correlated with this sharp event in Greenland. But the anomalies in many of these records span 400 to 600 years, start from about 8,600 years ago and form part of a repeating pattern within the Holocene. More sudden climate changes around 8,200 years ago appear superimposed on this longer-term cooling. The compounded nature of the signals implies that far-field climate anomalies around 8,200 years ago cannot be used in a straightforward manner to assess the impact of a slowdown of North Atlantic Deep Water formation, and the geographical extent of the rapid cooling event 8,200 years ago remains to be determined.     

基于最大和最小选择的两种新的恒虚警检测器

本文提出两种基于有序统计（ＯＳ）和单元平均（ＣＡ）产生局部估计，并分别应用最大选择（ＧＯ）和最小选择（ＳＯ）产生检测单元干扰功率水平估计Ｚ的新的恒虚警检测器（ＯＳＣＡＧＯ）和（ＯＳＣＡＳＯ）。我们推导出了它们在ＳｗｅｒｌｉｎｇⅡ型目标假设下的Ｐ＿（ｆａ），Ｐ＿（ｃｌ）和度量ＡＤＴ的解析表达式。分析了它们在均匀背景和强干扰目标环境之中的检测性能，并把它们与ＯＳ－、ＧＯＳＧＯ－、ＧＯＳＳＯ－ＣＦＡＲ进行了比较。结果表明，ＯＳＣＧＯ在均匀干扰背景和多目标情况下的检测性能与ＯＳ和ＧＯＳＧＯ相比，都有很明显的提高。在干扰目标数为某些值时，ＯＳＣＡＧＯ的ＣＦＡＲ损失比ＧＯＳＧＯ小近３ｂＢ。在均匀背景中，ＯＳＣＡＳＯ的检测性能与ＧＯＳＳＯ相近。但是，在多目标环境中，一定条件下的ＯＳＣＡＳＯ的检测性能甚至超过了ＯＳ和ＧＯＳＧＯ。     

Similar meltwater contributions to glacial sea level changes from Antarctic and northern ice sheets

The period between 75,000 and 20,000 years ago was characterized by high variability in climate and sea level. Southern Ocean records of ice-rafted debris suggest a significant contribution to the sea level changes from melt water of Antarctic origin, in addition to likely contributions from northern ice sheets, but the relative volumes of melt water from northern and southern sources have yet to be established. Here we simulate the first-order impact of a range of relative meltwater releases from the two polar regions on the distribution of marine oxygen isotopes, using an intermediate complexity model. By comparing our simulations with oxygen isotope data from sediment cores, we infer that the contributions from Antarctica and the northern ice sheets to the documented sea level rises between 65,000 and 35,000 years ago were approximately equal, each accounting for a rise of about 15 m. The reductions in Antarctic ice volume implied by our analysis are comparable to that inferred previously for the Antarctic contribution to meltwater pulse 1A (refs 16, 17), which occurred about 14,200 years ago, during the last deglaciation.     

Sea-level fluctuations during the last glacial cycle

The last glacial cycle was characterized by substantial millennial-scale climate fluctuations, but the extent of any associated changes in global sea level (or, equivalently, ice volume) remains elusive. Highstands of sea level can be reconstructed from dated fossil coral reef terraces, and these data are complemented by a compilation of global sea-level estimates based on deep-sea oxygen isotope ratios at millennial-scale resolution or higher. Records based on oxygen isotopes, however, contain uncertainties in the range of +/-30 m, or +/-1 degrees C in deep sea temperature. Here we analyse oxygen isotope records from Red Sea sediment cores to reconstruct the history of water residence times in the Red Sea. We then use a hydraulic model of the water exchange between the Red Sea and the world ocean to derive the sill depth-and hence global sea level-over the past 470,000 years (470 kyr). Our reconstruction is accurate to within +/-12 m, and gives a centennial-scale resolution from 70 to 25 kyr before present. We find that sea-level changes of up to 35 m, at rates of up to 2 cm yr(-1), occurred, coincident with abrupt changes in climate.