Palaeogeography, Palaeoclimatology, Palaeoecology Volume 163 Issue 1-2 2000 [Doi...

Palaeogeography, Palaeoclimatology, Palaeoecology 163 (2000) 6995www.elsevier.nl/locate/palaeo

Limnologic history of Lago de Patzcuaro, Michoacan, Mexicofor the past 48,000 years: impacts of climate and man

J. Platt Bradbury 1Geobotanical Institute, University of Bern, CH 3013 Bern, Switzerland

Received 8 December 1999; accepted for publication 21 June 2000

Abstract

Lago de Patzcuaro lies at an elevation of 2035 m (19 36 N 101 39 W ) on the west part of the volcanic MesetaCentral of Mexico, about 50 km east-southeast of Morelia. Pollen, diatom, and geochemical records from a 15 m coretaken in 6 m of water continuously track vegetation and limnological changes in the Patzcuaro basin for the past 48 ky(calendar years). Pine, oak, and alder pollen dominate both the Wisconsin and Holocene vegetation record, and indicatethat this region did not witness strongly arid climates during that time. Before the Holocene, Juniperus-type andArtemisia pollen document cooler climates of more effective moisture at least seasonally. Isoetes characterizes the fulland late glacial interval (2513 ka) and implies, along with the diatoms Aulacoseira and Stephanodiscus, cool, deep,freshwater conditions at Lago de Patzcuaro. The freshest and deepest lacustrine phase in the lake occurred 38 to 25 ka.During this time, Lago de Patzcuaro may have drained into the Lerma River system. Between 30 and 10 ka, diatomspecies that bloom in the winter and early spring imply moisture during those seasons. Evaluation of contemporaneousmarine and terrestrial records west and east of Lago de Patzcuaro suggests that this moisture probably originated fromwesterly storm tracks shifted south by the Laurentide ice sheet. After 10 ka, Lago de Patzcuaro became shallower andmore eutrophic. Rooted aquatic plants prospered under a climate regime of negative hydrologic budgets, increasedseasonality, and moisture arriving during the summer as it does today. The lake became alkaline enough by the mid-Holocene to deposit ostracode-rich marls at the core site for the first time in the past 48 ky. Indigenous agriculture inthe Patzcuaro drainage and around the lake appears about 4 ka, with the appearance of Zea and Chenopodiineaepollen. Iron and potassium track increased soil erosion, and planktic diatoms (Cyclostephanos) record nutrients flushedinto the lake from the drainage basin. 2000 Elsevier Science B.V. All rights reserved.

Keywords: diatoms; geochemistry; pollen; human impact; late Quaternary; Mexico; paleolimnology; Patzcuaro Lake

1. Introduction Limnologica de Patzcuaro provided critical limno-logical baseline information about the lake before

A millennium before its description by the cultural impacts became especially severe. By theSpanish in the 16th century, native Tarascans late 1940s, Lago de Patzcuaro had become theutilized Lago de Patzcuaro as a source of fish, best limnologically studied lake in Mexicotules and transportation. The pioneering work of (Deevey, 1957), a reputation that it probably stillDe Buen (e.g. De Buen, 1944a) at the Estacion enjoys today with the work of Chacon-Torres

(1993).From a paleolimnological perspective, the value1 Present address: 28551 Golden Gate Canyon Rd., Golden,

CO 80403, USA. of Lago de Patzcuaro lies in the fact that for much

0031-0182/00/$ - see front matter 2000 Elsevier Science B.V. All rights reserved.PII: S0031-0182 ( 00 ) 00146-2

70 J. Platt Bradbury / Palaeogeography, Palaeoclimatology, Palaeoecology 163 (2000) 6995

of the mid- and late Wisconsin and throughout flows and cinder cones such as the newly formed(1943) volcano Paricutn 50 km west of the lake.the Holocene, the lake has never been dry. ItsIndeed, volcanic activity, through the action ofdeposits, therefore, preserve an uninterrupted his-lava flows and uplift that dammed and compart-tory of past lake changes and of climate, unlikementalized pre-existing drainages of the Ro Lermamost lake basins in central Mexico, because ofsystem sometime during the Pleistocene, has beentheir arid surroundings and intermittent history ofimplicated in the formation of Lago de Patzcuarodesiccation.as well as lakes Zirahuen and Cuitzeo (De Buen,Pollen, geochemistry, and diatoms from short1943; Barbour, 1973). Faulted, tilted, and upliftedHiller cores in the southern part of the basin firstdiatomites and diatomaceous lake sediments,documented the paleoenvironmental value of theincluding numerous layers of volcanic ash, cropLago de Patzcuaro sediment record (Deevey, 1944;out especially along the southwestern side of theHutchinson et al., 1956). The stratigraphy wasbasin and testify to the pervasive effects of tectonicinterpreted as past fluctuations of temperature andand volcanic activity in controlling the morphologywater level, but a lack of radiocarbon dates allowedand depth of the basin. Evidence for surficialonly speculative correlations to the archaeologicalhydrologic linkage between Lago Chapala on thesequence from the Cuenca de Mexico.Ro Lerma and the now isolated (but presumedThis paper examines diatoms and geochemicalonce tributary) basins of Zirahuen, Patzcuaro, andproxies of past limnological change from a radio-Cuitzeo is based on the biogeographic distributioncarbon-dated, 14 m core from Lago de Patzcuaroof species of Chirostoma (pescado blanco), anthat encompasses the past 48 ky. These data com-endemic fish genus of central Mexico (Barbour,pliment and extend pollen analyses from this core1973).(Watts and Bradbury, 1982) that documented veg-

The present climate of central Mexico, includingetational and climate changes in Michoacan fromLago de Patzcuaro in Michoacan, is characterizedthe late mid-Wisconsin through the arrival of theby mid- to late-summer rainfall that results from

Spaniards in the early 16th century. A comparison the northward migration of the Inter-Tropicalof the Lago de Patzcuaro record with marine and Convergence Zone (ITCZ) and the expansion ofterrestrial records from central Mexico shows that the Bermuda/Azores and East Pacific Highsthe climatic effects of the Laurentide ice sheet (Metcalfe, 1987). The moisture has been presumedduring the full glacial extended at least to the to come from the east, in the Caribbean, althoughlatitude of 20 N, influencing vegetation, lake eastern Pacific and Gulf of California moisturedepth, productivity, and dynamics. The record of sources may be more important, especially forpollen, diatoms, and geochemistry also documents western Mexico (Douglas et al., 1993). During theinteractions between the lake and land-use strate- winter, stable, dry conditions occur as the ITCZgies by native, Hispanic, and modern cultures. returns towards the equator and the subtropical

high pressure belt with westerly flow aloft extendsover much of Mexico. However, cold outbreaks

2. Physiography, geology, and climate of polar air (nortes) associated with meridionalflow can bring winter rainfall and low temperatures

Lago de Patzcuaro (19 36 N 101 39 W ) lies to the Meseta Central at the latitude of Lago deat an elevation of 2035 m on the west part of the Patzcuaro and even farther south (Metcalfe, 1987).volcanic Meseta Central of Mexico (Fig. 1), about Mean annual precipitation at Lago de50 km east-southeast of the city of Morelia. Patzcuaro is about 950 mm, but varies significantlyVolcanic rocks, chiefly basalt, dominate the sur- from year to year. Between 1921 and 1986, therounding terrane at higher elevations, whereas lowest annual precipitation was 567 mm (1957),colluvial, fluvial, and lacustrine sediments, and whereas the highest value was 1636 mm (1935).thick, weathered, red soils characterize lower eleva- Mean annual evaporation (about 1500 mm/year)tions (Chacon-Torres, 1993). There is much evi- is greatest during the spring and early summer

when high temperatures and windy conditionsdence of recent volcanic activity, including basalt

71J. Platt Bradbury / Palaeogeography, Palaeoclimatology, Palaeoecology 163 (2000) 6995

