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Sta. Cruz Atizapa¤n: a 22-ka lake level record and climaticimplications for the late Holocene human occupation in the

Upper Lerma Basin, Central Mexico

Margarita Caballero a;, Beatriz Ortega a, Francisco Valadez b,Sarah Metcalfe c, Jose¤ Luis Macias a, Yoko Sugiura d

a Instituto de Geof|¤ sica, UNAM, Coyoaca¤ n 04510, Me¤ xico D.F., Mexicob Facultad de Ciencias, UNAM, Coyoaca¤ n 04510, Me¤ xico D.F., Mexico

c Department of Geography, University of Edinburgh, Edinburgh EH8 9XP, UK d Instituto de Investigaciones Antropolo¤ gicas, UNAM, Coyoaca¤ n 04510, Me¤ xico D.F., Mexico

Received 23 July 2001; received in revised form 15 July 2002; accepted 24 July 2002

Abstract

The Upper Lerma is a high altitude basin with three water bodies linked by the Lerma River. This basin has a

long archaeological history, characterised by the establishment of settlements within the lacustrine ecosystem itself

(man-made islands) during the late Classic to Epiclassic (AD 550^900), which were abandoned by the end of the

Epiclassic. The Upper Lerma is an ideal site to study climatic and environmental conditions during the period of

human occupation, as well as during the last full-glacial/interglacial cycle. Two sediment cores (STCRZ: 9.54 m and

Almoloya del R|¤o: 5.12 m) were recovered from the highest lake in the system (Chignahuapan). Ten radiocarbon

dates provide chronologies for these sequences in which the Tres Cruces Tephra (c. 8500 yr BP) and the Upper Toluca

Pumice (c. 11 600 yr BP) serve as stratigraphic markers. Magnetic properties, loss on ignition, and diatom analyses

were used to infer lake level fluctuations during the last c. 22 000 yr BP. The Late Pleistocene environment was

characterised by a freshwater lake. High sediment input and variable lake levels are recorded during the Last Glacial

Maximum (c. 19 000 1̂6 000 yr BP), while slightly higher water levels and reduced sediment input are recorded during

the Late Glacial (c. 16 000̂ 11000 yr BP). A short episode of shallow conditions is inferred by c. 12 400 yr BP.

Holocene lake levels were generally shallower, and three episodes of very shallow, slightly alkaline waters are

identified. The first dates to the early Holocene (c. 11 000 7̂000 yr BP). The second is centred at c. 4600/4500 yr BP.

The third occurred between c. 2000 (?) and 800 yr BP (c. 200 BC^AD 1100, calibrated ages) with very shallow water

after c. 1400 yr BP (AD 550, calibrated age). Lake level increased after c. 800 yr BP. These three shallow water eventsare also recorded at other sites in Central Mexico indicating regional climatic trends rather than local events. A deeper

water phase occurred between 7000 and 6400/6200 yr BP. The last shallow water phase correlates with the Classic and

Epiclassic periods (AD 200^900), and shallowest conditions occurred in the late Classic to Epiclassic (c. AD 550^900),

when the construction of man-made islands reached a peak. An increase in lake level after c. 800 yr BP (AD 1100

calibrated age) may have led to the abandonment of this life strategy.

2002 Elsevier Science B.V. All rights reserved.

0031-0182 / 02 / $ ^ see front matter 2002 Elsevier Science B.V. All rights reserved.

PII: S 0 03 1 - 0 1 82 ( 02 ) 0 0 5 02 - 3

* Corresponding author. Fax: +52-55-55509395.

E-mail address: [email protected] (M. Caballero).

Palaeogeography, Palaeoclimatology, Palaeoecology 186 (2002) 217^235

www.elsevier.com/locate/palaeo

mailto:[email protected]

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mailto:[email protected]



Keywords: palaeolimnology; palaeoclimatology; archaeology; Mexico; Quaternary; Late Pleistocene; Holocene; diatoms; magnetic

properties

1. Introduction

Palaeoenvironmental research in Central Mex-

ico started in the 1940s (Deevey, 1944; Sears,

1952) with the main objective of inferring climatic

conditions during the developmental stages of the

great Mesoamerican cultures. Later work focused

on climatic change during the last glacial/intergla-

cial cycle. Over the last decade, diatom, pollen,

ostracod, rock-magnetism, geochemistry, tephro-

chronology and palaeosol records have been pub-

lished (Metcalfe et al., 1991; O’Hara et al., 1993;Lozano et al., 1993; Caballero and Ortega, 1998;

Newton and Metcalfe, 1999; Bridgwater et al.,

1999; Caballero et al., 1999; Bradbury, 2000; Or-

tega Guerrero et al., 2000; Sedov et al., 2001).

Few sites, however, have a continuous record cov-

ering the last c. 20 000 yr, i.e. the last full-glacial/

interglacial cycle, or display su⁄cient preservation

and resolution for the late Holocene sediments to

allow clear correlation with the archaeological

record.

Many records for Central Mexico come fromthe Basin of Me¤xico. Due to urban development

and the explosive demographic increase in this

area, many sites where correlation between ar-

chaeological and palaeoenvironmental records

might have been possible have been destroyed or

buried under urban infrastructure. The nearby,

less disturbed, Upper Lerma Basin is therefore

an ideal site to study climatic and environmental

conditions during the period of human occupa-

tion, as well as during the last full-glacial/intergla-

cial cycle. In the Upper Lerma Basin, archaeolog-

ical studies indicate a long and uninterruptedoccupational history, which began as early as

3500 yr BP (Sugiura, 1992, 2000). The late Classic

to Epiclassic (AD 550^900), however, was charac-

terised by the establishment of man-made islands

within the lake, that were abandoned by the end

of the Epiclassic (c. AD 900). Natural variations

in water levels undoubtedly in£uenced this living

strategy. In this paper we present an integrated

view of the palaeolimnological evolution of Lake

Chignahuapan, in the Upper Lerma (Fig. 1), fromthe Last Glacial Maximum to the Holocene, and

emphasise possible correlations with the archaeo-

logical record, particularly at the end of the Clas-

sic period.

2. The study area

2.1. General setting

The Upper Lerma Basin (19‡10PN, 99‡32

PW ;

2570 m a.s.l.) is the source of the longest river

in Mexico, the Lerma. It is bounded to the east,

south and west by volcanic Sierras of Pliocene to

Holocene age and by smaller hills to the north,

through which the Lerma river £ows out of the

basin. The area has a tropical, high altitude cli-

mate (Cw2), with a summer rainy season (c. 1000

mm/yr, INEGI, 1980). Due to its high altitude,

frost is common during winter but summer tem-

peratures can be as high as 27‡C.

There are three water bodies, connected by theLerma River, from south to north: Chignahua-

pan, Lerma and Chiconahuapan (Fig. 1). Here

we present palaeolimnological data from Chigna-

huapan. At present, all the lakes are very shallow,

particularly during the dry, winter season, due to

water extraction for Mexico City. Springs used to

be an important source of water, but several have

gone dry. Lake level is maintained mainly by rain-

water, but inputs of sewage water are also com-

mon. The lakes are shallow (c. 3 m), with pH

between 7 and 8.5 and conductivity between 600

and 1000 WS/cm.

