QUATERNARY RESEARCH 12, 135-149 (1979) \C ^ On Time Scales and Causes of Abrupt Paleoclimatic Events HERMANN FLOHN Meteorologisches Institut der Universität Bonn, Universität Bonn, Auf dem Hügel 20, 53 Bonn l, West Germany Received May 24, 1978 During the last 7 x 105 years the occurrence of abrupt climatic variations, of an intensity probably reaching 5°C/50 yr and with a duration of the order of several centuries can be demon- strated; their frequency is of the order 10~4 (sometimes even 10^3) per year. Most impressive examples are sudden coolings in earlier interglacials; in some periods the variability of past climates was obviously much greater than now. Due to the effective spatial coherence of the atmospheric and oceanic circulation their extension, not necessarily of similar intensity, is probably hemispheric or even global. They are modified by feedback mechanisms within the geophysical climatic System; orbital changes play a selective role leading either to suppression or to growth. Any physical Interpretation of such abrupt paleociimatic events remain äs yet speculative. One of the most attractive models is the occurrence of clusters of major volcanic eruptions which is more frequent than expected in random series. This is similar to the clustering of severe earthquakes in recent years; both events are probably interrelated responses to the (apparently discontinuous) motions of tectonic plates. INTRODUCTION (Müller, 1965, 1974; Dansgaarde?«/., 1972; Many paleoclimatologists, especially Hecky and Degens, 1973; Osborne, 1974; nonmeteorologists, tend to be satisfied with Wijmstra, 1975; Coope, 1977); two different an Interpretation of past climatic fluctua- models for a geophysical interpretation of tions of the Late Cenozoic in terms of Or- such events have been proposed (Flohn, bital changes, i.e., in the Milankovich time 1974b, 1978). In this study it is intended to scale of 20,000-100,000 yr. Very long pa- interpret additional evidence (Woillard, leociimatic records, such äs investigated 1975, 1978 a,b; Müller, 1978), to obtain a by Shackleton and Opdyke (1973, 1976) more detailed time scale from continental from the equatorial Pacific, äs well äs the records and to discuss further possible loess profiles from Lower Austria and the causal mechanisms. In this context it is southern CSSR (Kukla, 1975; Fink and useful to distinguish between external and Kukla, 1977), indicate indeed a dominance internal causes: These terms are designed of fluctuations in this time scale during the from the viewpoint of the climatic System last 2 my. Based on the evaluation of two (GARP, 1975). The "almost-intransitive" cores from the southern Indian Ocean, behavior of this system (see E. Lorenz in Hays et al. (1976) have confirmed the role GARP, 1975, p. 132) presents one of the of orbital elements; this paper has, with its most challenging problems to scientists; critical attitude and its cautious analytical many geophysical sciences are more or less methods, convinced even skeptics. The involved. dominance of a nearly 100,000-yr cycle, which presents an enigma to climatologists, FLUCTUATIONS AT THE END OF THE has been differently (and more correctly) LAST GLACIATION interpreted by Wigley (1976) and Berger The peak of the last Ice Age (Würm- (1977, 1978). Wisconsin) occurred about 18,000 yr B.P. On the other hand much evidence for The melting of the continental ice domes of \arge-sca\ paleociimatic events of a shorter the northern hemisphere was finished about time scale (102-103 yr) has been presented 8000 yr B.P. (Fennoscandian) and finally 135 0033-5894/79/040135-15S02.00/0 Copyright © 1979 by the Universtty of Washington. All rights of reproduction in any form reserved. 136 HERMANN FLOHN about 4500 yr B.P. (Laurentide). Disappear- and lacking in the southern hemisphere. ance of the Fennoscandian ice was nearly Three readvances occurred after about contemporaneous with the disintegration of 13,000 yr B.P., about 11,900 yr B.P., and the Laurentide Ice Sheet into separate re- the last (Younger Dryas) after about 10,900 siduals, including a catastrophic incursion of yr B.P. They were separated by two warm the sea into Hudson Bay (Ives et al., 1975). interstadials: Bölling (around 12,500 yr After 6500 yr B.P. the climatic optimum B.P.) and Alleröd (around 11,400 yr B.P.). (Atlanticum, Hypsithermal) developed; The füll oscillations of the "Alleröd se- evidence for a eustatic rise of sea level of quence" together encompassed not more about 2-3 m above the present level is still than 2000 yr and the annual temperature controversial. During this period, sea ice in Variation reached, in southern Germany, the Canadian arctic archipelago disap- probably 5-6 C. Both warm interstadials peared, including the northern coast of were accompanied, in