Fig. 1. Index map showing location of Lago de Patzcuaro. J identifies the island of Janitzio, and the core location (cross) about1 km to the south-southeast. 5 and 10 m bathymetric contours are shown. Adapted from Chacon-Torres (1993).

co-occur during the dry season. Annual temper- of 11 km, a maximum depth of 12 m in the north-ern basin, and a mean depth of 5 m (Chacon-ature averages about 16.3C (Chacon-Torres,

1993). The seasonal variation of temperature and Torres, 1993). The lake covers an area of about130 km2 and contains approximately 628106 m3precipitation (Fig. 2) documents maximum

monthly temperatures (about 20C ) during the of water. Basalts and basalitic breccias form islandsin the central and southern parts of the lake basin.early summer (MayJune). Clouds and precipita-

tion significantly cool mid- and late-summer air Basalt also dominates the approximately 929 km2drainage basin (Chacon-Torres, 1993).temperatures. Winter temperatures average about

1213C and frost occurs occasionally in winter The lake has no significant surface inflow orany surface outflow, and appears to be largelymonths. Mean minimum winter temperatures (1

2C) in December and January are still too high maintained by shallow groundwater and localrunoff during the rainy season. Variation of dis-to promote freezing of the lake.solved oxygen, temperature, and pH with depth inthe southwestern part of the lake (theErongarcuaro arm; Fig. 1) suggests important3. Limnologysubaqueous groundwater discharge to the lake (DeBuen, 1944a, 1945). Springs at and above the lakeLago de Patzcuaro is a C-shaped lake (Fig. 1)

with a long axis of about 20 km, a maximum width margin also contribute to the lake. Lake level


from evaporation. Nevertheless, Lago dePatzcuaro never becomes particularly saline dueto evaporation because solutes gained by evapora-tion eventually leave the lake through recharge tothe groundwater system. A close correspondenceexists between the annual precipitationevapora-tion balance and lake level. The fact that observedlake level maxima are generally higher and laterthan maxima predicted by lake level models basedon precipitation and evaporation underscores therole of infiltration to the lake from precipitationrecharge within the drainage basin estimated tobe between 8 and 12% of the annual water budget

Fig. 2. Mean monthly temperature and precipitation at Lago (Chacon-Torres, 1993).de Patzcuaro for the period 19211944. Data from Chacon- The hydrochemistry of Lago de PatzcuaroTorres (1993). (Table 1) reflects the surrounding volcanic terrane,

as do the majority of lakes in the central Mexicanhighlands which are dominated by sodium andvaries seasonally 0.51 m as a result of evapora-bicarbonate ions (Bradbury, 1989). Deeveystion, and the balance between groundwater dis-(1957) water analyses imply minor concentrationcharge to the lake and lake recharge to theand loss of Ca and Mg by precipitation during thegroundwater table. Assuming that groundwaterdry years between 1939 and 1945, although overallchemistry shows little seasonal variation, thethe hydrochemistry of Lago de Patzcuaro hadincrease in lake conductivity between the wetbeen relatively constant between 1941 and 1973.season (September, 1973) and the dry seasonConductivity values of 850 mS/cm (Chacon-Torres,(May, 1974) (Table 1) indicates that the lake level

drop (~0.9 m) during that time resulted mostly 1993) measured in the lake in 19861987 may

Table 1Chemistry of Lago de Patzcuaro and Lago de Zirahuen, Michoacan, Mexico

Lake analysis Patzcuaro Zirahuen

September, 1973 July, 1941 September, 1973

Na 118.4 140.7a 4.4K 29.11 34.6a 2.8Ca 13.2 3.2 5.8Mg 27.2 2.5 4.6Cl 20.4 21.3 2.2SO4 3.9 0.2 0CO3 19.2 0HCO3 414.6 458.0 54.3Total P 0.0158 0.01 0.0054IX 1973 cond. (mmhos/cm) 524 67V 1974 cond. (mmhos/cm) 685 87pH 9.5 8.7b 8.08.3cSecchi (m) 1.25 1.20 4.0

Values in milligrams per liter except as indicated. 1941 data from Deevey (1957).a reported as na+K=175.3 mg/l. This value was partitioned according to the ratio of Na and K in the September, 1973 analysis.b De Buen (1944a).c De Buen (1943).


indicate progressive hydrochemical concentration become important dominants of the summer phy-toplankton as Lago de Patzcuaro has become morerelated to increased inflow of waste water as well

as dry climates after 1980 and progressive degrada- eutrophic due to human impact (Chacon-Torres,1993; Rosas et al., 1993).tion of the watershed. Nevertheless, water analyses

made in 1995 (Bridgewater et al., 1999) show no Although Aulacoseira predominates in Lago dePatzcuaro, modern sediment samples contain largesignificant differences in major ions from the 1941

and 1973 values. numbers of a small Stephanodiscus or Cyclo-stephanos species that may also reflect humanUrban, domestic and agricultural waste water

from the city of Patzcuaro and from surrounding impact on the lake system (Bradbury, 1997). Rosaset al. (1993) report large numbers (>3000 cells/l )towns and fields contributes considerable phos-

phorus to Lago de Patzcuaro, and increased defor- of a Stephanodiscus species in the southern part ofthe lake receiving phosphorus-rich outflows fromestation of the drainage has resulted in greater

transport of suspended solids to the lake since the the town of Patzcuaro. This diatom genus had notbeen reported living in the lake by earlier workers,initial limnological studies of Yamashita (1939)

and De Buen (1941). As a consequence, secchi and little is known about its seasonal dynamics.However, if like many small Stephanodiscus speciestransparency values have decreased and the lake

has become increasingly eutrophic (Chacon- (e.g. Kilham et al., 1986), the taxon in Lago dePatzcuaro is favored by low light conditions andTorres, 1993).

Lago de Patzcuaro does not consistently stratify high phosphorus levels, under natural conditionsit might be reasonable to expect this species toin any season and oxygen is generally abundant at

the bottom (Deevey, 1957). Complete circulation bloom in the fall or winter, after the rainy seasondelivers increased supplies of phosphorus to thethroughout the year ensures that lake turbidity is

comparatively high, especially after the rainy lake at a time of declining insolation and highturbidity (Chacon-Torres, 1993). The phosphorusseason (Chacon-Torres, 1993).

Despite a comparatively large number of studies levels in the southern part of the lake receivingPatzcuaro town euents are so high (up toand references on the limnology of Lago de

Patzcuaro, serial data on the seasonal distribution 0.48 mg/l ) that diatoms, including bothAulacoseira granulata and Stephanodiscus, grow inof phytoplankton are rare. Rioja (1940) reported

a massive bloom of diatoms [Melosira (= abundance here even during the summer (Rosaset al., 1993) and indicate that, as in many temper-Aulacoseira) granulata] accompanied by Ceratium

and Pediastrum in a sample taken in mid-March, ate lakes, diatoms are limited by phosphorus inLago de Patzcuaro.1940. Aulacoseira cells were fewer but still predomi-

nant in June and July of the same year, but fell to At this time the proper generic identity of thesmall Stephanodiscus species in Lago deminimum values by December (De Buen, 1944b).

Osorio-Tafall (1944) indicates that, in general, Patzcuaro is not certain. Specimens in modernsediment resemble tiny (ca. 5 mm) forms ofcyanobacteria bloom MayJune. Clearly seasonal

differences in weather patterns from year to year Cyclostephanos, planktic diatoms often associatedwith very high levels of eutrophication (Stoermerplay an important role in the development and

succession of algal blooms. The months of et al., 1987).Benthic and epiphytic diatoms (e.g. Surirella,February and March have the most wind at Lago

de Patzcuaro (Chacon-Torres, 1993), and increas- Synedra parasitica, and Diploneis) also enter theplankton, especially when low lake levels allowing light levels coupled with wind-generated turbu-

lence and nutrients probably account for the spring turbulence to detach epiphytic species from areasof submerged vegetation (Potamogeton,diatom blooms of Aulacoseira and other diatoms.

