Palaeoenvironmental evolution and sea-level fluctuations along the southeastern Pampa grasslands...

JOURNAL OF QUATERNARY SCIENCE (2006) 21(3) 227–242Copyright � 2005 John Wiley & Sons, Ltd.Published online 1 November 2005 in Wiley InterScience (www.interscience.wiley.com). DOI: 10.1002/jqs.953

Palaeoenvironmental evolution and sea-levelfluctuations along the southeastern Pampagrasslands coast of Argentina during the HoloceneISABEL VILANOVA,1* ALDO R. PRIETO1 and MARCELA ESPINOSA2

1 CONICET, Laboratorio de Paleoecologıa y Palinologıa, Departamento de Biologıa, Facultad de Ciencias Exactas y Naturales,Universidad Nacional de Mar del Plata, Mar del Plata, Argentina2 Centro de Geologıa de Costas y del Cuaternario, Facultad de Ciencias Exactas y Naturales, Universidad Nacional de Mar del Plata,Mar del Plata, Argentina

Vilanova, I., Prieto, A. R. and Espinosa, M. 2005. Palaeoenvironmental evolution and sea-level fluctuations along the southeastern Pampa grasslands coast of Argentinaduring the Holocene. J. Quaternary Sci., Vol. 21 pp. 227–242. ISSN 0267-8179.

Received 28 September 2004; Revised 25 April 2005; Accepted 31 May 2005

ABSTRACT: Holocene pollen and diatom analyses and complementary data from �18O and �13C,malacology and sedimentology have provided a detailed record of vegetation history and palaeoen-vironmental change at arroyo Las Brusquitas, on the southeastern coast of the pampas of Argentinaespecially in relation to past sea levels. Holocene palaeosalinity trends were estimated by DetrendedCorrespondence Analysis and by salinity indexes based on pollen and diatom data. As a consequenceof sea-level rise from the postglacial an extensive wave-cut platform formed over which Holoceneinfilling sequences were deposited unconformably. In these sequences, variation in pollen anddiatom assemblages occurred in agreement with changes in mollusc diversity and abundance, iso-tope values, and sediment deposits. Between ca. 6700 and 6190 14C yr BP (6279–6998 cal. yr BP)saline conditions predominated in an environment highly influenced by tides and salt water duringthe Holocene sea-level highstand. Between ca. 6200 and 3900 14C yr BP (4235–4608 cal. yr BP)shallow brackish water bodies formed surrounded by saltmarsh vegetation that became more wide-spread from 5180 14C yr BP (5830–6173 cal. yr BP) to 3900 14C yr BP in relation to a sea-levelstabilisation period within the regression phase. Less saline conditions marked by frequent variationsin salinity predominated between ca. 3900 and 2040 yr 14C BP (1830–2160 cal. yr BP). The intertidalsaltmarsh environment changed into a brackish marsh dominated by freshwater conditions andsporadic tidal influence. Halophytic vegetation increased towards ca. 200014C yr BP indicatingthat saline conditions may be due to either desiccation or an unusually high tide range with rarefrequency. After ca. 2000 14C yr BP the sedimentary sequences were buried by aeolian sand dunes.Changes in Holocene vegetation and environments in Las Brusquitas area are in agreement withdata obtained from various southeastern coastal sites of the Pampa grasslands. Copyright � 2005John Wiley & Sons, Ltd.

KEYWORDS: Pampa grasslands; palaeoenvironments; palaeosalinity; vegetation history; Late Quaternary; sea level; pollen; diatoms; Argentina.

Introduction

Palaeoenvironmental studies along the coast of the Pampagrasslands of Argentina (34 �–39 � S) demonstrate that climatechange and sea-level fluctuations were the main forcing factorsaffecting vegetation and coastal environments during theHolocene (e.g. Espinosa et al., 1984; Isla et al., 1986; Islaand Espinosa, 1995, 1998; Espinosa, 1994, 1998; Prieto,1996; Quattrocchio et al., 1998; Stutz et al., 1999, 2002; Stutz

and Prieto, 2003). In the southeastern region of the coast of thePampa grasslands (Fig. 1), marine influence is evident in out-crops (e.g., arroyo Las Brusquitas and arroyo La Ballenera),coastal lagoon deposits (Mar Chiquita lagoon), and mud-filleddepressions (Punta Hermengo).Sea-level changes proposed for the coast of the Pampa

grasslands in the past two decades indicate that a low, latePleistocene sea level (e.g. Guilderson et al., 2000; Violanteand Parker, 2000, 2004) and a mid-Holocene highstand (e.g.Gonzales et al., 1983, Isla, 1989, Isla and Espinosa, 1998,Violante and Parker 2000, 2004; Cavallotto et al., 2004)occurred in this region. Discrepancies do exist among recordsof relative sea-level highstand derived from different coastalsections, the largest variation in elevation appears to be morethan 8m between 6000 and 5000 14C yr BP. The height

*Correspondence to: Isabel Vilanova, CONICET, Laboratorio de Paleoecologıa yPalinologıa, Departamento de Biologıa, Facultad de Ciencias Exactas y Natur-ales, Universidad Nacional de Mar de Plata, Fures 3250, 7600 Mar del Plata,Argentina. E-mail: [email protected]

difference reduces towards the present day. In general, thesecurves show an increasing sea level from post glacial (20 000to 7000 14C yr BP), reaching the highest levels between ca.6500 and 6000 14C yr BP, followed by a regressive trendcharacterised by a stepped sea-level fall to the present position.Outcrops from arroyo Las Brusquitas yield the most

complete record of post glacial sea level available in theregion. These estuarine deposits are considered a good indica-tor of sea level change because wave effects and tidal actionhave not affected the deposits as compared to beach deposits.Arroyo Las Brusquitas has been studied since the beginning ofthe 20th century. Frenguelli (1921, 1945) has focused on dia-tom assemblages and stratigraphy, classifying the stratigraphicunits into ‘Lujanense’ (late Pleistocene), and ‘Querandinense’,‘Platense’ and ‘Aimarense’ (Holocene). Palaeoenvironmentalreconstruction and interpretations of the changes as a conse-quence of sea-level variations during the Holocene has beencarried out on the basis of sedimentology and palaeontology(Fidalgo and Tonni, 1983), micropalaeontology (Espinosaet al., 1984, Isla et al., 1986), palynology (D’Antoni et al.,1985), malacology (Tonni et al., 1999) and isotope analysis(Bonadonna et al., 1995). Some of the stratigraphic and palyno-logical interpretations have been reviewed by Prieto et al.(2003).Our aim here is to reconstruct a detailed vegetation history

and palaeoenvironmental evolution based on the combination

of pollen and diatom analyses at arroyo Las Brusquitas area.Accurate chronological records by radiocarbon dating andthe integration of complementary data from �18O and �13C,malacology and sedimentology contribute to a reliable inter-pretation of the environmental evolution, especially the rela-tionship with past sea levels. We compare arroyo LasBrusquitas sequences with other palaeoenvironmental recordsfrom the region, summarising the vegetation and environmen-tal evolution for the Holocene for the southeastern coast of thePampa grasslands.

