Holocene climatic and environmental change from pollen records of

Transcription

Review of Palaeobotany and Palynology 136 (2005) 63 – 92
www.elsevier.com/locate/revpalbo
Holocene climatic and environmental change from pollen records
of lakes Zempoala and Quila, central Mexican highlands
L. Almeida-Lenero a, H. Hooghiemstra b,*, A.M. Cleef b, B. van Geel b
a
Departamento de Ecologı́a y Recursos Naturales, Facultad de Ciencias, Universidad Nacional Autónoma de México,
C.P. 04510, México D.F. México
b
Institute for Biodiversity and Ecosystem Dynamics (IBED), Paleoecology and Landscape ecology, University of Amsterdam,
Kruislaan 318, 1098 SM Amsterdam, The Netherlands
Received 22 April 2004; received in revised form 20 April 2005; accepted 12 May 2005
Abstract
Pollen records of a 520-cm long core from Lake Zempoala (2800 m altitude) and a 884-cm long composite core from Lake
Quila (3010 m altitude), both located 65 km SW of Mexico City, show changes in vegetation and climate. The Zempoala record
covers the last c. 6320 cal yr BP, while the Quila record spans the last c. 11,610 cal yr BP. These lakes, with submerged aquatics
and hydroseral shore vegetation, lie today in the Abies religiosa-dominated forest belt.
From c. 6320 to 2530 cal yr BP Zempoala shows a mixed forest with mesophyllous taxa, indicating warmer and more humid
conditions than at present. From c. 2530 to 160 cal yr BP Abies forest and Pinus hartwegii forest dominated, indicating a
temperate climate. During the first part of this period, from c. 2530 to 1150 cal yr BP abundant Pinus forest is indicative of drier
climate conditions. During the last c. 310 cal yr of this period Abies-dominated forest prevailed, with a floral composition
resembling the modern Abies forests. This is indicative of temperate and humid climatic conditions. During the last c. 160 cal yr
BP the presence of Zea mays and Plantago reflect human impact.
From c. 11,610 to 10,650 cal yr BP Quila shows Alnus- and Pinus-dominated forest, reflecting humid climatic conditions.
From 10,650 to 5000 cal yr BP Pinus and Arceuthobium are indicative of a Pinus hartwegii forest, suggesting slightly lower
temperatures than at present. A 25-cm thick volcanic ash horizon at 380–355 cm core depth is the well-known dyellow ashT
marker with a known age of c. 4200–4800 14C yr BP. This age is supported by our bracketing dates of 5620 and 4650 14C yr BP.
From 4630 to 1000 cal yr BP mixed forest, including Pinus, Quercus, Carpinus, Hedyosmum, Juglans and Artemisia, is
indicative of mesophytic conditions. From 4630 to 1010 cal yr BP Abies forest was abundant, reflecting colder and more humid
conditions. From c. 1010 to 20 cal yr BP Pinus forest was very abundant suggesting, as in site Zempoala, dry climatic
conditions.
The pollen record from Quila documents four different types of forest: Pinus/Alnus forest (possibly Alnus jorullensis in zone
QUI-I), Pinus forest (most likely Pinus hartwegii forest in zone QUI-II), mixed forest in zone QUI-III), and Abies forest (Abies
religiosa forest in zone QUI-IV). The last c. 1000 years show a decline of mixed forest and increasing Abies forest, reflecting
cooler and more humid conditions. Quila also shows, during the most recent past, human impact by crop cultivation (Zea mays)
* Corresponding author.
E-mail addresses: [email protected] (L. Almeida-Lenero), [email protected] (H. Hooghiemstra).
0034-6667/$ - see front matter D 2005 Elsevier B.V. All rights reserved.
doi:10.1016/j.revpalbo.2005.05.001
64
L. Almeida-Lenero et al. / Review of Palaeobotany and Palynology 136 (2005) 63–92
and vegetation disturbance (Plantago) leading to erosion. Other microfossils, among which coprophilous fungi, were recorded
and illustrated.
D 2005 Elsevier B.V. All rights reserved.
Keywords: Mexico; environmental change; pollen analysis; Holocene; coprophilous fungi
1. Introduction
Pollen records from tropical mountains are sensitive recorders of climatic change as vertical migration
of the main vegetation belts occurred over only
hundreds of meters. Therefore, the source areas of
the pollen grains stay at a close distance to the archiving lake sediments all the time. Fine examples of such
pollen-based records of climate change come from
Colombian lake sediments. The lacustrine sediments
of Lake Fuquene (e.g. van Geel and van der Hammen,
1973; Mommersteeg, 1998) and the sediments of the
high plain of Bogotá, formerly a lake that drained
before the last glacial maximum (LGM) (e.g. Hooghiemstra, 1984; van ’t Veer et al., 2000) document
environmental history with a centennial to millennial
scale resolution.
The site inventory of the Latin American Pollen
Database (LAPD, 1996) provides a list of available
pollen records in Latin America; several are close to
our study area and relevant for comparison purposes
(Fig. 1). The region northwest of our study area
includes the sites Pátzcuaro (Michoacán State), Yuriria
and La Hoya (Guanajuato State). Closest to our study
area lies the Upper Lerma Basin, Texcoco and Chalco
lakes (Caballero and Ortega, 1998), and the Tlaloc
crater site. Relevant sites from the Yucatan Peninsula
are sites Lake Chichancanab and Lake Punta Laguna.
Bradbury (2000) documented environmental
change on the basis of pollen, diatoms and geochemical records from Lake Pátzcuaro. In this lake, climatic
conditions during the mid-Holocene were similar to
the present conditions. Evidence of the start of agricultural activities was found from c. 4000 cal yr BP
onwards. Based on the interpretation of magnetic,
geochemical and palaeoecological data from Lake
La Hoya, Lake Yuriria and Lake Pátzcuaro, Metcalfe
et al. (1994) reconstructed long-term variations in
water levels. Episodes with high water levels show
evidence of accelerated erosion, indicative of a change
in rainfall c. 3600 cal yr BP.
Closest to the lakes described here, in the Upper
Lerma Basin (Fig. 1) Metcalfe et al. (1991) documented climatic change and anthropogenic disturbance.
Their study was based on sediment chemistry, losson-ignition (LOI), mineral magnetism, and fossil diatom assemblages, and provided evidence for environmental changes since 11,580 F 70 14C yr BP (c.
13,800–13,400 cal yr BP). They showed that freshwater lakes developed since this date alternating, during
dry episodes, with acidic marshes. They attributed an
important phase of accelerated erosion around 3100 cal
yr BP to human disturbance, which culminated around
1400–700 cal yr BP. Caballero et al. (2002) studied site
Sta. Cruz Atizapan in the Upper Lerma Basin and
found, based on diatoms, a rise of the lake level at
7000 BP, while in the same basin site Almoloya Lake
showed lowering water levels at 6200 14C yr BP.
In the Basin of Mexico a reconstruction of late
Quaternary lake level changes in Lake Texcoco and
Lake Chalco (Fig. 1) was published by Bradbury
(1971, 1989). These studies were based on diatoms
and showed the development of marshes during the
last 10,000 years. González-Quintero and FuentesMata (1980) analyzed pollen from the sediments of
Lake Texcoco (Fig. 1) and from archaeological excavations. Evidence of human activity was recorded since
3000 14C yr BP. González-Quintero (1981, 1986) studied three archaeological profiles from the northern
border of Lake Chalco. Lozano et al. (1993) and
Lozano and Ortega (1994, 1998) used pollen, diatoms,
palaeomagnetic properties and LOI records from sediments of lakes Chalco and Texcoco. They documented
environmental change during the early and middle
Holocene and, again, reported human disturbance
since c. 3000 14C yr BP. In their pollen-based environmental reconstructions of the lakes Chalco and Texcoco, Lozano and Xelhuantzi-López (1997) reported a
strong impact of volcanism on the vegetation (Caballero et al., 1999). Finally, historical records were used
by O’Hara and Metcalfe (1995) to evaluate human
impact during the late Holocene and its relationships
25o
19o05’
N
L. Quila
20o
10
3
12 3
11
12
Temperate forest
Guatemala
110o
15o
100o
90o
Mexico City
7
V. Iztaccihuatl
5230 m 19o15’
3000
Toluca
City
2900
8
4000
3500
o
99 30’
o
V. Popocatepetl
5465 m
99 00’
3000
V. Nevado de Toluca
4590 m
19o00’
98o30’
L. Zempoala
3000
2900
3500
3000
4000
3500
9
5
6
19o00’
10 km
6
3000
4
99o18’
5
Gulf of Mexico
Pacific Ocean
Fig. 1. (A) Map showing the main vegetation types of central and southern Mexico and the location of relevant sites mentioned in the text. (B) Detailed geographical map of the area
south of Mexico City showing the selected sites and the relevant part of the Trans-Mexican Volcanic Belt where the study area is located; (C) Map showing the geographical location
of Lake Zempoala at 2800 m, and Lake Quila at 3010 m altitude in the National Park Lagunas de Zempoala, 65 km southwest of Mexico City. Explanation of the numbers: Michoacán
State: 1 = Lake Pátzcuaro, 2 = Zacapu Basin. Guanajuato State: 3 = La Piscina de Yuriria. Estado de Mexico: 4 = Upper Lerma Basin, 5 = Lake Quila, 6 = Lake Zempoala. Basin of
Mexico: 7 = Texcoco, 8 = Chalco. Tlaxcala State: 9 = Tlaloc crater. Yucatán: 10 = Lake Chichancanab, 11 = Lake Punta Laguna. Guatemala: 12 = Lake Peten–Itza.