The appearance of cyanobacteria during the warm- Myriophyllum) along the margins of the lake(Rioja, 1940; Osorio-Tafall, 1944). Epiphytic dia-est months of the year is also consistent with

observations in many eutrophic temperate lakes toms (Cocconeis placentula, Epithemia adnata,Gomphonema gracile, and Rhopalodia gibba) char-(e.g. Wetzel, 1983, p. 375). Cyanobacteria have


Table 2Radiocarbon dates from Lago de Patzcuaro, Michoacan, Mexico.

Depth (cm) Date (14C year B.P.) Sample number Calendar year (B.P.)

820823 289080 QL 1341 3001878881 364080 QL 1342 398010421045 9210150 QL 1343 10,32013461349 17,700300 QL 1344 21,15015501553 25,600750 QL 1345 29,92918551858 30,500+17001400 QL 1346 35,12921052115 44,100+30002400 QL 970 48,149

Depth in centimeters below water surface. Water depth=6.2 m

acterized Potamogeton beds at 3 m depth in 1973. 38.6 W ). The wooden plank raft, floated byattached 55 gallon steel drums, was part of a fleetMetcalfe (1985) reports abundant Staurosira con-

struens and Staurosirella pinnata growing on rocky used to remove nuisance populations of water-hyacinth (Eichornia crassipes) from shallow regionssubstrates in the lake.

Lago de Patzcuaro is rapidly changing in tro- of the lake. The water column and top 2 m ofsediment were cased with 10 cm I.D. aluminumphic status and size as human impacts combine

with climate change over the lake and its drainage pipe to facilitate coring. 1 m core sections wereextruded in the field, labeled, and wrapped inbasin. Limnologic observations made 50 years ago

are unlikely to be perfectly relevant throughout plastic sheeting and aluminum foil. The core pene-trated 14.8 m of diatomaceous gyttja and bottomedthe lake today, but can serve as analogs to past

conditions and processes useful for paleolimno- in sand at a depth of 15.2 m.logic and paleoclimatic interpretation. In thisregard, it appears that Aulacoseira species, particu- 4.2. Chronologylarly Aulacoseira ambigua and Aulacoseira granu-lata, represent the overall, unimpacted, Seven bulk radiocarbon dates were obtained

from the organic, diatomaceous gyttja of the corehydroclimatic environment of the basin in whichthe summer precipitation signal nourishes the lake (Table 2). The date sequence shows no reversals

and can be utilized to construct an age modelby surface runoff and particularly by groundwaterrecharge from the drainage for a large portion of (Fig. 3) that supports an assumed modern age for

the sediment surface and documents a reasonablythe following dry season. The abundance of epi-phytic and benthic diatoms should track the past continuous sediment accumulation rate of about

34 cm/ky. To facilitate correlations of diatom,extent of marginal submerged vegetation as itvaries in response to changes in lake level and geochemical, and pollen records with middle

Wisconsin marine records, the radiocarbon chro-transparency.nology has been converted to calendar yearsfollowing the protocols of Stuiver and Reimer(1993) and Bard et al. (1993). In this paper, ages4. Methodsare reported as calendar years beforepresent103 ka.4.1. Coring

The Lago de Patzcuaro sediment record was 4.3. Diatomscored in the spring of 1974 with a square-rod,Livingstone-type sampler (Wright, 1967) from an Diatoms were separated from 1 cm3 sediment

aliquots by digestion in hot (100C), concentratedanchored raft about 1 km south of Janitzio Island(Fig. 1) in 6.2 m water depth (19 33.7 N 101 nitric acid and rinsed with distilled water until


ceous gyttja that characterizes the core, except inthe lowermost sandy samples where coringstopped. Much of the variation in carbon andnitrogen reflects dilution by volcanic ashes.Carbon/nitrogen ratios >15 dominate the recordbetween 33 and 13 ka and suggest a relatively highinput of terrestrial organic matter (Meyers andLallier-Verge`s, 1999), coincident with glacial envi-ronments elsewhere. Lower values (


Fig. 4. Selected geochemical profiles for the Lago de Patzcuaro core. Data from Saporito (1975) plotted against an age model inthousands of calendar years before present (ka).

stratigraphic restriction to certain parts of the core Holocene. Other taxonomic possibilities for thispollen type include riparian Cupressus or Taxodiummust imply a coincidental accumulation of source

materials (iron, phosphorus, and organic matter) and (or) mesic Juniperus species from higher eleva-tions (Watts and Bradbury, 1982).that ultimately relate to distinctive past limnologic

conditions at those times. The stratigraphic distribution of herbaceouspollen, particularly Artemisia, and to a lesser extentAmbrosia, coincides with the distribution of5.2. PollenJuniperus, but with different trends and peaks. Forexample, maxima (~5%) of both Artemisia andSelected pollen types (Fig. 5) from the pollenAmbrosia occurred during the late glacial whenrecord of Lago de Patzcuaro (Watts and Bradbury,Juniperus values fell to less than 2%. Both1982) provide a background perspective ofArtemisia and Ambrosia were considered to indi-regional and local vegetation change for the pastcate drier and possibly cooler conditions than46 ky that augments and focuses paleolimnologicpresent (Watts and Bradbury, 1982), consistentinterpretations. Of the major tree taxa (Pinus,with the presence of presumed xeric JuniperusQuercus, and Alnus), Pinus and Quercus are thepollen. Ambrosia pollen returns to importance inmost common, but show few interpretable trends.the uppermost part of the diagram (Fig. 5), mostAlnus, on the other hand, consistently falls belowlikely indicating the abundance of agricultural10% in the late Holocene, probably in response toweeds in the region. The pollen record ofagricultural activities in the drainage basin andChenopodiineae is restricted to the upper part ofperhaps drier climate. The Cupressaceous pollen,the core. Although this herbaceous plant grouppossibly a xeric Juniperus species, has a distinctiveincludes native agricultural crops such asstratigraphy in which the highest values coincideAmaranthus and Chenopodium, the pollen likelywith Pleistocene levels in the core, but with virtu-

ally no important representation during the represents weeds colonizing exposed lake shore


Fig. 5. Selected pollen profiles for the Lago de Patzcuaro core. Data from Watts and Bradbury (1982) plotted against an age modelin thousands of calendar years before present (ka). Percentage of Isoetes calculated outside the pollen sum.

Fig. 6. Diatom ecological groups plotted against age in thousands of calendar years before present (ka). Planktic taxa include allcentric genera. Fragilaria includes Staurosira construens and varieties, Pseudostaurosira brevistriata, and Staurosirella pinnata.Attached taxa include species of the genera Achnanthes, Amphora, Cocconeis, Cymbella, Epithemia, Eunotia, and Gomphonema. Motiletaxa include species of the genera Anomoeoneis, Diploneis, Navicula, Nitzschia, Pinnularia, and Surirella.

habitats during drought (Deevey, 1944). The dis- Chenopodiineae, and also indicate closer approxi-mation of shallow water habitats and lower laketribution and abundance of Cyperaceae pollen

(probably mostly Scirpus) resemble that of the levels.


