S. - Stadial. (Heine, 1975, 1978, 1980, 1983, in press). Volcanoes of the Cor- dillera Neovolcänica with late Quaternary glacial deposits are shown in Fig. 2.
THE CLASSICAL LATE WEICHSELIAN CLIMATIC FLUCTUATIONS IN MEXICO
Klaus Heine Regensburg University, FRG
INTRODUCTION With respect to the c l a s s i c a l Late Weichselian c l i m a t i c f l u c t u a tions (Fig. 1) of the Netherlands and Western and Central Europe many s c i e n t i s t s t r i e d to establish s i m i l a r chronostratigraphies of c l i m a t i c variations for other parts of the world (e.g. C h i l e , Western North America, New Zealand, South Antarctica). Recent investigations give more and more evidence that the c l a s s i c a l West European chronostratigraphy of the Late Weichselian in many d e t a i l s is a regional stratigraphy and cannot be transferred to other areas of the world. Therefore i t is necessary to focus our attention on many d i f f e r e n t parts of the world, especially on the tropics and subtropics in order to establish a great number of seperate chronostratigraphies. I w i l l present here the results of investigations carried out during the l a s t two decades in the central Mexican highland (Puebla/Tlaxcala area, F i g . 2) by many s c i e n t i s t s which were involved in the German-Mexican "Mexico Project" of the DEUTSCHE FORSCHUNGSGEMEINSCHAFT. The conclusions concerning the Late Quaternary c l i m a t i c history of the central Mexican highland are based on the results of research in geomorphology, tephrochronology, palaeopedology, and palaeoclimatology. I. CHRONOLOGY OF GLACIAL DEPOSITS, EASTERN CORDILLERA NEOVOLVANICA, MEXICO In many papers I have reported on the late Quaternary chronostratigraphic Situation of the central Mexican volcanoes 95 N.-A. MOmer and W. Karlen (eds.), Climatic Changes on a Yearly to Millennial Basis, 95-115. © 1984 by D. Reidel Publishing Company.
O ü
Z CS
(0 C
2 o
UJ
s a
•
§
(0 u
ff £
•s z *
UJ (— flä
w y I
HOLOCENE WARM YD AL
:od: bo
•o
AL IG
BO
COLD
WARM
2
u.
"O
UJ
VALDERS
PB YD
NORTH BAY I.
AL
V A L D E R S S.
MD
TWOCREEK I.
BO
Pt. HURON S.
COOL
010
3 8
MACKIN AW I.
COLD UD
TWOCREEK W000FORD
Pt. B R U C E S. E E R I E I. NISSOURI S.
Fig. 1: Chronostratigraphy of the Late Weichselian according to various sources. PB - Preboreal; YD - Younger Dryas; AL - Allerjzid; OD - Ol der Dryas; MD - Middle Dryas; BO - Bölling; I. - I n t e r s t a d i a l ; ÄG - Ägard; LB - Low B a l t i c ; Vi - Vintapper; OLD - Ol dest Dryas; UD - Upper Dryas; S. - Stadial (Heine, 1975, 1978, 1980, 1983, i n press). Volcanoes of the Cord i l l e r a Neovolcänica with late Quaternary g l a c i a l deposits are shown in F i g . 2. Intensive investigations concerning the g l a c i a l history of the volcanoes numbered 2 to 10 ( F i g . 2) were carried out during 1971 - 1975. A summary of the palaeogeographic reconstruction of the eastern C o r d i l l e r a Neovolcänica area f o r the time 40,000 y r BP to present i s presented in F i g . 3. The area of investigation comprises the eastern part of the volcanic belt ( C o r d i l l e r a Neovolcänica) that i s composed of eroded Täte Tertiary and Quaternary volcanic and sedimentary rocks and extends f o r nearly 1000 kms along the 19th p a r a l l e l north. The huge volcanic massifs constitute barriers which separate the Valleys or basins from each other. Principal geographic features include f i v e volcanic massifs (from west to east: Nevado de Toluca, Ajusco, Sierra Nevada with Popocatepetl, I z t a c c i h u a t l , Tläloc and Telapön, Malinche, and Pico de Orizaba with Cofre de Perote) and four basins (basin of Toluca, basin of Mexico, basin of Puebla/Tlaxcala, and basin of El Seco/Oriental). The elevations of the basins ränge from about 2200 to 2600 m, whereas the
Fig. 2 : Index map of the C o r d i l l e r a Neovolcänica, Mexico. C i r c les indicate the location of the volcanoes: 1 - Nevado de Colima, 2 - Nevado de Toluca, 3 - Ajusco, 4 - Tläloc, 5 - Telapön, 6 - I z t a c c i h u a t l , 7 - Popocatepetl, 8 - Mal i nche, 9 - Cofre de Perote, 10 - Pico de Orizaba, 11 - Cerro Pefia Nevada, 12 - Durango mountaneous area, 13 - Tacanä, 14 - Tajumulco. peaks of the volcanoes have heights between 3952 m (Ajusco) and 5700 m (Pico de Orizaba). Different Täte Quaternary stratigraphic successions can be recognized within the area. The slopes of the volcanoes are d i s sected by barrancas (erosion g u l l i e s ) , radiating from the upper parts of the forest belt and descending to the basins. Thus the stratigraphic successions can e a s i l y be traced from one barranca to the other by the d i f f e r e n t layers of g l a c i a l and p e r i g l a c i a l deposits, palaeosols, debris, f l u v i a l gravels and sands, and l o e s s - l i k e so-called "toba -sediments which are interbedded with tephra, lava flows, and ignimbrite deposits. The term "tephra" pertains to a l l pyroclastic fragments, such as f i n e and coarse ash, l a p i l l i , volcanic bombs, and blocks. Different tephra layers as well as some palaeosols are of great use as stratigraphic markers within the l a t e Quaternary deposits of the volcanoes. In determining the stratigraphic succession of tephra, both f i e l d characteristics and laboratory examination were carried out. Radiocarbon dating of tephra layers has been r e s t r i c t e d to charcoal logs and branches imbedded in the ,l
Fig. 3: Scheme of three-dimensional development of the natural environment of the central Mexican highland during the Late Quaternary.