65
66
with climatic change in central Mexico. They concluded climatic change starting some 4000 years ago had a
considerable impact on sedentarism and agriculture.
The late Quaternary lacustrine records from sites
Tlaloc-I and Tlaloc-II, a crater of La Malinche volcano (Puebla-Tlaxcala; Fig. 1), also lie close to our
sites. These records, although with poor time control,
show the vegetation history from Lateglacial time to
the present (Ohngemach and Straka, 1983; Straka
and Ohngemach, 1989). During the last 5000 years
in particular, temperature and precipitation changed
repeatedly.
On the Yucatán Peninsula, climate variability and its
possible implications for the Mayan civilization were
studied on the basis of proxies (oxygen isotopes of
ostracods and gastropods) from Lake Punta Laguna
(Fig. 1). The Punta Laguna record is indicative of
multi-decadal and millennial scale changes in the oxygen isotope records during the late Holocene. These
changes reflect successive wet and dry episodes that
may have influenced the cultural evolution in Mesoamerica (Curtis et al., 1996; O’Hara and Metcalfe,
1997). At Lake Chichancanab (Fig. 1), a continuous
record of Holocene climatic change was reconstructed
by Hodell et al. (1995, 2001) using temporal variations
in oxygen isotopes and sediment composition. During
the middle and late Holocene the interval between 1300
and 1100 cal yr BP was driest and coincided with the
collapse of the Maya civilization at the end of its
dClassic PeriodT.
In Central America, south of Mexico, there are
pollen studies from Guatemala (Islebe et al., 1996a)
and Curtis et al. (1998) of Lake Peten–Itza (Fig. 1)
including pollen, stable isotope analyses of ostracods
and gastropods, geochemistry, elemental composition,
and magnetic susceptibility. Holocene climatic change
as well as human influence on the regional environment were recorded.
In Costa Rica, the bogs La Chonta (Hooghiemstra et
al., 1992) and La Trinidad (Islebe and Hooghiemstra,
1995) were studied. The record of La Chonta (at 2310
m altitude) shows that the bog was successively situated in the paramo belt, in the subalpine rainforest belt,
and finally in the upper montane forest belt, reflecting
vertical shifts of the upper forest line (UFL) of c. 1400
m during the last c. 80,000 years. The pollen record of
La Trinidad bog, at 2700 m altitude, shows the postglacial development of the montane oak forest belt
from c. 9500 to 1500 14C yr BP. Based on the analysis
of pollen and charcoal fragments, Horn (1993) studied
the vegetation history in the Costa Rican Chirripó
Mountains in relation to the fire history. Pollen, sediment and isotope data of Lake Quexil in Guatemala
(Fig. 1) have demonstrated a change from dry climatic
conditions during the late Pleistocene, to more mesic
conditions with higher temperatures during the Holocene (Leyden, 1984; Leyden et al., 1993, 1994).
Multi-site studies were successively carried out by
Brown (1985), Markgraf (1993), Bradbury (1997,
2000), Metcalfe et al. (2000) and Marchant et al. (in
review). For selected time windows since the LGM a
synthesis of vegetation and climatic change is available for the Mexican area.
The impact of humans and/or herbivores on the
vegetation in the past can often be inferred from the
presence of pollen grains of light-demanding plants
indicative of open vegetation and taxa characteristic of
eutrophic conditions. Eutrophication of lakes is often
caused by the input of excrement and other settlement-related organic waste. Humans and herbivores
may have been responsible for creating new habitats,
or at least changing the natural environment. Selected
taxa of fungal spores are good environmental indicators, such as the characteristic fungi on mammal dung,
mainly from herbivores. The spores of coprophilous
ascomycetes can be very common in sediments from
archaeological sites (Buurman et al., 1995; van Geel
et al., 1986a, 2003). Davis (1987) used records of the
coprophilous fungus Sporormiella to document the
abundance of large herbivores in North America.
The increase of Sporormiella in sediments of historic
age is indicative of the increasing grazing pressure
after the introduction of domesticated herbivores
(Davis, 1987). Not all fungal spores preserve as fossils. Preservation depends on the thickness of spore
wall and mycelium. As a consequence, normally only
spores of thick-walled ascomycetes and Dematiaceae
(Fungi Imperfecti with dark, relatively thick spore
walls) are preserved. The indicator value of non-pollen palynomorphs is not fully exploited by palynologists and it is our aim in this paper to attract attention
to these palynomorphs (van Geel, 1978, 2001; van
Geel et al., 1989, 1994, 1995, 2003).
The aim of the present study is to document the
pollen-based vegetation change from Zempoala and
Quila, and to compare the inferred climate history
with other relevant sites in the central Mexican mountains. We use pollen grains, spores and algae, and also
other microfossils rarely documented in Latin American palaeoecological studies. The most relevant nonpollen palynomorphs are illustrated and described. We
aim to assess the start of Mesoamerican agriculture
and deforestation in the study area, and we will compare the palynological evidence for early civilizations
with archeological records.
67
Tres Cumbres, Morelos, Mexico
(2810 m alt.)
o
C
(14-20)
o
9.9
C
1724.6
mm.
T-: 7
T+: 12
P-: 8.4
P+: 392
400
200
50
80
30
40
20
10
2. Environmental setting of the study area
2.1. Geography and climate
The lakes of Zempoala (2800 m altitude, 19803VN,
99818VW) and Quila (3010 m altitude, 19804VN,
99819VW) are located in the central part of the TransMexican Volcanic Belt (TMVB) (SPP, 1979; Metcalfe,
1997) 65 km southwest of Mexico City (Fig. 1). Both
lakes are located within the dParque Nacional Lagunas
de ZempoalaT at the base of the Cerro Chalchihuites
and are located in different endorheic basins (Bonilla,
1992). The surrounding mountains consist of volcanic
rock (Fries, 1960) and the soils around the lakes are
humid Andosols. Subterraneous discharge through volcanic ash layers in the valleys causes marked seasonal
differences in water levels (Sosa, 1935).
The climate of the study area is characterized by an
average annual temperature of 5–12 8C and monthly
precipitation of 1720 mm. Temperatures show a
strong daily fluctuation (Fig. 2). The rainy season
generally lasts from June to October and the dry
season lasts from November to May (Garcı́a, 1988).
As a consequence, the studied endorheic depressions
experience significant annual lake level fluctuations
(Sosa, 1935). During January and February the lakes
of Zempoala and Quila are frequently covered with
ice due to their high elevation. Based on a study along
the slope of the volcano Pico de Orizaba, some 250
km west of our study area, we use a temperature
gradient of 0.6 8C per 100 m displacement of the
UFL (Lauer and Klaus, 1975; Wille et al., 2001).
2.2. Regional and local vegetation
In the Zempoala National Park, forests are dominated by Quercus (oak), Pinus (pine) and Abies
0
J
F
M
A
M
J
J
A
S
O
N
D
0
Fig. 2. Climate diagram of Tres Cumbres, located at 2810 m altitude
and at 10 km distance from Lake Zempoala, Mexico.
(fir) (Miranda and Hernández-X, 1963). Several vegetation belts, with distinct altitudinal distributions
and characteristic arboreal composition, can be recognized in this area (Fig. 3). The lower montane
forest belt (1800–2800 m) includes dmesophyllous
forestT with Carpinus caroliniana, Garrya laurifolia,
Tilia houghii and Acalypha phleoides (Luna et al.,
1989). However, the same altitudinal interval on
exposed mountain ridges and on drier slopes
becomes a dmixed forestT (2400–2800 m) where
Quercus laurina, Arbutus xalapensis and Pinus
montezumae coexist with species of Salix and Viburnum. In the upper montane forest belt (2800–
3700 m) we can recognize two forest types at
different altitudinal intervals. From 2800 to 3550
m Abies religiosa-dominated forest occurs, with
Roldana angulifolia and Thuidium delicatulum in
the understory (Miranda and Hernández-X, 1963).
