We used 5704 14C, 10Be, and 3He ages that span the interval from 10,000 to 50,000 years ago (10 to 50 ka) to constrain the timing of the Last Glacial Maximum (LGM) in terms of global ice-sheet and mountain-glacier extent. Growth of the ice sheets to their maximum positions occurred between 33.0 and 26.5 ka in response to climate forcing from decreases in northern summer insolation, tropical Pacific sea surface temperatures, and atmospheric CO2. Nearly all ice sheets were at their LGM positions from 26.5 ka to 19 to 20 ka, corresponding to minima in these forcings. The onset of Northern Hemisphere deglaciation 19 to 20 ka was induced by an increase in northern summer insolation, providing the source for an abrupt rise in sea level. The onset of deglaciation of the West Antarctic Ice Sheet occurred between 14 and 15 ka, consistent with evidence that this was the primary source for an abrupt rise in sea level approximately 14.5 ka.

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letters to nature 344 NATURE | VOL 400 | 22 JULY 1999 | www.nature.com discrete Bragg peaks. This continuous pattern can therefore be sampled on a finer scale. That sufficient oversampling can lead to a reconstruction was pointed out by Bates 4 . To perform such a reconstruction, Chapman 2 devised a Fienup-type 17 iterative algorithm. Using a strengthened form of this, Miao et al. 5 were able not only to perform reconstructions of model data in two and three dimensions, but also to show that the degree of oversampling called for by Bates 4 can be relaxed somewhat for the higher-dimensional cases.In our experiment we made use of this reconstruction algorithm. The reconstruction from the diffraction pattern of Fig. 2 is shown in Fig. 4. Our phasing algorithm uses knowledge of a finite support which is defined as an enclosing boundary of the specimen. In this reconstruction, we chose a 5:7 m ϫ 5:7 m square as the finite support which is larger than the size of the image itself. The initial input to the iterative algorithm was a random phase set and, after about 1,000 iterations, a good reconstruction (Fig. 4) was obtained. The computing time of 1,000 iterations is ϳ30 min on a 450-MHz Pentium II workstation. Details of the reconstruction procedure are given elsewhere 5,16 . The reconstructed image is consistent with the resolution limit, ϳ75 nm, set by the angular extent of the CCD detector. The inner portion of the diffraction pattern could also be filled by Fourier processing of a moderate-resolution image of the specimen made with a scanning transmission X-ray microscope 1 , whereupon a reconstruction with an almost perfectly clean background was obtained.We believe that the successful recording and reconstruction of the test pattern reported here is the critical step that will open the way to high-resolution three-dimensional imaging of such structures as small whole cells, or large sub-cellular structures, in cell biology. Extension from two to three dimensions requires that a series of diffraction patterns be recorded as the specimen is rotated around an axis perpendicular to the beam. We have take the first steps in this direction. Model calculations indicate that the iterative algorithm used in this work is able to reconstruct such a data set 5 . To be able to collect the data set from a biological (or other radiation-sensitive) specimen, it would be necessary to keep the specimen near the temperature of liquid nitrogen. Experiments show that specimens at this temperature can withstand a radiation dose up to 10 10 Gy without observable morphological damage 18,19 . Finally, to improve the resolution without sacrificing specimen size, a CCD detector with more pixels would be needed: such detectors are now commercially available. Ⅺ

The spatio-temporal pattern of peak Holocene warmth (Holocene thermal maximum, HTM) is traced over 140 sites across the Western Hemisphere of the Arctic (0-180 W; north of B60 N). Paleoclimate inferences based on a wide variety of proxy indicators provide clear evidence for warmer-than-present conditions at 120 of these sites. At the 16 terrestrial sites where quantitative estimates have been obtained, local HTM temperatures (primarily summer estimates) were on average 1.670.8 C higher than present (approximate average of the 20th century), but the warming was time-transgressive across the western Arctic. As the precession-driven summer insolation anomaly peaked 12-10 ka (thousands of calendar years ago), warming was concentrated in northwest North America, while cool conditions lingered in the northeast. Alaska and northwest Canada experienced the HTM between ca 11 and 9 ka, about 4000 yr prior to the HTM in northeast Canada. The delayed warming in Quebec and Labrador was linked to the residual Laurentide Ice Sheet, which chilled the region through its impact on surface energy balance and ocean circulation. The lingering ice also attests to the inherent asymmetry of atmospheric and oceanic circulation that predisposes the region to glaciation and modulates the pattern of climatic change. The spatial asymmetry of warming during the HTM resembles the pattern of warming observed in the Arctic over the last several decades. Although the two warmings are described at different temporal scales, and the HTM was additionally affected by the residual Laurentide ice, the similarities suggest there might be a preferred mode of variability in the atmospheric circulation that generates a recurrent pattern of warming under positive radiative forcing. Unlike the HTM, however, future warming will not be counterbalanced by the cooling effect of a residual North American ice sheet. r ARTICLE IN PRESS

The Late Wisconsinan advance of the Laurentide