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Late Quaternary Vegetational and Climatic Changes in the PampaGrassland of Argentina

ALDO RAUL PRIETOLab. Palinologi´a. Dpto Biología, Centro de Geología de Costas y del Cuaternario, Facultad de Ciencias Exactas y Naturales, Universidad Nacional de

Mar del Plata, Funes 3250, 7600 Mar del Plata, Argentina

Received December 21, 1994

The vegetation and climate of the Pampa grassland, Argentina,during the late Quaternary are reconstructed from pollen recov-ered from dated stratigraphic sections from arroyo walls andfrom archaelogical excavations. Prior to 10,500 yr B.P., herba-ceous psammophytic steppe existed in the central part of thePampa grassland while xerophytic woodland associated withpsammophytic and halophytic steppe occurred in the southwest-ern part of the Pampa. These types of vegetation and the conti-nental conditions that prevailed in the area of the present-daycoast (38°S), indicate subhumid-dry climate and annual precipi-tation 100 mm lower than present. A subsequent change toward avegetation characteristic of ponds, swamps, and foodplains, ortoward environments with locally more effective moisture, oc-curred ca. 10,500 yr B.P. suggesting annual precipitation close tomodern levels or a higher availability of water in the central partof the Pampa grassland, this type of vegetation existed until 8000yr B.P., when it was replaced by grassland communities thatlasted until 7000 yr B.P. In the southwestern part of the Pampagrassland, this vegetation developed before 7000 yr B.P. and per-sisted until ca. 5000 yr B.P. Sea level higher than the present ca.6200 yr B.P. is consistent with sharp modification of the vegeta-tion and development of local halophytic communities dominantat 38°S. A return to subhumid-dry conditions occurred after 5000yr B.P. The late Holocene vegetation is characterized by pollenassemblages similar to the psammophytic and halophytic commu-nities of the Southern pampa grassland, associated with commu-nities with more edaphic conditions. At the same time, at 38°S asea level regression is suggested by the dominance of fresh-waterpollen assemblages and micropaleontological remains. The trendtoward humid, temperate conditions ca. 1000 yr B.P. suggested byvertebrate remains characteristic of temperate and humid condi-tions, as well as a very short but dry episode during the 18thcentury suggested by the geology, are not clearly evidenced in thepollen sequences. Vegetational and climatic changes are explainedby the latitudinal shifts and changes in intensity of the southernatmospheric circulation and changes in sea level. © 1996 University of

Washington.

INTRODUCTION

Several paleoclimatic models based primarily on pollenrecords from the Andean forest region have been proposed toexplain vegetational changes during late and postglacial time

southern South America (Markgraf, 1983, 1991; Markgrafetal., 1992; Villagrán, 1993). Pollen records from the Pampagrassland, however, are few and problematic. The few earlierrecords were either undated (e.g., Fernández and Romero,1984; Guerstein and Quattrocchio, 1984; Quattrocchioet al.,1988) or, if dated, the pollen sum was inadequate for the cal-culation of reliable relative frequencies (Nieto and D’Antoni,1985).Information on late Pleistocene and Holocene paleoenviron-

mental changes in the Pampa grassland has come from geo-morphologic and stratigraphic studies, including study of fossilvertebrate assemblages (e.g., Tonni and Fidalgo, 1978; Tonni,1992; Iriondo and García, 1993; Zárate and Blasi, 1993). Thisinformation, however, is in part controversial, inasmuch assome of the Holocene climatic fluctuations suggested by thegeologic evidence have not been detected in the faunal analy-ses (Tonniet al.,1988; Tonni, 1992). Also, paleoenvironmen-tal inferences for the late Pleistocene are largely problematicbecause many are based on studies of extinct taxa, and thereare few possibilities of verifying such inferences due to thelack of modern counterparts.The aim of this study is to reconstruct past plant communi-

ties in the Pampa grassland for the late Quaternary, to inferregional paleoclimatic trends, and to relate these trends to glob-al changes in atmospheric circulation. Several dated pollenrecords were analyzed and interpreted using modern pollenspectra. The comparison of fossil pollen sequences with mod-ern pollen provided information about changes in the compo-sition and distribution of late Quaternary vegetation.

GENERAL DESCRIPTION OF THE REGION

Vegetation

The phytogeographical province of the Pampa grassland islocated in eastern Argentina between latitudes 31° and 39°S(Cabrera, 1968) (Fig. 1). The present vegetation is the result ofnatural factors and of agricultural and grazing activities duringthe last 100 years. Today, the Pampa grassland is characterizedby crop farming similar to that of any temperate region of theworld and by small plantations of exotic forests. However, it is

QUATERNARY RESEARCH45, 73–88 (1996)ARTICLE NO. 0007

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still possible to find grassland relicts that probably resemblethe original communities (Leónet al.,1984).Until the 19th century, the dominant vegetation was a tree-

less grassland with edaphic shrub communities. The presentgrassland communities correspond to different stages of anold-field succession (Leónet al.,1984). The dominant vegeta-tion type is the grassland steppe, withStipa, Piptochaetium,Poa, Festuca,and Panicum (Cabrera, 1968). Although thePampa grassland is considered to be of uniform physiognomyand topography, it can be divided into four units based ongeomorphology, geology, physiography, soils, and vegetation(León, 1991). These units are: rolling pampa, inland pampa,flooding pampa, and southern pampa (Fig. 1).

Rolling pampa. In this humid and temperate grassland,

Gramineae (species ofStipa, Piptochaetium,andAristida) aredominant. Shrubs and suffrutices (several genera of Composi-tae) are poorly represented. Numerous small broad-leavedherbs and sedges are interspersed among the grasses. Naturalponds are lacking.

Inland pampa. Two subunits are recognized in this unit,the flat or central pampa, which occupies the eastern half of thearea, and west pampa, which is in a climate drier than the restof the unit. Floods occur during rainy periods in the flat pampa.Among other grasses, the inland pampa includes various spe-cies of Stipa. The most important edaphic communities arethose that develop on dunes (psammophytic steppe) with pre-dominance ofPanicum urvilleanum, Poa liqularis,andHyalisargentea.Species ofBaccharisand other shrubby members of

FIG. 1. Map showing location of stratigraphic pollen sections in relation to surface pollen sample sites (shown by numbers), vegetation units (León, 1991),and isohyets (mm) in the Pampa grassland.