From 3550 to 3700 m Pinus hartwegii-dominated
forest with Festuca tolucensis and Festuca amplissima is common (Lauer, 1978).
In and around the lakes submerged aquatic vegetation, swamp vegetation and shore vegetation is present. Today the shores and nearby surroundings of
lakes Zempoala and Quila are covered by grasslands,
surrounded by Abies religiosa forest. Under natural
conditions forest would probably reach very close to
the lake shore. The present-day vegetation around
Lake Zempoala includes representatives of 49 families, 107 genera and 153 species of vascular plants
(Islebe and ter Weijden, 1988; Bonilla and Novelo,
1995).
68
Vegetation Types
(m alt.)
Alpine Zacatonal (3700 m alt.)
Caryophyllaceae, Plantago
Pinus hartwegii forest (3550-3700 m alt.)
Arceuthobium, Brassicaceae, Eryngium,
Upper montane forest
Geraniaceae, Pinus, Poaceae
3500
Coniferous forest
2800-3700 m alt.
3000
Abies religiosa forest (2800-3550 m alt.)
Abies religiosa, Asteraceae, Cupressus, Salvia,
Symphoricarpos, Onagraceae
Lake Quila
Mixed forest (2400-2800 m alt.)
Lake Zempoala
Quercus, Alnus, Salix
2500
Lower montane forest
1800-2800 m alt.
Mesophyllous forest (1800-2400 m alt.)
Acalypha, Arbutus, Artemisia, Carpinus,
Hedyosmum, Juglans, Leguminosae,
Melastomataceae, Rubiaceae,
Scrophulariaceae, Thalictrum, Viburnum
2000
Fig. 3. Schematic cross-section from south (left) to north (right) through the study area showing the position of Lakes Zempoala and Quila. The
modern altitudinal distribution of the main forest types and the main taxa in each vegetation belt, as far as relevant for a palynological analysis,
are indicated.
The azonal swamp vegetation of Lake Quila is
dominated by aquatic and semi-aquatic elements,
such as Eleocharis densa, Cardamine flaccida, Isoetes mexicana and Typha latifolia (Bonilla, 1992;
Bonilla and Novelo, 1995). At a short distance
from the coring site, the submerged and heliophytic
vegetation of Lake Quila was described by Islebe
and ter Weijden (1988) and Islebe et al. (2003). At
the coring site in Lake Zempoala we recognized four
main vegetation types: 1) Agrostis bourgaei and
Glyceria lurida, 2) Carex lurica, 3) Eleocharis acicularis and Polygonum punctatum, and 4) Potamogeton pusillus and Ranunculus trichophyllus (Bonilla
and Novelo, 1995; Almeida-Leñero and Cleef,
1997). Aquatic communities occur in these lakes
with mainly Myriophyllum aquaticum (hydrophytes
with floating roots), Potamogeton illinoensis, Eleocharis densa (hydrophytes with submersed roots),
and heliophytic vegetation with Arenaria paludicola,
Ranunculus hydrocharoides and Juncus ebracteatus.
Along the lake shore, grasses such as Polypogon
viridis and Muhlenbergia repens occur, but also
Rumex obtusifolia (Bonilla, 1992; Bonilla and
Novelo, 1995; Islebe and ter Weijden, 1988; Islebe
et al., 2003).
3. Material and methods
The sediment cores were collected with a handoperated Dachnowsky corer using increments of 25cm length (Faegri and Iversen, 1989). Sediment cores
were wrapped in plastic bags and protected by rigid
pvc-tubing. An horizon with volcanic ash and sand at
355 cm core depth in Lake Quila meant that sediments
had to be collected in two intervals. At some 25 cm
from the first core we penetrated this horizon using an
Edelman corer (Faegri and Iversen, 1989) and removed these coarse sediments. Subsequently, we collected the sediment from 410 to 884 cm core depth. As
a consequence the Quila-I core represents the interval
from 5 to 380 cm, and the Quila-II core includes the
core interval from 411 to 884 cm. Sediment cores were
transported to Amsterdam and kept in cool and dark
conditions. Samples of a known volume of c. 2 cm3
were collected at 5-cm intervals along the core. For
pollen analyses, 100 samples were obtained from core
Zempoala and 101 from cores Quila.
Radiocarbon dating of selected organic horizons
provided time control. We dated 6 bulk samples
from core Zempoala and 4 bulk samples from core
Quila. The dates were calibrated using Calib 4.2
Botryococcus*
Fungal spores*
Moss (like) spores*
Pediastrum*
Spirogyra*
Zygnema-type*
69
Cyperaceae*
Hydrocotyle*
Isoetes*
Myriophyllum*
Potamogeton*
Typha*
Other elements
Cyatheaceae*
Hymenophyllum*
Lycopodium foveolate*
Monolete psilate*
Monolete verrucate*
Aquatics
Apiaceae
Chenopodiaceae
Dodonaea
Polygonaceae
Rumex
Solanaceae
Urticales
Zea mays
Pteridophytes
Arceuthobium
Brassicaceae
Eryngium
Pinus
Poaceae
Caryophyllaceae
Plantago
Taxa marked with an asterisk are not included in the pollen sum.
Pinus hartwegii Abies religiosa
Zacatonal
Abies
Alnus
Asteraceae
Quercus
Cupressus
Salix
Lamiaceae
Symphoricarpus
Acalypha
Anacardiaceae
Arbutus
Artemisia
Carpinus
Euphorbiaceae
Hedyosmum
Leguminoseae
Melastomataceae
Rubiaceae
Scrophulariaceae
Thalictrum
Viburnum
Mixed forest Mesophytic forest Human impact
Fir forest
Alpine bunch grassland Pine forest
(Stuiver and Reimer, 1993) and the calibration curve
INTCAL98 (Stuiver et al., 1998). We have not tried to
calibrate dinterpolatedT or dextrapolatedT radiocarbon
ages as substantial uncertainties make such a procedure meaningless.
Prior to processing, one tablet of exotic Lycopodium spores was added to each sample for calculation of
the pollen concentration values. All samples were
prepared using standard pretreatment techniques including solution in sodium pyrophosphate, acetolysis,
and heavy liquid separation by bromoform (Faegri
and Iversen, 1989). Pollen residues were mounted in
a glycerin gelatin medium, sealed with paraffin, and
analyzed with a Zeiss microscope at 400 magnification. For identification of pollen grains and spores
we used the pollen reference collection of the University of Amsterdam, in combination with the pollen
morphological publications of Ohngemach and Straka
(1983), Hooghiemstra (1984), and van der Hammen
and González (1963). Results of the pollen analysis
were graphed as a pollen percentage diagram. The
pollen sum consists of all arboreal and herbaceous
taxa of the regional vegetation. Most pollen spectra
have a pollen sum of 400 grains. Pollen grains from
aquatic taxa and spores of ferns, mosses, fungi, colonies of algae and other microfossils were excluded
from the pollen sum. We arrived at the following
ecological groups: (1) Pinus (representing the altitudinal zone of pine forests), (2) Abies (representing the
altitudinal zone of fir forest), (3) mixed forest taxa, (4)
mesophyllous forest taxa, (5) taxa characteristic of
human impact, (6) aquatics, (7) pteridophytes, (8)
algae, (9) other microfossils (Table 1). The group of
other microfossils includes various fungal spores
types, Rivularia-type (Cyanobacteria), charred epidermis fragments of grasses, leaf-spines of the hydrophyte Ceratophyllum (the pollen grains of this plant
do not fossilize), and remains of Azolla. Horizons in
which sediments showed abundant fragments of volcanic ash (tephra) under binocular analysis, also
revealed fine ash fragments in the microscope slides.
Pollen diagrams were plotted with the software
TILIAGRAPH. For calculations we used TILIA and
for the cluster analysis of terrestrial pollen taxa we
used CONISS (Grimm, 1987). Important changes in
the composition of the pollen spectra and/or significant changes in representation of the dominant taxa
characterize the boundaries between pollen zones.