volcanic deposits. Many samples from charcoal were dated by M.A. Geyh. In addition to these data palaeosols, wood fragments from gravels, peat, and calcrete (caliche) deposits were dated by radiocarbon. Other age determinations of the l a t e Quaternary deposits are being attempted through studies in archaeology and prehistory. Pollen studies in cores of Sediments in small volcanic craters,maars, and lakes (Ohngemach & Straka, 1983) cover the time span of approximately 35,000 to 0 y r BP. The results are compared with the chronology of the glacial. deposits. Furthermore, r e l a t i v e age-dating methods have been used to demonstrate age differences in the t i l i sequence; such r e l a t i v e dating methods include topographic p o s i t i o n , morphologic shape of the moraines, rock-weathering parameters, thickness of aeolian "toba"-sediments, s o i l properties, and Vegetation cover (including liehen data of Holocene g l a c i a l and p e r i g l a c i a l deposits). A scheme of the three-dimensional development of the natural environment of the central Mexican highland during the late Quaternary is given in F i g . 3. Periods of normal and of catastrophic processes, s t a b i l i t y , and erosion during the past 36,000 yr BP are immediately recognizable. The processes which are c o n t r o l led by the b i o c l i m a t i c milieu create d i s t i n e t i v e s o i l s and landforms. Düring the past 36,000 yr BP three major periods with c l i matically induced high erosion i n t e n s i t i e s and g l a c i e r advances can be distinguished: ( i ) 36,000 to >32,000 y r BP, ( i i ) around 12,000 y r BP, and ( i i i ) 10,000 to 8,500 y r BP. These periods with high erosion i n t e n s i t i e s and g l a c i e r advances coincide with c l i matic changes from r e l a t i v e a r i d i t y to greater humidity. Figure 3 shows that there is no synchroneous development of the trend of the temperature on the one hand and of the trend of the humidity on the other hand. The temperature curve f o r the l a s t 36,000 yr BP is marked in the tropics of the central Mexican highland by an increase between 36,000 and 32,000 y r BP, a decrease between ca. 26,000 and ca. 16,000 y r BP (with the l a s t g l a c i a l temperature minimum around 17,000 - 16,000 y r BP), and an increase of the temperature with minor fluetuations during the period 16,000 to 8,000 y r BP. The postglacial c l i m a t i c Optimum is reached 8,000 to 5,000 yr BP. During the Holocene, only between ca. 2,000 and 3,000 y r BP the climate was s l i g h t l y cooler and wetter, producing a minor g l a c i e r advance. Chronology of Late Weichselian Glacial Deposits The chronology of the Late Weichselian g l a c i a l deposits is best known at the Malinche volcano. There, most of the l a t e g l a c i a l t i l l s are related to volcanic rocks and tephra as well as to organic materials that can be dated by radiocarbon. By means of tephrochronologic methods v a l l e y - t o - v a l l e y correlation of t i l l s is possible. The general d i s t r i b u t i o n and dimensions of the late Quaternary glaciers of the Malinche volcano and problems associated with the i d e n t i f i c a t i o n and d i f f e r e n t i a t i o n of each g l a c i e r
advance and t h e i r age relationship were discussed by the author in previous papers (Heine, 1975, 1978, 1980, 1983 ). T i l l s with poorly preserved moraine forms are those of the M I-glacier advance between 36,000 and >32,000 y r BP and of t i l l s deposited during the M I I - g l a c i a t i o n about 12,000 yr BP. Lateral and end moraines deposited during the M I l l - g l a c i a t i o h between 10,000 and 8,500 yr BP show well preserved morainal forms, so do the Holocene neoglacial deposits which are d i v i s i b l e into two advances (M IV: 2000 - 3000 yr BP; M V: L i t t l e Ice Age). A map of the summit area of the Malinche volcano with the locations of the p r i n cipal g l a c i a l and p e r i g l a c i a l deposits is presented in Fig. 4. An outline of the local stratigraphy of the g l a c i a l deposits at the eastern slopes of the Malinche volcano (Fig. 5), based mainly on palaeosols ( f B o l , fBo2, fBo3) and tephra layers (marker horizon