Study area

The study region is located on the Atlantic coast, in the south-eastern part of the Pampa grasslands of Argentina (Fig. 1).Arroyo Las Brusquitas (38 � 140 S, 57 � 460 W) has a drainagebasin of 58 km2 (Isla and Espinosa, 1998) on a lowlanddepression formed by a low gradient valley, which devel-oped during the Pleistocene. Along the coastal area low-altitude cliffs have developed beneath cliff-top dunes (Islaet al., 1996). Between the arroyo Las Brusquitas outcropand the present shoreline a wave-cut platform is sporadicallypresent on the Pliocene aeolian sediments or on late Pleisto-cene debris flows. The modern tidal regime has an amplitudeof less than 1m (microtidal) and is dominated by episodicstorms (Isla and Espinosa, 1998). Tides have little influenceon creek-water salinity, which varies from 1.3% to 0.4%from the outlet to 2 km upstream.Prior to European settlement there was open vegetation char-

acterised by dune-building species on the coast and by inlandgrasslands. The present vegetation is strongly affected by differ-ent agricultural land-use practices and anthropogenic activ-ities. It is characterised by several kinds of grass, weeds andherbs along with exotic trees. This vegetation is distributed oversand dunes next to the shoreline, the fixed sand dunes and thecreek banks (Table 1).

Materials and methods

Pollen and diatom sampling

Fossil pollen and diatoms were sampled from a 4m thicksedimentary sequence near the Arroyo Las Brusquitas outlet(Profile 1, Fig. 2). Eighty-five pollen samples were taken fromthe top outcrop surface down to water level every 10, 5 or2 cm following a regular sampling interval. Sixteen diatomsamples were collected from sedimentary layers defined bymajor changes in the sediment (colour, grain size, sedimentarystructures).Other shorter sequences were recognised in the area a few

metres downstream from Profile 1 (Profile 2) and 1 km southof the creek, in the intertidal zone (Profile 3) (Figs 1 and 2).Eight samples from Profile 2 were collected for additional pol-len analyses. Mollusc deposits from Profile 3 (Fig. 2) in thesame stratigraphic position as that in Profile 1 and 2 (Tonniet al., 1999; Prieto et al., 2003) were considered for correlationand interpretation of the coastal evolution.Two surface sediment samples for pollen analysis were col-

lected with a spatula from the upper soil of the creek bank andfrom fixed sand dunes, at a depth between 2 and 5 cm belowthe surface.

Figure 1 Location of sites discussed in the text in the southeasternPampa grasslands coast

228 JOURNAL OF QUATERNARY SCIENCE

Copyright � 2005 John Wiley & Sons, Ltd. J. Quaternary Sci., Vol. 21(3) 227–242 (2006)

Pollen analysis

Standard palynological techniques (Gray, 1965; Faegri andIversen, 1989) were employed to process pollen samples. FiveLycopodium clavatum spore tablets were added to calculatepollen concentration in grains per gram. (NaPO3)6 was usedinstead of hot KOH and no acetolysis was performed. Thesemodifications were introduced in order to preserve palaeomi-croplankton. HCl was used to eliminate carbonates. Heavyliquid separation using ZnCl2 and HF was used to eliminatesilicates. The organic residues were stained with Safranine2% and sieved through a 10-mm mesh to eliminate fine parti-cles. These residues were mounted in glycerine. Pollen pre-parations were examined under magnifications of 1000�with an Olympus BH2 microscope. Identification was basedon comparison with the pollen reference collection at thePaleoecology and Palynology Laboratory at the UniversidadNacional de Mar del Plata. Other palynomorphs besides pollengrains were identified: algae (Pediastrum), copepod or anostra-cod resting eggs (Cobricosphaeridium, Head et al. (2003)),dinocysts of Peridinoideae, Spiniferites and Operculodinium,and acritarchs. The total count of surface samples was 961palynomorphs from the creek bank and 789 from fixed sanddunes. In Profile 1 pollen sum varied between 215 and 4301palynomorphs, and between 2264 and 4682 in Profile 2.Twenty samples from Profile 1 were sterile; palynomorphpreservation was relatively poor between the depths of 160and 190 cm. Many samples contained aggregates or clumpsof pollen grains of the families Poaceae, Chenopodiaceae,Asteraceae, Asteraceae subf. Cichorioideae and Brassicaceae.Pollen clump percentages were calculated following Prieto(2000).Relative pollen frequencies were calculated in relation to a

total sum which excluded algae, Cobricosphaeridium, dino-cysts and acritarchs. Zonation of the Profile 1 was carried outby stratigraphically constrained cluster analysis (CA). Pollentypes selected for this analysis were those whose mean valuewas at least 1% or whose maximum value was higher than5%. These types are Poaceae, Chenopodiaceae, Ruppia (ditch

grass-tasselweed), Typha, Myriophyllum (water-milfoil), Aster-aceae, Cyperaceae and Apiaceae. Although Brassicaceae,Asteraceae subf. Cichorioideae and Polygonaceae-typeaccomplished the above requirements they were excludedbecause of their local over-representation. CA divided the pol-len diagram from Profile 1 into seven zones (P1 to P7, Fig. 3).