Table 1
Identified pollen and spore taxa from the sediments from the cores of lakes Zempoala and Quila (central Mexico highlands) arranged according to present-day altitudinal and
ecological affinity
70
Table 2
Lithological sequence of core Zempoala, and composite core Quila-1/
Quila-2
Depth
(cm)
Description
Zempoala
0–80
80–120
120–180
180–260
Red brown organic sediments
Peat, suspected ash at 110–115 cm
Clay
Sandy peat (very fibrous macros in the bottom), ash
at 260–243 cm
260–330 Clay
330–415 Peat
415–520 Dark clay, rich in organic material
Quila-1
0–100
Clay, with a volcanic ash horizon at 70–75 cm
100–355 Peat, rich in organic material with a volcanic ash
horizon at 110 cm
355–380 Volcanic ash
Quila-2
411–530 Gray clay with dark horizons
530–830 Peat, with thin intercalations of sandy clay at 530 cm
and 600 cm, and water-logged sediments at 730 cm
830–884 Dark sandy clay
of the Pico de Orizaba, on the Nevado de Toluca from
2600 to 4200 m, and on the volcano Malinche from
3000 to 4000 m. We also used the results from the
Central American Guatemalan highlands (Islebe and
Hooghiemstra, 1995) where this relationship was
studied along a transect from 2800 to 4200 m. The
interpretation of the pollen records representing the
local vegetation has been based on the studies of
modern plant communities by Islebe and ter Weijden
(1988) and Islebe et al. (2003). For ecological ranges
of taxa we also used Marchant et al. (2002).
4. Results
4.1. Stratigraphy
The main changes in the lithology of the sediment
cores are described in Table 2 and are shown in the
summary of pollen diagrams.
4.2. Chronological control
The interpretation of the pollen records is supported by modern pollen rain studies along altitudinal
gradients from comparable settings not farther than
200 km from our lakes, which show the relationship
between pollen representation and vegetation cover.
We used the results from Ohngemach and Straka
(1983) in particular; they showed the relationship
between vegetation and modern pollen rain along an
altitudinal transect from 3000 to 4600 m on the slopes
In the Zempoala core three depth intervals were
selected for time control where the organic carbon
content of the sediments was most promising. In
each interval two bulk samples from a 3 cm core
section were collected for dating (Table 3). The
three sets of two samples revealed ages that differed
by less than 200 radiocarbon years, and all 6 samples are in chronological sequence. We conclude
Table 3
Radiocarbon dates, calibrated age ranges (at 1 S.D.), and type of dated sediment from the cores of lakes Zempoala and Quila
Depth
(cm)
Age:
14
C yr BP
Age: cal yr BP (1 S.D.)
Sediments
Lab. number
Lake Zempoala
190–186
195–191
370–366
375–371
490–485
495–491
580 F 70
620 F 70
2110 F 200
2110 F 130
4460 F 100
4580 F 120
540–650
550–650
1870–2340
1930–2300
5290–4890
5460–5050
Peat
Peat
Peat
Peat
Clay
Clay
GrN-18727
GrN-18728
GrN-18729
GrN-18730
GrN-18731
GrN-18732
Lake Quila
324–321
430–427
780–777
860–857
4650 F 60
5620 F 38
8480 F 80
9713 F 45
5470–5310
6310–6440
9330–9540
11,090–11,200
Peat
Peat
Peat
Clay
UtC-5343
UtC-5344
UtC-5400
UtC-5401
y13CLake Zempoala
24.6
24.3
22.5
24.1
that sediment accumulation rate is close to linear
(Fig. 4).
Time control of the sediments from Lake Quila is
based on four bulk radiocarbon dates collected between 860 and 320 cm depth. Obtained ages are in
chronological sequence and the plot of the depth vs.
age (Fig. 4) shows a more or less linear accumulation rate for the lower and upper parts of the Quila
core, separated by a volcanic ash horizon. The age of
71
this well known volcanic ash marker bed is c. 4200–
4880 14C yr BP (Limbrey, 1986) and fits well the
interpolated age based on our bracketing ages of
5620 F 38 14C yr BP and 4650 F 60 14C yr BP. On
the basis of the pollen concentration record, 14C ages
and lithology, intervals with different sediment accumulation rates can be recognized. These differences
relate to clay accumulation in the lower part of the
core, and accumulation of peat and clay, intercalated
Fig. 4. Graph showing depth vs. calendar age of sediments from Lake Quila (top) and Lake Zempoala (bottom), central Mexico. Bars indicate
the possible range in calendar age (1r). For core Quila, the interval in between dotted lines represents the volcanic ash horizon, which separates
the upper and lower parts of the sequence.
72
with volcanic ash in the upper part (above 415 cm)
of the core.
4.3. Records of other microfossils
A number of microfossil types, rarely registered
during routine analysis, have been documented in this
study. These microfossils were very well preserved.
Their descriptions are given in Appendix A and
photographs are presented in Plates I and II.
4.4. Characterization of the pollen diagrams
4.4.1. Lake Zempoala
The pollen diagrams from Lake Zempoala show the
most important fossil pollen and spore taxa out of the
42 different types that have been identified (Figs. 5 and
6). Pollen concentration values vary mainly from
20.103 to 150.103 grains cm 3 of sediment. Based on
the CONISS cluster analysis, two main zones could be
recognized, each with subdivisions. The ages of the
zones are given in calendar years and based on the
calculated dcal yr BPT-scale shown in Fig. 6.
Zone ZEM-IA
520–422.5 cm
20 samples
Zone ZEM-IB
422.5–392.5 cm
6 samples
Zone ZEM-IIA
392.5–282.5 cm
22 samples
Zone ZEM-IIB
282.5–102.5 cm
37 samples
Zone ZEM-IIC
102.5–27.5 cm
13 samples
Zone ZEM-IID
27.5–10 cm
3 samples
c. 6320–3140
cal yr BP
c. 3140–2530
cal yr BP
c. 2530–1150
cal yr BP
c. 1150–310
cal yr BP
c. 310–180
cal yr BP
c. 180–160
cal yr BP
Zone ZEM-I is characterized by Pinus (34–63%,
max. 72%), Quercus (2–22%), Salix (0.6–14%, max.
20%), Alnus (0.5–15%), and by a low representation
of Poaceae (0.6–9%), Abies (0–5%, max. 9%),
Asteraceae (0.2–4%), and Rubiaceae (0–5%). Aquatic taxa have very low values. In zone ZEM-1 two
subzones (ZEM-1A and ZEM-1B) were distinguished. In zone ZEM-IA Pinus shows lowest values
compared to the remaining part of the record, whereas Quercus, Alnus and Salix show highest percentages, and Artemisia and Brassicaceae are also
important. Zone ZEM-IA includes two radiocarbon
dates of 4580 F 120 14C yr BP and 4460 F 100 14C
yr BP. In zone ZEM-IB Cupressus is represented for
the first time and temperate elements show higher
percentages. Potamogeton is better represented than
in ZEM-IA.
Zone ZEM-II is characterized by Pinus (28–88%),
Abies (0–17%, max. 29%), Salix (9–13%, max. 20%),
Alnus (0–9%, max. 18%) and Quercus (0–16%) and
by the consistent presence of Asteraceae and Poaceae.
Taxa belonging to the local vegetation, such as Cyperaceae, Anthoceros–Hymenophyllum, bryophyte spores
and fungal spores show marked fluctuations. Compared to zone ZEM-I, the most important change is
the significant increase of Pinus and Abies, a minor
increase of Arceuthobium, Melastomataceae and
Euphorbiaceae, and a decrease in representation of
Quercus, Alnus and Salix. In zone ZEM-II four subzones (ZEM-IIA, ZEM-IIB, ZEM-IIC and ZEM-IID)
were distinguished. Zone ZEM-IIA shows a relatively
high representation of Arceuthobium and Salix.
Values of Alnus, Abies and Anthoceros–Hymenophyllum are low. This zone includes a radiocarbon age of
2110 F 130 14C yr BP. Zone ZEM-IIB shows higher
representation of Abies, Melastomataceae, and Anthoceros–Hymenophyllum than in ZEM-IIA, and shows
low values of Salix and Rubiaceae. The local vegetation is mainly characterized by Cyperaceae (3–120%,
Plate I.
1–3:
4–7:
8:
9–11:
12–18:
19–20:
21–22:
23–25:
26–30:
Ascospores of Podospora-type, 1000 (1, 2: LZ ; 3: LZ-I/90)
Ascospores of Cercophora-type, 1000 (4, 5: LQ-II/60; 6, 7: LZ-I/43)
Ascospore of Diporotheca spec., 1000 (LQ-II/290)
Heterocysts of Rivularia-type, 1000 (LZ-I/54)
Separate ascospore-cells of Sporormiella spec., 1000 (12: LZ-I/43; 13–18: LZ-I/250)
Chlamydospores of Glomus spec., 1000 (19: LZ-I/50; 20: LZ-II/215)
Hyphopodia of Gaeumannomyces spec., 1000 (LZ-I/250)
Conidia of fungus type 200, 1000 (LQ-II/220)
Charred epidermis fragments of Poaceae ( 500); 26, 27, 28 and 30 show characteristic alternating long and short epidermis cells. 28
and 30 show prickles and 29 shows a stoma, 500 (26 and 29: LQ-II/215; 27, 28, 30: LZ-I/215).
73
74
Plate II.