2.2. Volcanic stratigraphy

The Nevado de Toluca, of Pleistocene^Holo-

cene age, is the main volcano in the basin. Two

major Plinian-type eruptions occurred in the Ne-

vado during the last 25 000 years (Bloom¢eld and

Valastro, 1974, 1977; Bloom¢eld et al., 1977;

Mac|¤as et al., 1997): the Lower Toluca Pumice

M. Caballero et al. / Palaeogeography, Palaeoclimatology, Palaeoecology 186 (2002) 217^235218



(LTP, V24000 yr BP) and the Upper TolucaPumice (UTP, V11 600 yr BP). Two more events

were documented recently: (1) a ¢ne grey massive

sand^silt ash £ow deposit with a minimum age of

14 000 yr BP (Caballero et al., 2001; Garc|¤a-Palo-

mo et al., 2002), and (2) the so-called White Pum-

ice £ow deposit (Mac|¤as et al., 1997), dated at

12 414+290/3280 and 1204092 yr BP (Garc|¤a-

Palomo et al., 2002).

Monogenetic activity has also been important

in the area (Bloom¢eld, 1975; Garc|¤a-Palomo etal., 2002), such as that of the Tres Cruces volca-

no that produced the Tres Cruces Tephra (TCT)

by c. 8500 yr BP (Bloom¢eld, 1975). Newton and

Metcalfe (1999) reported other tephras in the

area: the Lower Almoloya Tephra (LAT), dated

at c. 12 400 yr BP, the Techuchulco tephras (1

and 2), between the UTP and the TCT and the

Upper Almoloya Tephra (UAT), overlying the

TCT.

Fig. 1. Location maps. (a) Central Mexico with the location of Lake Chignahuapan, Upper Lerma Basin, and of other sites with

previously published palaeoenvironmental records. (b) Upper Lerma Basin, with the location of palaeolimnological study sites:

1. Pit 2 (Metcalfe et al., 1991); 2. Isla II core (Caballero et al., 2001); 3. Almoloya del R|¤o core (Newton and Metcalfe, 1999,

this paper); 4. Pit 1 (Metcalfe et al., 1991) ; 5. STCRZ core (this paper).

M. Caballero et al. / Palaeogeography, Palaeoclimatology, Palaeoecology 186 (2002) 217^235 219



2.3. Archaeology

Humans were present in the Upper Lerma (or

Toluca) Basin from the early Formative (1500^

1000 BC), and increased in numbers during themiddle Formative (800^500 BC) (Sugiura, 1992,

2000). Occupation then decreased until the early

Classic (AD 200^450) when re-population started.

Population increased during the subsequent peri-

ods, reaching a peak during the Postclassic (AD

1000^1520). Late Classic (AD 550^650) to Epi-

classic (AD 700^900) sites, however, are numer-

ous both around the lakes and on man-made is-

lands (or mounds) of variable size within them.

During the Epiclassic period, mound construction

accelerated and lacustrine life reached its peak.All mounds were abandoned by the end of the

Epiclassic (c. AD 900).

Archaeological excavations (by Y. Sugiura) on

some of these mounds have uncovered evidence

that sheds light on the end of the Classic period

in this area. Most of the smaller arti¢cial islands

were probably domestic habitation units, while

some larger ones were apparently used for public

gatherings. Although island use was intensive anduninterrupted, inhabitants had to deal with the

problems of sinking and the e¡ects of water level

£uctuations by raising the £oor levels using rocks

and sediments. Natural water level variations un-

doubtedly a¡ected local living conditions.

2.4. Previous palaeoenvironmental work

Chemistry and diatom analysis of two pit se-

quences (Pit 1 and Pit 2, Figs. 1 and 2) were

studied by Metcalfe et al. (1991). Sugiura et al.(1994) also studied a series of pits for which pol-

len, diatom and soil analyses are in progress. Most

of these sequences are only c. 2 m deep, many with

the 11 600 yr BP UTP at their base. They largely

Fig. 2. Stratigraphic columns of palaeolimnological sequences from Lake Chignahuapan, Upper Lerma Basin, Central Mexico.

Sequences are presented from west to east. Tephra abbreviations as follows: TCT= Tres Cruces Tephra, UTP = Upper Toluca

Pumice, LAT = Lower Almoloya Tephra, UAT = Upper Almoloya Tephra.




cover the Holocene, and frequently have a discon-

tinuous diatom record. Caballero et al. (2001) re-

port diatom analysis from a core sequence (Isla II)covering the last c. 14 000 yr BP (Figs. 1 and 2),

but chronostratigraphic resolution of this core,

particularly for the Holocene, is relatively poor,

as the sequence is dominated by volcanic material.

3. Methods

Here we present results from two sequences:

STCRZ (9.54 m) and Almoloya del R|¤o (5.12 m)

(Figs. 1 and 2). The STCRZ sequence is locatednear an archaeological research site (Santa Cruz

Atizapa¤n) where a late Classic to Epiclassic arti-

¢cial island was excavated by Y. Sugiura (Fig. 1).

The STCRZ sequence shows a clear horizon of

Epiclassic pottery shards (Fig. 2). It consists of

a 2-m trench and a 7.54-m core drilled at the

base of the trench. The Almoloya del R|¤o core

was taken about 4 km west of the STCRZ site

(Fig. 1). All cores were drilled with an Eijkelkamp

percussion-type soil sampler.

The STCRZ core was sampled for diatoms and

pollen at 10-cm intervals. The STCRZ trench was

sampled every 5 cm, to provide high resolutiondata for the late Holocene. The uppermost 0.5 m

were not studied because the soil was reworked by

agricultural activities. Six samples for radiocarbon

dating were collected at selected horizons (Table

1, Fig. 2).

The Almoloya del R|¤o core was sampled pri-

marily for tephra studies (Newton and Metcalfe,

1999) and diatom analyses. Three radiocarbon

dates were published in Newton and Metcalfe

(1999) and one new 14C date is presented here

(Table 2, Fig. 2). Samples for diatom analysiswere taken only from non-tephra units. Sampling

resolution was every 2 cm in the bottom 50 cm of

the sequence and every 5 cm elsewhere, except the

top peat unit.