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the Compositae are frequent. In flat, low-lying areas and alongthe margins of ponds, halophytic communities very similar tothose of the flooding pampa appear.

Flooding pampa. This grassland is dominated by grassescomparable to communities of the rolling pampa, includingBothriochloa, Paspalum,andBriza; however, other grass spe-cies are characteristic of this low and flat area, such asSpo-robolus, Aristida,and some species ofStipa.Pond, swamp, andfloodplain communities are frequent, with reed-like speciessuch asScirpus, Typha,andSpartina.Common broad-leavedherbs areAlternanthera, Vicia,and Eryngium.Halophyticcommunities with some Chenopodiaceae,Distichlis, and Li-moniumare important in this part of the Pampa grassland andin low-lying areas.

Southern pampa.The dominant steppe grasses areStipaandPiptochaetium.This unit includes the only two low-altitudemountain systems of the Pampa grassland, the Tandilia andVentania ranges. The dominant species in these areas are thegrassPaspalum quadrifariumor several species ofEryngium.Colletia paradoxaand Dodonaea viscosaare the shrubbyedaphic dominant community. In the Tandilia ranges this com-munity is accompanied byEupatorium buniifoliumor Baccha-ris tandilensis and in the Ventania ranges byDiscarialongispinaassociated with various endemic grasses.The boundary of the Pampa grassland is determined by its

contact with xerophytic woodland vegetation (Fig. 1) known as“Espinal” (Cabrera, 1968). Due to more than 100 years offarming, it is impossible to recognize clearly the xerophyticwoodland-grassland boundary at many points along the borderof the Pampa grassland (León, 1991).The contact between southern pampa and xerophytic scrubs

is a broad ecotone between the isohyets of 500 and 550 mmannual precipitation.Prosopidastrum globosum, Condalia mi-crophylla, andDiscaria longispinamake up the thorn scrubcommunities, together with fewProsopis caldeniaandP. flex-uosa.There are also psammophytic communities and alkalinesoils with shrubs and halophytic steppe near the coast (Sali-cornia ambigua, Heterostachys ritteriana,andSuaeda divari-cata).To the east of the Pampa is the Tala district. It is character-

ized byCeltis talaand it is developed on paleo-dunes or onHolocene beach ridges. To the north, west, and south of thePampa is the Caldén district, which is characterized byProso-pis caldeniaandP. flexuosa(León and Anderson, 1983).

Climate

The climate of the Pampa grassland is temperate. In contrastto other regions located at comparable latitudes in the NorthernHemisphere, the Argentina Pampa is characterized by an east–west moisture gradient and increasing continentality toward thenorthwest (Burgos, 1968). Average temperatures in the north-east are between 24°C in January (summer) and 10°C in July(winter); for the same months in the southwest, they are be-tween 20°C and 7°C, respectively.

Winds and precipitation are influenced by the seasonal shiftof the Atlantic and South Pacific anticyclones and by frontalsystems related to the southern westerlies. Because of theseinfluences, the area is crossed by air masses that travel in aSW–NE direction all year round. In the warmest months, fron-tal storms give rise to heavy precipitation. In winter, the north-ward displacement of the subtropical high-pressure belt allowsthe westerlies to reach the southern part of the Pampa grass-land, and cold and dry air breaks out.The total annual rainfall increases in a west–east direction.

The highest precipitation is reached in the NE (>900 mm/yr),and the lowest is in the SW (<500 mm/yr) (Fig. 1). This is theresult of the interaction of masses of humid Atlantic air enter-ing from the NE and E, with drier and colder masses movingfrom the SW. The north and south potential evapotranspirationvalues are between 850 and 750 mm/yr, respectively. Whencomparing these values with those of precipitation, the area canbe classified as subhumid-humid in the east, with a NE humidborder, and as subhumid-dry in the west, with a southern semi-arid border (Burgos and Vidal, 1951).According to Lemcoff (1991), this climatic classification is

the one that best relates to the complex distribution of nativevegetation. The flat pampa, the flooding pampa, and the east-ern part of southern pampa are related to a subhumid-humidclimate and the rolling pampa with a humid climate. Thesezones have water balance or incipient water excess. Instead, thewest pampa, the major part of southern pampa, and the xero-phytic scrubs-southern pampa ecotone are related to a dry-subhumid climate and incipient water deficit.

MATERIALS AND METHODS

Modern pollen samples were collected from surface soils(except 33, 22, and 17, which are from ponds) between 34° and39°S and 59° and 63°W (Fig. 1). Fossil pollen sections weresampled from arroyo walls and archeological excavations. Allsedimentary sequences are the result of episodic sedimentationand contain unconformities related to soil development anderosive surfaces. Their locations are shown in Figure 1.Sediment samples were processed using standard techniques

(Gray, 1965). Warm 10% KOH was used to dissolve humidacids and to deflocculate clays, HCl to eliminate CaCO3,heavy-liquid separation with ZnCl2 (d 4 2.2 g/ml) and HF toeliminate silicates, and an acetolysis mixture to reduce organicmatter. To every weighted sample, 200 ml of exotic pollensuspension containing 48,140 pollen grains ofFagus grandi-folia or five Lycopodiumtablets was added to calculate pollenconcentration (grains/g dry weight).Pollen preparations were examined under magnifications of

1000×. Identifications were based on comparison with the pol-len reference collection at the Laboratory of Palynology of theUniversity of Mar del Plata and with published pollen atlasesand keys (Heusser, 1971; Markgraf and D’Antoni, 1978).