1–2:
3–4:
5–8:
leaf-spines of Ceratophyllum spec., 500 (LZ-I/380)
Glochidia and massulae of Azolla spec., 500 (LQ-II/110)
Volcanic ash particles (tephra), 500 (LQ-III/350).
max. 175%), Hydrocotyle (0–2.5%) and Isoetes (0–
4%). The interval 260–243 cm includes volcanic ash.
ZEM-IIB shows high percentages of Pinus (42–86%),
moderate percentages of Salix (0–9%, max. 20%),
Quercus (0–12%, max. 16%) and Abies (1–14%),
and low percentages of Alnus (1–10%), Asteraceae
(0.4–8%), Poaceae (0–7%), Rubiaceae (0–5%, max.
8%) and Melastomataceae (0–3%). There is a marked
increase of Anthoceros–Hymenophyllum (0–14%,
max. 200%). Percentages of fungal spores (60–
700%, max. 3500%) and bryophyte spores (0–9%,
max. 48%) are high. The top of this zone is radiocarbon dated at 620 F 70 14C yr BP. Volcanic ash occurs
in the intervals of 243 to 226 and 115 to 110 cm.
Pollen zone ZEM-IIC is characterized by Pinus (37–
89%), Abies (0–22%, max. 30%), Asteraceae (0–9%,
max. 22%), Alnus (0–6%, max. 17%), Quercus (0–
8%, max. 12%), and low percentages of Salix (0–5%,
Fig. 5. Pollen percentage diagram of core Lake Zempoala (2800 m altitude), central Mexico. From left to right are shown: AMS 14C ages, depth, a selection of the most important
pollen records arranged by ecological preference, and pollen zones. The estimated abundance of fungal spore types is shown as bars.
75
Fig. 5 (continued).
76
Fig. 6. Summary pollen diagram of core Lake Zempoala. From left to right: AMS 14C ages, age scale in dcal yr BPT, depth, lithology, summary records of the main ecological groups
(mixed forest = Quercus, Alnus and Salix; mesophyllous forest = Hedyosmum, Carpinus, Juglans, Carya, Ilex, Tilia, Celtis, Arbutus, Rubiaceae and Eryngium), pollen sum values,
pollen concentration record, pollen zones, and the CONISS dendrogram.
77
78
Fig. 7. Pollen percentage diagram of core Lake Quila (3010 m altitude) in central Mexico. From left to right are shown: AMS 14C ages, depth, a selection of the most important pollen
records arranged by ecological preference, and pollen zones.
Fig. 7 (continued).
79
80
Fig. 8. Summary pollen diagram of core Lake Quila. From left to right: AMS 14C ages, age scale in dcal yr BPT, depth, lithology, summary records of the main ecological groups
(mixed forest = Quercus, Alnus and Salix; mesophyllous forest = Hedyosmum, Carpinus, Juglans, Rubiaceae and Eryngium), pollen sum values, pollen concentration record, pollen
zones, and the CONISS dendrogram.
max. 10%), Apiaceae (0–6%, max. 10%), Melastomataceae (0–6%, max. 9%) and Poaceae (0–4%,
max. 8%). Compared to the previous zone, this
zone is marked by an increase of Abies and Asteraceae and by a decrease of Pinus and Quercus. The
local vegetation is represented by Cyperaceae (60–
290%, max. 525%) and Isoetes (0–10%, max. 23%).
In the middle part of this zone abundant spores of
the coprophilous fungus Podospora occur. Zone
ZEM-IID is characterized by the increase of Alnus
and the start of the record of Zea mays (0–2%),
Dodonaea (0–2%) and Eryngium (0–5%). There is
also an increase in the representation of aquatic
taxa, such as Hydrocotyle (0–3%) and Zygnema
(0.5–2%). Compared to the previous zone, zone
ZEM-IID is most marked by the presence of Zea
mays, and by decreasing percentages of Pinus and
Abies.
4.4.2. Lake Quila
The composite pollen diagram of cores Quila-1 (on
top of the volcanic ash horizon) and Quila-2 (below
the volcanic ash horizon) shows the most important
fossil pollen and spore taxa out of the 46 that have
been identified (Figs. 7 and 8). Pollen concentration
values vary mainly from 20.103 to 150.103 grains per
cm 3 of sediment. Based on the CONISS cluster
analysis, four main zones could be recognized. The
ages of the zones are given in calendar years and
based on the calculated dcal yr BPT-scale shown in
Fig. 8.
Zone QUI-I
884–811.5 cm
6 samples
Zone QUI-II
811.5–383.5 cm
27 samples
Zone QUI-III
383.5–80.5 cm
19 samples
Zone QUI-IV
80.5–5 cm
12 samples
c. 11,610–10,650
cal yr BP
c. 10,650–5000
cal yr BP
c. 5000–1010
cal yr BP
c. 1010–20
cal yr BP
Zone QUI-I is mainly characterized by high representation of Pinus (15–50%), Alnus (8–55%), Quercus (4–9%) and Poaceae (0–10%), and by low
representation of Abies, aquatics and ferns. Local
taxa show low percentages: Myriophyllum (0–1%)
and Spirogyra (0–1%). Radiocarbon dating near the
base yielded an age of 9713 F 45 14C yr BP.
81
Zone QUI-II is characterized by Pinus (35–80%),
Quercus (with highest percentages in the core of 0–
16%, max. 25%), Alnus (2–23%), Abies (3–18%,
max. 23%) and Arceuthobium (0–1.7%). Melastomataceae (0–9%, max. 21%) is recorded for the first time
in the middle of this zone. There are marked percentages of Cyperaceae (0–20%, max. 30%) and Hydrocotyle (max. 10%). Compared to the previous zone
the representation of Pinus and Abies is higher,
Arceuthobium appears for the first time and Alnus is
decreasing substantially. The base and the top of this
zone have been radiocarbon dated at 8480 F 80 14C yr
BP and 5620 F 38 14C yr BP, respectively.
Zone QUI-III follows after the volcanic ash horizon of 380 to 355 cm and is dominated by Pinus
which fluctuates between 46% and 96%. Abies also
shows its highest percentages (1–36%). Moderate
percentages occur for Quercus (0–13%, max. 19%),
Alnus (0.7–9%), Poaceae (0–12%, max. 30%), and
Arceuthobium (0.2–0.8%). Taxa of local significance
are Cyperaceae (0–15%, max. 20% ). Compared to the
previous zone, Pinus shows higher percentages, Abies
shows marked fluctuations, and Asteraceae continuously show relatively high percentages. The lower
part of this zone has been radiocarbon dated at
4650 F 60 14C yr BP.
Zone QUI-IV is characterized by significant percentages of Pinus which decrease towards the top (58–
36%), Abies (7–15%), Quercus (4–12%), Alnus (1–
10%), Poaceae (5–17%, max. 23%) and Asteraceae
(1–8%). In this zone the highest percentages of Eryngium are found (0.9–4%). Taxa related to human impact are significant: Plantago (0–0.3%), Rumex (0–
0.4%) and Zea mays (0–0.3%). Cyperaceae (9–70%,
max. 95%) show the highest percentages of the record.
Compared to the previous zone, increase of Cyperaceae
and the abundance of aquatics such as Myriophyllum
(0–0.3%), Potamogeton (0–5%), Typha (0–5%), and
the alga Botryococcus (0–18%) is most significant.
5. Reconstruction of environmental changes from
the Zempoala and Quila records
5.1. Lake Zempoala
During the period c. 6320–2530 cal yr BP (zone
ZEM-I) the basin first accumulated clay (zone ZEM-
82
IA). Low abundance of aquatics (mainly Potamogeton) and the presence of algae (Fig. 6) suggest open
water in which fine grained sediments accumulated in
a low energy regime. Around 3000 cal yr BP, open
water disappeared (indicated by the absence of algae)
and peat formation initiated in the basin, suggesting a
lowered water table (zone ZEM-IB).On a regional
scale, the presence of Quercus, Alnus, Salix, and
the moderate values of Pinus indicate the existence
of a mixed forest, including some elements from the
mesophyllous forest, at the elevation of Lake Zempoala. Apparently, lower montane forest had a higher
altitudinal position than today. This mid-Holocene
oak forest represents the last phase of a temperate
and dry period. During the following period c. 2530–
160 cal yr BP (zone ZEM-II) lithological changes
suggest a variable water level. For several hundreds
of years accumulation of peat continued (zone ZEMIIA), pointing to continuous low water levels. Around
1600 cal yr BP, a significant increase of cyperaceous
reedswamp, and later an increase of algae, suggest
higher water levels resulting in clay accumulation.
Another period of peat formation occurred c. 1000 to
600 cal yr BP (in the middle of zone ZEM-IIB), with
a sand component. Aquatics demonstrate low representation indicating low water levels. During the
period sand was supplied to the peat bog, high representation of pioneer taxa with such as Salix and
fern spores suggest local disturbance of the vegetation cover. The beginning and end of sandy peat
accumulation is characterized by peaks of fungal
spores indicative of the active decomposition of organic material during low water level stands. From c.
