No abstract

No abstract

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

The Greenland ice core from NorthGRIP (NGRIP) contains a proxy climate record across the Pleistocene-Holocene boundary of unprecedented clarity and resolution. Analysis of an array of physical and chemical parameters within the ice enables the base of the Holocene, as reflected in the first signs of climatic warming at the end of the Younger Dryas/Greenland Stadial 1 cold phase, to be located with a high degree of precision. This climatic event is most clearly reflected in an abrupt shift in deuterium excess values, accompanied by more gradual changes in d18 O, dust concentration, a range of chemical species, and annual layer thickness. A timescale based on multi-parameter annual layer counting provides an age of 11 700 calendar yr b2 k (before AD 2000) for the base of the Holocene, with a maximum counting error of 99 yr. A proposal that an archived core from this unique sequence should constitute the Global Stratotype Section and Point (GSSP) for the base of the Holocene Series/Epoch (Quaternary System/Period) has been ratified by the International Union of Geological Sciences. Five auxiliary stratotypes for the Pleistocene-Holocene boundary have also been recognised.

[1] High-resolution sediment cores from the Vøring Plateau, the North Iceland shelf, and the East Greenland shelf have been studied to investigate the stability of major surface currents in the Nordic Seas during the Holocene. Results from diatom assemblages and reconstructed sea-surface temperatures (SSTs) indicate a division of the Holocene into three periods: the Holocene Climate Optimum (9500-6500 calendar (cal) years BP), the Holocene Transition Period (6500-3000 cal years BP) and the Cool Late Holocene Period (3000-0 cal years BP). The overall climate development is in step with the decreasing insolation on the Northern Hemisphere, but regional differences occur regarding both timing and magnitude of SST changes. Sites under the direct influence of the Norwegian Atlantic Current and the Irminger Current indicate SST cooling of 4-5°C from early Holocene to present, compared to 2°C recorded under the East Greenland Current. Superimposed on the general Holocene cooling trend, there is a high-frequency SST variability, which is in the order of 1-1.5°C for the Vøring Plateau and the East Greenland shelf and 2.5-3°C on the North Iceland shelf.

No abstract