LATE QUATERNARY PAMPA GRASSLAND OF ARGENTINA 75


MODERN POLLEN SPECTRA

The frequency distribution of modern pollen taxa from 36surface sites was arranged according to NE–SW vegetation andclimate trends (Fig. 2). Pollen introduced by human activitiessince the early 20th century, e.g.,Pinus, Eucalyptus,Cupres-saceae,Carduus sp., were excluded. On the other hand,Gramineae, Compositae (Tubuliflorae and Liguliflorae), Che-nopodiineae, and Cruciferae contain both native and intro-duced taxa. Because many of these cannot be differentiated atthe generic level, it is impossible to distinguish which of themare not associated with weeds and ruderal species. For thisreason, these taxa were included in the pollen sum.Samples from the humid-subhumid climate region corre-

spond to the rolling pampa, the flat pampa, the flooding pampa,and the eastern part of the southern pampa. Sites (1, 19, and25–33) show high proportions of Gramineae with variableamounts of hydrophytic and halophytic indicators. Highamounts of Cyperaceae and Chenopodiineae associated with

Alternanthera,Umbelliferae (includingEryngium), Monocoty-ledoneae, andPlantagorepresent either edaphic communitieswhere floods last for long periods, or ponds, swamps, anddepressions which are inundated nearly all year round and arecolonized by various reed-like species. In the flooding pampa,usually only one species dominates each habitat; e.g., “juncal”is dominated byScirpusand “totoral” byTypha(León, 1991).They are represented by high proportions of Cyperaceae andTypha in modern pollen samples (A. R. Prieto, unpublisheddata). Irregular percentages of Liguliflorae, reaching 55%, andCaryophyllaceae (ArenariaandSpergulariatypes) are present.Woody plants, such asCeltis andCondalia,are indicative ofthe Tala district which reaches the southern limit near Mar delPlata (Fig. 1).Samples from the subhumid-dry climate region (2–8, 11–18,

34–36) are for the most part related to the southern pampa-xerophytic scrubs ecotone and the west pampa.Within the southern pampa-xerophytic scrubs ecotone, the

edaphic communities that developed in the high areas between

FIG. 2. Frequency of pollen surface samples collected at sites indicated by number in vegetation units shown in Figure 1. The pollen data in percentageswere plotted from NE to SW. Vegetation units are as follows: R, rolling pampa; F, flooding pampa; If, inland pampa (flat pampa); Iw, inland pampa (west pampa);S, southern pampa; e, southern pampa-xerophytic scrubs ecotone; c, xerophytic woodland.

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the pampean valleys on poor sandy soils with calcareous crustsnear the surface are represented by high proportions of Cruci-ferae (up to 60%) and are differentiated from the halophyticand psammophytic communities of the southern pampa and thenorthwestern psammophytic steppe communities (westpampa), represented by Tubuliflorae and Chenopodiineae.Gramineae appear within a range of percentages, in generalbelow 30%; Cyperaceae have values lower than 2% in mostsamples, or disappear entirely. Low percentages of arborealand scrubby taxa, e.g.,Prosopis, Schinus, Condalia, Ephedra,and Prosopidastrum globosum,reflect the thorn-scrub plantcommunities. The pollen assemblages of the stations in semi-arid climate (9–10) are similar to those of the southern pampa-xerophytic scrubs ecotone, except forEphedra,which reacheshigher percentages.Although the inability to identify Gramineae pollen below

family level, the relatively small number of modern pollensamples from some units and the introduced taxa from humandisturbance have been an obstacle to the separation of thePampa grassland into different vegetation units; modern pollensamples are representative of plant communities existing overthe Pampa grassland along the NE–SW gradient of decreasingrainfall and evapotranspiration (Fig. 2).

PROXY RECORDS

Only five pollen records of late Quaternary age were se-lected for paleoenvironmental reconstruction. Fossil–pollen

spectra were zoned using the CONSLINK method (Birks andGordon, 1985).

Arroyo Sauce Chico(38°058S, 62°168W)

This record is a 340-cm-long section from an arroyo SauceChico cut (Prieto, 1989). Two distinct lithologic units are rec-ognized. From 0 to 2.50 m is fine sandy silt (loessic silt) andfrom 2.50 to 3.40 m is clayey silt. This unit shows coarselaminae of alternating dark and light layers and abundant dia-toms and mollusk remains, such asPseudosuccinea columella,Ancylussp., andBiomphalaria peregrina.The section spansmore than 7000 yr, based on one radiocarbon date of 6170 ±170 yr B.P. (Buschiazzo and Peinemann, 1985) and one TLdate of 4400 ± 300 yr (D. T. Rodbell, written communication,1995). Comparable dates were obtained from two other nearbysections which can be correlated lithostratigraphically (7240 ±170, 5320 ± 100, and 5350 ± 130 yr B.P., Table 1 and Fig. 3).The section can be divided into four pollen zones (Fig. 4).

The whole record is dominated by Gramineae and Tubulifloraepollen without significant changes during the entire ca. 7000yr. However, changes in other pollen taxa suggest vegetationalchanges.In the lowest zone (SCH 4), older than 5000 yr B.P., Cypera-

ceae,Typha, Plantago,and Ranunculaceae (Ranuculusssp)pollen reach their highest percentages. The pollen assemblageis comparable to the modern vegetation of the flooding and flatpampa. The alternation ofTyphaand Cyperaceae percentages

TABLE 1Radiocarbon Ages of the Pollen Sections and the Nearby Sections Lithostratigraphically Correlateda

Ages Lab.(yr B.P.) no. Typeb Material dated References

1950 ± 100 AC-0715 C Collagen González, M. A. (personal communication, 1989)3395 ± 107 LP-86 C Inorganic CaCO3 Huarteet al. (1983)3630 ± 60 CSIC-593 C Collagen Crivelli Monteroet al. (1987/1988)3900 ± 70 LP 317 C Shells Tonni, E. (personal communication, 1994)4400 ± 300 OTL 494 T Eolic sediment Rodbell, D. T. (written communication, 1995)4540 ± 550 alpha 2330 T Eolic sediment Zárate and Flegenheimer (1991)5180 ± 80 LP 307 C Shells Tonni, E. (personal communication, 1994)5320 ± 100 AC-0322 C Terrestrial mollusc González,et al. (1983)5350 ± 130 AC-0346 C Terrestrial mollusc Gonzálezet al. (1983)6010 ± 400 LP 88 C Collagen Crivelli Monteroet al. (1987/1988)6170 ± 170 HV 10239 C Organic matter Buschiazzo and Peinemann (1985)6190 ± 160 AC-522 C Shells Espinosaet al. (1984)7240 ± 170 AC-320 C Terrestrial mollusc Gonzálezet al. (1983)7560 ± 160 AC 717 C Organic matter González, M. A. (personal communication, 1989)9070 ± 140 AC 714 C Organic matter González, M. A. (personal communication, 1989)9100 ± 150 AC 434 C Organic matter González, M. A. (personal communication, 1989)9330 ± 195 AC 716 C Organic matter González, M. A. (personal communication, 1989)9490 ± 150 AC 996 C Inorganic Ca CO3 González, M. A. (personal communication, 1989)