Fig. 4: Geomorphological map of the Malinche summit area. 1 - Talus debris, 2 - slope with volcanic s.ands, 3 - rock g l a c i e r , 4 - M IV-rock g l a c i e r , 5 - M III 3-moraine, 6 M III 2- and M III 1-moraine, 7 - U-shaped Valley, 8 rock, 9 - crest, 10 - crater (age i n C years BP), 11 gravel, 12 - s o l i f l u c t i o n t e r r a c e t t e , 13 - t u r f e x f o l i a t i o n , 14 - upper timber l i n e (ca. 3900 - 4000 m), 15 Valley, barranca, 16 - edge versus Valley. 14
CLASSICAL LATE WEICHSELIAN CLIMATIC FLUCTUATIONS IN MEXICO 4461m 5 (rock debris) Shoulders um . . • ^--volcanicsands M IV rock glacier— ^^—4150m 3
l280(W-1912f -3950m fc--7450i250(W-1923r
7405 ±145 (Hv4757)-r: 76901100 (Hv4758)-^
82401300(W-1909) M III 2 moraine 2970m M llllmoraine #
7645 t 80(Hv4243). 77151 80(Hv4884)~i: 12060! 165 (Hv4244 £ 20735±460 (Hv4245" 81101300 W-1927 23940H000(W-19O3 25920110001W-1911 ; 17350t 550IW-1913 P-
^176501550(W-1925f
_2750m
3889511200 (Hv4241)
Fig. 5: Scheme of the stratigraphy of the Late Quaternary depos i t s at the eastern slopes of the Malinche volcano. The C dates marked with stars refer to the western flank of the volcano. 14
rB), provide s u f f i c i e n t Information of how d i f f e r e n t late Quaternary stratigraphic successions have been elaborated. From the other volcanoes of the C o r d i l l e r a Neovolcänica, deposits belonging to the major glaciations of the late Quaternary are described i n detail by Heine (1975). Figures 6 to 8 give e v i dence of the location of the g l a c i a l and p e r i g l a c i a l deposits on Pico de Orizaba, the southern part of I z t a c c i h u a t l , and the Nevado de Toluca volcano. Comparable to the late Quaternary s t r a t i graphic successions of the Malinche volcano are the g l a c i a l deposits of these other volcanoes. Apart from the M I I - g l a c i a t i o n that did not e x i s t at the Nevado de Toluca volcano (see Heine, 1983), f i v e g l a c i e r advances of the late Quaternary can be t r a ced on the slopes of Pico de Orizaba and I z t a c c i h u a t l , whereas on the flanks of the less high volcanoes of Nevado de Toluca and Malinche the Holocene g l a c i a l deposits of the M IV and M V phase do not occur. Instead of the g l a c i a l deposits p e r i g l a c i a l forms and deposits developed here (rock g l a c i e r s , ice-cored moraines, rock debris). In the correlation diagramm (Fig. 9) most geologic-climatic unit boundaries are based either on maximum or minimum radiocarbon dates or on r e l a t i v e c r i t e r i a . In most cases radiometric cont r o l s are very r e l i a b l e and the presented boundaries w i l l l i k e l y
not be shifted on the time bar. The Period about 16,000 to 12,000 y r BP During the period of 16,000 to 12,000 yr BP there were no major g l a c i e r advances on the central Mexican volcanoes. The s o i l deve-
K HEINE 1975. ergänzt 1982
lopment of the palaeosol fBol terminated about 16,000 y r BP according to radiocarbon dates of charcial from pumice layers
Fig. 7: Geomorphological map of the southern part of the I z t a c c i huatl massif. 1 - g l a c i e r , 2 - M V-moraine, 3 - cirque, 4 - U-shaped Valley, 5 - s t r i a t e d ground, 6 - röche moutonee, 7 - crest, 8 - upper timber l i n e , 9 - brook (periodic). Fig. 6: Geomorphological map of the Pico de Orizaba. 1 - g l a c i e r , 2 - f o s s i l ice beneath debris, 3 - M V-moraine, 4 - M IVmoraine, 5 - M III 3-moraine, 6 - M III 2- and M III 1moraine, 7 - cirque, 8 - U-shaped v a l l e y , 9 - röche moutonee, 10 - i c e - f r e e crest during maximum g l a c i a t i o n , 11 - rock g l a c i e r ( L i t t l e Ice Age), 12 - s t r i a t e d blocks on moraine, 1 3 - t a l u s , 1 4 - d e b r i s flows, 1 5 - t h u f u r , 16c r a t e r , 17 - steep w a l l s , 18 - lava flow, 19 - c r e s t , 20 - v a l l e y , barranca, 21 - edge versus v a l l e y , 22 - Upper timber l i n e (ca. 4000 m), 23 - well with brook, 24 path.
covering the Ah-horizon of the palaeosol f B o l . No traces of erosion on the slopes of the volcanoes are v i s i b l e . If the preservation of the topsoil of the palaeosol fBol during the period 16,000 to 12,000 yr BP is taken to be an evidence for r e l a t i v e l y weak erosion and denudation on the mountain f l a n k s , then the c l i mate must have been r e l a t i v e l y dry and cool compared with today.
j j
1 L
MV
Radiocarbon dates $ valsequillo area + RioFroPass x Malinche volcano O Iztacclhuatl, Popocatepetl • Nevado äe Toluca volcano A El Seco/Oriental area • Tlaxcala area
|
MIV
RH X
•
j
O
XX 1 e x » !