Diatom samples were prepared by treating

them successively with 10% HCl and concentrated

H2O2 to eliminate carbonate and organic matter

respectively. Slides for microscope analysis were

prepared with 200 Wl of the ¢nal solution, using

Naphrax as a mounting medium. Microscope

Table 1

AMS 14C dates for the STCRZ sequence, Upper Lerma Basin, Mexico

Laboratory code Radiocarbon date N13CPDB Calibrated datea Depth

(yr BP) (x) (calendar years) (m)

A-9701 1200+195/3

190 3

26.8 AD 655^1020 (1295^930 BP) 0.90^0.95NSRL-12051 4560 45 328.1 3495^3098 BC (5314^5073 BP) 1.95^2.00

A-9702 8630 80 325.9 7750^7580 BC (9700^9530 BP) 3.76^3.78

A-9703 9950 180 328.8 9750^9240 BC (11 695^11 185 BP) 4.42^4.45

NSRL-10408 11 850 110 327.3 12 1 10^11 8 50 BC (14 0 60^13 800 BP) 5.49^5.51

NSRL-10934 21 500 160 326.1 ^ 8.87^8.89

A = Geochronology Laboratory, University of Arizona; NSRL= INSTAAR ^ Laboratory for AMS Radiocarbon Preparation

and Research, University of Colorado.a Radiocarbon Calibration Program 2000, rev. 4.3 (Stuiver et al., 1998).

Table 2

AMS 14C dates for the Almoloya del R|¤o sequence, Upper Lerma Basin, Mexico

Laboratory code Radiocarbon date N13CPDB Calibrated datea Depth

(yr BP) (x) (calendar years) (m)

Beta-102337 6180 60 310.9 5258^5005 BC (7208^6955 BP) 2.52^2.54

Beta-94127 9180 50 325.9 8516^8289 BC (10 465^10 238 BP) 3.38^3.40

Beta-94128 12 060 60 327.8 13 0 72^11 8 94 BC (15 0 22^13 844 BP) 4.58^4.60

Beta-94129 12 400 60 327.4 13 4 08^12 2 28 BC (15 3 58^14 178 BP) 4.80^4.82

Beta= Beta Analytic Inc., Florida.a Radiocarbon Calibration Program 2000, rev. 4.3 (Stuiver et al., 1998).




analyses were carried out at 1000U magni¢cation

with Olympus BH2 and BX50 microscopes. Mini-

mum counts of 400 valves were done, and species

identi¢cations were made with reference to stan-

dard £oras (Hustedt, 1930, 1959, 1960^66; Gasse,1986; Kramer and Lange-Bertalot, 1986, 1988,

1991). Pollen analysis for the STCRZ sequence

is currently under way.

Samples for loss on ignition (LOI), i.e. organic

matter content, were collected every 5 cm

throughout the sequence. Samples were ignited

at 550‡C for 2 h (Bengtsson and Enell, 1986).

Samples for rock-magnetism analyses (see Table

3) were collected in 8-cm3 plastic cubes at inter-

vals of 5 cm. Mass speci¢c low ¢eld magnetic sus-ceptibility ( M ) was measured in a Bartington sys-

tem. Anhysteretic remanent magnetisations

(ARM) were measured in half of the samples, at

10-cm intervals. ARMs were imparted in a 50-WT

bias ¢eld, superimposed on a peak alternating

Table 3

Magnetic parameters and their environmental signi¢cance in lacustrine sediments

Parameter Interpretation

Magnetic susceptibility M . Ratio of the induced magnetisation

to the strength of a weak ¢eld applied. Commonly expressedin mass speci¢c units (m3/kg).

Is a measure of the concentration of magnetic minerals. If

concentration of ferrimagnetic minerals (e.g. magnetite) is low,it re£ects the presence of antiferrimagnetic (haematite,

goethite), paramagnetic (Fe-Mg silicates) and diamagnetic

(silica, carbonates) minerals. High M can be indicative of

higher erosion, airfall products of basaltic-andesitic volcanism.

Anhysteretic remanent magnetisation (ARM). Magnetisation

acquired in a biasing DC ¢eld (50 WT) within an alternating

decreasing ¢eld (100 mT).

Depends on the concentration of ferrimagnetic material, but is

highly sensitive to smaller magnetite grain sizes ( 610 Wm).

Isothermal remanent magnetisation (IRM). Magnetic rema-

nence acquired in a strong DC ¢eld ( s 5 mT). The remanence

acquired at the highest ¢eld applied (usually 1 T) is expressed

as saturation remanent magnetisation (SIRM or M R).

Depends on the concentration of magnetic material. Only

responds to ferrimagnetic grains and is not a¡ected by

paramagnetic content (such as pyroxenes, amphiboles, biotites,

etc.).

Saturation magnetisation (M S). The highest magnetisation

achievable in the presence of the saturating ¢eld.

Depends on the concentration of magnetic material.

Coercive force (B O)C, and coercivity of remanence (B O)CR.

The inverse ¢eld necessary to rotate to zero either a saturation

magnetisation, in the presence of the applied ¢eld [(B O)C], or a

saturation remanence magnetisation, in the absence of the

applied ¢eld [(B O)CR].

The ratio (B O)CR/(B O)C is diagnostic of grain size in Ti-

magnetites. Plotted against the ratio M R/M S, helps to

discriminate between the ¢ne single domain magnetic grain

size (SD), the medium pseudo-single domain size (PSD), and

the coarse multidomain grain size (MD) (Day et al., 1977).

ARM/SIRM ratio. Is a guide of grain size variations. Increases with the presence

of ¢ne, SD grains.

S 300 ratio. De¢ned as S 300 = (IRM300/SIRM)U

100, where IRMis an inverse ¢eld of 0.3 T, imparted on a sample previously

saturated.

Is an indicator of changes in magnetic mineralogy. Lowervalues indicate the presence of a high coercivity phase, such as

haematite or goethite, while higher values indicate low

coercivity values, characteristic of ferrimagnetic mineralogy.

Curie temperatures. The temperature at which ferrimagnetic

minerals become paramagnetic, and their remanence and

susceptibility drops.

Is diagnostic for the composition of minerals: 580‡C for

magnetite, 680‡C for haematite. In Ti-magnetites, it decreases

as Ti content increases.

Low temperature transitions. The changes in remanence during

heating (or cooling) between 10 and 300 K, due to phase

transitions.

Useful for the identi¢cation of mineralogy. Magnetite shows a

marked drop at c. 120 K.




¢eld of 100 mT in a Schonstedt GSD-1 demagnet-

iser, and remanences measured in a 2G three-axis

cryogenic magnetometer. Isothermal remanent

magnetisation (M R) in all samples was imparted

with a pulse magnetiser at a forward ¢eld of 1 T

and at a backward ¢eld of 300 mT, and measured

in a Molspin £uxgate magnetometer. S 300 ratios

were calculated (Table 3). Saturation magnetisa-tion (M S) and coercivity parameters, B CR, B C,

were obtained from hysteresis loops measured

with a Princeton Measurement Corporation Mi-

cro-Vibrating Sample Magnetometer (WVSM) at

room temperature in selected samples. Magnetic

mineralogy was determined from the Curie tem-

peratures, estimated from the variations of M be-

tween 20 and 700‡C, measured in a Kappa bridge

in £owing helium gas for an inert atmosphere.