Diatom analysis

For diatom analysis, each sediment sample was treated withHCl (10%) to remove carbonates, and oxidised with H2O2

(30%) to remove organic matter. After several washings withdistilled water, slides were mounted with Pleurax. A minimumof 300 frustules were counted for each sample using a magni-fication of 1000� . Identification was carried out mainly withreference to Hustedt (1930, 1937–1938, 1959–1966), Germain(1981), Archibald (1983) and Krammer and Lange-Bertalot(1991). Diatom species were grouped in relation to salinity tol-erances and life-forms, following the ecological classificationof De Wolf (1982) and Vos and De Wolf (1993). Fifty-nine dia-tom taxa were identified. Species with less than 1% representa-tion were not graphed. Diatoms are shown as present between285 and 310 cm since preservation was poor, and it was notpossible to obtain a reliable sum to calculate the percentages.Based on stratigraphically constrained cluster analysis, the dia-tom diagram is zoned into six zones (D1 to D5, Fig. 4).

Pollen and diatom salinity index

Pollen Salinity Index (PSI) and Diatom Salinity Index (DSI) wereestimated. PSI represents halophytic pollen types (Chenopodia-ceae, Limonium brasiliense (sea lavender) and Ruppia) as apercentage of halophytic plus hydrophytic pollen types (Typha,Myriophyllum, Cyperaceae, Alternanthera) and freshwateralgae (Pediastrum). Higher percentages represent more saline

Table 1 Plant species in arroyo Las Brusquitas area

Sand dunes next Fixed sand dunes Creek banksto the shoreline

Weeds-grasses Weeds–grasses Exotic trees Sedges, bulrush, aquatic and semi-aquatic herbsSpartina coarctata Cortaderia selloana (Pampas grass) Myoporum laeatum Scirpus americanus (bulrush)(cordgrass) (myoporum)Cakile marıtima Spartina ciliata (cordgrass)(sea rocket) Stipa neesiana (needle grass) Typha latifolia (cat-tail)

Poa sp. (meadow grass) Acacia longifolia(Sydney golden wattle)

Aster squamata (starwort) Senecio bonariensis (groundsel-ragwort)Ambrosia tenuifolia (ragweed)Grindelia discoidea (gum-weed) Tamarix gallica Hydrocotyle ranunculoides (water pennywort)

(tamarisk)Pluchea saggitalis(wingstem-camphorweed)Solidago chilensis (goldenrod) Ludwigia peploides (false loosestrife)Hydrocotyle bonariensis(water pennywort)Rhapanus sativus (radish) Rorippa nasturtium aquaticum (mustard family)Margyricaprus pinnatus (pearl berry)Glandularia dissecta (vervain family) Azolla filiculoides (fern)Marrubium vulgare (white horehound)Melilotus albus and M. indicus (melilot)Dipsacus sativus (teasel)Cynara cadunculus (cynara)

HOLOCENE EVOLUTION OF PAMPA COAST, ARGENTINA 229


conditions. DSI was calculated according to Byrne et al.(2001).

Statistical analysis

A Detrended Correspondence Analysis (DCA) was performedto study pollen and diatom groups ordination between ca.6200 and 2000 14C yr BP from Profile 1. In order to have thesame number of pollen and diatom samples, an arithmeticalaverage was calculated for pollen samples from each sedimen-tary layer recognised and sampled for diatom analysis (Fig. 2).Diatom taxa were grouped according to salinity tolerances andlife-forms, while the same pollen types selected for the clusteranalysis were used for DCA. The complete display of the trajec-

tory (path) resulted from the DCA of 14 records of 18 pollenand diatom types was examined and interpreted in terms ofpast salinity changes (Fig. 5). For plotting, calculations, CAand DCA the TILIA and TILIAGRAPH software (Grimm,1992) were used.

Results

Stratigraphy

A summary of the field description of Profile 1 and its correla-tion with Profile 2 and 3 is provided in Table 2. Pliocene depos-its are mainly composed of loessoid sediments (Zarate, 2003)

Figure 2 Studied profiles. Stratigraphic units are described in Table 2



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Malva

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Diatom

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1



Figure 5 DCA biplot of axes 1 and 2. (a) Pollen and diatom variable scores. (b) Sample scores represented by stratigraphic units and radiocarbondates according to their position

Table 2 Sedimentary description of stratigraphic units from Profile 1 and the correlation with Profiles 2 and 3 of arroyo Las Brusquitas area

Unit Subunit Sedimentary description of stratigraphic units (after Frenguelli, 1921; Fidalgo and Tonni,1983; Espinosa et al., 1984, Prieto et al., 2003)

6 Aeolian deposits and sand dunes partially modified by modern developed soil. AimarenseFragments of shells.

f Clayey silt, light grey with lens of sand very friable. Massive to coarse stratified.e Clayey silt, light grey. Concentration of organic matter at the top, dark greyish brown.

Massive to coarse stratified.5 d Clayey silt with allochthonous and episodic concentration of organic matter at the top.

Basal contact abrupt and irregular. Upper contact abrupt. Massive to coarse stratified.c Clayey silt, brown to greenish yellow. Bioturbated at bed contact. Platenseb Clayey silt, yellowish. Silty sand at the base. Abrupt and irregular basal contact.a Clayey silt, wavy bedding.e Silty sand, yellow brown.d Sandy clay, light grey.

4 c Clayey silt, wavy bedding.b Clayey silt, wavy bedding.a Clay with intercalation of silt. Correlated to Profiles 2 and 3.c Clayey fine sand, light brown. Coarse stratified.

3 b Silty sand, light brown. Root traces with oxidation haloes. Querandinensea Clayey fine sand, greenish grey.

2 Debris flow deposits (Profiles 2 and 3). Lujanense (Late Pleistocene)1 Loessoid sediments. Chapadmalal Fm (Pliocene)



and unconformably underlie the late Pleistocene debris flowdeposits (Fig. 2). The Holocene sequence starts with very fine,quartz sand without molluscs, above which very fine clayeysand with marine molluscs are deposited. The sequence con-tinues up-section with well-stratified silt, clay and fine sand,and towards the top, clayey silt, and alternating sand andmud with brackish/freshwater molluscs. These deposits areoverlain by coarse-grain sand.

Radiocarbon dates and chronological control

Eight radiocarbon dates were obtained from total organicmatter and inorganic carbon from shells. The results arepresented in Table 3. All ages reported are in uncalibratedradiocarbon years before present (14C yr BP) and calibratedages (cal. BP� 2�). They indicate that the studied sectionswere deposited from ca. 6700 14C BP to present time. Ifsome reservoir effect exists on the shell dates, it has not beencorrected because there has been no study of the reservoireffect on modern molluscs that could be used to correct thedates.