•
•
Daring by archaeology, historical sources
c
r
1 fBo2
i
i l j
Mll
fBol
X
V u
A
X
j
M
X
A
1 •1
1
1 1
2 3 ^ 5
6
7 8 1 9 1D
11 12 13 % 15
2D
25
30
3 x10 yrBP 3
Pinus/Mixed forest Abies Pinus/Mixed forest ö
iiiniii cold irrm cool
more humid H-t-H alternating more humid - more arid
DXO more arid
Fig. 9: Correlation diagram. Geologic-climatic unit boundaries are based on radiocarbon ages, tephrochronologic c o r r e l a t i o n s , s o i l development, pollenanalyses, sedimentologic c r i t e r i a , and topographic position. The pollen zones r e f e r to Ohngemach & Straka (1983). Fig. 8: Geomorphological map of the Nevado de Toluca volcano. 1 - t a l u s , 2 - 'Glatthang' (straight smooth slope), 3 rock g l a c i e r (phase I ) , 4 - rock g l a c i e r (phase I ) , 5 rock g l a c i e r (phase I I ) , 6 - rock g l a c i e r (phase I I I ) , 7 - Valleys in debris, 8 - debris flow, 9 - debris flow lobe, 10 - M III-moraine, 11 - M III-moraine, not c l e a r l y i d e n t i f i e d , 12 - U-shaped v a l l e y , 13 - t i l i , 14 - röche moutonee, 15 - rock, 16 - volcanic plug, 17 - c r e s t , 18 v a l l e y , barranca, 19 - upper timber l i n e (ca. 4000 m), 20 - crater lake, 21 - dry lake, 22 - road, 23 - hut with a l t i t u d e in meter.
Relative dry conditions are indicated by the preservation of the palaeosol f B o l . Relative cool conditions are indicated by the termination of the development of the palaeosol f B o l , because the s o i l formation and the s o i l properties only depend on climate and a l t i t u d e (Miehlich, 1974). It is s i g n i f i c a n t that the period 16,000 to 12,000 yr BP is not represented by any radiocarbon date obtained from deposits of the slopes of the volcanoes (Fig. 9). The M II-Glacier Advance at about 12,100 yr BP The M I I - g l a c i e r advance occured around 12,100 y r BP, but only on the high volcanoes east of the basin of Mexico M II-moraines extend down to lowest elevations of less than 2750 m i n huge Valleys of the Malinche and Pico de Orizaba. Above 3000 to 3300 m they are covered by a mantle of volcanic tephra, debris, and younger g l a c i a l and p e r i g l a c i a l deposits, so that the erosion of the barrancas do not reach the M II-morainic m a t e r i a l ; therefore nothing is known about the upper l i m i t of M I l - t i l l s on the mountain slopes. The M I I - g l a c i a l episode is characterized by a short duration of presumably less than 200 years according to i n v e s t i gations of the moraines and other g l a c i a l , p e r i g l a c i a l , and f l u v i a l Sediments (Heine, 1983). The M I I - g l a c i e r advance was caused by intense p r e c i p i t a t i o n rather than by a temperature decrease. This increase in p r e c i p i t a t i o n has been observed in various Sediments from sites of the Sierra Nevada, the Malinche, and the Pico de Orizaba. It i s not represented by any deposits east of the basin of Mexico, e.g. at the Nevado de Toluca, which I believe marks the influence of the Gulf of Mexico in respect to the short but intense increase in p r e c i p i t a t i o n around 12,100 yr BP. The Period from 12,000 to 10,000 y r BP The period between about 12,000 and 10,000 y r BP is characterized by s o i l formation on the slopes of the high volcanoes. The palaeosol fBo2 was developed. According to the a l t i t u d i n a l belt of the formation of the palaeosol fBo2 the climate must have been r e l a t i v e l y cool compared with today. The M III-Glacier Advances between 10,000 and 8,500 yr BP During the period between 10,000 and about 8,500 yr BP the complex of the M III-moraines were formed. The f i r s t and second advances of the M I I I - g l a c i a t i o n led to the formation of huge l a t e ral and end moraines (see Fig. 4 - 8 ) . These moraines extend from below the present tree l i n e about 4000 m down to around 2900 m ( f i r s t advance: M III 1) and 3000 m (second advance: M III 2) respectively. Recessional moraines of the M I I I - g l a c i a t i o n (M III 3) are found at about 4000 m a l t i t u d e at the Malinche, whereas moraines of this advance reach farther downvalley on the fTanks of the Sierra Nevada and the Pico de Orizaba, because of the
greater extent of the zone of net accumulation above the f i r n l i n e during the M III 3-glacier advance (Fig. 10). It i s i n t e r esting to note that during the M III 3-glaciation the formation 6 -p CD
E in o m o:£
211 altrtude. in m 5500 c
£ o iBP_ \
4000-
•
Vi
um 12100yrBP 36000yrBP 3500«
\>.\
iii
Ii \\\ Iii
3000-
f 2500climatic snow-line
\ ! MV rock glacier \ /
MIV rock glacier
^
ice-cored moraine
T
ice-cored moraine and/or end moraine
•^Ns/^'mean altitude ^ 7
°* 9
, a c i e r
tongues
Entwurf: HEINE
Fig. 10: Climatic snow lines reconstructed for the d i f f e r e n t Late Quaternary g l a c i e r advances. of ice-cored moraines was very common on the Nevado de Toluca (Fig. 8) (Heine, 1976). The M I I I - g l a c i e r advances r e f l e c t a l o wering of the c l i m a t i c snowline during the period 10,000 - 9,000 y r BP of approximately 800 m in reference to the snowline of 1850 AD. Sinee about 9,000 y r BP the mean annual temperatures increased so much that the relationship between temperature and p r e c i p i t a t i o n led to a rapid s h i f t of the c l i m a t i c snowline to higher