10,610 ± 180 AA 1328 A Charcoal Zárate and Flegenheimer (1991)10,730 ± 150 I-12741 A Charcoal Zárate and Flegenheimer (1991)10,750 ± 160 AC 995 C Shells González, M. A. (personal communication, 1989)10,790 ± 120 AA 1327 A Charcoal Zárate and Flegenheimer (1991)

a See Figure 3 for location.bC, conventional14C date; A, AMS date; T, TL date.



suggests fluctuating water levels, as does the alternation of thesediment layers. Plumbaginaceae (Limonium) and Malvaceaepollen at the base of the zone could represent halophytic com-munities that developed in soils with high alkalinity related tothe high soluble salt concentration (NaCl) (Buschiazzo andPeinemann, 1985), suggesting poor drainage conditions. Thehighest concentrations of Fe and Mn and organic matter (2–4%) of the whole profile (Buschiazzo and Peinemann, 1985)are found in this zone, which suggests redox conditions. On theother hand, the presence of a 3- to 5-cm-thick diatomite layernear the top of the zone indicates shallow, but draining water.This, as well as pollen and sediment chemistry, indicates lo-cally more-abundant moisture.Unusually high percentages ofNothofagusand traces ofPo-

docarpusand Proteaceae suggest a westerly wind component.At present, these taxa grow in the Subantarctic forest region ofthe Andes, between 36° and 54°S, located ca. 750 km west ofthe study area. Xerophytic woodland pollen, although of lowfrequency, is represented bySchinusandEphedra.Beginning 5000 yr B.P., the predominant pollen types cor-

respond to taxa of the southern pampa, with a sharp reduction

and/or disappearance of humidity indicator taxa (Cyperaceae,Ranunculus, Plantago,andTypha). In pollen zone SCH 3 Le-guminosae (Papilionatae), Liguliflorae, and Chenopodiineaepredominate, associated with taxa related to sandy or alkalinesoils (e.g.,Limonium,Malvaceae, andOenothera). The ab-sence of time control does not allow an estimate of the age ofthe SCH2 pollen zone. The pollen assemblages are much thesame as those of SCH3. The differences are indicated by higherpercentages of some particular pollen type (e.g., Gramineae).Pollen representing long distance transport in both zones isrepresented by traces ofNothofagus, Podocarpus,and Protea-ceae. Both pollen zones resemble the southern pampa vegeta-tion. Accordingly, climatic conditions until 5000 yr B.P. musthave been wetter, with a humid-subhumid climate, while afterthis time it must have become drier with a subhumid-dry cli-mate.SCH1 is characterized by Chenopodiineae, Cruciferae, and

Ephedra reaching their highest relative percentages of thewhole sequence. The spectra are analogous to modern pollensamples from the area of the profile. Increase in Cruciferaeassociated with high proportions of Tubuliflorae and an abrupt

FIG. 3. Comparative stratigraphy of sediment sections analyzed for pollen. Pollen zone boundaries are bold lines with14C ages (Table 1) shown by trianglesand circles.

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FIG.4.

ArroyoSauce

Chico

profile.Diagram

ofpollenfrequencyandtotalpollenconcentration,subdivided

stratigraphicallyby

pollenzones.



decrease in Gramineae in the uppermost levels of the recordsuggest a strong disturbance. Whether this disturbance had anatural cause, or resulted from agriculture and pasture activitymust be evaluated from coincidental evidence. The earliestsettlement documented for this region dates to the late 19thcentury (Verettoni and Aramayo, 1976).Pollen concentration of the whole profile is highly variable.

These shifts are probably not the result of pollen productionchanges but rather of variable sedimentation rates related toepisodic sedimentation and the development of paleosols.

Fortín Necochea (37°23*S; 61°08*W)

This record is a 110-cm-long section from an archeologicalexcavation, located close to a pond. The section comprises thelast 6000 yr, with two radiocarbon dates of 6010 ± 400 and3630 ± 60 yr B.P. (Crivelli Monteroet al., 1987/1988). Thesedimentary profile consists of fine sand and very fine sand andcoarse silt (Fig. 5). The paleoenvironmental information fromthis record addresses the last 3600 yr, because no pollen couldbe recovered from the lower part of the section (Nieto andPrieto, 1987). Lack of pollen implies post-depositional pro-cesses that caused pollen destruction, perhaps related to car-bonate accumulation. Two pollen zones are recognized in thepollen stratigraphy (Fig. 5).

Zone FN2. The pollen assemblage is dominated by Cruci-ferae resembling the vegetation of the poor sandy soils of thesouthern pampa-xerophytic scrubs ecotone.

Zone FN1b. A change in environmental conditions is indi-cated by a rapid increase in both Gramineae (up to 50%) andTubuliflorae, and an abrupt decrease in Cruciferae pollen. Theappearance of Caryophyllaceae and Ranunculaceae (Ranuncu-

lus) associated withPlantago,Cyperaceae, and Umbelliferaesuggests increase in moisture conditions.