4. Results

4.1. Stratigraphy

4.1.1. STCRZ sequence

The base of the sequence (9.54^9.43 m) is a

pumice-rich lapilli layer of unknown source, older

than 21500 yr BP (Fig. 2, Table 1). The layerconsists of subangular to subrounded lapilli pum-

ice fragments, embedded in a ¢ner matrix, sug-

gesting that the pumice layer represents a re-

worked horizon and not a primary tephra fall.

Lacustrine silt is present between 9.43 and

5.47 m (Fig. 2). The 8.87^8.89 m level was dated

at 21500160 yr BP (Fig. 2, Table 1) giving a

minimum age for this record. The UTP is well

represented between 5.47 and 4.46 m, overlying

Fig. 3. Time scale, lithology, LOI and several rock-magnetic parameters for the Santa Cruz core. Magnetic susceptibility M , an in-

dicator for the concentration of magnetic minerals in sediments, is plotted along with frequency dependence M fd% values for se-lected samples (scale on the bottom). High values of M fd% indicate the presence of ultra¢ne SP grains. Higher ARM/SIRM ratios

indicate the presence of ¢ne SD grains. Lower S ratio values are indicative of a high coercivity magnetic mineralogy, commonly

haematite or goethite.




an organic-rich level (Fig. 3) dated at 11 850 110

yr BP (Fig. 2, Table 1). Lacustrine silt is present

between 4.46 and 3.75 m. The TCT is present at

3.75^3.52 m, above sediments dated to 8630

8 0 y r B P (Fig. 2, Table 1). Lacustrine silt ispresent between 3.52 and the top of the sequence.

Two thin (6 2 cm), dark grey tephras composed

of ¢ne sand scoria occur at 1.70 and 1.40 m. Or-

ganic-rich sediments (Fig. 3) underlying the 1.70 m

tephra were dated at 4560 45 yr BP (Fig. 2,

Table 1). The 1.69^1.62 m and 1.33^0.98 m levels

contain horizontal bands of beige and dark brown

silt. The archaeological horizon, containing pot-

tery shards, occurs between 0.98 and 0.77 m.

Charcoal fragments, also abundant at this level,

dated to 1200+195/3

190 yr BP (AD 655^1020,calibrated age, Stuiver et al., 1998, Table 1,

Fig. 2). The STCRZ sequence apparently did

not reach the LTP (c. 24 000 yr BP) and except

for the UTP and the TCT, no other tephras were

identi¢ed. The two thin tephras near the top of

the sequence have not been reported previously

and provide evidence of the complex regional

tephra stratigraphy.

4.1.2. Almoloya del R|¤ o core

The stratigraphy of this core is described inNewton and Metcalfe (1999) and is presented

here in Fig. 2. In this sequence the UTP and the

TCT are clearly identi¢ed and the Lower and

Upper Almoloya tephras were described from

this core. Chronology is based on four radiocar-

bon dates (Fig. 2, Table 2), spanning c. 12 500 yr

BP.

4.2. Rock magnetism


Rock-magnetic parameters in lake sedimentsare useful guides to environmental changes (e.g.

Thouveny et al., 1994). Processes such as catch-

ment erosion, soil development, volcanic activity

and lake chemistry can yield di¡erent assemblages

of magnetic properties. Rock-magnetic techniques

discriminate the magnetic content in three main

variables: concentration, magnetic grain size andmineralogy. A summary of rock-magnetic param-

eters and their environmental signi¢cance in sedi-

ments is presented in Table 3.

Curie temperatures between 490‡C and 572‡C

show (Table 3) that the main magnetic phases are

titanomagnetites with Ti content from TM20 to

almost pure magnetite, and in some samples an

additional phase with Curie temperatures between

350‡C and 400‡C (Table 3). Low temperature de-

magnetisation experiments, where a well-marked

drop of remanence below 120 K is present in mostsamples, con¢rmed the presence of magnetite.

Presence of a magnetically hard phase, goethite

or haematite, is indicated by relatively low S ra-

tios (Table 3).

Rock-magnetic parameters show a mixture of

characteristics (Fig. 3). The magnetic concentra-

tion is very low for most of the record, where

values are around 0.1 Wm3/kg and a diamagnetic

component is present, except in the tephra at

1.40 m and in two zones of lake sediments: be-

tween 8.20 and 7.00 m (c. 19 000^16 000 yr BP, M s 0.3 Wm3/kg) and above 1.35 m (c. 1500 yr BP,

M s 0.5 Wm3/kg). However, di¡erent magnetic

mineral and grain size assemblages are present

in each of these two zones. The deepest has the

coarsest magnetic grains and high coercivity min-

eral content, presumably haematite/goethite. In

contrast, the topmost record has low coercivity

minerals of relatively smaller grain sizes. Sedi-

ments between the two tephras at 1.70 and

1.40 m (c. 3500^2500 yr BP) have the same char-

acteristics as the 8.20^7.00-m zone.

Although the very low magnetic concentrationzones below 8.20 m (c. 19 000 yr BP) and between

7.00 and 5.78 m (c. 16 000^12 000 yr BP) possess

Fig. 4. Diatom stratigraphy from STCRZ core, Upper Lerma Basin, Central Mexico, selected taxa (as percentages of total

counts). (a) Aerophilous/benthic and epiphytic varieties with chrysophyte statocyst (as total counts) and sponge spicules (as total

counts). (b) Tychoplanktonic varieties with sums of aerophilous and alkaliphilous taxa and total diatom abundance as valves per

gramme of dry sediment (v/gds). Shaded areas correspond to tephra layers, tephra abbreviations as follows: UTP = Upper Toluca

Pumice, TCT = Tres Cruces Tephra.




relatively coarse and low coercivity minerals, the

zones at 5.78^5.47 m (c. 12 500^11 850 yr BP) and

4.30^2.80 m (c. 9500^6500 yr BP) have coarse

grains, but the highest coercivity, indicating sig-

ni¢cant haematite/goethite content. In contrast,the low concentration zone between 2.8 and

1.6 m (c. 6500^3300 yr BP) has smaller magnetic

grain size, magnetically soft, low coercivity min-

erals.

The UTP, and the sediments above up to 4.30 m

(c. 9500 yr BP), are characterised by moderate

magnetic concentrations with diamagnetic content

and low coercivity coarse magnetic grains. The

TCT is characterised by high magnetic concentra-

tion, higher coercivity content and coarse (MD)

grains.

4.2.2. Almoloya del R|¤ o

Only magnetic susceptibility was measured for

this sequence. This shows high values ( s 1 Wm3/

kg) in the TCT, the UAT and parts of the UTP.

There is a lower peak (c. 0.5 Wm3/kg) in the LAT.

A peak of 0.8 Wm3/kg was recorded in a thin unit

at 2.50 m. It was thought that this might corre-

spond to the Yellow Ash identi¢ed by Metcalfe et

al. (1991) and dated to 4750 60 yr BP. However,

no volcanic glass was found at this level in theAlmoloya sequence, so its identi¢cation remains

uncertain.