Modern and fossil pollen spectra

The pollen diagram in Fig. 3 illustrates percentages of the lead-ing taxa from the two surface samples and from fossil samples,along with the cluster analysis dendrogram from Profile 1.Figures 6 and 7 illustrate palynomorph concentrations and pol-len clump percentages, respectively. Figure 8 represents pollenpercentages, total concentration and the mollusc records fromProfile 2.

Description of the modern pollen spectra

Modern pollen spectra reflect present local vegetation asso-ciated mainly with topography features and hydrology, whichis evidenced in taxa proportions (Fig. 3). Poaceae, Asteraceaeand Brassicaceae are more abundant on sand dunes (50%,20% and 13%, respectively) than on creek banks. In contrast,Cyperaceae and Typha show higher percentages on creek bank(25% and 10%, respectively) than in sand dunes because theyare related to the higher water table near the creek. Chenopo-diaceae percentages are low in both spectra (less than 15% onthe creek bank and 5% in sand dunes) compared to percen-tages of these taxa from halophytic communities (Stutz andPrieto, 2003). These spectra have a taxa composition similar

to the fossil spectra but differ in pollen proportions and in algaeand cyst content (Fig. 3).

Description of the fossil pollen and diatom zones

Diatom zones (D1 to D5, Fig. 4) covary along with pollenzones (P1 to P7, Fig. 3), consequently, for purposes of discus-sion, they are described together as LB1 to LB7 zones.Zone LB1 (P1, 395–316 cm) is characterised by Chenopodia-

ceae and Poaceae with a mean of 50% for the former and 30%for the latter (Fig. 3). Asteraceae, Cyperaceae and Apiaceaehave percentages lower than 10% with Ruppia, Typha andRanunculus registered in traces. Acritarchs have a mean of8%. Traces of Peridinoideae and the presence of Spiniferitesare also observed. Total pollen concentration is up to 350grains per gram. Few pollen clumps of Poaceae are registered.Zone LB2 (P2, 316–282 cm) Chenopodiaceae and Poaceae

have means of 34% and 25%, respectively, Asteraceae 29%and Asteraceae subf. Cichorioideae 11.5%. Acritarchs average18% and Spiniferites and Peridinioideae are registered as pre-viously. Total pollen concentration increases to 3500 grainsper gram because of the increase in most of the taxa concentra-tion (Fig. 6). Few pollen clumps of Poaceae, Asteraceae andAsteraceae subf. Cichorioideae are registered. Diatom abun-dance is very low (Fig. 4); the presence of marine/planktonActinoptychus splendens (Shadbolt) Ralfs, marine/brackishbenthos Nitzschia compressa (W. Smith) Grunow, brackish/freshwater tychoplanktonic Fragilaria construens (Ehr.) Gru-now and marine/plankton Actinoptychus vulgaris Schumannis observed.Zone LB3 (P3, D1, 282–268 cm) is marked by similar percen-

tages and taxa composition to the previous zone but the meansof Ruppia and Brassicaceae increase to 9% and 15%, respec-tively. Also acritarchs increase, having a mean of 35%. Totalpollen concentration is between 2250 and 5000 grains pergram (Fig. 6). Pollen clumps mainly from Brassicaceaeincrease, averaging 7%. Diatom assemblage is dominated byRhopalodia gibberula (Ehr.) O. Muller, epiphyte in brackishwater; accompanied by Synedra platensis Frenguelli and Coc-coneis placentula Her, both epiphytes as well (Fig. 4).This zone is stratigraphically correlated to the pollen spectra

from Profile 2 (Fig. 8), which show the same taxa composition.The spectra of the Profile 2 are characterised by both Chenopo-diaceae and Poaceae with means of 47% and 38%, respec-tively, and Ruppia (6%). Acritarchs have higher percentagescompared with their values in the zone LB3, averaging 76%.Total pollen concentration is also much higher than in zoneLB3, ca. 35 000 grains per gram.Zone LB4 (P4, D2, 268–228 cm) Chenopodiaceae is

dominant, with a mean of 57% accompanied by Poaceae(20%), Asteraceae and Cyperaceae (6%). Asteraceae subf.

Table 3 Radiocarbon dates obtained from the Profile 1 and 3 of arroyo Las Brusquitas area

Profile Depth (cm) 14C yr BP Cal BP 2� Laboratory no. Material References

1 120 –125 2040�80 2260–2300 Beta 152124 Organic matter Prieto et al. (2003)1830–2160

1 161– 163 2210�70 2010–2350 Beta 157779 Organic matter Prieto et al. (2003)1 225– 230 3900�70 4235–4608 LP 317 Heleobia sp. Bonadonna et al. (1995)1 250– 260 5180�80 5830–6173 LP 307 Heleobia sp. Bonadonna et al. (1995)1 280– 290 6190� 160 6279–6998 AC 434 Tagelus plebeius Espinosa et al. (1984)1 325– 330 6380�60 6710–6990 Beta 156145 Mytilus platensis Prieto et al. (2003)3 70–80 5830�80 6485–6867 LP 297 Ostrea equestris Tonni et al. (1999)3 110 36130�620 Beta 162051 Bulimulus bonariensis Prieto et al. (2003)



Figu

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Pollen

concentrationdiagram

from

Profile

1



Cichorioideae, Brassicaceae and Polygonaceae-type average5%, Apiaceae Ruppia and Typha, 3%. Acritarchs averageabout 50% with traces of Cobricosphaeridium. Total pollenconcentration increases to more than 10 000 grains per gram(Fig. 6). Diatom assemblages are dominated by marine/brack-ish epiphytes and benthic Cocconeis scutellum Ehr., Naviculapennata Schmidt, Navicula zosteretiiGrunow, Amphora salinaW. Smith, Campylodiscus clypeus Ehr., Surirella striatula Tur-pin, Rhopalodia gibberula (Ehr.) O. Muller, Synedra platensisFrenguelli and Cocconeis placentula Ehr. (Fig. 4).Zone LB5 (P5, D3, 228–192 cm) Poaceae and Chenopodia-

ceae decrease, averaging about 36% and 26%, respectively.