Zone FN1a.The increase in the amount of Chenopodiineae,Cruciferae, and Cyperaceae pollen contrasts with the decreasein Gramineae. This change suggests that locally the environ-mental conditions were similar to the pollen samples of thesouthwestern Pampa grassland. The increase in Chenopodi-ineae and Cyperaceae, and traces ofTyphaandAzolla,suggestthat salt flat communities in low areas near the edges of pondshad developed.These changes are difficult to interpret in climatic terms and

most probably reflect changes in the local environment, withinthe regional subhumid-dry climate. As some of these taxa arecurrently associated with human impact, the vegetation couldhave developed in habitats subject to more natural forms ofdisturbance (Grime, 1979) that resulted from some local fac-tors such as seasonal drought.Pollen concentration increases from FN2 to near the top of

FN1a, reflecting a decrease in the sedimentation rate. Thechange at the top of FN1a may be related to modern distur-bance.

Empalme Querandies(37°008S; 60°078W)

This pollen record is the result of the integration of threesections from an arroyo Tapalqué cut (Prieto, 1989). Four li-thologic units are recognized. The section from 3.00 to 1.80 mconsists of sandy silt and clayey silt with powdery CaCO3 inthe last 0.30 m. Silt is seen from 1.80 to 1.30, from 1.30 to 0.90m fine sandy silt, and from 0.90 to 0 m sandy silt to fine sand.The 300-cm-thick integrated record covers the late Quaternaryperiod. Seven radiocarbon dates provide chronological control:

FIG. 5. Fortín Necochea profile (modified from Nieto and Prieto, 1987). Diagram of pollen frequency and total pollen concentration, subdivided strati-graphically by pollen zones.

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1950 ± 100 (AC-0715); 7560 ± 160 (AC-717); 9070 ± 140(AC-714); 9100 ± 150 (AC-434); 9330 ± 190 (AC-716); 9490± 150 (AC-996), and 10,750 ± 200 yr B.P. (AC-995). Fourpollen zones are recognized (Fig. 6):EQ4 occurs prior to 10,700 yr B.P. The sedimentary deposit

represents the upper part of the “green” unit of the GuerreroMember of the Luján Formation (Fidalgoet al., 1986). Thechronological limits of this member are unknown. Only a fewradiocarbon ages of the remains of extinct megafauna and ofLittoridina parchapei from profiles close to the EQ pollensequences are available. The Guerrero Member basal unit(“brown”), which shows the highest record of grazer verte-brates, is dated at 28,900 ± 800, and the upper unit (“green”),which shows an increase in megaherbivores, dates between21,040 ± 450 and 13,070 ± 120 yr B.P. (Carbonariet al.,1992;Tonni and Figini, 1994). These faunal changes have been in-terpreted as representing a shift to cooler and drier conditions(Tonni and Cione, 1994).Gramineae and Chenopodiineae pollen dominate this zone.

The presence of other indicator taxa such as Rubiaceae (Rubiaand Relbuniumtype), Malvaceae,Calycera crassifolia,andEphedraassociated with typical communities of the interdunes(Alternanthera, Triglochin striata, Sysirinchium,and Cypera-ceae) (Cabrera, 1941, Verettoni, 1965) suggests a psammoph-ytic steppe. This zone has no analogues in the modern pollenspectra, but the association resembles the psammophyticsteppe of the northwestern part of the inland pampa (San Luisand La Pampa Province, Fig. 1), where agriculture is almostnonexistent and apparently natural grassland has been pre-served (León, 1991). In order to test this hypothesis it is nec-essary to study the modern pollen–vegetation relationship. Thesuccessional process of the community cannot be fully recon-structed because near the top of the zone the record is sterile orcontains pollen grains with evidence of microbial decay relatedto postdepositional processes. The pollen concentration is lowthroughout the zone, decreasing from the base to the top, sug-gesting open vegetation and/or rapid accumulation of the de-posit.EQ3 corresponds approximately to the interval from

10,000–8000 yr B.P. It is dominated by Cyperaceae (ScirpusandCarex) and Gramineae, with Umbelliferae (Hydrocotyletype), Ranunculaceae (Ranunculus), Althernantera,Chenopo-diineae, Leguminosae, and traces ofTypha.The pollen assem-blage is comparable to the flooding and flat pampa, suggestinga subhumid-humid climate. Increase in pollen representing hy-drophytic types suggests the transformation of ponds intoswamps, indicating an advanced stage of eutrophication. Thedecrease in the total pollen concentration near the top suggestsa progressive increase in the sedimentation rate.EQ2 begins at ca. 8000 yr B.P. and extends later than 7000

yr B.P. Gramineae begin to dominate, and most of the taxa thatcharacterized the previous zone disappear or have values ofless than 5%. This suggests a change toward the establishmentof humid grassland, such as the rolling pampa. This zone likelyconstitutes the culmination of a cycle initiated in the previous

zone. The hydrophytic vegetation that almost entirely coveredthe pond, suggested by the abundance of aquatic taxa in theprevious zone, allowed the subsequent filling-in of the pond.The low sedimentation rate at the base of this zone suggeststhat the process was probably very slow. Under these condi-tions, a soil would form in a hydromorphic environment.Judging from the succession of the hydrophytic communities

(EQ3) to a grassland community dominated by Gramineae(EQ2), it seems likely that the local environment represents ahydrosere succession, a phenomenon similar to the one thatpresently occurs in most of the flooding pampa. Due to diffi-culties encountered in determining the period of time involvedin the pedogenetic process, the upper age limit of this zonecannot be established. Moreover, truncation caused by an ero-sive episode precludes reconstruction of the successional pro-cess of the community in its entirety.The EQ1 zone includes the last ca. 3000 yr B.P. It is char-

acterized by Cruciferae,Ephedra,and Compositae tubulifloraepollen. The pollen assemblages are analogous to the modernsamples from the inland pampa and to the halophytic andpsammophytic communities of the southern pampa-xerophyticscrubs ecotone, suggesting a subhumid-dry climate. Abun-dance of Cyperaceae is probably related to poorly drained de-pressions. The vegetational characteristics of this pollen zoneare related to the modern geomorphic conditions that showlandforms typical of an arid-semiarid climate (Fidalgo,et al.,1986), such as paleodunes and low-lying areas, in spite of thepresent subhumid-humid climate.On the other hand, the palynological evidence suggests a

delay in the vegetation response to the establishment of thepresent environmental conditions.