4.3. Diatoms


In the STCRZ sequence diatoms were preserved

throughout the record, even though severely bro-

ken at several levels (Fig. 4). Diatom associations

can be grouped into three main assemblages

(Table 4), similar to those found in the Isla II

core (Caballero et al., 2001) :(1) Shallow alkaline marsh: Indicates the pres-

ence of shallow, slightly alkaline waters with some

aquatic vegetation. The abundance of Nitzschia

amphibia suggests a relatively high nutrient level.

An Aulacoseira species (A. sp1) is present, which

is characterised by dome-shaped valves (c. 50%)

that are taken to be resting spores (Edlund et al.,

1996; Caballero et al., 1999). Chrysophyte stato-

cysts are also present. The abundance of benthic

and aerophilous species, together with the spore-

bearing Aulacoseira, indicates that at least season-

ally the marsh was nearly dry.

(2) Freshwater pond : This assemblage repre-

sents an intermediate water level stage betweenthe marsh and lake environments. It is character-

ised by circumneutral (pHW7) waters, and abun-

dant aquatic and subaquatic vegetation.

(3) Freshwater lake: Represents the deepest

water level in the STCRZ record, but the absence

of true planktonic species in this sequence indi-

cates that the lake was not particularly deep,

maybe only a few metres. It was characterised

by circumneutral waters (pHW7) with the pres-

ence of some aquatic vegetation.

Cluster analysis (CONISS, Grimm, 1991^1992)divides the record into two main zones, below and

above the UTP level (5.47^4.46 m), which corre-

sponds roughly to the late Pleistocene/Holocene

transition (Fig. 4). The lower half of the core

shows higher total diatom abundance and is dom-

inated by the freshwater lake assemblage. Never-

theless, three sub-zones can be discriminated

in this lower part of the core suggesting slight

lake level £uctuations (Fig. 4). Below 7.00 m

(c. 16 000 yr BP) the freshwater lake assemblage

is associated with small numbers of Cocconeis pla-centula, indicating the in£uence of littoral envi-

ronments. A peak of an Aulacoseira species (Au-

lacoseira. sp2, a form close to Aulacoseira distans

or Aulacoseira alpigena) is present between 8.81

and 8.57 m (c. 20 000 yr BP), which might indicate

a short episode of slightly more dilute, deeper

waters. Above 7.00 m (c. 16 000 yr BP) the fresh-

water lake assemblage is overwhelmingly domi-

nated by Fragilaria pinnata; also present are

very low, but constant, numbers of small Cyclo-

tella species (mainly Cyclotella pseudostelligera),

suggesting more open and slightly deeper waterconditions.

In the upper half of the core (6 11 600 yr BP),

diatom assemblages show lower total abundance

and higher species diversity. In the ¢rst sample

above the UTP, diatom abundance is low, domi-

nated by Fragilaria brevistriata, a species that

seems to be pioneering after the intense perturba-

tion of the system by the tephra input. This pat-

tern is repeated after the TCT, where F. brevi-




striata also reaches high values. Between the UTP

(c. 11 600 yr BP) and the TCT (c. 8500 yr BP) the

alkaline shallow marsh assemblage is present, but

in association with some Fragilaria spp. Above

the TCT and up to 3.00 m (c. 7000 yr BP), thefreshwater lake assemblage is present, but in asso-

ciation with the aerophilous Navicula minima.

These assemblages indicate that between c.

11 600 and c. 7000 yr BP Chignahuapan had var-

iable water levels, £uctuating between marsh and

lake conditions (Fig. 4). Between 3.00 m (c. 7000

yr BP) and 1.40 m (c. 2500 yr BP) the freshwater

pond assemblage is dominant, but the interval

between 2.20 and 1.80 m (c. 5000^4000 yr BP)

shows the presence of the shallow, alkaline marsh

assemblage (Fig. 4). Between 1.40 m (c. 2500 yrBP) and 0.65 m (c. 800 yr BP) the shallow, alka-

line marsh assemblage is dominant. The base of

this interval (1.35^1.25 m, c. 2500^2000 yr BP),

however, shows the presence of small Cyclotella

spp. (mainly Cyclotella pseudostelligera) which

suggests the in£uence of slightly deeper water.

Table 4

Diatom assemblages present in the STCRZ and Almoloya del R|¤o cores, Upper Lerma Basin, Central Mexico

Assemblage Main diatoms Habitat pH

Shallow alkaline marsh (STCRZ) Nitzschia amphibia E Ak

Rhopalodia gibba B Ak

Rhopalodia gibberula B Ak

Cocconeis placentula E C/Ak

Epithemia turgida E C/Ak

Hantzschia amphioxys A C/Ak

Eunotia formica B C

Eunotia naegelii A/B C/Ac

Pinnularia spp. A/B C/Ac

Aulacoseira sp1 B ? ?

Shallow freshwater marsh (Almoloya) Eunotia formica B C

Eunotia glacialis A/B C/Ac

Eunotia soleirolii E C

Pinnularia maior A/B C/Ac

Hantzschia amphioxys A C/Ak

Navicula (Luticola) mutica A/B C/Ak

Nitzschia amphibia E Ak

Aulacoseira italica T C

Freshwater pond (STCRZ) Cocconeis placentula E C/Ak

Cymbella aspera E C/Ak

Cymbella cistula E C/Ak

Rhoicosphenia curvata E C

Fragilaria (Punctastriata) pinnata T C

Fragilaria pinnata var. lancettula T C

Freshwater lake (STCRZ) Fragilaria (Punctastriata) pinnata T C

Fragilaria pinnata var. lancettula T C

Fragilaria (Pseudostaurosira) brevistriata T CFragilaria (Staurosira) construens f. venter T C

Cyclotella pseudostelligera T/P C

Freshwater lake (Almoloya) Fragilaria pinnata var. lancettula T C

Fragilaria (Pseudostaurosira) brevistriata T C

Stephanodiscus neoastrea P C

Aulacoseira ambigua P C

Aulacoseira granulata var. angustissima P C

Fragilaria (Staurosira) construens f. venter T C

Species pH and habitat preferences are included.

Habitat: A = aerophilous, B = benthic, E = epiphytic, T = tychoplanktonic, P = planktonic.

pH: Ac = acidophilous, C= circumneutral, Ak = alkaliphilous.




Fig. 5. Diatom stratigraphy from Almoloya del R|¤o core, Upper Lerma Basin, Central Mexico, selected taxa (as percentages of

spond to tephra layers, tephra abbreviations as follows: LAT= Lower Almoloya Tephra, UTP = Upper Toluca Pumice, TCT =Almoloya Tephra.



The diatom sequence indicates, therefore, that the

lake became generally more alkaline after c. 2500

yr BP, with a brief episode of slightly deeper

waters (c. 2500^2000 yr BP) before generally shal-

low water environments were established (Fig. 4).The most recent samples studied (0.60^0.50 m)

show a mixed diatom association with species of

the alkaline, shallow pond and the freshwater lake

assemblages.

