Although the dramatic climate disruptions of the last glacial period have received considerable attention, relatively little has been directed toward climate variability in the Holocene (11,500 cal yr B.P. to the present). Examination of ?50 globally distributed paleoclimate records reveals as many as six periods of significant rapid climate change during the time periods 9000"8000, 6000"5000, 4200"3800, 3500"2500, 1200"1000, and 600"150 cal yr B.P. Most of the climate change events in these globally distributed records are characterized by polar cooling, tropical aridity, and major atmospheric circulation changes, although in the most recent interval (600"150 cal yr B.P.), polar cooling was accompanied by increased moisture in some parts of the tropics. Several intervals coincide with major disruptions of civilization, illustrating the human significance of Holocene climate variability.

Holocene climatic instability: A prominent, widespread event 8200 yr ago Email alerting services cite this article to receive free e-mail alerts when new articles www.gsapubs.org/cgi/alerts click Subscribe to subscribe to Geology www.gsapubs.org/subscriptions/ click Permission request to contact GSA http://www.geosociety.org/pubs/copyrt.htm#gsa click viewpoint. Opinions presented in this publication do not reflect official positions of the Society. positions by scientists worldwide, regardless of their race, citizenship, gender, religion, or political article's full citation. GSA provides this and other forums for the presentation of diverse opinions and articles on their own or their organization's Web site providing the posting includes a reference to the science. This file may not be posted to any Web site, but authors may post the abstracts only of their unlimited copies of items in GSA's journals for noncommercial use in classrooms to further education and to use a single figure, a single table, and/or a brief paragraph of text in subsequent works and to make GSA, employment. Individual scientists are hereby granted permission, without fees or further requests to

We discuss ice core dating, the difficulties connected with trace measurements, and the significance of the ionic composition of snow. We examine temporal (from the last decades back to the last climatic cycle) and spatial (including examples from coastal as well as central areas of Greenland and Antarctica) variations in the ionic budget of the precipitation and evaluate ice core studies in terms of the chemical composition of our past atmosphere. We review (1) how Greenland and Antarctic ice cores that span the last few centuries have provided information on the impact of human activities and (2) how the chemistry of deep ice cores provides information on various past natural phenomena such as climatic variations (glacial-interglacial changes, E1 Nifio), volcanic eruptions, and large boreal forest fires. Glaciochemical studies are of importance for obtaining proxy atmospheric data at high latitudes, since direct atmospheric measurements are rare and limited in time. They can also provide valuable data concerning paleovolcanism, the response of our atmosphere to other natural phenomena such as major and/or rapid changes in climate, and the response of the high-latitude atmosphere to human activities (e.g., acidification of precipitation, ozone depletion, and oxidative capacity of the atmosphere). Such information can also increase our knowledge of the natural variability of major biogenic cycles. Dating of Snow and IceThe establishment of reliable chronologies for ice cores is the first necessary step in glaciochemical studies. Depending on the required accuracy and the time period and the location (high or low annual accumulation rate) under consideration, various methods can be used to provide dating of depth profiles. These include stratigraphic studies, reference horizons, radioactive decay of some radionuclides, and comparison with other records.In principle, numerous stratigraphical methods based on seasonal changes in the isotopic composition of the ice or in the concentration of impurities, as well as in physical properties of snow, may be used to establish year-by-year dating of ice cores. Summer deposits are often composed of coarser-grained crystals (hoar layers) and packed more loosely than are the fine-grained, more homogeneous, and hard-packed winter deposits [Gow, 1965].The dating of ice layers based on the seasonality of the stable isotope content [Dansgaard, 1964] has been applied to numerous ice cores. One advantage of this method is that no special precautions are necessary to prevent sample contamination. However, a damping effect related to diffusion processes occurring during firn formation limits the stable isotope method to the dating Figures 2 and 3). MSA is a special case, for which the seasonality exhibits a complex picture. Indeed, while no clear seasonal feature is found incentral Antarctica (Figure 3a), a strong summer maximum has been detected in surface snow layers in coastal Through the entire paper, two kinds of units are used to express the concentration 6f species prese...

A carbon-rich black layer, dating to Ϸ12.9 ka, has been previously identified at Ϸ50 Clovis-age sites across North America and appears contemporaneous with the abrupt onset of Younger Dryas (YD) cooling. The in situ bones of extinct Pleistocene megafauna, along with Clovis tool assemblages, occur below this black layer but not within or above it. Causes for the extinctions, YD cooling, and termination of Clovis culture have long been controversial. In this paper, we provide evidence for an extraterrestrial (ET) impact event at Х12.9 ka, which we hypothesize caused abrupt environmental changes that contributed to YD cooling, major ecological reorganization, broad-scale extinctions, and rapid human behavioral shifts at the end of the Clovis Period. Clovis-age sites in North American are overlain by a thin, discrete layer with varying peak abundances of (i) magnetic grains with iridium, (ii) magnetic microspherules, (iii) charcoal, (iv) soot, (v) carbon spherules, (vi) glass-like carbon containing nanodiamonds, and (vii) fullerenes with ET helium, all of which are evidence for an ET impact and associated biomass burning at Ϸ12.9 ka. This layer also extends throughout at least 15 Carolina Bays, which are unique, elliptical depressions, oriented to the northwest across the Atlantic Coastal Plain. We propose that one or more large, low-density ET objects exploded over northern North America, partially destabilizing the Laurentide Ice Sheet and triggering YD cooling. The shock wave, thermal pulse, and event-related environmental effects (e.g., extensive biomass burning and food limitations) contributed to end-Pleistocene megafaunal extinctions and adaptive shifts among PaleoAmericans in North America.comet ͉ iridium ͉ micrometeorites ͉ nanodiamonds ͉ spherules

Abstract. The Greenland Ice Sheet Project 2 glaciochemical series (sodium, potassium, ammonium, calcium, magnesium, sulfate, nitrate, and chloride) provides a unique view of the chemistry of the atmosphere and the history of atmospheric circulation over both the high latitudes and mid-low latitudes of the northern hemisphere. Interpretation of this record reveals a diverse array of environmental signatures that include the documentation of anthropogenically derived pollutants, volcanic and biomass burning events, storminess over marine surfaces, continental aridity and biogenic source strength plus information related to the controls on both high-and low-frequency climate events of the last 110,000 years. Climate forcings investigated include changes in insolation of the order of the major orbital cycles that control the long-term behavior of atmospheric circulation patterns through changes in ice volume (sea level), events such as the Heinrich events (massi've discharges of icebergs first identified in the marine record) that are found to operate on a 6100-year cycle due largely to the lagged response of ice sheets to changes in insolation and consequent glacier dynamics, and rapid climate change events (massive reorganizations of atmospheric circulation) that are demonstrated to operate on 1450-year cycles. Changes in insolation and associated positive feedbacks related to ice sheets m•y assist in explaining favorable time periods and controls on the amplitude of massive rapid climate change events. Explanation for the exa•t timing and global synchroneity of these events is, however, more complicated. Preliminary evidence points to possible solar variability-climate associations for these events and perhaps others that are embedded in our ice-corederived atmospheric circulation records.