Asteraceae and Asteraceae subf. Cichorioideae have meansof 14% and 11%, respectively, and Brassicaceae, Polygona-ceae-type, Typha and herbs 5%.Myriophyllum and Pediastrumappear in traces. Cobricosphaeridium has a mean of 25%,Peridinioideae 14% and acritarchs 16%, while Operculodi-nium dinocysts appear in traces. Total pollen concentrationdecreases to 1500 grains per gram toward the top. Pollenclumps from most taxa average less than 5% (Fig. 7). All paly-nomorphs from the upper part of this zone are poorly pre-served. The marine/brackish diatoms begin to decrease whilethe brackish/freshwater ones become important. The assem-blages are dominated by Cocconeis scutellum and Navicula

Figure 7 Pollen clumps diagram from Profile 1. Asteraceae include undifferentiated Asteroideae and Mutisiae

Figure 8 Pollen percentage, total concentration and mollusc diagram from Profile 2



zosteretii (epiphytes) and Surirella striatula (benthic), accompa-nied by Cocconeis placentula (epiphyte) and Fragilaria con-struens (tychoplankton) (Fig. 4).Zone LB6 (P6, D4, 192–164 cm) Poaceae is dominant, aver-

aging 60%. It is accompanied by Asteraceae, 22%, along withChenopodiaceae, average 7% and Cyperaceae and Apiaceae,4%. Both Pediastrum and Cobricosphaeridium increase, withmeans of 7% and 30% respectively, whereas Peridinioideaereach a percentage of 70% at the base to disappear at thetop. Acritarchs decrease, having a mean of 7%. Traces ofOper-culodinium dinocysts are registered at the base. Total pollenconcentration increases to 2500 grains per gram mainly dueto the increase in Poaceae concentration (Fig. 6). At the base,pollen grains and cysts show poor preservation. The domi-nance of Fragilaria construens (tychoplankton) accompaniedby Cyclotella meneghiniana Kutz. (plankton) and Denticulaelegans Kutz. (benthos) is observed. Rhopalodia gibberulaand Campylodiscus clypeus are registered as in the previouszone but only one marine taxa Navicula zosteretii is present(Fig. 4).Zone LB7 (P7, D5, 164–120 cm) is characterised by fluctuat-

ing values of Chenopodiaceae and Poaceae, averaging 33%and 30%, respectively. Asteraceae 12%, Apiaceae 11%, Aster-aceae subf. Cichorioideae 8%, Myriophyllum 3% and Ranun-culus in traces are registered. Cobricosphaeridium averagesabout 20%. At the base, high values of Pediastrum (up to50%) are in agreement with high relative abundance (62%)of the planktonic and freshwater diatom Aulacoseira granulata(Ehr.) Ralfs, which gradually decreases while the relative abun-dance of brackish/freshwater taxa Cyclotella meneghiniana,Fragilaria construens, F. construens var. venter and Epithemiaadnata increase (Fig. 4).

Pollen–diatom ordination

The pollen and diatom DCA ordination in Fig. 5 represents asalinity gradient. Samples display a trajectory (path) from moresaline at ca. 6200 14C yr BP to less saline conditions at ca. 2000yr 14C BP (Fig. 5). Samples between ca. 6200 and 3900 14C yrBP are dominated by Typha, Ruppia, Chenopodiaceae andCyperaceae, along with polyhalobous and mesohalobous dia-toms, associated to benthos, aerophilous and epiphytes life-forms. Poaceae, Asteraceae, Apiaceae, Myriophyllum andPediastrum dominate samples between ca. 3900 and 200014C yr BP along with oligohalobous, halophilous and indiffer-ent tychoplankton and plankton life-forms.

Discussion

Vegetation history and environmental changes

Late Pleistocene debris flows (ca. 36 000 14C yr BP) suggestmassive land removal and an increase in creek water dis-charge. Continental environments dominated during the latePleistocene since sea level was � 70m at ca. 36 000 14C yrBP and at � 105m during the Last Glacial Maximum (LGM)(Guilderson et al., 2000), and consequently the shoreline waslocated ca. 150 km to the east of its present location (Fray andEwing, 1963).Sea-level rise following the LGM caused an extensive wave-

cut platform. Holocene infilling sequences were depositeduncomformably over this platform during the ensuing marine

transgression affecting the coastal vegetation. Variations in pol-len and diatom assemblages occurred during the Holocene inagreement with changes in mollusc diversity and abundance,�18O and �13C records and the lithology (Fig. 9).From ca. 6700 to 6400 14C yr BP (LB1) increasing Chenopo-

diaceae percentages and low values of Ruppia suggest salt-marsh vegetation, and traces of Spiniferites indicate brackish-water influence in an environment with slight freshwater input,as indicated by Cyperaceae, Apiaceae and traces of Typha,Myriophyllum and Ranunculus. Acritarchs and Peridinoideaecysts indicate a brackish environment with salinity variationswhen they are considered in conjunction with the rest ofthe assemblage. Pollen clumps of Poaceae indicate localdeposition. Sea-level was increasing above the modern level.The absence of mollusc remains suggests that the environmentin which vegetation developed was far from the supratidalzone, and that the tidal effects were insufficient to transportmollusc shells. Low values of total pollen concentration couldbe related to rapid sedimentation in an environment with sud-den changes in water level.From 6400 to 6200 14C yr BP (LB2) Chenopodiaceae indi-

cate salt marsh vegetation with brackish-water influence sug-gested by traces of Ruppia and low values of acritarchs. Thepollen spectra are similar to those obtained from modern sam-ples from bottom of the Mar Chiquita coastal lagoon, whichrecord pollen that come not only from the low saltmarsh butalso from halophytic vegetation from middle and high salt-marsh communities (Stutz and Prieto, 2003). The fluctuatingvalues of Chenopodiaceae and Poaceae, accompanied by adiscontinuous increase of Asteraceae, Asteraceae subf. Cichor-ioideae, Brassicaceae and Polygonaceae-type suggest varia-tions in water level due to flooding events. Some of thosetaxa that develop above the upper tidal limit prevail afterrepeated and severe disturbances such as flooding becausethey have the ability to grow rapidly during the relatively shortintervals between the flooding (Grime, 1979). Floodingoccurred during the sea-level rise, which forced the creek baselevel to change through progressive infilling. Pollen clumpsfrom Poaceae, Asteraceae and Asteraceae subf. Cichorioideaeindicate local deposition which could be increased due toflooding events. Higher pollen concentrations of Poaceae,Asteraceae and Asteraceae subf. Cichorioideae suggest thelocal presence of these taxa adjacent to halophytic vegetation.Few diatom frustules from taxa with different life-forms andsalinity tolerance are found. The presence of Actinoptychusvulgaris, a conspicuous and widely distributed marine diatom,which at present is common in coastal waters (De Wolf andDenys, 1993) indicates the marine influence ca. 6200 14C yrBP.Mollusc deposits mostly composed of gastropods and