Cerro La China(37°578S; 58°378W)

This site corresponds to an archaeological locality with threelevels of human occupation, the oldest corresponding to paleo-indians. Three radiocarbon dates of 10,730 ± 150, 10,790 ±120, and 10,610 ± 180 yr B.P. and one TL date of 4540 ± 550yr provide chronological control (Zárate and Flegenheimer,1991). The pollen analysis is based on three discontinuousloess sequences and the detailed palynological sequences aregiven in Prieto and Paez (1989) and Paez and Prieto (1993).Only the major events are summarized here. As in the ArroyoSauce Chico sequence, the pollen assemblages are dominatedby Gramineae and Tubuliflorae, while other pollen taxa permitenvironmental changes to be distinguished.The earliest pollen assemblage between 10,500 yr B.P. and

the early Holocene is characterized by substantial amounts ofpollen from hydrophytic plant communities, Cyperaceae,Plan-tago, Solanum,and Monocotyledoneae, suggesting an environ-ment with locally more effective moisture. Long-distance pol-len is represented by taxa from the Subantarctic forest region(high percentages ofNothofagusand Myrtaceae and traces ofPodocarpus, Lomatia,and Quinchamalium chilense) and issimilar to the one registered in the Sauce Chico profile, sug-



FIG.6.

EmpalmeQuerandíesprofile.Diagram

ofpollenfrequencyandtotalpollenconcentration,subdivided

stratigraphicallyby

pollenzones.

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gesting a westerly wind component. Based on the relationshipbetween percentages and concentration of long-distance pollenversuslocal pollen, Paez and Prieto (1993) proposed the de-velopment of a soil related to an interval of greater environ-mental stability and whose upper part was tuncated. Zarate andFlegenheimer (1991) link this pedogenetic event with a humidregime which may have existed at the time of the Holocenemarine ingression. Due to an erosive episode, the successionalprocess of the community could not be reconstructed for thefinal early Holocene. This erosive episode was correlated byZarate and Flegenheimer (1991) with drier conditions that pre-vailed during the time of the marine regression.After 4500 yr B.P. hydrophytic communities were replaced

by herbaceous taxa characteristic of the southern pampa(Gramineae, Caryophyllaceae,Plantago, Leguminosae, andLiguliflorae) and Eryngium edaphic communities. This hasbeen interpreted as a shift to subhumid-dry conditions and/or aslocal variations of the relief surface reflecting changes in thewater table. The increase of pollen concentration toward thetop of this assemblage has been interpreted as a buried soil.This would imply a climatic shift towards more stable andhumid conditions than those of existing at the beginning. Thisassemblage spans an unknown period of time.The last assemblage is separated from the previous one by a

slight erosive episode that has been related to new drier con-ditions. This occurred during historical times and the pollenspectra represent a steppe of grasses and show an over-representation of local taxa.

Arroyo Las Brusquitas(38°148S; 58°528W)

This record is a section from the arroyo Las Brusquitas. Thesedimentary profile consists of well-stratified silts, sands, andclays with several unconformities buried by eolian sand depos-its (Espinosaet al.,1984). Contrary to what has been publishedby D’Antoni et al. (1985), the period of time spanned by thepollen stratigraphy does not reach the present, since the eoliansand deposit from the upper section of this profile (ca. 1–1.6 m)has not been sampled. Four radiocarbon dates provide chrono-logical control: 3900 ± 70 (LP-317), 5180 ± 70 (LP-307) (E.Tonni, personal communication, 1994); 6190 ± 160 (AC-522)(Isla et al., 1988), and 3395 ± 107 yr B.P. from a nearbylitostratigraphically correlated profile (Table 1 and Fig. 3).Consequently, the pollen sequence extends only until ca. 2500–3000 yr B.P. The macro and micropaleontological and sedi-mentological analysis provide the paleoenvironmental infor-mation (Fidalgo and Tonni, 1983; Espinosaet al.,1984). Threepollen zones (LB3 to LB1), the upper and lower divided intosubzones, are recognized in the pollen stratigraphy (Fig. 7).LB3b may be assigned to the end of the late Pleistocene

based on the remains of extinct vertebrates and because it is anupper part of the Guerrero Member (Fidalgo and Tonni, 1983).The regional vegetation is dominated by taxa of the Pampagrassland and the local vegetation by Chenopodiineae, Polygo-naceae, and Cyperaceae.

In LB3a, the decrease of Chenopodiineae and Gramineaewith peaks of local populations of Cyperaceae, Ligulifloraeand Cruciferae suggests a modification of the vegetation. Thetop of this subzone containsin situ mollusk shells (Tagelusplebeius) in articulated trophic position, indicating a maximumsea level 2.2–2.5 m higher than at present and dated at 6190 ±160 yr B.P. Thus, sea water formerly occupied the valley ofthis creek.As a whole, the LB3 zone shows a change, from base to top,

from continental conditions to an environment with the strong-est marine influence of the entire profile. For the top of thiszone, polyhaline conditions are inferred from the micropaleon-tological analysis (M. Espinosa, personal communication,1994).LB2 begins at ca. 6000 yr B.P. and extends until after 3900

yr B.P. Chenopodiineae pollen reach their highest percentages,suggesting locally dominant halophytic communities similar tothose presently developing between the Atlantic coast and theMar Chiquita lagoon (37°449S; 58°259W), which are domi-nated byDistichlis spicataand Salicornia ambigua(Prieto,1993). The appearance ofTyphaand Hydrocharitaceae (Elodeatype) at 3900 yr B.P., as well as the increase in Gramineae anddecrease in Chenopodiineae, suggests a reduced marine influ-ence. A mixing of water is also indicated by wavy and len-ticular bedding (Islaet al.,1988). The change from oligohalineassociations to associations with higher contents of fresh-waterspecies coincides with the first occurrence of fresh-water pol-len indicators.LB1b is dominated by Hydrocharitaceae (Elodeatype) and

LB1a is dominated by Gramineae and Chenopodiineae andfresh-water communities (Myriophyllum, Hydrocharitaceae,Typha,Cyperaceae, and Ranunculaceae). The pollen assem-blages and micropaleontological remains suggest a fresh-waterenvironment that coincides with the basin filling since ca. 3900yr B.P.Pollen concentration is low for the whole profile, with the

exception of two peaks that coincide with horizons rich inorganic matter. The first of those is related to an episodicdeposition of organic matter assigned to a coastal marsh(Fidalgo and Tonni, 1983) and the second to a paleosol.