4.3.2. Almoloya del R|¤ o sequence

In this core diatom preservation was discontin-

uous (Fig. 5). Diatom associations can be

grouped into two assemblages (Table 4), similar

to those found in the STCRZ sequence: (1) Shal-

low freshwater marsh : It indicates the presence of rather shallow, circumneutral and relatively open

waters. It di¡ers from the shallow alkaline marsh

assemblage present in the STCRZ record by the

dominance of benthic and aerophilous varieties

and the lower abundance of alkaliphilous and ep-

iphytic taxa (Table 4). (2) Freshwater lake : It dif-

fers from the freshwater lake assemblage in the

STCRZ record by the presence of true planktonic

taxa (Table 4), indicating somewhat deeper and

more open environments.

The base of the Almoloya core (below theLAT, c. 12 400 yr BP) is dominated by the shal-

low, freshwater marsh assemblage. Between the

LAT and the UTP the only countable sample is

dominated by Fragilaria construens var. venter,

indicating a slight increase in lake level. The ¢rst

sample above the UTP is rich in Fragilaria brevis-

triata, a pattern also recorded in the STCRZ se-

quence. Between the UTP (c. 11 000 yr BP) and

the TCT (c. 8500 yr BP) the shallow, freshwater

marsh assemblage is present. This assemblage

continues in the ¢rst two samples above the

TCT and UAT, indicating that shallow waterconditions existed at Chignahuapan before, dur-

ing and after the fallout of these tephras. The

freshwater lake assemblage then dominates. Be-

tween 2.72 and 2.57 m the large centric Stephano-

discus neoastrea is abundant, dominating the as-

semblage (47%) at 2.72 m (Fig. 5). This indicates

an increase in water level and the establishment of

relatively deep water conditions. The part of the

core between 2.70 and 2.25 m is also marked by

the highest diatom abundances. Centric forms

then decrease in abundance, suggesting a gradual

decrease in lake level. In the upper part of the

core (6 c. 6000 yr BP) only two samples yielded

diatom counts. These both show higher percen-tages of Cocconeis placentula var. lineata, which

dominates in the top sample (0.51 m).

5. Discussion

The STCRZ and the Almoloya del R|¤o records

show a higher accumulation of lacustrine sedi-

ments for the Holocene than the previously

studied sequences (Pits 1 and 2, Metcalfe et al.,

1991, and Isla II core, Caballero et al., 2001)(Fig. 2). This re£ects their position in a more cen-

tral part of the basin, with relatively deeper water.

The STCRZ sequence is also the longest studied

for the area, covering the period from the last full

glacial to the present, with continuous diatom

preservation. Given the presence of late Classic

to Epiclassic pottery dated to 1200+195/3190 yr

BP (calibrated age AD 655^1020, Stuiver et al.,

1998, Table 1) the STCRZ core enables palaeoen-

vironmental correlation with the archaeological

record. Fig. 6 summarises the lake level curvefrom Chignahuapan and compares it with records

from other sites from Central Mexico.

The STCRZ record indicates that the Late

Pleistocene (c. 22 000 to 11 600 yr BP) lacustrine

environment in Chignahuapan was dominated by

a freshwater lake that experienced some water

level variability (Fig. 6). Between c. 22 000 yr BP

and c. 19 000 yr BP (s 8.00 m) the magnetic rec-

ord, characterised by a low concentration of

coarse Ti-magnetite grains, indicates low sediment

input, while diatoms suggest the presence of a

freshwater lake, with a slightly deeper, more di-lute phase around c. 20000 yr BP. Between c.

19 000 and 16 000 yr BP (8.20^7.00 m) magnetic

data suggest an increase in the input of sediments,

clearly indicated by a rise in concentration ( M ) of

coarse magnetic grains. The magnetic mineralogy

inferred for this interval is a variable mixture of

Ti-magnetite and a high coercivity phase, presum-

ably haematite. The content of haematite is par-

ticularly high around 19 000 yr BP (8-m level)




Fig. 6. Lake level reconstructions from sites in Central Mexico: Pa¤tzcuaro (Bradbury, 1997, 2000), Zacapu (Metcalfe, 1995 andYuriria (Davies, 1995; Metcalfe and Hales, 1994 and unpublished data), Chignahuapan (Upper Lerma Basin, this study); Ch

Texcoco (Bradbury, 1989; Lozano and Ortega, 1998), Tecocomulco (Caballero et al., 1999). UTP = Upper Toluca Tephra, TCT=



which suggests that at this time the sediments

were partly oxidised as the result of aerobic con-

ditions caused by lake level £uctuations. During

this interval diatoms re£ect a slight change in the

dominant Fragilaria species (peak in Fragilariaconstruens f. venter followed by dominance of

Fragilaria pinnata v. lancettula, Fig. 4), but do

not suggest extended dry periods. The STCRZ

magnetic and diatom data (Figs. 3 and 4) together

indicate that between c. 19 000 and 16 000 yr BP

the shallow freshwater lake at Chignahuapan

showed a slight decrease in water depth, with var-

iable levels, reaching particularly low levels (sea-

sonally ?) by c. 19 000 yr BP. During this interval,

which corresponds to the Last Glacial Maximum,

sediment input to the lake was relatively high,probably as a result of more open vegetation con-

ditions and lowering of the tree line.

Between c. 16 000 and 11 000 yr BP (7^5.5 m)

the STCRZ magnetic data show a return to a low

content of coarse Ti-magnetite grains, indicating

low sediment input. Diatoms show a slight in-

crease in water level, suggesting a more stable,

slightly deeper, open lake (Fig. 4). In the Isla II

core, taken at the western shores of lake Chigna-

huapan (Fig. 1), comparable Fragilaria-rich assem-

blages are also present between c. 14 000 and11 000 yr BP suggesting that a freshwater lake ex-

tended over the whole area. At the top of the Late

Pleistocene sequence in STCRZ, below the UTP

tephra, there is a shift to very low magnetic con-

centrations where diamagnetic minerals dominate

M and haematite content rises, possibly a conse-

quence of drier conditions. This magnetic signal

in the STCRZ record correlates with particularly

high LOI values and, in the Almoloya del R|¤o

core, with the shallow water assemblage at the

base of the sequence (c. 12 400 yr BP). These re-

sults all indicate a short episode of generally shal-lower, marshy conditions, with lower pH in some

areas, by c. 12 400 yr BP. The Almoloya del R|¤o

sequence, however, suggests a recovery of lake lev-

el just prior to the fallout of the UTP (Fig. 5).

Holocene conditions in Chignahuapan appear

to have been generally shallower and more vari-

able, with episodes of very shallow and slightly

alkaline waters. The ¢rst of these events dates to

the early Holocene, between c. 11 000 to c. 7000

yr BP. In STCRZ, the magnetic record shows a

low concentration of ferrimagnetics (Ti-magne-

tite) and high haematite content, while the dia-

toms indicate the presence of shallow, variable

water levels. In the Almoloya del R|¤o and IslaII (Caballero et al., 2001) cores, diatoms also in-

dicate shallow water conditions during this inter-

val (Fig. 5). In Pit 2 (Metcalfe et al., 1991) shal-

low marsh conditions are followed by a higher

water table after c. 8000 yr BP. In this sequence,

as well as in the Isla II core, there is evidence that

the TCT was exposed to subaereal conditions, in-

dicating very shallow or dry environments at the

western shore of the lake for some time after the

tephra fallout (c. 8500 yr BP).