bivalves from supratidal and intertidal environments (Fig. 9,stratigraphic unit 3c) record that a sea-level highstand occurredbetween ca. 6400 and 6200 14C yr BP. These deposits extendover 1000m along the present coast indicating that this deposi-tional environment was laterally continuous. At ca. 6400 14C yrBP levels of valves and fragments of the molluscsOstrea eques-tris andMytilus platensis indicate the maximum tide range in asupratidal environment in coincidence with the highstand. Atthese levels the presence of articulated valves of Ostrea eques-tris indicate that these molluscan remains were not significantlyreworked. Incrustations of the mollusc Brachydontes rodri-guezi in Pitar rostratus valves suggest varying degrees of sub-aerial exposure during the Holocene, which provides furtherevidence for the development of an intertidal environment(Prieto et al., 2003). A short distance upstream, gypsum rosettesreveal evaporation and slight freshwater input, from a coastalmarsh. At ca. 6200 14C yr BP, the autochthonous concentration



Figu

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Pollen

SalinityIndex

(PSI),palyn

omorphproportions,Diatom

SalinityIndex

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ollusc

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of infaunal bivalve Tagelus plebeius in living position at 2.4ma.s.l. indicates high sea-level position and an intertidal estuar-ine environment.Heleobia australis andH. parchappii in thesedeposits suggest marine–brackish conditions since the formerinhabits environments with tidal influence and its up-estuarypenetration is coincident with the upper limit of tides, andthe latter is able to tolerate high salinity variations (De Fran-cesco and Isla, 2003). �13C and �18O isotope data from H.parchappii (averaging � 6% and � 1%, respectively) (Fig. 9)indicate diverse environments coexisting (coastal marine–brackish) in response to an interplay between fluvial and mar-ine waters (Bonadonna et al., 1995).Between ca. 6200 and 5200 14C yr BP (LB3) brackish shal-

low water bodies as ditches or ponds were established in thearea, as suggested by the increased values of Ruppia, whichis recorded in the spectra from both Profiles 1 and 2. Pollenclumps of Asteraceae, Brassicaceae and Polygonaceae-typeindicate local deposition in Profile 1 area related to floodingupstream only. Areas with freshwater influence are suggestedby traces of Typha and Alternathera and low values of Cyper-aceae and Apiaceae. Very low total pollen concentration fromProfile 1 (< 5000 grains per gram) compared to Profile 2 (up to35000 grains per gram) suggests local differential sedimentfocusing. Diatom records are abundant and diverse; the assem-blages are represented by brackish and epiphytic diatomRhopalodia gibberula accompanied by Amphora salina, Syne-dra platensis and Cocconeis placentula. They indicate ashallow depth brackish environment (lagoon) and associatedvegetation. Abundant shells of Heleobia parchappii and theabsence of H. australis indicate reduced tidal influence, per-haps by the development of a sand barrier. This situation isrecorded in both Profiles 1 and 2. Values of �13C and �18Oare relatively high, in particular oxygen (2.5%) (Fig. 9) indicat-ing the establishment of a lagoonal environment and isotopicenrichment due to evaporation (Bonadonna et al., 1995).Between ca. 5200 and 3900 14C yr BP (LB4) Chenopodia-

ceae is dominant in the spectra accompanied by Poaceae, sug-gesting a more widespread saltmarsh similar to the present onesurrounding the Mar Chiquita lagoon where Salicornia ambi-gua (Chenopodiaceae) is conspicuous at the lagoon borders(Stutz and Prieto, 2003). The presence of Limonium brasiliensesuggests saltmarsh high zones. This vegetation would haveexpanded occupying more saline areas, colonised when sealevel was stable within the sea-level regression (Isla andEspinosa, 1998). Increased Typha suggests shallower watersthat are characterised by acritarchs (with a mean of 50%)(Fig. 9). Increases in concentrations of the hydrophyticsCyperaceae and Apiaceae and also of Asteraceae subf.Cichorioideae, Brassicaceae and Polygonaceae-type reflectareas with freshwater influence near the creek. Diatom assem-blages indicate an estuarine lagoon environment with brackishshallow water characterised by epiphytes and benthic taxaSurirella striatula, Nitzschia compressa, Synedra platensisand Rhopalodia gibberula, accompanied by marine taxa asNavicula zosteretii and Cocconeis scutellum. Estuarine condi-tions and tidal influence are suggested by the transported mol-luscan valves of Tagelus plebeius andHeleobia australis shells,as well as by the alternating well-stratified silts, clays andrippled fine sands with lenses, flaser and wavy bedding struc-tures, which are related to ephemeral currents in the intertidalenvironment (Isla et al., 1986).After ca. 3900 14C yr BP, all proxy data record a relative sea-

level regression (Fig. 9). Between ca. 3900 and 3000 14C yr theincrease of herbs, Typha, Apiaceae and Cyperaceae in marshyzones, along with Pediastrum, suggests freshwater influence(LB5). Replacement of Ruppia by Myriophyllum indicates adilution and gradually salinity decrease (Verhoeven, 1979).

These spectra suggest a transition from saltmarsh to a brackishmarsh with sporadic marine influence and freshwater input,evidenced as well by lower �13C and �18O values (Bonadonnaet al., 1995). The decreased marine/brackish epiphytes Cocco-neis scutellum andNavicula zosteretii and the benthic Surirellastriatula and the increased brackish/freshwater planktonic andtychoplanktonic Cocconeis placentula and Fragilaria con-struens are characteristic of the supralittoral area subjected toperiodic desiccation such as a brackish marsh in coincidencewith pollen assemblages. A less saline environment is alsoshown by Heleobia parchappii and Biomphalaria peregrina(Fig. 9), the latter inhabiting freshwater environments, butbeing able to tolerate slightly elevated salinity (Espinosa et al.,2003). Scarce shells of Heleobia australis, Mytilidae fragmentsand traces of Operculodinium dinocysts suggest sporadic tidalinfluence. Sediments change to clayey silts with massive strati-fication suggesting fluvial input. Toward ca. 3000 14C yr BP anincrease of Cobricosphaeridium and Peridinioideae cysts(representing almost 80% of the spectra) suggest desiccationperiods of an unstable water body with variable depth and fluc-tuating salinity (Borel et al., 2003). Desiccation is inferred frompoor preservation and reworked palynomorphs as well. Thelowest total pollen concentration is related to the increase inPeridinoideae cysts and Cobricosphaeridium; whereas, theincreased pollen clumps from Poaceae, Chenopodicaceaeand Apiaceae and total pollen clumps are related to water-levelchanges.Between ca. 3000 and 2200 14C yr BP (LB6), pollen spectra