elevations. Only a few recessional moraines give evidence of minor gläcier fluctuations during the period 9,000 to 8,500 y r BP. The Period from 8,500 to 5,000 yr BP The rapid deglaciation temrinated ca. 8,500 y r BP. According to the radiocarbon dates of the palaeosol fBo3 that developed either on the M - I I I - t i l l s and/or on volcanic tephra of post-Pleistocene age (Fig. 5) the period between 8,000 and 5,000 yr BP was s l i g h t ly warmer than recent times; the development of andosols took place even in areas up to 4200 m a l t i t u d e (today andosol development is r e s t r i c t e d by low temperatures over 4000 m a l t i t u d e ) . The postglacial c l i m a t i c Optimum is documented in the central Mexican highland by the palaeosol fBo3. The Period from 5,000 yr BP to Recent Times During the l a s t 5,000 years, two periods with g l a c i e r advances occurred. The M IV-glaciation is dated to about 2,000 to 3,000 yr BP. A minor g l a c i a t i o n can be correlated with the L i t t l e Ice Age. Since about 1850 AD to 1972, the ice caps and glaciers of the I z t a c c i h u a t l , Popocatepetl, and Pico de Orizaba were reduced to small remnants of ice (Heine, 1983b). Palynologic Analyses Palynologic, sedimentary, and radiocarbon analyses of g y t t j a , c l a y , sand, ash, lake marl, and other deposits, formed in small craters and maars, provide a palaeoenvironmental and p a l a e o c l i matic record of approximately 35,000 y r BP for the eastern Cord i l l e r a Neovolcänica (Ohngemach & Straka, 1983; Heine & Ohngemach, 1976). Of special interest is a pollen sequence of the T l a loqua crater which is situated at an a l t i t u d e of about 3100 m on the western slope of the Malinche volcano (Fig. 11). During the M I I I - g l a c i a t i o n (lower part of the diagram, F i g . 11) the upper zone of the forest c e r t a i n l y consisted of pines j u s t as today, f o r pine pollen mostly accounts for more than 80 % of the pollen during the time rieh in NAP. Ainus and Quercus play a comparatively minor part (Ohngemach 1977). Of p a r t i c u l a r i n t e r est is the occurrence of Picea, a genus encountered in Mexico t o day only at a few r e l i c t stations. The nearest s i t e of Picea c h i huahuana is situated 900 km north-west of the Malinche volcano, picea mexicana is found some 700 km to the north. The pollen of picea occurs below about 2 m in every sample, above the 2 m l e v e l , Picea is no longer found. The extinetion of Picea coincides with the rapid decrease of the NAP curve (Ohngemach, 1977). According to Heine (1975, 1980, 1983a; see Fig. 3) the climate was cooler and more humid than today during the M I I I - g l a c i a l advances. Such a climate may have been the precondition for the former presence of spruce at the Malinche volcano and the basin of El
Fig. 11: Pollen diagram of the Pleistocene/Holocene boundary at the Malinche volcano, Mexico (after Heine & Ohngemach, 1976 and Ohngemach & Straka, 1983). Seco/Oriental (Ohngemach & Straka, 1983). When the climate became d r i e r and warmer after the M I I I - g l a c i a t i o n , Picea was not able to persist any longer and dies out (Ohngemach, 1977; Heine & Ohngemach, 1976). The level marked by the extinction of Picea i s t a ken as the boundary between Pleistocene and Holocene Vegetation. Provided that the diagram (Fig. 11) S t a r t s shortly after the beginning of M I I I - g l a c i a t i o n at 10,000 y r BP, one reaches an age of some 8,500 y r BP f o r this t r a n s i t i o n , i f one takes further i n to account a radiocarbon date (ca. 8,000 yr BP) f o r the Ainus maximum at 1.7 m together with the supposition of a constant Sedimentation rate (Ohngemach 1977). This agrees well with the f i x a tion of the Pleistocene/Holocene boundary based on geomorpholog i c , palaeopedologic, and palaeontologic evidence (Heine & Ohngemach, 1976). The region around the Tlaloqua crater was only a t r a n s i t Station to higher elevations for the Pinus hartwegii forest that succeeded the late Weichselian alpine grassland a f t e r about 8,500 y r BP (see also Fig. 9). Ohngemach (1977) and Ohngemach & Straka (1983) i n f e r the presence of Pinus hartwegii from the occurrence
O f the parasite Arceuthobium globosum S S p . graudicaule
by which