VEGETATION AND CLIMATIC HISTORY

None of the records individually provide a complete vegeta-tional history of the Pampa grassland for the Late Quaternarysbut considered jointly together they allow a reconstruction ofthe changes that occurred during this time.Prior to 10,500 yr B.P., herbaceous psammophytic vegeta-

tion similar to the one prevailing today in the northwestern partof the Pampa grassland had developed (EQ4). This paleo-vegetation suggests that a subhumid-dry climate formerly ex-isted in the area where the flooding pampa is presently found(Fig. 1). Previous to its reduction by agriculture and grazing,psammophytic steppe was found on Entisols with precipitationof ø450 mm, and on Mollisols with precipitation between 600



and 700 mm (León and Anderson, 1983) and moisture stress.According to Quattrocchioet al. (1993), late Pleistocene veg-etation near Sauce Chico is represented by xerophytic wood-land associated with psammophytic and halophytic steppe.None of the pollen assemblages from the Pampa grassland forthe final late Pleistocene represents “associations of Patagoniantypes” as defined by Iriondo and Garcia (1993).

On the other hand, Quattrocchioet al. (1993) suggest dryconditions for the late Pleistocene in the southwestern part ofPampa grassland based on ostracod analysis, with seasonalrains and shallow temporary waterbodies with low energy andhigh evaporation. This is in agreement with the low-lying in-terdune communities found in EQ4.In agreement with the pollen data, the climate for the late

FIG. 7. Arroyo Las Brusquitas profile (modified from D’Antoniet al.,1985). Diagram of pollen frequency and total pollen concentration, subdivided intopollen zones.

ALDO RAÚL PRIETO84


Pleistocene is characterized by precipitation ca.100 mm lowerthan at present, suggesting a northeasterly shift of the isohyets.This shift could be due to the more northernly location of theSubtropical circulation (but not as extreme as at the last glacialmaximum) or, according to Villagrán (1993), a stronger At-lantic Anticyclone and an asymmetry with the Pacific Anticy-clone that would imply a meridional circulation predominatelyfrom the southwest.Based on assemblages of modern vertebrate remains of Pata-

gonian and/or Central Argentina origin associated with extinctmammals, it has been suggested that during the latest Pleisto-cene, the environmental conditions were arid or semiarid andthat mean temperatures were lower than at present, or at leastwinter mean temperature was lower by 10°C (Tonni andFidalgo, 1978; Pradoet al.,1987).Related to lower sea levels, the coastline in the Buenos Aires

Province was 150 km to the east of its present location ca.11,000 yr B.P. (Fray and Ewing, 1963; M. Zárate, personalcommunication, 1994) and hence continentality must havebeen higher than today. At this time, regional vegetation in thearea close to the modern coast was dominated by Pampa grass-land taxa (LB3b), indicating that conditions were more conti-nental than today.Stabilization of the sandy areas possibly began with a gen-

eral amelioriation of climate starting ca. 13,000 yr B.P. (Lauerand Frankenberg, 1984; Clapperton, 1993) and a greater mois-ture content in the atmosphere related to warmer oceans (Mark-graf, 1993).The subsequent shift toward a vegetation characteristic of

ponds, swamps, and floodplains (Empalme Querandíes) or to-ward environments with locally more effective moisture (CerroLa China) occurred ca. 10,500 yr B.P. Since there are no avail-able dates for the base of the Sauce Chico pollen sequence, itis not possible to infer a synchronous change. Here, grasslandwith hydrophytic communities began to develop before 7000yr B.P. and continued until ca. 5000 yr B.P., whereas in Em-palme Querandíes, these hydrophytic communities were re-placed by grassland communities ca. 8000 yr B.P. and devel-oped only after 7000 yr B.P. The modern vegetation is char-acteristic of the flooding, flat, and rolling pampa with mesicconditions and precipitation above 800 mm, or of the edaphiccommunities where inundations can occur nearly all yearround. This suggests a subhumid-humid to humid climate anda mean annual precipitation close to modern levels, or a higherwater availability (excess precipitation over evaporation).Higher precipitation could be related to a poleward shift of

the Atlantic Convergence Zone, with a predominant circulationfrom the northeast, or to a weakening of the South AtlanticAnticyclone (Villagrán, 1993).During this period of subhumid-humid climatic conditions,

sea level reached its modern level by ca. 8000 yr B.P., and rose2.2 to 2.5 m higher than present at ca. 6000 yr B.P. in BuenosAires Province (Isla, 1989). Local marine influence in the Ar-royo Las Brusquita pollen section at 6200 yr B.P. is also sug-gested by a sharp modification in the local vegetation (LB3a)

and by the fact that halophytic communities became locallydominant (LB2) (Fig. 7).A temperate climate for the southeastern part of the Pampa,

starting at the beginning of the Holocene, was inferred byTonni et al. (1988) from the increase of Brazilian vertebrateremains and a decrease in those of Patagonian and/or CentralArgentina origin. However, Tonni (1992) and Iriondo and Gar-cía (1993) argue that arid conditions extended until 8500 yrB.P., based on the appearance of vertebrate remains similar tothose of the Guerrero Member associated with Pleistocene ex-tinct megafauna at 38°S and dated between 8890 and 7320 yrB.P. This interpretation cannot be accepted uncritically, espe-cially because the dates are unreliable (Dillehayet al.,1992).Two significant events occurred in the Pampa grassland at

about the time of the change from psammophytic steppe toflooding and humid grassland, between ca. 11,000 and 10,000yr B.P.; one is the appearance of paleoindians (Flegenheimerand Zárate, 1993) and the other, the extinction of Pleistocenemegafauna. Both events may have been related at least indi-rectly, to the type of climatic change examined here.A return to subhumid-dry conditions is suggested by the