By c. 7000 yr BP lake levels at Chignahuapanwere higher (probably the highest in the Holo-

cene), with the presence of a freshwater lake

that had areas of deeper, more open conditions.

In Pit 1 a mixed signal is recorded between c. 8000

and c. 6000 yr BP, with layers dominated by shal-

low water species and one sample dominated by

centric forms (Aulacoseira granulata, Cyclotella

meneghiniana, Stephanodiscus subtilis, Stephano-

discus niagarae) indicating relatively deep water

conditions. The presence of large centric species

(Stephanodiscus) is consistent with the £ora in theAlmoloya core above the UAT (Fig. 5). In the

STCRZ record there are no planktonic species

but diatoms suggest a slight increase in lake level

by c. 7000 yr BP (Fig. 6). According to the Al-

moloya core, lake levels started to fall by c. 6200

yr BP.

A striking change in magnetic characteristics is

recorded in STCRZ between c. 6400 and 3300 yr

BP (2.80 and 1.60 m); as the concentration re-

mains very low, the magnetic grain size results

indicate an assemblage of smaller grains (PSD)

and (Ti) magnetites with low to nil haematite con-tent. This change in grain size does not corre-

spond to an increase in susceptibility, which could

re£ect dilution by high organic content. During

part of this interval (2.80^2.20 m, c. 6400^5000

yr BP) the STCRZ diatom record indicates the

presence of a freshwater pond, rich in aquatic

vegetation. Taking the magnetic data into ac-

count, this pond must have had relatively stable

water levels. At Pit 2 diatoms suggest the presence




of a shallow lake, however, in the Almoloya del

R|¤o core and in Pit 1 diatoms are not preserved

(Fig. 5). Towards the end of this interval the

STCRZ diatom record (2.20^1.80 m, c. 5000^

4000 yr BP) indicates a second event of very shal-low water environments dated at 4560 45 yr BP

(3495^3098 BC, calibrated ages, Stuiver et al.,

1998, Table 1). This shallow water episode is as-

sociated with the highest LOI values of the record

(Fig. 3), supporting the interpretation of a marsh

environment. Shallow conditions are also re-

corded in Pit 2, where a hiatus in sedimentation

is inferred after c. 4600 yr BP (Metcalfe et al.,

1991). The data indicate that by 4600/4500 yr

BP (3495^3098 BC, calibrated ages) very shallow

conditions prevailed in lake Chignahuapan.In the STCRZ sequence, this very shallow water

episode is followed by a recovery to freshwater

pond conditions that lasted until c. 2500 yr BP

(1.35 m). The magnetic data, however, show (as-

sociated with a sharp decrease in LOI) an increase

in the magnetic grain size and an increase in hae-

matite, which suggests variability in lake levels.

Diatom data from Pits 1 and 2 also show a recov-

ery in lake level after the c. 4600 yr BP hiatus. The

data indicate that after the c. 4600/4500 shallow

episode and until c. 2500 yr BP (c. 800 BC, cali-brated ages), a freshwater pond of £uctuating lake

levels, rich in aquatic vegetation, was present at

Chignahuapan (Fig. 6). If there were no sedimen-

tation hiatus in the STCRZ sequence, then the end

of the shallow episode was around c. 4000/3700 yr

BP (c. 2700 BC, calibrated age).

After c. 2500 yr BP the diatom record at

STCRZ indicates generally shallower, more alka-

line environments (Fig. 6). A brief period of

slightly deeper water levels between c. 2500 and

2000 yr BP (1.40^1.24 m, c. 800^200 BC, cali-

brated ages) is followed by the establishment of very shallow conditions between c. 2000 and 800

yr BP (1.25^0.65 m, c. 200 BC^AD 1500 cali-

brated ages). This upper section of the STCRZ

record (above 1.4 m, 62500 yr BP) is character-

ised by a sharp increase in M , where coarse, low

coercivity grains dominate the ferrimagnetic frac-

tion and there is no evidence of high ultra¢ne SP

content. This remarkable rise in M starts where

pottery shards appear in the record, and contin-

ues to the top of the sequence, beyond the stra-

tum with artefacts. Human activities such as slash

and burn agriculture, as well as pottery ¢ring and

meal cooking can form Fe-bearing minerals, in-

cluding magnetite, and can also lead to increasedsurface erosion, particularly by deforestation.

Correlation with Pit 1 is di⁄cult as there is no

clear signal in the diatom record, while relating

these events to Pit 2 is problematic because the

sequence lacks dates for the late Holocene. In Pit

2, however, a relatively deep water phase (Aula-

coseira ambigua, Cyclotella meneghiniana, Nitz-

schia amphibia and Cocconeis placentula) is re-

ported by Metcalfe et al. (1991), and thought to

date to c. 1600 yr BP. It is possible that the rel-

atively deeper episode recorded at STCRZ corre-lates with this deeper water phase at Pit 2. There

is also an indication of deeper water in the Almo-

loya core at 1.36 m, but unfortunately this is not

dated. The later reduction in lake levels is clearly

recorded in both Pits as a ‘root mat’ dated at

1380 50 yr BP (AD 617^688, calibrated age,

Stuiver et al., 1998). As a whole, the data suggest

that after a short, deeper water phase (c. 2000(?)

yr BP, c. 200 BC, calibrated age), generally shal-

low environments were established at Chignahua-

pan with particularly shallow conditions presentafter c. 1400 yr BP (c. AD 550, calibrated age).

Very shallow water conditions at c. AD 550 cor-

relate with development of man-made islands dur-

ing the late Classic to Epiclassic (AD 550^900).

In the STCRZ sequence, sediments above the

archaeological horizon contain a mixed diatom

assemblage that is di⁄cult to interpret. In Pits 1

and 2 the sediments above the ‘root mat’ indicate

a clear increase in lake level. In Pit 1 these sedi-

ments were dated at 870 50 (AD 1059^1221,

calibrated ages, Stuiver et al., 1998).

6. Conclusions

The STCRZ sequence represents a continuous

record of lake level changes for the past s 22 000

yr in the Upper Lerma Basin. This sequence al-

lows good correlation with the archaeological rec-

ord of the area, particularly during the late Clas-

sic to Epiclassic. The integrated data from the




Upper Lerma Basin give a valuable lake level

curve (Fig. 6) for regional palaeoclimatic studies

as many sequences from other basins in Central

Mexico show shorter or discontinuous records

(Metcalfe et al., 2000).Late Pleistocene environments in Chignahua-

pan were characterised by the presence of a fresh-

water lake that showed some stage variability

(Fig. 6). The lake was slightly deeper by c. 20 000

yr BP, followed by lower levels at c. 19 000 yr BP.