600 to 310 cal yr BP water levels rose again and clay
accumulated in open and relatively deep water. During the period c. 310 to 160 cal yr BP (zones ZEMIIC and ZEM-IID) the water body changed into a
shallower lake; the sediments became richer in organic material. The presence of Ceratophyllum
spines, Cyperaceae and Typha are indicative of shallow water. Isoetes may have grown on dynamic
shores in the drainage system, and/or at places with
several meters of water. Such vegetation types indicate that the lake had developed a hydrosere along
the shores which is comparable to the present-day
situation (Bonilla and Novelo, 1995; Almeida-Leñero
and Cleef, 1997). The entire zone ZEM-II is characterized by the presence of coprophilous fungi which
suggests the frequent presence of cattle around the
lake/bog.
On a regional scale, during the full period from c.
2530 to 160 cal yr BP (zone ZEM-II), Abies-dominated forest prevailed in the surroundings of the lake,
and the forested areas experienced altitudinal migrations. The presence of various types of mesophyllous
forest and mixed forest are indicative of a temperate
climate varying from humid to subhumid (McDonald,
1993; Almeida-Leñero, 1997). During the period c.
2530–1150 cal yr BP (zone ZEM-IIA) the abundance
of Pinus, in combination with Arceuthobium, suggests
the presence of a Pinus hartwegii forest close to the
lake (Ohngemach and Straka, 1983). During this period the lake was situated in the Abies forest, close to
its uppermost altitudinal limit where it is transitional
to Pinus hartwegii forest. The altitudinal position of
the forest belts was possibly some 400 m lower than
today, indicative of a lower temperature. Using a
temperature gradient of 0.6 8C/100 m we estimate
average temperatures of c. 2.5 8C lower than today
for this period. During the period from c. 1150 to 310
cal yr BP (zone ZEM-IIB) Pinus hartwegii forest
dominated around the lake. Pinus forest is indicative
of dry climatic conditions and this conclusion corroborates the reported dry period from 2200 to 1200 cal
yr BP. The driest interval of this period was from 1300
to 1100 cal yr BP. In the last part of this period the
lake was located in the Abies forest belt pointing to
climatic conditions similar to today. During the period
c. 310 to 160 cal yr BP (zones ZEM-IIC and ZEMIID) Abies forests predominated under conditions
similar to the present. The increasing contribution of
non-arboreal taxa reflects progressive deforestation.
Taxa such as Zea mays and fungal spores of the
coprophilous genus Podospora indicate human occupation of the study area. The lake had open and
relatively deep water, but changed into a shallower
lake during these two centuries; the sediments became
richer in organic material.
5.2. Lake Quila
During the period from c. 11,610 to 10,650 cal yr
BP (zone QUI-I) the accumulation of organic rich
clay, and an almost complete absence of aquatics
(some Myriophyllum and Hydrocotyle), indicate the
presence of an open water body. The supply of sand
suggests erosion in the surroundings of the lake. At
the end of this period the water table lowered and peat
formation started. Presence of shallow water plants
and algae did not increase before the end of this zone.
The pollen spectra show the presence of montane
Alnus and Pinus forest, similar to the type of forest
occurring today in the central Mexico highlands
(Almeida-Leñero et al., 1997). The presence of this
type of forest suggests humid conditions.
During the period c. 10,650 to 5000 cal yr BP
(zone QUI-II) there was a relatively long interval
with peat formation: aquatics and algae were common
and the water table was lower. Around 7000 cal yr BP
(530 cm core depth) clay started to accumulate suggesting a rise of the water table. At the end of this
period a volcanic ash horizon was deposited, reflecting a volcanic eruption in or near the study area.
In the surrounding area, temperate forest with
Pinus, Alnus and Abies was present. Pollen grains of
Pinus and Arceuthobium indicate that Pinus hartwegii
forest may have been close to Lake Quila. Compared
to the present-day situation, this altitudinal distribution
reflects a downward shift of the Pinus/Abies transition
of some 400 m. We therefore estimate a temperature
depression of some 2.4 8C compared to today.
The period c. 5000 to 1010 cal yr BP (zone QUIIII) was characterized mainly by peat formation, suggesting a low water table; aquatics and algae also
occurred.
In the surroundings of Quila, temperate forest with
Pinus, Alnus and Abies was present. Pollen grains of
Pinus and Arceuthobium are present at the same time
and indicate that Pinus hartwegii forest continued to
be present, pointing to continuing dry conditions. This
is supported by the low water table.
During the period of c. 1010 cal yr BP to the
present (zone QUI-IV) aquatics and algae became
abundant and clay was accumulated, pointing to
higher water levels compared to the previous period.
Cyperaceous reedswamp with Typha and Potamogeton was abundant at the shores of the lake. In the
surroundings of the lake Abies religiosa forest dominated. According to a comparison with the modern
ecological envelope of this forest type (Rzedowski,
1978), temperate subhumid climatic conditions prevailed. Lower representations of Pinus, Abies and
Quercus indicate a downward shift to present-day
conditions of the belt, with a mixed forest. For the
83
first time in the record, human occupation of areas
close to Quila is indicated by the occurrence of Zea
mays and Rumex (Almeida-Leñero et al., 1990).
6. Discussion
Multi-event volcanic ash depositions may detract
significantly from the quality of a pollen record, but
our sites were not intensively affected. Pollen records
from the Valley of Mexico suffer from frequent and
irregular ash depositions that severely damage the
vegetation cover in a large area, and from tectonically
induced irregularities in the stratigraphic column
(Lozano and Xelhuantzi-López, 1997). The sediment
sequences of lakes Zempoala and Quila offered evidence on lithology, fossil pollen, spores and nonpollen palynomorphs, local changes in the lake basins,
regional scale climate change, and human impact.
Lakes Zempoala and Quila are altitudinally and latitudinally close together.
In the pollen spectra we identified 27 taxa of
vascular plants at the family-level, and 19 taxa at the
generic level. In the fossil pollen spectra of both cores,
as well as in the spectra of the recent pollen rain, the
same arboreal taxa are dominant in decreasing order of
abundance: Pinus, Alnus, Abies, and Quercus (Palacios Chávez, 1977; Tovar-González, 1987; Ohngemach and Straka, 1983). These studies also illustrate
that Pinus and Alnus, and to a lesser degree Quercus,
are over-represented in the pollen rain, but their pollen
signals closely relate to the altitudinal interval where
these forests occur. Pinus shows an optimal representation in the belt with Pinus hartwegii forest (Ohngemach and Straka, 1983). Abies is under-represented in
the pollen rain, which was also reported from a Guatemalan study (Islebe and Hooghiemstra, 1995), and
its maximum representation is in the Abies forest belt.
Where Alnus is dominant in the vegetation, this might
be considered to be an indication of disturbance.
Alnus shows its maximum representation in the
mixed forest. High percentages of Cupressus and
Quercus in the pollen spectra are indicative of local
presence of these taxa in the vegetation. The most
important genera of Poaceae in our study area are
Calamagrostis and Festuca and these taxa are
under-represented in the pollen rain (Ohngemach
and Straka, 1983).
84
According to the calibrated time scale, the pollen
record of Lake Zempoala includes the last c. 6320 cal
yr BP. The uncalibrated ages of the two samples close
to the base of the Zempoala core differ by only a few
hundreds of years from the age of the dyellow ash bedT
(4880 F 120 14C yr BP). We therefore assume that the
Lake Zempoala record probably started after deposition of this important marker. An alternative interpretation that the record started after deposition of the
dupper fine pumiceT horizon of the Pomez volcanic
ash, dated 4250 F 110 14C yr BP, is less plausible
(Bloomfield and Valastro, 1977; Lorenzo, 1991; Lambert, 1986; Limbrey, 1986); both beds are associated
with a dry period (Metcalfe et al., 1991; Metcalfe,
1995). According to Metcalfe et al. (1991) most important Holocene climatic change occurred above this
marker bed and this corresponds with the Zempoala
record.
The Zempoala (2800 m) core records two types of
forest: mixed forest (today occurring below the altitude of this lake) at the base of the record, later
replaced by Abies forest (today occurring from 2800
to 3500 m at and above the altitude of the lakes). This
is clear evidence of altitudinally shifting vegetation
belts forced by climate change. Changes in the floristic composition within periods of dominance by particular vegetation belts, mainly reflect changes in
humidity, but changes in the seasonality of precipitation makes the relationship between total precipitation, moisture availability and vegetation complex
(Street-Perrott and Perrott, 1990; O’Hara and Metcalfe, 1995). The record of environmental change
now available may be further improved by analysing
additional proxies (e.g. diatoms), and by developing a
pollen morphological key to the different species of
Pinus.