PLATE I


Isoetes spores became particularly important in outflow (2100 m), although it remains speculativein this tectonically active region. The presence ofthe late Pleistocene record. This submerged aquatic

fern of cool, oligotrophic lakes on mineral-rich, native fish in the lake today (Barbour, 1973), andof fish scales and bones at prehistoric levels in theoften sandy, littoral habitats documents climate-

related limnological change in Lago de Patzcuaro. core, implies some sort of past connection to theLerma drainage, however.Increased extent of coarse-grained, mineral-rich

habitat would accompany a rise in lake level, Some of the major planktic diatoms in the Lagode Patzcuaro record have not been identified withparticularly in the southern part of the basin where

the shore slopes are low and rocky (Chacon- any certainty, probably some are undescribedspecies with unknown ecology, nutrient require-Torres, 1993). Cooler climates and reduced evapo-

ration coupled with increased precipitation could ments and successional dynamics. Speculationsabout such matters can be made from morphologi-produce a higher, fresher Lago de Patzcuaro with

a more neutral pH and lower levels of HCO3, and cally similar modern species, but resulting paleo-limnological interpretations must be made withwould be consistent with growth of Isoetes

(Hutchinson, 1975, pp. 149, 370, 379, 424 ff.). caution (Plates 13). [The plates document severalimportant centric diatom taxa from the Lago dePatzcuaro core that are new to Mexico or imper-5.3. Diatomsfectly known. The light photomicrographs show auniform enlargement (magnification) of 1200.Planktic freshwater diatom species of

Aulacoseira, Stephanodiscus, Cyclostephanos, and The tentative identifications on the plate captionsfollow the protocol used in the text. Paired figuresCyclotella dominate the Lago de Patzcuaro record

along with benthic species of Fragilaria (taxa now of the same specimen show first high, and thenlow focus views to illustrate the nature of valverecognized as Staurosira construens, Staurosirella

pinnata, and Pseudostaurosira brevistriata; Fig. 6). ornamentation on undulate specimens ofStephanodiscus and Cyclostephanos and toAttached and motile benthic diatoms are important

in the record, but only dominate in the sandy illustrate the valve mantle surface and opticalsection, respectively, of Aulacoseira valves.]sediments at the base of the core. Neither planktic

nor benthic, saline-alkalibiontic diatoms appear in In general, species of Stephanodiscus (Plate 1),with the possible exception of Stephanodiscus nia-large numbers anywhere in the record, indicating

that for the past 48 ky Lago de Patzcuaro was garae, bloom in the spring during conditions oflow but increasing levels of light and abundantessentially fresh, as it is today. The predominance

of freshwater planktic diatoms implies lake levels nutrients, particularly phosphorus (e.g. Bradbury,1988; Bradbury and Forester, 2000). In the westernand open-water areas at least as high and extensive

as today. Possibly lake levels rose high enough in United States, S. niagarae often blooms in the fall,and this may also have been the case in Lago dethe past to allow the lake to drain by surface

PLATE I

15. Stephanodiscus oregonicus. 1a, b: a comparatively large specimen (14.62 m, ~45.6 ka). 2a, b: a slightly eccentric form ofthe taxon. 3, 4, 5a, b: intermediate to small-sized specimens (5.12 m, ~13.0 ka).

68. Stephanodiscus sp. aff. Stephanodiscus oregonicus. Specimen series shows variable density of loose areolation in the centralpart of the valve disk (7.88 m, ~24.5 ka).

911. Cyclotella stelligera (9.32 m, ~29.2 ka). These specimens show a form or variety of the taxon that does not have thecharacteristic central flower or star.

1214. Cyclostephanos sp. 12a, b, c: a comparatively large specimen at progressively deeper depths of focus. 13, 14: small, typicalspecimens (0.45 m, ~0.6 ka). This taxon is characterized by scattered areolae in the central part of the valve disk.

1521. Cyclostephanos sp. cf. Cyclostephanos tholiformis. 15, 16: comparatively large, heavily silicified individuals. 1721: small,weakly silicified individuals (13.22 m, ~37.7 ka). This taxon has dense, regular areolation in the valve disk center.


PLATE II


Patzcuaro in the past. If so, it implies some precipi- poor environments of eutrophic lakes such as nearthe base of the epilimnion during stratification. C.tation and storms as well as cooling temperatures

at that time. stelligera can be a common planktic diatom inLago de Zirahuen (elevation 2075 m, maximumAulacoseira granulata and Aulacoseira ambigua

were likely spring and early summer species, as depth 46 m, conductivity 79105 mS, Ca>Mg>Na, secchi 4 m) only 12 km southwest of Lago dethey are today in the lake and in many temperate

lakes (Brugam, 1983). Of the two, A. granulata Patzcuaro, although nothing is known of its sea-sonal dynamics in that system.apparently has the highest trophic requirements.

The heavily silicified and large-pored A. ambigua Little is known about the nutrient and lightrequirements of Cyclostephanos sp. aff.v. robusta (Plate 2), by virtue of its similarity to

some species living in the great lakes of Africa, Cyclostephanos tholiformis (Plate 1), except that intemperate North America C. tholiformis favorsmay document high lake levels and conditions of

deep circulation and abundant silicon fluxes as highly eutrophic lakes and ponds and warmsummer temperatures (Stoermer et al., 1987).hypothesized by Kilham et al. (1986). The variety

has been found in the Holocene record of theMeseta Central lakes Zacapu and Zirahuen(Metcalfe, personal communication, 1999). Like 6. Paleolimnological synthesis of Lago de

Patzcuarothe nominate variety, it is presumed to be a highlight species blooming during the spring andsummer. The taxon referred to as Aulacoseira Detailed diatom analyses (Fig. 7AD), supple-

mented by pollen and geochemical data form the(Plate 3) is apparently a heavily silicified form ofAulacoseira islandica or A. ambigua v. robusta. In basis of a paleolimnological synthesis for Lago de

Patzcuaro. With the exception of the sandy sedi-north temperate lakes A. islandica proliferatesduring the early spring in low temperature environment at the base of the core, the uniform diatoma-

ceous gyttja that accumulated during the pastments (ca. 6C) and high nutrient (especially sili-con) fluxes (Stoermer et al., 1981). 48 ky implies a generally stable depositional envi-

ronment without large changes in basin configura-Rhizosolenia eriensis was widely distributed butrare in Lago de Patzcuaro during August, 1982 tion and morphometry. Faulted diatomites along

the southern margin of the lake and 10 to >50 m(Rosas et al., 1993). Perhaps its presence reflectsa greater tendency towards stratified conditions above the present shore elevation testify to an

earlier basin configuration and geologic settingbecause this diatom often occurs during thesummer and early fall in fresh, stratified, moder- that included active volcanism and tectonic dis-

placements. Nevertheless, the diatoms characteris-ately deep temperate lakes (Bradbury, 1988;Huber-Pestalozzi, 1942). Cyclotella stelligera has tic of those deposits (Stephanodiscus niagarae,

Stephanodiscus oregonicus, Aulacoseira granulata,a wide distribution in low alkalinity lakes, andoverall, it appears to have comparatively low nutri- Aulacoseira ambigua) are the same as the species

of the past 48 ky found in the core and support aent requirements (Brugam, 1983, 1993). It canappear during circulation in oligo-mesotrophic Quaternary age and environment for the outcrops

in comparison with late Tertiary diatomaceouslakes (Bradbury, 1988) or perhaps in nutrient-

PLATE II

2226. Aulacoseira pfaffiana (2.62 m, ~4.4 ka). 22, 23: mantle views. 2426: disk views.2731. Aulacoseira 27a, b, 28, 29: mantle views. 30, 31: disk views (0.8 m, ~1.1 ka). This taxon resembles Aulacoseira species 9

of the PRILA Iconograph. Disk views may represent different taxa or growth stages.3234. Aulacoseira ambigua v. robusta Gasse. 34b shows a valve section focus of the lower left double valves of 34a (12.22 m,

~35.0 ka).35, 36. Aulacoseira granulata (13.22 m, ~37.7 ka). 36a shows terminal cell spine.


PLATE III

3746. Aulacoseira sp. 3744: form with wide, hollow (?) sulcus in both high- and mid-focus views (13.22 m, ~37.7 ka). 4546:form with narrow, knife-like sulcus and morphology similar to A. islandica (6.82 m, ~20.3 ka). The two forms integrate.

deposits nearby (Israde Garduno, 1993). Benthic motile and attached diatoms dominatethe basal sand (Fig. 6). The genera Pinnularia andDistinctive volcanic ashes in the diatomite and

lake sediment outcrops have potential for correla- Eunotia (Fig. 7A) are generally too broken forreliable identification, but probably representtion and absolute dating.