Pinus hartwegii i s preferably infected. According to the pollen diagrams of Ohngemach & Straka (1983) no n o t i c i b l e deterioration in temperature occurred during the Late Weichselian and early Holocene trend of warming (16,000 to 8,000 y r BP). On the other hand, the M I - g l a c i a t i o n , the M I I I - g l a c i e r advances, and the M IV-glaciation are well recorded in the pollen diagrams. II. PALAEOCLIMATIC INTERPRETATION In the central Mexican highland, the Late Weichselian (ca. 16,000 to 8,500 y r BP) i s characterized by increasing temperatures without the well-known sudden deteriorations of temperature during the 01 der and Younger Dryas periods. Apart from the 12,000 y r BP event (Heine, 1983a) that seems to be triggered by an enormeous increase in p r e c i p i t a t i o n rates mainly i n the regions facing the Gulf of Mexico-coast, only the period 10,000 to 8,500 y r BP shows c l i m a t i c conditions favorable f o r greater glaciations on the high mountains. This Late Weichselian c l i m a t i c f l u c t u a t i o n i s caused by higher p r e c i p i t a t i o n during a period with increasing temperatures between the r e l a t i v e l y cold phase of oxygen isotope stage 2 and the warm postglacial c l i m a t i c Optimum of oxygen isotope stage 1. The palynologic evidence indicates, too, that the M e i stocene/Holocene boundary i s dated to about 8,500 y r BP i n the central Mexican Highland. Different pollen diagrams from the area prove that no abrupt c l i m a t i c fluctuations in temperature comparable to the Older and Younger Dryas periods occurred. Even the 12,000 y r BP event cannot be traced i n the pollen diagrams, presumably because of the Short duration of the 12,000 y r BP event of less than 200 years (Heine, 1983a). The c l a s s i c a l Late Weichselian c l i m a t i c fluctuations of the central Mexican highland are ( i ) with respect to the temperature; increasing temperatures without any major variations and/or abrupt f l u c t u a t i o n s , and ( i i ) with respect to the p r e c i p i t a t i o n : a short but great increase of p r e c i p i t a t i o n about 12,000 y r BP and a period with higher p r e c i p i t a t i o n between 10,000 and 8,500 y r BP. Yet, u n t i l today we cannot estimate the absolute amount of p r e c i p i t a t i o n during the various Late Weichselian g l a c i a t i o n s . The pollen studies show that the Late Weichselian fluctuations i n p r e c i p i t a t i o n i n the basins must have been of no s i g n i f i c a n t i n fluence f o r the Vegetation. We might conclude, because of t h i s , and because of the shrinkage of the glaciers since the L i t t l e Ice Age maximum g l a c i e r extent i n 1850 AD that took place without any s i g n i f i c a n t variations i n temperature and/or p r e c i o i t a t i o n , that the Late Weichselian glaciations were due to only s l i g h t l y a l t e red conditions i n humidity.
III.
CORRELATION WITH LATE QUATERNARY EVENTS IN OTHER REGIONS
The Täte Quaternary environmental history of Lake Valencia/Venezuela (Bradbury et a l . , 1981) obtained by chemical, palaeontolog i c a l , and mineralogical analyses of a 7.5 m core from the lake allows a tentative correlation with the Situation in central Mexico. The data show that dry climates existed in the Lake Valencia region from at least 13,000 yr BP (= the base of the core) u n t i l about 10,000 y r BP. The Lake Valencia Basin was occupied by intermittent saline marshes at that time. Düring the period 13,000 to 10,000 y r BP only the pöllen record gives evidence f o r a minor increase in moisture by a small amount of arboreal pollen [spondias and Bursera) at a depth of 6.4 m (Bradbury et a l . , 1981). If one takes into account a constant Sedimentation rate of the 13,000 to 10,000 y r BP deposits, the increase in moisture in what otherwise was an a r i d c l i m a t i c period can be compared to the M I I - g l a c i a t i o n of Mexico. About 10,000 y r BP, a permanent Lake Valencia of fluctuating s a l i n i t y formed and arboreal plant commun i t i e s replaced the e a r l i e r dominant x e r i c herbaceous Vegetation and marsh plants. This zone 2 of the Lake Valencia core c o r r e l a tes with the M I I I - g l a c i a t i o n period of central Mexico. By 8,500 y r BP, Lake Valencia reached moderate to low s a l i n i t i e s and d i s charged water; the modern Vegetation became established at that time. A f t e r 8,500 y r BP, the lake twice ceased discharging as a result of reduced watershed moisture (Bradbury et a l . , 1981). The stratigraphic record from Lake Valencia is important because i t contains a continuous, well-dated, and wel1-documented palaeoenvironmental history of a löw-latitude, low elevation s i t e from the late Pleistocene to the present. Table 1 i s a tentative corr e l a t i o n of palaeoenvironmental interpretations from the Lake Valencia record with the Mexican data. The most important conclusions from Table 1 are the c o i n c i dence of the central Mexican late Quaternary chronostratigraohy Central Mexico tNspaper
Lake Valencia Bradbury etat. 1981
m'v MIV zone 3
SKTBiar TOTnaT
of today
Mllt
?
zone 2
Humid
zone 1
Mae^than
zone 1 ? ?
Morearid than today
Table 1: Tentative correlation of palaeoenvironmental i n t e r pretations from the Lake Valencia record with the Mexican data
with the Lake Valencia core and the evidence that the Younger Dryas period did not e x i s t , neither in Mexico nor in Venezuela.