In the following section we compare the results of
the records of Lake Zempoala and Lake Quila with
reconstructions from other sites. A synthesis is shown
in Table 4.
With respect to Zempoala, the dry conditions during the period from c. 6320 to 2539 cal yr BP (zone
ZEM-I) were also reported for the central Mexican
region by González-Quintero and Fuentes-Mata
(1980). They found an increase in Quercus, as well
as the first appearance of Zea in Lake Texcoco.
González-Quintero and Fuentes-Mata (in Niederberger, 1987) also showed the first occurrence of maize
in this period. In Lake Chalco Watts and Bradbury
(1982) showed the existence of shallow freshwater
and marshes from 5000 years ago onward, also pointing to dry conditions. Lozano et al. (1993) and Lozano
and Ortega (1994, 1998) documented similar environmental change during the early and middle Holocene
and human disturbance from c. 3000 14C yr onward.
In central region of Mexico Metcalfe et al. (1991) also
reported a phase of accelerated erosion around 3100
cal yr BP. On the basis of diatom studies in the Zacapu
Basin Metcalfe (1995) inferred dry periods between c.
6000 and 4500 14C yr BP, and around 1000 14C yr BP.
During the last part of this period (zone ZEM-1B) in
particular, water levels were low and peat formation
started in the lower parts of the basin.
The lower temperatures during the period from
2530 to 1150 cal yr BP (zone ZEM-IIA) are supported
by Vázquez-Selem (1997), who described a terminal
moraine at the northern part of Iztaccihuatl, dated at c.
2000 14C yr BP. The same study assumed that at La
Malinche volcano climatic conditions during the development of La Morrena-M-IV were colder than at
present. Based on geomorphology, palaeopedology
and tephrastratigraphy of glacier advances on the
volcano La Malinche, Heine (1988) inferred a cool
period from c. 3000 to 2000 14C yr BP. Also the
Mexican pollen and diatom records of Brown
(1985), Straka and Ohngemach (1989), Metcalfe et
al. (1991), and the records from Central American
lakes (e.g. Hodell et al., 1995; 2001) show lower
temperatures for this period.
During the period from c. 1150 to 310 cal yr BP
(zone ZEM-IIB) the interval from 1300 to 1100 cal yr
BP was driest and represents an extreme since the midHolocene (Hodell et al., 1995; 2001). The latter interval of 200 years coincides with the collapse of the
Maya civilization. The sequence of climatic change
between 1800 and 1100 14C yr BP (assuming errors of
50 years this reflects the period of c. 1800–1600 to
1050–950 cal yr BP) has also been documented in
marine records (Brown, 1985) and in a climate record
from Michoacan State (Metcalfe, 1995). In the last part
of the period, represented by zone ZEM-IIB, the lake
was located in the Abies forest belt pointing to climatic
conditions similar to today. In the Valley of Mexico
and surroundings there is a well documented record of
agricultural activities which started 3000 14C yr BP
(Brown, 1985). There is little doubt that evidence
Table 4
Synthesis of Holocene vegetation development in the Central Mexican highlands and inferred climatic conditions for the altitudinal interval from 2800 to 3000 m
Time
scale
Cal yr
BP
20
(QUI–IV)
1100
Lake zempoala, Mexico
(3010 m alt.)
(2800 m alt.)
Actual vegetation
(grassland and Abies religiosa
forest)
Actual vegetation
(Abies religiosa forest)
Abies forest and elements of
human impact
(Zea mays)
Abies forest
2000
Dry high salinity
160
Forest and elements of
human impact
Yucatan, Central America
and the Caribbean
(ZEM–II)
(Zea mays)
Very dry
significant anthropogenic impact
Dryer; but variable
saline lakes
Significant local
anthropogenic impacts
2530
Drying
Mixed forest
5000
6000
Central Mexican Volcanic Belt
(Ruter et al., 2004)
Hiatus
(QUI–III)
4000
Cal yr
BP
(ZEM–I)
Pinus forest,
few pollen and volcanic ash
Variable regionally
Short dry interval–wet;
Generally lower lake levels
but high variable
Highest lake levels
Brosimum decline
Abies forest
6320
Drying begings
high lake levels
Wet, rising lake levels
8000
Rising lake levels
(QUI–II)
Glacial advances?
0
Lake Quila, Mexico
10,000
Very arid and cold
10,650
(QUI–I)
11,610
Lowest lake levels
Alnus forest
85
The sequence of environmental change from lakes Quila and Zempoala (with pollen zones) are compared with the synthesis study by Ruter et al. (2004) for central Mexico and
Yucatan.
86
reflects the establishment of an indigenous civilization
with a dense population for Mesoamerica (Niederberger, 1987; Metcalfe et al., 1989). Agricultural development was registered slightly later in the peripheral
parts around the Valley of Mexico (Brown, 1985).
With respect to Quila, the period from 11,610 to
10,650 cal yr BP (zone QUI-I) shows a dominance of
Alnus and this period is likely to correlate with zone III
in the sections Tlaloc-I and Tlaloc-II (3100 m altitude)
from the area of volcano La Malinche, east of our
study area (Fig 1). Ohngemach and Straka (1983)
and Straka and Ohngemach (1989) assumed that
Alnus forests found in this area resulted from local
fires, but may also represent successional stages towards an Abies forest. Madrigal (1967) interpreted the
present Alnus–Pinus forest in this way. High percentages of Alnus during the same period are also reported
by Xelhuantzi-López (1994) and Watts and Bradbury
(1982) for the area northwest of our study area, and by
Lozano and Ortega (1994) for the area of the Valley of
Mexico. For this period an increase in humidity, related to increased precipitation, was also reported by
González-Quintero (1981, 1986).
For the period from c. 10,650 to 5000 cal yr BP
(zone QUI-II) Ohngemach and Straka (1983) and
Straka and Ohngemach (1989) also report slightly
cooler conditions than at present. High percentages
of Pinus possibly correlate with zone IV of sites
Tlaloc-I and Tlaloc-II (Straka and Ohngemach,
1989). Between 7000 and 5000 14C yr BP (assuming
errors of 50 years this reflects the period of 7950–
7750 to 5850–5650 cal yr BP) changes in the extension of Pinus forest was also reported by Brown
(1985). For the Upper Lerma Basin, Toluca, Metcalfe
et al. (1991) registered an important episode characterized by environmental disturbance, desiccation of
Almoloya Lake and accelerated erosion in the surrounding valleys. These events are plausible under dry
climatic conditions. During the same time the dyellow
ashT was deposited, an event that supposedly corresponds to the upper part of our zone QUI-II.
Coinciding with our period from c. 5000 to 1010 cal
yr BP (zone QUI-III) González-Quintero (1981, 1986)
reported an arid period from c. 4850 to 4200 14C yr BP
(assuming errors of 50 years this reflects the period of
5650–5500 to 4850–4650 cal yr BP) associated with
significant erosion in the Basin of Mexico. Niederberger (1987) and Bradbury (1989) reported similar cli-
matic conditions on the basis of diatoms in the Basin of
Mexico, so did Watts and Bradbury (1982) in Lake
Pátzcuaro. The expansion of Abies forest might correspond to zone VII of pollen record La Malinche (Ohngemach and Straka, 1983; Straka and Ohngemach,
1989). Significant climatic change during this period
is also reported by Hodell et al. (1995; 2001) for
Mexico, by Curtis et al. (1996; 1998) for the Maya
Lowlands and for other Mesoamerican sites, and by
Islebe et al. (1996b) for Costa Rica. Abundant Pinus
forest during the last part of this period (uppermost
part of zone QUI-III) reflects the widespread occurrence of Pinus hartwegii forest, indicative of the most
arid period reported during the middle to late Holocene from c. 1300 to 1100 cal yr BP. Climatic aridity
during this time was also noted by Metcalfe et al.
(1991) for the Lerma Basin where lakes changed
between 900 and 1400 AD. Dry climatic conditions
were also reported from Lake Pátzcuaro, central Mexico by Watts and Bradbury (1982), who inferred low
water levels based on archaeological and lacustrine
features. Dry conditions were also reported for Zacapu
Basin (Metcalfe, 1995) and for the Yucatán Peninsula
(Curtis et al., 1996, 1998; Hodell et al., 1995, 2001).
The presence of Abies forest since c. 1010 cal yr
BP (zone QUI-IV) was reported earlier by GonzálezQuintero (1986) for the Basin of Mexico, by Metcalfe
et al. (1991) for the Upper Lerma Basin.