Pinnularia maior and Eunotia monodon for the (Fig. 7C and D). The pattern of shifting predomi-nance therefore probably documented a subtlemost part and indicate local environments of fresh,

neutral to mildly acidic pH, but the common limnologic response to nutrient fluxes (includinglight) and turbulence, all under the seasonal cli-presence of Cocconeis placentula (Fig. 7D) indi-

cates alkaline water generally. Common valves of matic control that governed the timing andstrength of circulation and stratification, and per-Hantzschia amphioxys and Navicula mutica docu-

ment the close proximity of moist terrestrial habi- haps the timing of overland nutrient delivery byprecipitation.tats, whereas the presence of megascopic root

molds in the sandy sediment, large numbers of At 47 ka Aulacoseira pfaffiana became rathercommon (Fig. 7B), indicating a transition tophytoliths and sponge spicules (Fig. 7A) indicate

shallow, marshy conditions with emergent vegeta- cooler and probably more humic-rich wateraccording to the modern distribution of this taxontion and well-oxygenated water. The high phos-

phorus and iron content (Fig. 4) of the basal sandy (e.g. Lange-Bertalot and Metzeltin, 1996). If Lagode Patzcuaro was a through-flowing system atsediment reflects vivianite associated with fish

bone. At 48 ka, this part of the Patzcuaro basin 48 ka, this transition suggests impoundment andconsiderable macrophyte growth and decay.probably represented a marsh-margin environ-

ment, perhaps with through-flowing drainage to Between 47 and 38 ka Navicula seminulum andStaurosira construens dominated the Lago dethe Ro Lerma or one of its tributaries. The basin

geometry implied by this environment was unsuit- Patzcuaro record with Stephanodiscus oregonicus(Fig. 7A and B). This freshwater diatom assem-able for a large, comparatively deep lake and

implies that the tectonic, volcanic, and related blage indicates shallow water, but with enoughnearby open-water habitats for seasonal phyto-geomorphic changes that produced the modern

Lago de Patzcuaro postdated 48 ka. plankton development. S. oregonicus blooms inthe spring in Klamath Lake, Oregon (BradburyBetween 47 and 10 ka, a period that preceded,

included, and followed the maximum extent of and Forester, 2000) after ice out and coincidentwith surface and groundwater recharge to thiscontinental glaciers in the northern hemisphere,

the Lago de Patzcuaro diatom record was charac- lake. Windy conditions during this season supplynutrients and turbulence to suspend this diatom,terized by planktic freshwater diatoms (Figs. 6 and

7B). Throughout this long period of time, the and low light levels allow it to successfully competeagainst Aulacoseira species and cyanobacteria indistribution of major planktic diatoms followed a

stepwise pattern of appearance approaching the Klamath Lake.Modern Fragilaria-dominated diatom commu-interval correlating with the glacial maximum (25

20 ka). In the succession of planktic diatoms that nities along rocky, littoral habitats in Lago dePatzcuaro (Metcalfe, 1985) suggest the water mayled into the full glacial interval, first came

Stephanodiscus oregonicus (4740 ka), followed by have been less than 4 m deep at the core site.Abundant Quercus and comparatively commonCyclostephanos (4038 ka), then Aulacoseira ambi-

gua v. robusta (3734 ka), Cyclotella stelligera Ambrosia pollen (Fig. 5) imply warm, seasonallydry summer climate. Low values of Cyperaceae(30 ka), and finally Rhizosolenia eriensis (29

25 ka). The early full glacial was dominated by pollen probably relate to fluctuating lake levelsthat made inappropriate habitats for marsh vegeta-Stephanodiscus sp. (2522 ka). Afterwards,

Aulacoseira sp. dominated (2320 ka) followed by tion, whereas abundant Alnus pollen probablycame from riparian habitats surrounding the lake.Cyclostephanos (1815 ka) and finally again by S.

oregonicus (1510 ka). There were enough species Geochemical evidence (high iron and phosphorusvalues) for observed vivianite may track localoverlaps and fluctuations within this sequence of

changing diatom dominants to indicate an overall accumulations of fish bone in the sediment. Algalproductivity was high, as evidenced by the gen-limnologic (e.g. hydrochemical ) compatibility of

the species involved. Other species maintained erally low C/N ratios during this time (Fig. 4).Cyclostephanos sp. aff. Cyclostephanos tholi-stratigraphic continuity throughout the record


(a)

(b)

Fig. 7. (A)(D) Percentages of selected diatom genera and species plotted against age in thousands of calendar years before present(ka) from the Lago de Patzcuaro core. Percentages of phytoliths and sponge spicules (A) plotted as percentages outside the diatom sum.

formis (Plate 1) became sporadically abundant Cyclostephanos sp. aff. C. tholiformis at or neartephra layers (Fig. 4) may implicate volcanism asafter 41 ka (Fig. 7B). It presumably indicates an

especially rich nutrient supply and warm summer a source of nutrients or turbidity ( low light condi-tions?) for the development of this species. If so,temperatures. The occurrence of most peaks of


(c)

(d)

Fig. 7. (continued).


its paleoclimatic significance may not be great. fresh (cond. ~400 mS) lake fed and drained by theRo Lerma system in Jalisco, Mexico. Its presenceNevertheless, it is also possible that as lake levels

began to rise, increased fluxes of nutrients stored in Chapala suggests that Lago de Patzcuaro mayhave shared some limnological characteristics within flooded marginal wetlands and marshes became

available to support Cyclostephanos. this turbid lake before the glacial maximum. Likeother Stephanodiscus species, this taxon probablyContinued lake level rise and influx of silicon-

rich ground and surface water may have promoted flourished under low light winter conditions withabundant nutrients. Potassium reached very highthe abundant development of Aulacoseira ambigua

v. robusta 3833 ka. Low C/N ratios (Fig. 4) indi- values during this episode (Fig. 4), indicating largedetrital influx from winds and precipitation, andcate a predominance of algal production in the

fluxes of organic matter to the sediments, and low high Isoetes percentages (Fig. 5) may have relatedto extensive, coarse-grained, mineral-rich littoralvalues of iron and phosphorus may imply a greater

efficiency of nutrient cycling throughout this time habitats maintained by high-energy lake marginwaves and currents. Storms and precipitationinterval. Low percentages of Artemisia and

Ambrosia, but rising and comparatively high per- almost certainly arrived during the winter monthsduring this time, and may have characterized muchcentages of Pinus and Alnus, respectively (Fig. 5),

are consistent with increased moisture. of the year although prominent, episodic represen-tation of Aulacoseira granulata (Fig. 7C) implies aCyclotella stelligera and then Rhizosolenia

eriensis characterize the Lago de Patzcuaro record summer seasonality with warm, nutrient-rich, tur-bulent conditions at times.between 34 and 25 ka. In 1973, the surface sedi-

ments of Lago de Zirahuen contained abundant Very high percentages of Aulacoseira sp. charac-terized the full glacial period at Lago de PatzcuaroC. stelligera, indicating that this diatom was a

common element of the phytoplankton of this (2220 ka). The distribution of the possibly relatedtaxon A. islandica today suggests that the lakefresh, comparatively deep and rather oligotrophic

lake. R. eriensis, although rare today in Lago de may have regularly frozen in the winter, and thatprecipitation occurred as snow. This enhancedPatzcuaro (Rosas et al., 1993), more commonly

characterizes the phytoplankton of cold, deep, low recharge to groundwater in the drainage basin thatconsequently discharged large concentrations ofalkalinity temperate lakes (Stoermer and Yang,

1970). The co-occurrence of R. eriensis and moder- silicon to the lake during the spring to fosterblooms of this heavily silicified diatom. Otherate percentages of Isoetes spores 2925 ka (Fig. 5)

further suggest fresh, oligotrophic limnological paleolimnological indicators remain largelyunchanged except for iron, which reached itsconditions.

C/N ratios became high and potassium lowest values in the entire record, and marginallylower C/N ratios. Low iron values are generallyincreased (Fig. 4) during the time when these

diatoms predominated, respectively indicating consistent with implied oligotrophic lake condi-tions, as are continued high percentages of Isoetes,increased deposition of terrestrial organic matter

and mineral detritus. High values of iron may also whereas lower C/N ratios could be taken to indi-cate somewhat reduced input of terrestrial organicbe ascribed to increased erosion of iron-rich soils.

