The Younger Dryas (YD) was the most significant rapid climate change event that occurred during the last deglaciation of the North Atlantic region. Previous ice core studies have focused on the abrupt termination of this event [ Dansgaard et al., 1989] because this transition marks the end of the last major climate reorganization during the deglaciation. Most recently the YD has been redated--using precision, subannually resolved, multivariate measurements from the GISP2 core--as an event of 130070 years duration that terminated abruptly, as evidenced by an 7C rise in temperature and a twofold increase in accumulation rate, at 11.64 kyr BP [ Alley et al., 1993] (Figure 2). The transition into the Preboreal (PB), the PB/YD transition, and the YD/Holocene transition were all remarkably fast, each occurring over a period of a decade or so [ Alley et al., 1993]. Fluctuations in the electrical conductivity of GISP2 ice on the scale of <5-20 years have been used to reveal rapid changes in the dust content of the atmosphere during the same periods and throughout the last glacial [ Taylor et al., 1993b]. These rapid changes appear to reflect a type of ``flickering'' between preferred states of the atmosphere [ Taylor et al., 1993b], which provides a new view of climate change. Holocene climates are by comparison stable and warm.
High resolution (mean: 3.48 years/sample), continuous measurements of GISP2 major anions (chloride, sulfate and nitrate) and cations (sodium, magnesium, potassium, calcium and ammonium) were used to reconstruct the paleoenvironment during the YD because these series record the history of the major soluble constituents transported in the atmosphere and deposited over central Greenland [ Mayewski et al., 1993c]. These multivariate glaciochemical records provide a robust indication of changes in the characteristics of the sources of these soluble components or changes in their transport paths, in response to climate change. A dramatic example is provided by the calcium series (Figure 2) covering the last 10-18 kyr BP. Prominent periods of increased dustiness have been observed in the record, peaking approximately every 500 years (see figures in Mayewski et al. [1993c]): during the early PB at 11.4 kyr BP; throughout the YD at 11.81, 12.22 and 12.64 kyr BP; during the Bolling/Allerod (B/A) at 13.18, 13.65, and 14.02 kyr BP; and during much of the Glacial. Such events have been attributed by Mayewski et al. [1993c] to changes in the size of the polar atmospheric cell and in source regions (e.g., growth and decay of continental biogenic and terrestrial source regions).
The climate change that accompanied the YD was not restricted to Greenland. The record of variations in the CH concentration of trapped gases in the GRIP ice core [ Chappellaz et al., 1993] shows that tropical and subtropical climates were colder and drier during the YD and also earlier cold events. The major natural source region of CH is low-latitude wetlands [ Chappellaz et al., 1993]; higher atmospheric concentrations are presumably due to the greater areal extent of tropical and subtropical wetlands [ Chappellaz et al., 1993].
The ammonium flux record from GISP2 provides an estimate of continental biogenic source strength [ Mayewski et al., 1993a] during the YD. Although at the onset of the Bolling/Allerod ammonium flux levels and outliers rose dramatically, during the YD ammonium flux levels dropped only minimally and the number of ammonium outliers decreased slightly. Since ammonium concentrations are highest near continents [ Logan, 1983] and decrease with transport as a consequence of deposition, it appears that continental sources close to Greenland (North America and Europe) were not as dramatically affected during the YD as were low-latitude wetland regions, as evidenced by the CH record. This may indicate the continued importance of ice sheets and permafrost in limiting the growth of vegetation at higher latitudes until the end of the YD. Both low-latitude source CH and ammonium rise at the end of the YD [ Chappellaz et al., 1993; Mayewski et al., 1993c].