Sauce Chico and Cerro La China pollen sequences, beginningca. 5000 yr B.P. In Empalme Querandíes and Fortín Necochea,these conditions began sometime before ca. 3000 (EQ1) and atabout 4000 yr B.P. (FN2), respectively. As in the previousperiods, it is not possible to infer a simultaneous change in allthe Pampa grassland.The late Holocene vegetation is characterized at Sauce

Chico, Empalme Querandíes, and Fortín Necochea by pollenassemblages similar to the psammophytic and halophytic com-munities of the southern pampa, associated with communitieswith moister edaphic conditions, which suggests changes in thelocal environment. At Cerro La China, a steppe replaced thehydrophytic communities. Some pollen zones show that veg-etation could have developed during this time in habitats sub-ject to more “natural” disturbances. This is also observed insome profiles for the Pleistocene–Holocene limit (A. R. Prieto,unpublished data.). These communities are different from thoseof the late Pleistocene, although they also suggest subhumid-dry conditions, but not as extreme. This is in agreement withthe suggestion of Zárate and Blasi (1993) that although aridconditions prevailed during the late Halocene they were notnearly as severe as those of the late Pleistocene. These authorssuggest that the reworking and redeposition of local sedimentswere significant and that these erosive episodes were the causeof the truncation of the soils that developed in many areasduring early Holocene period of stability. This is the cause ofsome of the gaps that are found in the pollen sequences referredto above.This vegetational change suggests a northward shift of the

anticyclonic centers, but in a situation not as extreme as in thelate Pleistocene. The shift may have been seasonal, like themodern situation, equatorward during winter and polewardduring summer, as suggested by Markgraf (1993) for otherlatitudes. This may be indicated by an environmental signal



that appears mixed in the records, e.g., vegetation from humidlocal environments that co-occur with more xeric vegetation,or associations of ecologically incompatible fauna (Tonni,1992).After ca. 5000 yr B.P., a marine regression began (Isla,

1989). In the Arroyo Las Brusquitas, it is suggested by thepreponderence of micropaleontological remains and pollen re-lated to a fresh-water environment that coincides with the basinfilling (LB1).According to Tonni (1992), the semi-arid and arid condi-

tions lasted until ca. 1000 yr B.P. when a fauna characteristicof temperate and humid conditions were reestablished. This, aswell as a very short but dry episode that occurred during the18th century during the Little Ice Age (Rabassa, 1987), is notevident in the pollen sequences. The SCH1 pollen zone and theupper part of the Cerro La China pollen sequences could berelated to this latter trend, but it is necessary to analyze agreater number of dated profiles to test this hypothesis. Thislack of definition may be the result of the circulation patternsthat remain in a transitional or intermediate stage between theextreme positions (Markgraf, 1983).Human settlements since the early 20th century are indicated

by the presence ofPinus and Myrtaceae (Eucalyptustype)associated with some ruderal taxa in the uppermost samples ofeach profile (except Las Brusquitas).

FINAL REMARKS

Although these broad intervals of climatic changes andtrends are only approximations based on the pollen evidenceand limited dates currently available, it has been possible tosuggest that vegetation reconstruction, based solely on shifts innonarboreal pollen, provides reliable paleoclimatic informa-tion. Accordingly, since the late Pleistocene, different types oftreeless grassland have dominated the vegetation of the Pampa.The primary climate-forcing factors have been the location andintensity of the Atlantic and Pacific anticyclones and changesin sea level. The former mainly influenced the distribution ofprecipitation, and the latter, the degree of continentality. Thesechanges caused significant modifications in the vegetation.Major drawbacks affecting interpretation of the late Quater-

nary vegetation and climate include the chronostratigraphy ofthe records and the inability to resolve problems of pollenidentity in some families. Although the modern pollen model isrepresentative to large extent of the Pampa grassland, “no-analogue” situations, as in late Pleistocene, exist. These couldbe related to the increasing influence of man in modern com-munities of the Pampa grassland or lack of modern pollensamples from particular communities in the regional model.However, we should not dismiss the possibility that modernvegetation communities analogous to those of the latest Pleis-tocene simply do not exist.The fossil pollen records were obtained from sedimentary

sequences that resulted from episodic sedimentation and con-tain unconformities. Therefore, the gaps resulting from erosion

and pedogenesis are variables that must be kept in mind wheninterpreting the data. Clearly there is a need for rigourousgeomorphic and stratigraphic analysis in combination with thestudy of pollen remains in order to place them in a regionalstratigraphic framework. It is necessary to improve dating con-trol, as well to obtain further high-resolution late Quaternaryrecords from the Pampa grassland.This study provides new data for the general model of the

atmospheric circulation for the midlatitudes temperate regions,thus contributing to the global understanding of the climaticchanges that occurred in the last 18,000 years in southernSouth America.

ACKNOWLEDGMENTS

This paper is an expanded version of part of my doctoral thesis. Thanks aredue to M. Zárate, who critically reviewed a previous draft of this paper, and M.Najjar for their help in the English translation of the manuscript. I am speciallyindebted to V. Markgraf for her valuable suggestions and constructive criti-cism. Part of the financial support was provided by CONICET and StiftungVolkswagenwerk to H. L. D’Antoni, and the rest by the Comision de Inves-tigaciones Científicas de la Provincia de Buenos Aires (CIC) to the author andM. E. Quattrocchio, and by the Universidad Nacional de Mar del Plata (Grant086/92). M. A. González and E. P. Tonni kindly provided the14C datings ofthe Empalme Querandíes and Arroyo Las Brusquitas, respectively. D. T. Rod-bell provided the TL dating of the Sauce Chico. For his help in the field, Iexpress my gratitude to M. Quattrocchio. Figures were drafted by V. Ber-nasconi, M. Farengas, and M. Tomás. I thank Chalmers Clapperton and ananonymous reviewer for their helpful comments on an early version of themanuscript.

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