A general increase in the input of sediments to the

lake is recorded between c. 19 000 and 16 000 yr

BP, this was probably related to more open veg-

etation conditions and a lower tree line. This in-

terval correlates with the Last Glacial Maximum

in the Northern Hemisphere. For the Late Glacial(c. 16 000^11 000 yr BP), lower sediment input

and moderately higher lake levels were inferred.

There was a short episode of shallow lake levels

around 12400 yr BP, and a later rise in water

level. By c. 11 000 yr BP the Upper Lerma record

is dominated by the presence of the UTP.

In Central Mexico there are few records cover-

ing this period; those available are summarised in

Fig. 6. In lakes Texcoco and Tecocomulco, in the

neighbouring Basin of Mexico (Fig. 1), late Pleis-

tocene resolution is poor, with a hiatus that ex-tends from c. 15 000/14 000 yr BP into the mid to

early Holocene (Bradbury, 1989; Lozano and Or-

tega, 1998; Caballero et al., 1999). Lake Chalco,

also in the Basin of Mexico (Figs. 1 and 6), how-

ever, displays similarly shallow levels during the

Last Glacial Maximum (c. 18 000 yr BP) with a

moderate increase in water levels during the Late

Glacial (Caballero and Ortega, 1998). A sequence

from La Piscina de Yuriria, in southern Guana-

juato (Fig. 1), also shows dry conditions around

the Last Glacial Maximum, with short-lived epi-

sodes of wetter conditions in the Late Glacial(Fig. 6) (Davies, 1995 ; Metcalfe, unpublished

data). At Lake Pa¤tzcuaro (Figs. 1 and 6) relatively

constant high lake stands are present throughout

the period around the Last Glacial Maximum

(c. 37 000 to 12 500 yr BP (uncalibrated), Brad-

bury, 1997, 2000). The Terminal Glacial

(c. 12 500 to 10 000 yr BP (uncalibrated ages)) is

marked by lower lake levels (Fig. 6). The results

from Chignahuapan therefore appear to be con-

sistent with the idea that enhanced glacial mois-

ture from displaced mid-latitude westerlies was

con¢ned to northern and western Mexico (Brad-

bury et al., 2001).

Holocene water levels in Lake Chignahuapanappear to have been generally shallower, possibly

as a consequence of the ¢lling of the basin after

the UTP fallout. Water level was more variable,

marked by three episodes of shallow marsh con-

ditions. The ¢rst dry episode dates to the early

Holocene (c. 11000 to 7000 yr BP), and the

Tres Cruces volcanic eruption occurred during

this interval. This shallow water episode is also

recorded at other sites in Central Mexico. In lakes

Tecocomulco and Texcoco it correlates with a

sedimentation hiatus (Fig. 6). In Chalco, shallow,alkaline, saline environments dominated between

c. 10 000 and c. 5000 yr BP (Fig. 6) and in lake

Zacapu (Figs. 1 and 6) a shallow marsh was

present prior to c. 8000/7000 yr BP (Metcalfe,

1995). In Pa¤tzcuaro and La Piscina de Yuriria,

however, relatively high lake levels are recorded

(Fig. 6). These trends suggest that a negative

water balance dominated over part of Central

Mexico during the early Holocene.

A relatively deep water period is inferred at

Chignahuapan between c. 7000 and 6400/6200 yrBP, followed by the establishment of a freshwater

pond, with stable water levels. Wetter conditions

are also recorded in La Piscina de Yuriria in the

early mid-Holocene (Fig. 6). After c. 5000 yr BP,

a second event of shallow conditions is recorded

(c. 4600/4500 yr BP) at Chignahuapan which can

be correlated with similar trends in the records

from La Piscina de Yuriria (Metcalfe and Hales,

1994), Zacapu (Metcalfe, 1995) and Pa¤tzcuaro

(Bradbury, 1997, 2000). It is clear that generally

lower lake levels and a negative water balance

were present in Central Mexico around 4500 yrBP.

The presence of a freshwater pond, rich in

aquatic vegetation, with variable water level, is

inferred for Chignahuapan between c. 4000(?)

and 2500 yr BP (c. 2700^800 BC). These condi-

tions seem to have prevailed at the beginning of

the archaeological record in the Upper Lerma Ba-

sin during the Early Formative (c. 1500^1000

BC). More alkaline, and nutrient-rich environ-




ments were established at Chignahuapan after

c. 2000 yr BP (200 BC), corresponding to the

third shallow marsh phase. Particularly shallow

conditions are recorded by c. 1400 yr BP (AD

550) and a clear correlation (stratigraphical andchronological) can be made between this dry

phase and the occupation of the late Classic

(AD 550^650) to Epiclassic (AD 700^900) arti¢-

cial islands characteristic of this area. Shallow

lake levels by the end of the Classic period are

clearly recorded from La Piscina de Yuriria (c.

1400^900 yr BP, Metcalfe and Hales, 1994),

Lake Zacapu (c. 1000 yr BP, Metcalfe, 1995)

and Lake Pa¤tzcuaro (c. 1200^800 yr BP, Bridg-

water et al., 1999; Bradbury, 2000). The record

from Chignahuapan suggests that the Classic wasrelatively dry, but that drying became particularly

intense by the end of this period (AD 550). Low

lake levels were, nevertheless, favourable for the

development of a lacustrine ‘lifestyle’ at Chigna-

huapan. Correlation with the available records

from Central Mexico indicates that this late Clas-

sic dry episode was widespread across the area. It

is probably the clearest climatic signal in the re-

gion. At Chignahuapan, a recovery in lake level is

recorded by c. 870 yr BP (AD 1059^1221). This

date correlates well with the end of the Epiclassic,and it is inferred that an increase in lake level

could have been a factor related with the aban-

donment of the man-made islands that occurred

at this time.

Acknowledgements

This research was supported by the National

University of Mexico (DGAPA, IN104797) and

CONACyT (G-28528-T). S.E.M. thanks the Lev-

erhulme Trust for ¢nancial support (F/158/AWand F/158/BL) and Dr Anthony Newton for the

susceptibility measurements on the Almoloya

core. Three radiocarbon dates (NSRL codes)

were funded by the National Science Foundation

(NSF) Grant ATM-9809285 to the University of

Colorado INSTAAR ^ Laboratory for AMS Ra-

diocarbon Preparation and Research. Dr Cecilia

Caballero (Institute of Geophysics, UNAM) per-

formed the rock-magnetism measurements sup-

ported by the Visitors Fellowship Program of

the Institute for Rock Magnetism (IRM), Univer-

sity of Minnesota. The IRM is funded by the

Earth Sciences Division of the NSF and the

W.M. Keck Foundation. A. Soler, T. Hernandezand S. Sosa provided technical support. We thank

Dr. M. Brenner and an anonymous reviewer for

their thoughtful comments on the original manu-

script.

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