In conclusion, the results from the pollen records of
Quila and Zempoala appear to reflect climatic change,
evidenced by vertical migration of the main vegetation belts. The pollen records show local fluctuations
in the basin of the lakes, reflecting changes in the
hydrological balance, climate change on a regional
scale, and human impact on the landscape. Environmental changes correspond to those reported for the
central Mexican region and, as such, support the
regional sequence of climate fluctuations.
The composite record of environmental change
start with a period of more humid and cooler conditions than at present. During the period of c. 6320–
2540 cal yr BP dry conditions are inferred, which
supports the evidence from other central Mexican
records, showing a general trend of increasing dryness
from early to mid-Holocene time. This dry episode
was widespread in central Mexico. During the mid
and late Holocene there are indications of human
impact, as is the case in other sites in the study area.
Lower temperatures during the period from 2530 to
1150 cal yr BP are supported by moraine records. The
period from 1300 to 1100 cal yr BP was driest and
represents extreme conditions since mid-Holocene
time; this period coincides with the collapse of the
Maya civilization. We started to use non-pollen microfossils, rarely documented in Latin American palaeoecological studies, as an unexplored source of
information.
Acknowledgements
Peter Kuhry is thanked for his assistance in the
field. Elly Beglinger and Annemarie Philip (Amsterdam) are thanked for the preparation of the pollen
samples. The late Gerald Noldus assisted with identification of pollen grains. Gerald Islebe and Sonia
Salamanca are acknowledged for the constructive
comments on an earlier draft of the manuscript. Maarten Blaauw is thanked for assistance with chronology.
Juan Carlos Berrio supported with using TILIA software. Veronica Aguilar is thanked for continuous
support. Financial support from the Universidad
Nacional Autónoma de México (UNAM) and FUOS
(University of Amsterdam) made this study possible.
We thank Sarah Metcalfe and an anonymous reviewer
for constructive comment on an earlier draft of this
paper.
Appendix A
Morphological descriptions and ecological/environmental preferences of other microfossils recorded
in the sediments of Lake Zempoala.
Podospora-type (Type 368); Plate I, 1–3.
Ascospores, 29–46 16–23 Am. One-celled, ellipsoidal, smooth, dark brown, with one protruding pore
2 Am in diameter, directly below the apex. Pore
surrounded by an annulus. Basal end bluntly conical,
and originally bearing a hyaline appendage (which
does not fossilise). Fossil spores of the Podosporatype (Sordariaceae) were recorded and illustrated for
the first time by van Geel et al. (1981) in a sediment
deposited directly beside an Iron Age settlement in the
Netherlands. In later studies Podospora-type spores
were observed also in material from other archaeolog-
87
ical sites where it could be used as an additional
indicator for (extra) dung and the presence of dense
populations of herbivores. Lundqvist (1972) described
and illustrated many species of Sordariaceae and his
well-illustrated study can be used as a reference for
identification of fossil spores of that family of predominantly coprophilous Ascomycetes.
Cercophora-type (Type 112); Plate I, 4–7.
Ascospores, 16–22 9–12 Am. One-celled,
smooth, brown, truncate at the basal side (originally
bearing a hyaline appendage, which does not fossilise)
and tapering at the apical end. One sub-apical pore c.
0.7 Am in diameter. Sometimes Cercophora spores
show one septum, and if so, there is no constriction at
the septum (van Geel, 1978). According to Lundqvist
(1972), representatives of the Sordariaceous genus
Cercophora are coprophilous or occur on decaying
wood, on herbaceous stems and leaves.
Sporormiella-type (Type 113); Plate I, 12–18.
Separate cells of ascospores, 12–20 9–11 Am.
Cells smooth and brown, showing a slightly curved
germ slit, extending the entire length of the cell. When
Sporormiella-type is present in the fossil record, there
are always two types of cells. There is a category
showing two flattened ends (Plate I, 12–14) and a
category of cells showing one flattened end, in combination with a rounded end (Plate I, 15–17). The
reason is that complete ascospores of Sporormiella
species are three to many septate. The septa are transverse and ripe ascospores split up in separate cells,
each one being able to germinate and form a new
individual fungus. In the fossil state no observations
of complete ascospores can be made, but the presence
of the two types of cells (dterminal cellsT and dmiddle
cellsT) is an extra feature for identification. Identification to species level of Sporormiella-type is not possible because fruit bodies, asci and complete
ascospores are not available (compare Ahmed and
Cain, 1972). The representatives of the related, coprophilous genus Sporormia are without germ slits.
However, the descriptions of Ahmed and Cain
(1972) are based on non-germinated spores, and a
slit may appear after germination. Therefore we cannot exclude that also Sporormia is among our fossil
spores, and thus we refer to the fossil spores as
Sporormiella-type instead of Sporormiella. Fossil
spores of Sporormiella were distinguished by Davis
et al. (1977), Davis and Turner (1986), Davis (1987),
88
and van Geel et al. (2003) they used the spores as
indicators for dung produced by herbivores.
Diporotheca spec. (Type 143); Plate I, 8.
Ascospores 50–55 21–24 Am, biseptate (not visible in illustration), fusiform, both ends truncate with
a germ pore c. 3 Am in diameter. Surface of brown
spores showing dark brown anastomosing ribs. For a
more complete fossil record (illustrations including
fruit bodies) reference is made to van Geel et al.
(1986b). Originally the genus Diporotheca was classified as a representative of the Meliolaceae by Gordon and Shaw (1960), but Mibey and Hawksworth
(1995) introduced the family Diporothecaceae to accommodate the genus Diporotheca. Gordon and
Shaw illustrated D. rhizophila and mentioned roots
of Solanum species as the substrate of the parasitic
fungus.
Glomus spec. (Type 207); Plate I, 19 and 20.
Chlamydospores globose to egg-shaped, 17–138
Am, light to dark brown, with hyphal attachment.
Representatives of the genus Glomus are vesicular–
arbuscular endomycorrhizal fungi and they occur on a
variety of host plants. The presence of Glomus in lake
deposits can be used as an indication for erosion of
soils, because the spores are formed below the soil
surface and normally not transported. For additional
information about the fossil record of Glomus see van
Geel et al. (1989).
Gaeumannomyces spec. (Type 126), Plate I, 21 and
22.
Hyphopodia dark brown, roughly circular to oval
or irregular in outline, with blunt lobes. Greatest
diameter 12–39 Am. A prominent clear spot 1.8–2.8
Am in diameter in the central area, representing the
point of host penetration. In Europe the fossil hyphopodia of Gaeumannomyces appeared to be indicative
for local Carex in the peat-forming vegetation (Pals et
al., 1980; van Geel et al., 1989).
Type 200; Plate I, 23–25.
Fungal spore: coiled cluster of 5–10 globose cells;
greatest diameter of the clusters 18–45 Am. Individual
cells variable in size, the smaller cells concentrated at
one side of the cluster. One of the smaller cells
tapering into a hypha, broken off at the end (former
connection with mycelium). van Geel et al. (1989)
recorded Type 200 spores in a Lateglacial deposit
from the Netherlands, where they were common in
the terrestrialisation phase (with Equisetum, Phrag-
mites and Carex) of a shallow pool. A direct relationship with certain host plants or specific ecological
conditions could not be revealed yet.
Rivularia-type (Type 170; Cyanobacteria); Plate I,
9–11.
Heterocysts almond-shaped, 10–12 6–8 Am, with
a c. 0.8 Am wide pore at the narrow end, which often
bears an up to 2 Am long appendage (the sheath of the
trichome which did not become fossilised). Similar
heterocysts were recorded and illustrated by van Geel
et al. (1983, 1989). Cyanobacteria can play a pioneer
role in aquatic habitats thanks to their nitrogen-fixing
ability.
Gramineae, charred epidermis fragments; Plate I,
26–30.
Grasses show a characteristic pattern of epidermis
cells: adjacent cells have dzip-fastenerT connections
with each other and, apart from long cells, there are
also short cells. Some of the observed fragments
showed prickles and stomata. A more complete record
and the study of modern reference material might
result in identification of genera or even species.
Considering the fact that grass pollen has to be
lumped (apart from Cerealia, like Zea), identification
of the gramineous taxa would be most welcome.
Ceratophyllum spec. (Type 137); Plate II, 1 and 2.
Leaf spines, consisting of a single thickened cell,
narrowly elongate and sharply pointed, bearing at its
base the impression of attachment to adjacent cells.
Records of the spines are important because the pollen
of Ceratophyllum does not preserve.
Azolla spec.; Plate II, 3 and 4.
Glochidia and massulae of Azolla could be recognised at various levels in the cores.
Volcanic ash fragments (tephra); Plate II, 5–8.
In various pollen slides we observed the characteristic small, hyaline fragments of volcanic ash. The
usual preparation method of tephra in peat samples
is given by Pilcher and Hall (1992).
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Holocene climatic and environmental change from pollen records of

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