Climates of increased storminess and precipitation matter and consequently less wind and overlandrunoff.were probably ultimately responsible for these

changes. The diatom assemblages between 34 and Aulacoseira sp. continued to characterize thepost-glacial period, although it is joined by25 ka probably document the deepest and freshest

episode in the recorded Quaternary history of Aulacoseira granulata, Stephanodiscus niagarae(Fig. 7C), and Cyclostephanos sp., all of whichLago de Patzcuaro.

A Stephanodiscus species allied to indicate greater seasonality of nutrient supplies,light, and turbulence. Even so, Isoetes impliesStephanodiscus oregonicus (Plate 1) characterized

the period 2522 ka. This taxon occurs in the generally oligotrophic lacustrine environmentsuntil 15 ka. Very high amounts of Alnus pollensurface sediments of Lago de Chapala, a large,


and maximum ratios of C/N (Figs. 5 and 4) indi- production as Lago de Patzcuaro became moreeutrophic. The abundance of planktic Synedracate abundant moisture in local riparian habitats

within the drainage and transportation of ter- species chiefly Synedra acus and Synedra rum-pens especially about 5 ka (Fig. 7C) impliesrestrial organic material to the lake. Stephanodiscus

oregonicus reappeared during late glacial times episodes of stronger stratification and high Si/Pratios (Tilman et al., 1982; Bradbury, 1988) that(1510 ka) when high percentages of Artemisia

and then Ambrosia suggest drier terrestrial environ- could reflect the hot, dry climates of the modernprecipitation regime during the early summer. Thements. High percentages of Fragilaria species

during 1310 ka (Figs. 6, 7A and C) also docu- shallow-water Aulacoseira species, Aulacoseira dis-tans and Aulacoseira pfaffiana (Fig. 7B and C),ment generally lower lake levels. However, the

occurrence of S. oregonicus indicates continued indicate local environments of neutral to low pHresulting from decomposition of lake margin vege-input of winter season moisture, although biseaso-

nal moisture regimes cannot be discounted. tation and production of humic acids. Overall, themid-Holocene diatom assemblages imply a fresh-Elsewhere in Mesoamerica, especially to the east

and southeast of Lago de Patzcuaro, the late water and persistent low lake stand under balancedhydrologic conditions. To some extent this couldglacial period is dry until about 12 ka, indicating

that the easterly, summer moisture sources that reflect a progressive shallowing of the lake bysedimentation.influence the area today did not become effective

until that time (Bradbury, 1997). After 5 ka and throughout the late Holocene,large, abrupt fluctuations of diatoms, pollen, andHigh but fluctuating percentages of Aulacoseira

ambigua and Fragilaria species characterize the geochemical indicators of paleolimnology, vegeta-tion, and climate (Fig. 8) relate to strong environ-Holocene at Lago de Patzcuaro. These diatoms

live in the lake today (Osorio-Tafall, 1941), mental variation, in part caused by human andpossibly volcanic impacts around the lake and inalthough A. ambigua is much less common than

Aulacoseira granulata. Generally high percentages its drainage. Low lake levels documented by ostra-code marls and high weight percentages of calciumof attached and motile benthic diatoms in the

Holocene (Fig. 6) testify to overall shallower lake between 4.6 and 3.8 ka precede the first indicationof agriculture in the basin identified by the appear-depths, and low C/N ratios indicate high algal

Fig. 8. Selected diatom, pollen, and geochemical profiles from the Lago de Patzcuaro core for the past 10 ka plotted against age inthousands of calendar years before present. Z denotes the occurrence of Zea pollen grains in the core.


ance of Zea pollen (Fig. 8). Tephra layers preceded By 2.4 ka Aulacoseira ambigua again signaled atemporary rise in lake level.and followed the calcium-rich, low-lake interval

and suggest a potential interaction between cli- Large amounts of iron-rich clay and phytolithsentered the basin between 1.8 and 1.2 ka, coinci-mate and volcanism. Nevertheless, the role of man

in affecting the water balance of Lago de Patzcuaro dent with increases of eutrophic planktic diatoms(Aulacoseira granulata and Cyclostephanos sp.) thatcannot be ruled out. Alnus pollen had rapidly

declined and Chenopodiineae pollen had already suggest a significant influx of nutrients to thebasin. These changes correspond to a major epi-begun to rise above background levels by 4 ka,

both events unprecedented in the 48 ky vegetation sode of indigenous agriculture related to earlyClassic occupation in the area (OHara et al.,history of the lake. Watts and Bradbury (1982)

suggested human manipulation of the watershed 1993). Overall, however, Lago de Patzcuaroremained shallow and somewhat saline comparedas a possible cause for the Alnus decline, although

abnormally dry climates could have been responsi- with the preceding millennium. It is possible thatindigenous water management technology hadble as well as human impact.

Lake changes potentially attributable to agricul- improved to a level that consistently deprived thelake of water. The flooding of clay to the basintural practices in the basin occurred after 3.6 ka

when Zea first appeared. Erosion and deposition decreased somewhat after 1.2 ka, and ostracodesflourished in shallow-water habitats until 0.8 kaof iron-rich red clay in the lake basin after 3.2 ka

(Fig. 8) coincides with a large increase in percen- (Fig. 8). Episodes of flooding and marginallyhigher lake levels indicated by pulses of Aulacoseiratage of Aulacoseira ambigua that indicates higher

lake levels in response to increased precipitation ambigua and (or) A. granulata briefly punctuatedthe Fragilaria- and Cyclostephanos-dominatedafter the mid-Holocene low stand. The influx of

detrital material was minor compared with later lake system at 1.00.6 ka and again during the last200 years of the record. These intervals probablyepisodes of erosion, however (OHara et al., 1993),

and it seems likely that climate-related limnologic relate to short-term climate changes within asystem progressively more impacted by man. Thechange may have acted in concert with agriculture

to produce this record. Possibly the favorable dominance of a tiny Cyclostephanos species duringthe past millennium testifies to greatly increasedmoist climates at that time led to the development

of agricultural practices in the region. trophic levels as erosion and hydrochemical con-centration prevailed in Lago de Patzcuaro. VeryLow lake levels, again indicated by ostracodes

and high calcium values as well as by Fragilaria large numbers of phytoliths washing into the basinduring the past 200 years (Figs. 7A and 8) proba-species and other benthic diatoms (Fig. 6), charac-

terized the interval between 2.8 and 2.0 ka. The bly relate to the progressive deforestation of thedrainage that continues today (Chacon-Torres,benthic diatoms Amphora veneta and Anomoeoneis

sphaerophora and the planktic species Cyclotella 1993).meneghiniana (Fig. 8) show that during part ofthis arid interval Lago de Patzcuaro became moresaline than any time previously. Low percentages 7. Discussionof Chenopodiineae pollen, to the extent that thispollen type represents agricultural weeds, may The Lago de Patzcuaro sediment record repre-

sents a continuous chronicle of Quaternary lacus-imply reduction of tillage and other forms ofhuman impact at this time. However, increased trine and climate changes for the past 48 ky.