the Colombian Cor- Greenland and Ellesmere Island, with simi- dillera (5°N; 2850 m) by humid phases (van lar traces in the European and Siberian Arc- Geel and van der Hammen, 1973), in con- tic (Vasarief al., 1972). trast to the Wurm peak and the Younger The retreat of the Continental ice was by Dryas, which were both arid. Of special no means uniform. Several glacial read- interest are the high (400 m!) lake-level vances occurred nearly simultaneously fluctuations (Hecky and Degens, 1973) of (Mörner, 1973); evidence for their role in Lake Kivu (Fig. 1). the tropics is until now scanty (e.g., Fig. 1) The catastrophic readvance of the ice after the Alleröd destroyed, "wholesale," years B.P Lake Level full-grown forests. This readvance, to- -300 *• *100 11000 gether with marked cooling at Camp Cen- tury, Greenland (76°N) and at Byrd Sta- tion, West Antarctica (80°S) (Johnson et 1)500 - al., 1972), along with desiccation in Colum- bia occurred, from all indications, in a time 12000 - span of 200-350 yr. Similar time spans are reported from many cores; the variations of the Vegetation between temperate forest and forest—tundra in a peat bog north of 12500 - Beifort (southeast France) (Woillard, 1975) is shown in Figure 2. It should be realized, however, that Vegetation immigration needs more time than the true climatic variations; in contrast to this, an abrupt climatic 13000 - change may kill a nonadapted Vegetation in a few years or decades. An immediate response to the climate is demonstrat- ed by the abrupt sequence of two differ- ent beetle faunas from near Birmingham, n,Fe)C03 England. Here a typical arctic fauna was Facies found at a layer dated 10,025 ± 100 yr B.P., AI2 03 Facies 13700 while no such fauna survived 10 cm higher 102 ICH1Ö7' IOW —>H1GH where an age of 9970 ± 110 yr (Osborne, Manganese (ppm) Thermocli ne FIG. 1. Lake-level fluctuations and bottom chemis- 1974) indicates a time most probably at the try of Lake Kivu (2°S), 13,700-11,000 yr. B.P. (after end of Younger Dryas cold period. Taking Hecky and Degens, 1973). the data literally, this "very rapid" warm- ABRUPT PALEOCHMATIC EVENTS 137 BOREAL - 8600 PREBOREAL -10200 Y. DRYAS ALLERÖD -10800 H 1700 M. DRYAS BÖLLING ^12400 13200 U. DRYAS C—r- —*• W FIG. 2. Vegetation History (forest versus tundra) during the Late Glacial, northeast France, Dept. Hte. Saone, Grands Pr6s Core II (after Woillard, 1975); AP = tree pollen, NAP = nonarboreal pollen, C(W) = cold (warm). ing should have occurred in a time span of last glaciation (about 25,000 yr B.P.) must about 50, but not more than 150 yr. The have been "nearly instantaneous" (Lamb July temperature change during this event and Woodroffe, 1970). has been estimated (Coope, 1977) to be Since we are now living in an interglacial 8-9 C. Within this complex "Alleröd se- period without knowing its end, the most quence" the warm phases coincide with interesting events are the cooling phases high humidity in the African tropics within the last interglacials; the following (Butzer, 1976). Figure l indicates an earlier discussion shall be concentrated on these. humid phase around 13,000 yr B.P., lasting Much evidence has been presented in re- only a few centuries (Bölling ?). cent years, including the monographs pub- The Alleröd complex was accompanied lished by Frenzel (1967, 1968). This is espe- by a rise of the world's sea level, from cially true for the last interglacial, now about -50 to about -35 m (Mörner, 1973). commonly defined äs stage 5e (Emiliani and At -38 m the Bering Strait reopened and Shackleton 1974), lasting approximately Arctic waters again entered the Pacific. The from 127,000 until about 115,000 yr ago latter had never been äs cold äs the North (Appendix). Convincing evidence from Atlantic, where 90% of the meltwater from Barbados (Matthews, 1973) and New the continental ice domes formed a cool, Guinea (Bloom et al., 1974) has been pre- low-saline surface layer. During and after sented for the occurrence of two significant the peak of the glacial phases, Atlantic cool periods (5b, 5d) between three warm surface water was less saline than the phases (5a, 5c, 5e), with eustatic lowering Pacific, not äs a cause (Weyl, 1968), but äs a of sea level of nearly 80 m within about 5000 consequence of the glaciation of the north- yr. This is roughly equivalent to an average ern continents. annual storage of 20 cm water equivalent at a glacial area near 30 x l O6 km2. This is not EXAMPLES OF ABRUPT unrealistic, if one assumes that the annual INTERGLACIAL COOLINGS storage at the center may be äs low äs 5 At the beginning of the last glaciation, the cm/yr, while it may rise at the outer margins climatic fluctuations seem to have been äs to 50—60 cm/yr, similar to the present ac- rapid äs at its end (Appendix).