probably represent surrounding sandy areas close to the marshsince Poaceae and Asteraceae are the main components of themodern pollen spectra from fixed sand dunes (Fig. 3). Therecord of high values of Cobricosphaeridium suggest that fluc-tuating salinity continue in the water body; whereas, Cypera-ceae, Apiaceae and Pediastrum, along with the lowest valuesof Chenopodiaceae, suggest freshwater influence. The diatomrecord is characterised by the dominance of the tychoplank-tonic Fragilaria construens accompanied by Cyclotella mene-ghiniana (plankton), Denticula elegans and Campylodiscusclypeus (benthos), Rhopalodia gibberula (epiphyte), and theonly one marine taxa Navicula zosteretii (Fig. 4). This assem-blage indicates a salinity decrease since brackish taxadecrease, and brackish/freshwater and tychoplanktonic/planktonic diatoms are the main ecological groups. Heleobiaparchappii and Biomphalaria peregrina records indicate thecontinuity of brackish–freshwater environment. Scarce shellsof Heleobia australis, Mytilidae fragments and traces of Oper-culodinium dinocysts suggest that tidal influence is sporadic.Between ca. 2200 and 2000 14C yr BP (LB7) brackish–

freshwater marsh is indicated by pollen spectra. An increasein frequencies and concentration values of hydrophytic ele-ments suggests freshwater conditions. Sudden and episodicfreshwater flooding, probably after an increase in stream inflowis indicated by an abrupt development of Pediastrum (up to50%) and the diatom Aulacoseira granulata (plankton/fresh-water) that reaches a high percentage (62%), accompaniedby the tychoplanktonic Fragilaria construens var. venter andthe epiphyte Epithemia adnata (representing almost 80% ofthe assemblage; Fig. 9). The freshwater mollusc Gundlachiaconcentrica appears at this time, as carbon and oxygen iso-topes (Fig. 9) indicate a dominant freshwater environment.However, towards ca. 2000 14C yr BP Chenopodiaceae andCobricosphaeridium increase, which suggests brackish condi-tions, and Heleobia australis reflects sporadic tidal influence(Fig. 9). The causes for these saline conditions may be due toeither desiccation processes or an unusually high tide rangewith rare frequency. Also, at ca. 2000 14C yr BP �13C and�18O values follow a trend similar to that occurring between



ca. 6200 and 5200 14C yr BP when �18O higher valuessuggested isotopic enrichment due to evaporation (Bonnadonaet al., 1995).After ca. 2000 14C yr BP, the sedimentary sequences were

buried by aeolian sand dunes. At present, the sand dunes char-acterise the arroyo Las Brusquitas area, mainly as cliff-topdunes which are part of a southern sand barrier (Isla et al.,1996).

Holocene palaeosalinity trends

DCA ordination of pollen and diatoms shows the same basictrends in Holocene salinity conditions, from more saline con-ditions after ca. 6200 14C yr BP to less saline toward ca. 200014C yr BP (Fig. 5).Between ca. 6200 and 3900 14C yr BP more saline condi-

tions are indicated by halophytic vegetation and marine/brack-ish diatoms with epiphytic and benthic life-forms, which arerelated to shallow water environments and the presence ofassociated vegetation. Between ca. 3900 and 2000 14C yrBP, less saline conditions are indicated not only by thedecrease of halophytic vegetation and the association of hydro-phytic elements but also by brackish/freshwater diatoms withplanktonic and tychoplanktonic life forms. Typha scores inthe DCA (Fig. 5) suggest that this taxa can tolerate brackish con-ditions in environments such as interdune depressions domi-nated by brackish conditions due to marine influence (Stutz,2000). Between ca. 2200 and 2000 14C yr BP conditions offreshwater predominance are reflected in increased Pediastrumand diatoms with plankton life-form, which suggest floodingand a deeper water body.PSI indicates more saline conditions between ca. 6700 and

3900 14C yr BP, as a consequence of the sea-level highstand,and a period of stabilization between ca. 5200 and 3900 14Cyr BP (Fig. 9). DSI reveals a mixture of marine, brackish andfreshwater taxa, related to the presence of Cocconeis placen-tula, which is euryhaline and may be considered of brackishenvironment if it is accompanied by other brackish taxa (Vosand De Wolf, 1993). After ca. 3900 14C yr BP high fluctuationsin salinity conditions are indicated by PSI and DSI, related to asporadic marine influence and freshwater input during theregressive sea-level phase at different rates (Fig. 9). After ca.3000 14C yr BP, DSI shows a decrease trend of marine brackishtaxa. More saline–brackish conditions at ca. 2000 14C yr BPmay be due to either desiccation or an unusually high tiderange of 4m a.s.l. This altitude is abnormally high for salt-waterinfluence compared to the altitude in other sequences at thesoutheastern coast of Pampa grasslands, assuming the sametidal range for this part of the coast during the Holocene asoccurs today (e.g. Mar Chiquita lagoon, Punta Hermengo,arroyo La Ballenera, Fig. 1). In those sequences the altitudefor salt-water influence has a similar position between 2 and2.5m a.s.l. Consequently, this apparent difference in tidalrange may suggest a tectonic uplift of Las Brusquitas area as firstproposed by Isla et al. (1986).The marine influence on palaeosalinity trends is the domi-

nant forcing between ca. 6700 and 3900 14C yr BP, in relationto the sea-level highstand. However, between ca. 6200 and5200 14C yr BP evaporation water inferred by high isotope shellvalues could have influenced palaeosalinity trends as well,related to climatic forcing that controlled the moisture regime.Between ca. 3900 and 2000 14C yr BP freshwater had greaterinfluence on palaeosalinity trends, related not only to a moresporadic marine influence due to sea-level fall but also to fresh-water flooding during periods of increased precipitation at ca.