Although the lake has no surface outlet, it is notconsumption of water for irrigation in drainagesaway and upstream from the lake might produce a closed basin inasmuch as it receives and dis-

charges significant amounts of groundwater.a similar paleolimnological response. High C/Nratios indicate increased deposition of organic Consequently it has always been essentially a fresh-

water lake. Only in the past 3 ky has water diver-matter from terrestrial vegetation in response tolower lake levels and (or) watershed management. sion to agriculture led to any significant chemical


concentration of this lake which allowed precipita- the swamp cypress, Taxodium mucronatum, as apossible source for the pollen. According totion of calcium carbonate and abundant ostra-

codes. The pollen record (Fig. 5) as well as the Gonzalez Quintero (1986), this Taxodium suggestslocal climates with precipitation of 1500 mm/yearlong stratigraphic distribution of several diatom

species (Fig. 7D) indicate that the lake has not and mean annual temperatures of 20C based onits current distribution in Mexico around lakeswitnessed climates dry enough to convert the basin

into a shallow, saline lake like Lago de Cuitzeo, and extensive marshes.Because the Juniperus and Cupressus species inor the historic lakes in the Cuenca de Mexico

(Bradbury, 1989). Today, under the influence of the Meseta Central of Mexico grow today in asummer-wet precipitation regime, they may beclimate and human impact, the lake is as low,

saline, and productive as it has ever been. inappropriate candidates for the source ofJuniperus-type pollen during the Pleistocene atHowever, climate and hydrologic change before

the mid-Holocene and throughout the past 48 ky Lago de Patzcuaro, at least when winter seasonprecipitation was more important. Juniperus calif-combined to produce long periods when Lago de

Patzcuaro was probably higher and certainly ornica, a winter-wet species that occurs in lateglacial-aged packrat middens in Baja Californiafresher than it is at present.(Penalba and Van Devender, 1998), might be apossibility, although whether it extended as far7.1. The pollen recordsouth as Michoacan would require macrofossilrecords of appropriate age to determine.From the perspective of the pollen record, eco-

logically silent pollen types (Pinus, Quercus, and The disappearance of Juniperus-type pollencharacterizes the early Holocene vegetation record,Alnus) dominate the terrestrial vegetation record

of the Lago de Patzcuaro region. Only the modest and by mid-Holocene time Chenopodiineae pollenmay indicate the presence of early agriculture inpresence of Cupressaceae (Cupressus or Juniperus)

or Taxodiaceae (Taxodium) pollen (up to 20%) the region. The Chalco sub-basin has a very similarrecord (Lozano-Garcia and Ortega-Guerrero,differentiates the Wisconsinan environment. The

Juniperus-type pollen in the Patzcuaro record was 1994). At Tlapacoya, the presence of cultigens inthe pollen record by 5 ka (Gonzalez Quintero,assumed, by virtue of its co-occurrence with low

percentages of Artemisia and Ambrosia pollen, to 1986) clearly implicates a human impact onthe vegetation that effectively compromises therepresent a xeric juniper species (Watts and

Bradbury, 1982), but this assessment conflicts with pollen record for paleoclimatic interpretation(Bradbury, 1982).diatom evidence of lake conditions fresher than

today during most of Wisconsin time.Unfortunately, Juniperus-type pollen cannot 7.2. Moisture sources

be ascribed to a particular taxon. This pollen typeis not important today nor in the Holocene, either Today central Mexico experiences a summer-

wet (monsoonal ) moisture regime. Based on aat Lago de Patzcuaro or in the Cuenca de Mexico(e.g. Lozano-Garcia et al., 1993). Nevertheless, as review of a number of sites throughout tropical

America, Bradbury (1997) argued that during thein the Lago de Patzcuaro pollen record,Cupressaceae/Taxodiaceae pollen also charac- full and late glacial periods, westerly moisture

sources predominated in west-central Mexico. Theterized the full and late glacial periods in theChalco record of the Cuenca de Mexico (Lozano- evidence for this conclusion came from the fact

that sites in southeastern Mexico, Central AmericaGarcia and Ortega-Guerrero, 1994). The discoveryof a Taxodium log (Mirambel, 1986), radiocarbon- and northern South America, which today are

under the influence of easterly tropical moisturedated at 23,150 14C [=~26.4 ka] (GX-0959)(Garcia-Barcena, 1986), from the excavations at during the summer, were dry during the full and

late glacial periods. Consequently, easterly mois-Tlapacoya in the Chalco sub-basin (elevation of2240 m) in the Cuenca de Mexico could implicate ture sources were presumed to have been ineffective


Fig. 9. Organic carbon, biogenic opal, and 15N from Mexican ocean margin core NH15P and diatom ecological groups in thePatzcuaro core plotted against calendar year age for the past 50 ka. Marine geochemical and age data from Ganeshram (1996).Diatom percentages smoothed by five-level averaging. Fragilaria species as in Fig. 6. Fallwinter diatoms include all Stephanodiscusspecies and Aulacoseira islandica.

or to have not existed at all at that time. In Support for westerly, winter season moisturereaching western Mexico during the full and latecontrast, sites west of the Cuenca de Mexico

contained records of increased moisture or westerly glacial periods also comes from a marine record(NH15P) off the coast of Sinaloa (Mazatlan 22.5winds during the full and late glacial. However,

with the exception of some plant and animal taxa N ) (Ganeshram, 1996). Here, low values oforganic carbon, opal, and 15N indicate reducedfrom packrat middens in northern Mexico (e.g.

Van Devender, 1990a,b), no moisture proxy of the upwelling during the full and late glacial periods(Fig. 9). Ganeshram and Pedersen (1998) attributerecords examined specifically indicates a winter-

wet seasonal moisture regime brought about by the reduction of upwelling in this region toincreased westerly, on-shore winter winds whichwesterly precipitation.

The distribution of the diatom Stephanodiscus brought moisture to western Mexico but canceledshore-parallel currents and Ekman transport thatin full and late glacial records of northern Mexico

(e.g. Metcalfe et al., 1997), as well as in Lago de drives upwelling today. Like others, they ascribethe ultimate cause of wind changes to growth ofPatzcuaro, might indicate winter season moisture

because this low-light species tends to bloom in the Laurentide ice sheet and displacement of stormtracks to the south as the North Pacific Highthe fall or winter months in relatively freshwater

lake systems. The same is probably true for and Aleutian Low pressure cells weakened andstrengthened, respectively.Aulacoseira sp. Although moisture from summer

precipitation that persisted into the winter under Two remaining questions concerning the magni-tude and extent of westerly moisture in Pleistocenecooler climates might possibly satisfy the ecological

needs of these diatoms, winter precipitation can records of central Mexico are: (1) did significantwesterly moisture reach the Cuenca de Mexicomore effectively maintain high lake stands in arid,

subtropical regions (Prentice et al., 1992). during the full and late glacial? and (2) to what


extent was westerly moisture important in main- 1993). The Cuenca de Mexico during this timesupported a large and comparatively deep, but stilltaining Lago de Patzcuaro before the full glacial

period, between 50 and 30 ka? alkaline lake that united at least the Texcoco andChalco sub-basins between 34 and 23.5 kaIt is probable that the Cuenca de Mexico

received some westerly moisture during the full (Bradbury, 1989; Caballero-Miranda, 1997).Finally, the 33 ka mammal fauna at Tlapacoya,and late glacial periods. Records from lowland

Guatemala, although poorly dated, indicate cold which contains such species as Hydrochoreus (cap-ybara) and Lutra (river otter), indicates riverineand dry conditions during this time (Leyden et al.,

1993). The implied absence of easterly precipita- terrestrial environments in the Cuenca de Mexicowith warmer and more humid climates than duringtion there would also have deprived central

Mexican lakes of this source of moisture. Diatom the full glacial (Alvarez, 1986; Lorenzo, 1986).Together these late OIS 3 records suggest thatrecords from the Cuenca de Mexico indicate that

the Chalco sub-basin was a shallow but fresh lake effective moisture in the Cuenca de Mexico mayhave exceeded full glacial and also Holocene levelsor marsh during the full and late glacial (Caballero-

Miranda, 1997), that contained significant percent- where the summer monsoon dominates today.Possibly easterly moisture was even greater andages of Stephanodiscus species. Nevertheless,

insufficient moisture reached the Cuenca de more effective during late OIS 3 than during theHolocene. However, the fact that most marineMexico to unite the sub-basins into one lake and

overall, the full and late glacial periods were upwelling proxies are, with the exception of 15N,lower between 50 and 30 ka than during theapparently drier than before (Bradbury, 1989).

This might suggest a decreasing spatial trend (west Holocene (Fig. 9) implies that upwelling duringlate OIS 3 was not as strong as during theto east) of westerly moisture across central Mexico

during the late Wisconsin, although colder mean Holocene, and argues against a climate dynamicthat produced stronger than modern easterlyannual temperatures may have been equally or

more responsible for marginally increasing effec- precipitation. Nevertheless, Lago de Patzcuarowas predominantly fresh 3830 ka (Fig. 6).tive moisture.