The stratigraphic records from Mexico and Venezuela cannot be correlated with l a t e Quaternary glaciations in Central and South America; the principal d i f f i c u l t y is the lack of adequate radiocarbon time control in most studies (Bradbury et a l . , 1981; Heine, 1975, 1983c). According to Mercer (1983), in the South American t r o p i c s , interpretations of past variations of glaciers in terms of climate is d i f f i c u l t ; owing to the i n t e n s i t y of solar radiation year-round, the glaciers are very sensitive to changes in cloudiness and albedo of the g l a c i e r surface. Mercer (1983) reports that in Peru by 12,200 yr BP the Quelccaya Icecap was not much 1 arger than i t is today and that a l a t e r readvance that was in progress soon a f t e r 11,500 yr BP culminated a f t e r 11,000 yr BP. The icecap then shrank rapidly and shortly a f t e r 10,000 y r BP was l i t t l e or no larger than i t is today. The dating of the readvance a f t e r 11,500 y r BP suggests that i t preceded the European Younger Dryas Stade by a few centuries, but i t does not preclude the p o s s i b i l i t y that they were simultaneous (Mercer, 1983). In southern South America no g l a c i a l geological evidence has been found anywhere for a readvance at the time of the Younger Dryas period (Mercer, 1983), although severe cooling and a r i s e in prec i p i t a t i o n have been inferred from the pollen record (Heusser et. a l . , 1981). The analyses of f o s s i l beetle assemblages through the Late Glacial of southern Chile implies a r e l a t i v e stable climate, too, and does not support, l i k e the geomorphological evidence, the assumption of a major c l i m a t i c deterioration about the same time as the Younger Dryas in Europe (Ashworth & Hoqanson, 1983). IV. C0NCLUSI0NS The palaeonvironmental data from Mexico, Venezuela, and Chile support the hypothesis that Late Weichselian climates in the t r o pics of America were more a r i d than today. The c l a s s i c a l Late Weichselian c l i m a t i c fluctuations of the 01 der and Younger Dryas periods did not occur. There is evidence that these Late Weichsel i a n c l i m a t i c deteriorations are best documented in Scandinavia (Berglund et a l . , 1983; Berglund & Mörner, 1983) although even for middle Sweden Björck & Digerfeldt (1983) postulate that either many s i g n i f i c a n t events occurred during Younger Dryas or hardly nothing at a l l , perhaps not even the 'famous' drainage of the B a l t i c Ice Lake at the northern point of Mt. B i l l i n g e n . If we move away from Scandinavia to the Alps then the 01 der Dryas nearly disappers (de Beaulieu et a l . , 1983; Ammann et a l . , 1983) and the Younger Dryas cooling cannot have been very strong (Ammann et a l . , 1983). Observations from the Alps and Switzerland show that the Older Dryas Stade can only be traced by 'reworked m a t e r i a l ' and not by pollen evidence (de Beaulieu et a l . , 1983) or as a somewhat " d r i e r phase (Ammann et a l . , 1983). In the Alps, the Younger Dryas was more pronounced by palaeoenvironmental changes 1
at greater heights, whereas on the Swiss Plateau the Younger Dryas c l i m a t i c deterioration was less severe. From the ' t r o p i c a ! point of view' we should not only look for evidence f o r the Ol der and Younger Dryas Stades, but also for evidence f o r the non-existence of these climatic f l u c t u a tions. The low-latitude stratigraphies experience a Late Weichsel i a n t r a n s i t i o n from füll g l a c i a l dry and cold/cool climate to postglacial conditions without s i g n i f i c a n t cold temperature anomal i es. ACKNOWLEDGEMENTS The author i s grateful to M. A. Geyh f o r radiocarbon age determinations, H. Heide-Weise f o r mineralogical analyses, and the DEUTSCHE FORSCHUNGSGEMEINSCHAFT f o r f i n a n c i a l support. REFERENCES Ammann, B., Chaix, L., Eicher, U., E l i a s , S., G a i l l a r d , M.-J., Hofmann, W., Siegenthaler, U., Tobolski, K. & Wilkinson, B., 1983. Vegetation, insects, molluscs and oxygen isotopes in Late-Würm deposits at Lobsigensee (Swiss Plateau). Abstracts 2nd Nordic Symp., Stockholm May 16 - 20, 1983, Danish Meteor. Inst., Kopenhagn, pp. 2-3. Ashworth, A. C. & Höganson, J . W., 1983. Was there a climatic change in southern Chile about the same time as the Younger Dryas in Europe? Abstracts Intern. Symp. on Late Cainozoic Palaeoclimates of the Southern Hemisphere, Swaziland 29 Aug. - 2 Sept. 1983, p. 31. Beard, J . H., 1973. Pleistocene-Holocene Boundary and Wisconsinan Substages, Gulf of Mexico. Geol. Soc. amer. Memoir, 136, 277-316. Berglund, B. E., Dearing, J . , Lemdahl, G., Liedberg-Jönsson, B. & Persson, T., 1983. The Kullen Peninsula in SW Sweden - a palaeoecological reference s i t e f o r Late Weichselian. Abstracts 2nd Nordic Symp., Stockholm May 16 - 20, 1983, Danish Meteor. Inst., Kopenhagn, pp. 14-15. Berglund, B. E. & Mörner, N.