2200 14C yr BP. Pollen records from the Mar Chiquita lagoonarea at the southeastern coast (Fig. 1) suggest increased preci-pitation during several periods between 2100 and 400 14C yrBP and drier conditions after 400 14C yr BP (Stutz et al.,2002). Evaporation and desiccation processes in arroyo LasBrusquitas area, especially between ca. 3000 and 2200 14Cyr BP, could also have influenced the palaeosalinity trends dur-ing drier climatic periods, in relation to subhumid–dry climaticconditions that prevailed in the Pampa grasslands after ca.4000 14C yr BP (Prieto, 1996). These inferences suggest thatdry and variable climatic conditions for the late Holocene inthe southeastern coast of the Pampa grasslands can be consid-ered as forcing palaeosalinity trends.

Sea-level and marine influence in the southeasternregion of the Pampa grasslands

The changes in the Holocene vegetation and environmentspresented here are in agreement with data obtained from var-ious southeastern coastal sites of the Pampa grasslands.Between ca. 6700 and 2000 14C yr BP the effect of the sea-levelfluctuations on the Atlantic coast of the southeastern Pampagrasslands occurred at different times and magnitudes accord-ing to the characteristics of each basin: depth, distance fromshoreline, geomorphology and freshwater input (Isla et al.,1986). Evidence of high sea-level position are recorded inarroyo Las Brusquitas, arroyo La Ballenera and PuntaHermengo with the establishment of infilling estuaries andmud-filled depressions. Further north, in the Mar Chiquitalagoon area, the development of a coastal lagoon along withsaltmarshes, beaches, barriers and shallow lakes is related tothe Holocene transgressive–regressive sea-level history (Stutzet al., 2005). Southward, in arroyo La Ballenera, a similarpalaeoenvironmental evolution to arroyo Las Brusquitas isrecorded (Stutz et al., 1999; Espinosa et al., 2003). However,differences in timing and magnitude of the marine and tidalinfluence are evident. Between ca. 6800 and 6200 14C yr BP,lagoons with small or no tidal range formed in response to ahigh sea-level stand that occurred during a transition fromfreshwater to a halophytic vegetation in arroyo La Ballenera(Stutz et al., 1999; Espinosa et al., 2003); whereas, in arroyoLas Brusquitas, halophytic vegetation was already developed,in relation to a shorter distance from the shoreline. In the PuntaHermengo area, a tidal channel infilling occurred at ca. 670014C yr BP (Espinosa, 2001).From 6200 to ca. 4800 14C yr BP an estuarine lagoon devel-

oped in arroyo La Ballenera (Espinosa et al., 2003); while inarroyo Las Brusquitas estuarine shallow water bodies alsodeveloped between ca. 6200 and 3900 14C yr BP. Prior to5000 14C yr BP in the Mar Chiquita lagoon area, pollen dataindicate psammophytic–halophytic vegetation mosaic andsand barriers circumscribing the lagoon area and delimitingan estuarine environment (Stutz et al., 2005).Between ca. 4700 and 4120 14C yr BP and between ca. 5200

and 3900 14C yr BP pollen spectra from arroyo La Balleneraand arroyo Las Brusquitas indicate the development of salt-marsh vegetation while diatom assemblages suggest brackishestuarine environment for both areas.In Punta Hermengo at ca. 3300 14C yr BP, diatom records

indicate a brackish, shallow and stable environment such asa swamp with a restricted connection to the sea (Espinosa,2001). In the Mar Chiquita lagoon area, a gradual replacementfrom psammophytic–halophytic mosaic to halophytic salt-marsh community occurred between ca. 5000 and 2500 14Cyr BP (Stutz et al., 2005). In arroyo Las Brusquitas, brackish



marsh with less saline conditions and freshwater input devel-oped between 3900 and 2000 14C yr BP, related to a sea-leveldecrease at different rates and sporadic tidal influence. Thereare no records from arroyo La Ballenera.Arroyo Las Brusquitas palaeoenvironmental evolution show

that a brackish–freshwater environment predominated at ca.2000 14C yr BP followed by the formation of a sand barrier.Further north, for this last period of the sea-level regressionphase, pollen data reveal that the present halophytic vegetationcharacteristic from saltmarsh began in the Mar Chiquita lagoonarea at ca. 2500 14C yr BP (Stutz et al., 2005). As previouslymentioned, freshwater conditions predominate in arroyo LasBrusquitas today; even tides are not significant at the creek out-let. Sand beaches characterise the southern area near the creekwith the mollusc deposits from the marine transgression lyingunderneath the sand and subjected to sea storms.The changes in the vegetation and environments along the

southeastern coast of Pampa grasslands are consistent withthe evolution of the coastline of Argentina in relation to sea-level changes, which has been related to regional responsesto glacio-eustatic effect and continental uplift (Isla, 1989;Codignotto et al., 1992). Recent studies propose that glacialisostatic adjustment in the southern Argentina could have beeninfluenced by the broad continental shelf, but the influence ofsignificant neotectonic uplift could not be dismissed (Rostamiet al., 2000). On the other hand, Milne et al. (2005) suggest theeustatic sea-level equivalent after ca. 7000 cal. yr BP was nomore than ca.� 1m. In the coast of Pampa grasslands, tectonicuplift rates were two times lower than those observed in Pata-gonia (Codignotto et al., 1992) but sufficient to determine dif-ferent responses to the Holocene sea-level fluctuations in theeastern and southeastern parts (Isla et al., 1996). On the south-eastern coast sea-level fluctuations modified the undulatinglandscape where estuarine sequences formed during the max-imum transgression, whereas in the eastern (36 � S) sea-levelfluctuations modified the extensive low-gradient areas whereprograding barriers and spits were the dominant forcing onvegetation community development after ca. 5400 14C yr BP(Vilanova et al., 2005). The evolution along the southeasterncoast of the Pampa grasslands shows how sea-level variationsaffected vegetation and the environment, changing the land-scape and salinity trends, which could be useful for modellingpossible future scenarios related to sea-level rise attributed toclimate changes and global warming.

Acknowledgements This study was financed by Mar del PlataNational University (15/E138) and FONCYT PICT 7-6477. We aregrateful to A. S. Miquel for determination of Bulimulus bonariensisand to A. M. Borel for determination of Cobricosphaeridium and Peri-dinoideae. Special thanks to J. L. Toney and to the anonymous reviewerfor their helpful comments to improve the content and presentation ofthis manuscript and to the editor for reviewing the English.

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