The second question relates to the first inasmuch Consequently, westerly moisture may have playedan additional, seasonal role, especially in westernas its answer requires an evaluation of records east

and west of Lago de Patzcuaro for the time under central Mexico at that time. Perhaps other oceanand atmospheric circulation patterns affected thisconsideration. The marine record off the coast of

northwest Mexico (NH15P) shows that proxies of late OIS 3 record that were absent or reduced inthe Holocene.coastal upwelling (Fig. 9) reached intermediate to

equivalent values compared with the Holoceneduring oxygen isotope stage (OIS) 3 (5030 ka), 7.3. Zirahuen as an analog for late OIS 3and that the North Pacific High and its relatedshore-parallel ocean currents seasonally affected Aulacoseira ambigua v. robusta and Cyclotella

stelligera prospered in Patzcuaro between 35 andthe west coast of Mexico during this time. Byimplication, easterly precipitation related to the 25 ka. They both lived in Lago de Zirahuen histori-

cally, suggesting that from hydrochemical andBermudaAzores High, and (or) monsoonalsummer precipitation related to the heat low in limnological perspectives, this lake could be a

reasonable analog for Lago de Patzcuaro prior tonorthwestern Mexico (e.g. Ganeshram andPedersen, 1998; Douglas et al., 1993), must have the full glacial period. Zirahuen lies 40 m above

and 12 km to the south of Patzcuaro, but is anbecome important sources of moisture for Mexico.Southeast of central Mexico, on the Yucatan order of magnitude smaller (1045 ha), less saline

than Patzcuaro (Table 1), and about three timesPeninsula, the pre-full glacial part of the Lago deQuexil, Guatemala record documents cool and deeper. Zirahuen has a mean depth of 18 m and a

volume of about 188106 m3. Precipitation atmoist montane vegetation that would also haverequired an easterly moisture source (Leyden et al., Zirahuen (11801234 mm/year) is about 20%


greater than at Patzcuaro, but the mean annual lowering of mean annual temperature by 7C atLago de Patzcuaro would independently suffice totemperatures of the two lakes are similar (16.1

and 16.3C, respectively). Topographically, equate evaporation and precipitation. Of the twoindependent possibilities, a precipitation increaseZirahuen is the highest lake in the area and proba-

bly lies near the top of the groundwater flow would seem more likely, especially because suchlevels of annual precipitation fall within the rangesystem. Although the distribution of endemic fish

(Chirostoma) supports a past surface hydrologic of variability today (5571636 mm). Nevertheless,years of above average precipitation are also coolerconnection between Zirahuen, Patzcuaro, Cuitzeo

and ultimately the Ro Lerma (De Buen, 1943; at Lago de Patzcuaro (Chacon-Torres, 1993, figs.4.3 and 4.4) and the high, fresh lake stage ofBarbour, 1973), the probability of a groundwater

flow system between these basins remains late OIS 3 must have resulted from a combinedeffect of lowered temperature and increasedunevaluated.

Annual evaporation at Zirahuen (1103 mm) precipitation.is significantly less than at Patzcuaro(1481 mm/year), and results in a positive 7.4. The limnological impact of manhydrologic balance for Zirahuen in contrast toLago de Patzcuaro where evaporation exceeds The settlement of indigenous inhabitants in the

immediate area of Lago de Patzcuaro had andprecipitation on an annual basis. Nevertheless,lake levels at Zirahuen fluctuate about 1 m in continues to have profound effects on the lake

inter-related with and exacerbated by climateresponse to wet and dry season moisture (Bernal-Brooks and MacCrimmon, 2000) as rapid infiltra- change. The effects of catchment erosion and

deposition of iron-rich clay in the basin as a resulttion to the groundwater table removes water fromthe lake. Such rapid seepage from the lake plays of prehistoric agricultural practices along inflowing

drainages have been well documented (OHaraan important role in maintaining its very freshhydrochemistry and contributes significantly to the et al., 1993). In addition to the effects of erosion

and sedimentation, by using surface water forregional groundwater system. In addition, thegreater depth and smaller area of Zirahuen relative irrigation, agriculture promotes evaporative losses

that ultimately resulted in increased hydrologicto Lago de Patzcuaro means that its volume reduc-tion by evaporation would be considerably less deficits for the lake. What water did return to

Lago de Patzcuaro via recharge and runoff wouldthan at Patzcuaro. Based on the relative volumeloss, Patzcuaro would need about five times more have been more saline after its use for irrigation,

where evaporative losses are high. Possibly theeffective precipitation (cooler temperatures andincreased rainfall ) to become as fresh as Zirahuen diatom evidence for shallow water and increased

salinity in Lago de Patzcuaro after 3.6 ka docu-is today. Unless seepage from Lago de Patzcuaroincreased proportionally under this regime of ments this effect, although coincident climate

change cannot be ruled out.increased effective moisture, the lake would soonhave risen to the level of overflow and drained The object of irrigation agriculture is to facili-

tate the conversion of soil nutrients into croptowards the Ro Lerma. In either case, it becamefresh enough to support diatoms currently or plants. Although pre-Hispanic agriculturalists may

have fertilized their fields, apparently over-fertiliza-recently living in Zirahuen.A combination of lower mean annual temper- tion did not occur to the extent that indigenous

irrigation would have transported significant nutri-atures and increased precipitation is consistentwith the intermediate character of OIS 3 marine ents to the lake during the early phases of agricul-

ture (3.61.0 ka) around the basin. Theand terrestrial records (Ganeshram, 1996;Bradbury and Forester, 2000). The Zirahuen stratigraphic distribution of Cyclostephanos sp.,

the diatom most likely to represent strong eutro-analog allows the possibility that late OIS 3 tem-peratures were similar to Holocene temperatures, phication of Lago de Patzcuaro, suggests that

significant and persistent fertilization of the lakehowever. A precipitation increase of 1.5 or a


did not begin until about 0.8 ka (Fig. 8) when the about the limnology, climate, and physiographyof Lago de Patzcuaro and Lago de Zirahuen.post-Classic Purepecha inhabited the region.

Spanish accounts suggest that the Purepecha, Helpful reviews by Henry Hooghiemstra and T.C.Johnson were much appreciated. I gratefullywhose ceremonial and secular use of wood was

large (OHara et al., 1993), may have initiated acknowledge the Geobotanical Institute of theUniversity of Bern for facilities occupied and usedextensive deforestation within the Patzcuaro basin.

With deforestation, minerals, elements, and nutri- during the preparation of this paper.ents once involved in biologic cycling entered therunoff and groundwater to Lago de Patzcuaro andincreased the nutrient input to the lake. The arrival Referencesof the Spaniards about 400 years ago (A.D. 1541)caused a precipitous population decline in the Alvarez, T., 1986. Fauna pleistocenica. In: Lorenzo, J.L., Mir-

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Barbour, C.D., 1973. A biogeographical history of ChirostomaLago de Patzcuaro to recover from its highly (Pices: Atherinidae): a species flock from the Mexican Pla-eutrophic state (Fig. 8). Nevertheless, 200 years teau. Copeia 3, 533556.

Bard, E., Arnold, M., Fairbanks, R.G., Hamlelin, B., 1993.ago unprecedented numbers of phytoliths washed230Th234U and 14C ages obtained by mass spectrometry oninto the lake as the drainage basin suffered contin-corals. Radiocarbon 35, 191199.ued deforestation under Spanish and Mexican

Battarbee, R.W., 1973. A new method for the estimation ofdominion, and neolimnological studies document absolute microfossil numbers, with reference especially tocontinued and increased impacts on the limnology diatoms. Limnol. Oceanogr. 18, 647653.

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720-004500 and the Limnological Research Center ostracods: an example from Lake Patzcuaro, centralat the University of Minnesota. Mark S. Saporito Mexico. Palaeogeogr., Palaeoclimatol., Palaeoecol. 148,

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