-A., 1983. Late Weichselian deglaciation and chronostratigraphy of Southern Scandinavia: Problems and present "state of a r t " . Abstracts 2nd Nordic Symp., Stockholm May 16 - 20, 1983, Danish Meteor. Inst., Kopenhagn, pp. 9-13. Björck, S. & Digerfeldt, G., 1983. Shore displacement and deg l a c i a t i o n chronology around the boundary Pleistocene-Holocene in the Middle Swedish endmoraine zone. Abstracts, 2nd Nordic Symp., Stockholm May 16 - 20, 1983, Danish Meteor. Inst., Kopenhagn, pp. 17-19. Björklund, K. R. & Göll, R. M., 1979. Ice Age Climates of the
Norwegian-Greenland Sea. GeoJournal, 3.3, 273-286. Bowen, D. Q., 1978. Quaternary Geology. A Stratigraphic Framework f o r M u l t i d i s c i p l i n a r y Work. Oxford, pp. 1-221, Pergamon. Bradbury, J . P., Leyden, B., Salgado-Labouriau, M., Lewis, W. M., Schubert, C , Binford, M. W., Frey, D. G., Whitehead, D. R. & Weibezahn, F. H., 1981. Late Quaternary Environmental History of Lake Valencia, Venezuela. Science, 214, 12991305. de Beaulieu, J . - L . , Clerc, J . & R e i l l e , M., 1983. Late Weichsel i a n fluctuations in the French Alps and Massif Central from pollenanalyses. Abstracts, 2nd Nordic Symp., Stockholm May 16.- 20, 1983, Danish Meteor. Inst., Kopenhagn, p. 8. Dreimanis, A., 1977. Late Wisconsin Glacial Retreat in the Great Lakes Region, North America. Ann. New York Acad. Sciences, 288, 70-89. Evenson, E. B., Farrand, W. R., Eschmann, D. F., Mickelson, D. M. & Mäher, L. J . , 1976. Greatlakean Substage: A Replacement for Valderan Substage in the Lake Michigan Basin. Quaternary Research, 6, 411-424. Heine, K., 1975. Studien zur jungquartären Glazialmorphologie mexikanischer Vulkane - mit einem Ausblick auf die Klimaentwicklung. Das Mexiko-Projekt der DFG VII, Wiesbaden, pp. 1178, Steiner. Heine, K., 1976. Blockgletscher- und Blockzungen-Generationen am Nevado de Toluca, Mexiko. Die Erde, 107, 330-352. Heine, K., 1978. Neue Beobachtungen zur Chronostratigraphie der mittelwisconsinzeitlichen Vergletscherungen und Böden mexikanischer Vulkane. E i s z e i t a l t e r u. Gegenwart, 28, 139-147. Heine, K., 1980. Quartäre P l u v i a l z e i t e n und klimamorphologischer Formenwandel in den Randtropen (Mexiko, Kalahari). Arb. Geogr. Inst. Univ. Saarland (Festschr. C. Rathjens), 29, 135-157. Heine, K., 1983a. Ein aussergewöhnlicher Gletschervorstoss in Mexiko vor 12,000 Jahren. Ein Beitrag zum Problem der spätglazialen Klimaschwankungen. Catena, 10, 1-25. Heine, K., 1983b. Mesoformen der P e r i g l a z i a l s t u f e der semihumiden Randtropen, dargestellt an Beispielen der C o r d i l l e r a Neovolcänica, Mexiko.~Abh. Akad. Wiss. Göttingen, Math.-Phys. K l . (in press). Heine, K., 1983c. Führt die Quartärforschung zu n i c h t - a k t u a l i s t i schen Modell Vorstellungen in der Geomorphologie? Colloquium Geographicum, 16, 93-121. Heine, K., in press. Spät- und postglaziale Gletscherschwankungen in Mexiko: Befunde und paläoklimatisehe Deutung. In: Lateand postglacial o s c i l l a t i o n s of g l a c i e r s : Glacial and p e r i g l a c i a l forms (ed. H. Schroeder-Lanz), in press, Balkema. Heine, K. & Ohngemach, D., 1976. Die Pleistozän/Holozän-Grenze in Mexiko. Münster. Forsch. Geol. Paläont., 38/39, 229-251. Heusser, C. J . , Streeter, S. S. & Stuiver, M., 1981. Temperature and p r e c i p i t a t i o n record in southern Chile extended to
-43,000. y r ago. Nature, 294, 65-67. Mercer, J . H., 1983. Late Cainozoic Glacier Variations in South America South of the Equator. Abstracts Intern. Symp. on Late Cainozoic Palaeoclimates of the Southern Hemisphere, Swaziland 29 Aug. - 2 Sept. 1983, pp. 19-21. Mörner, N.-A., 1976. Paleoclimatic records from South Scandinavia, global c o r r e l a t i o n s , o r i g i n and c y c l i c i t y . Paleolimnology of Lake Biwa and the Japanese Pleistocene, 4, (ed. S. Horie), pp. 499-528. Miehlich, G,, 1974. Klima- und altersabhängige Bodenentwicklung von Vulkanascheböden der Sierra Nevada de Mexico. M i t t . Deutsch. Bodenkundl. Gesellsch., 18, 360-369. Ohngemach, D., 1977. Pollen Sequence of the Tlaloqua Crater (La Malinche Volcano, Tlaxcala, Mexico). Bol. Soc. botan. Mexico, 36, 33-40. Ohngemach, D. & Straka, H., 1983. Beiträge zur Vegetations- und Klimageschichte im Gebiet von Puebla-Tlaxcala. Pollenanalysen im Mexiko-Projekt. Das Mexiko-Projekt der DFG XVIII, Wiesbaden, pp. 1-161 + Suppl., Steiner. Woillard, G., 1975. Recherches palynologiques sur l e Pleistocene dans l ' E s t de l a Belgique et dans les Vosges Lorraines. Acta Geographica Lovanensia, 14, 1-168.
