Journal of Paleolimnology (2005) 34: 445–469 DOI 10.1007/s10933-005-5792-8
Springer 2005
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Holocene vegetation history and palaeoenvironmental conditions on the temperate Atlantic coast of Argentina, as inferred from multi-proxy lacustrine records Sonia L. Fontana Department of Earth Sciences, Palaeobiology, Uppsala University, Villava¨gen 16, SE 752 36 Uppsala, Sweden (e-mail:
[email protected]) Received 3 July 2004; accepted in revised form 2 April 2005
Key words: Pollen, Ostracods, Vegetation history, Coastal dunes, Holocene, Buenos Aires Province, Argentina
Abstract Holocene vegetation history and palaeoenvironmental conditions are investigated at the south coast of Buenos Aires Province, Argentina. La Olla 1 and Laguna del Sauce Grande sediment sequences are analysed for pollen, calcareous microfossil (ostracods and foraminifers) and plant macrofossil remains (mainly seeds and charophyte oospores). Supplementary information is provided by sedimentological analysis. Modern surface sample data are used to assist in the interpretation of the fossil records. La Olla 1 sequence covers the period 7890 to 7630 cal. BP. The microfauna recovered is characteristic of a shallow marginal-marine environment such as a coastal lagoon. The microfossils indicate a marine connection between 7850 and 7800 cal. BP. Plant macrofossil remains and pollen analyses indicate an extension of the water body after 7780 cal. BP. The pollen record reveals the development of a halophytic plant community in a coastal environment. The sediment record from Laguna del Sauce Grande comprises the last 3000 years. Microfossils and macrofossil remains indicate that the lake history begins with a temporary brackish-water phase. More stable conditions and higher salinity values occur between 1940 and 900 cal. BP. Periods of water level fluctuations occur after 900 cal. BP, with high water levels between 660 and 270 cal. BP. The uppermost samples of the sequence show similar conditions to present day. Pollen spectra indicate a relatively stable vegetation composition throughout the last 3000 years. Pollen assemblages reflect the present regional grassland vegetation with taxa characteristic of the surrounding dune communities.
Introduction The temperate Atlantic coast extends along the eastern part of South America from Porto Alegre, Brazil to Bahı´ a Blanca, Argentina (between 28 and 39 S). This coastal area experienced considerable relative sea level fluctuations during the Holocene. A large number of investigations were undertaken to understand its evolutionary history (e.g., Isla and Espinosa 1995; Isla 1998; Angulo et al. 1999; Garcı´ a-Rodrı´ guez et al. 2004;
Bracco et al. 2005). The present coastal features were formed during and after the last marine transgression. The coastal relief is characterized by flat sand beaches and extended dune systems. Pleistocene and Holocene deposits occur within the coastal dunes as barriers, marshes, coastal lagoons, tidal flats, and chenier plains. Farther from the coastal strip, a vast plain extends into the interior of the continent. Characteristic sand-requiring vegetation grows on the dunes (Cabrera 1941; Pfadenhauer 1993) and temperate
446 subhumid grasslands cover the inland region (Soriano et al. 1991). Precipitation and temperature in the region decrease from the north-east to the south-west. Thus, the southernmost area, around Bahı´ a Blanca, is the most arid and coolest (547 mm annual precipitation; 15.6C mean annual temperature). Here, the ‘pampa’ grassland abuts on the xerophytic forest known as ‘espinal’ (Cabrera 1994). The late Quaternary vegetation history of the region has been inferred mainly from pollen records recovered from outcrop sections along river valleys and from loess sequences (e.g., Prieto and Paez 1989; Paez and Prieto 1993; Borromei 1995, 1998; Quattrocchio et al. 1995, 1998; Prieto 1996, 2000; Grill 1997, 2003; Stutz et al. 1999). These records cover the period from late Pleistocene or early Holocene to the present. The sections present truncated paleosols and erosional discontinuities, suggesting depositional hiatuses, which together with a complex stratigraphy, complicate their interpretation (Quattrocchio and Borromei 1998). Palynological studies based on lacustrine records for this region are few (e.g., Zavala et al. 1992; Prieto 1993; Mancini 1994; Stutz and Prieto 2001; Stutz et al. 2002). Although lakes usually provide continuous fossil sequences for the reconstruction of palaeoenvironments the present pampean lakes have so fare only yielded sediment sequences going back to the mid-Holocene ca. 6000 PB. Information from microfossil studies in the region originates mostly from coastal environments and fluvial sections (e.g., Whatley and Moguilevsky 1975; Bertels and Martinez 1990, 1997; Zavala et al. 1992; Ferrero 1996; Whatley et al. 1997, Bertels-Psotka and Laprida 1998a, 1998b, 1998c; Laprida 1998, 2001; Espinosa et al. 2003). In these studies, ostracods, foraminifers, diatoms and gastropods have been successfully used to infer salinity changes, water level variations and sea level fluctuations, and thus to reconstruct the palaeoenvironment of the coastal area. The aim of this study is to reconstruct the vegetation history and environmental changes during the Holocene at the south coast of Buenos Aires Province, Argentina. La Olla 1 and Laguna del Sauce Grande sediment sequences were analysed for pollen, calcareous microfossils and plant macro fossil remains; supplementary information is provided by sedimentological analysis. Additionally, surface samples from temporary water
bodies located in the area of the Balneario Sauce Grande are included in order to assist interpretation of the fossil records (Figure 1).
Description of sites studied La Olla 1 La Olla 1 (3860¢ S; 6121¢ W) is an archaeological site that contains significant information about the use of marine resources by hunter-gatherers during the early Holocene (Politis and Bayo´n 1995; Johnson et al. 2000). It is located in the coastal intertidal zone (Figure 1). The site consists of lacustrine sediments that have been deposited during periods of lower sea level. Today, the site is usually covered by beach sand. It has been exposed only a few times and for brief periods, during exceptionally low tide. La Olla 1 is spatially and temporally associated with Monte Hermoso I, a site located 200–1000 m west (Zavala et al. 1992; Bayo´n and Politis 1996). Human footprints together with other prints of birds and an artiodactyl are preserved in the lacustrine sediments of this site.
Laguna del Sauce Grande Laguna del Sauce Grande is a shallow fresh-water lake located 8 m above sea level and approximately 4 km from the Atlantic coast (Figure 1). It covers an area of 23 km2, with mean water depth of 1.1 m. The lake occupies an aeolian depression, receiving waters from the Sauce Grande River. The river originates in the Sierras Australes, a mountain range rising up to 1250 m a.s.l., located 200 km north of the lake, and enters the Atlantic Ocean ca. 20 km southeast of the study site. The lake is elongated east–west. The northern shore is characterized by a low escarpment, while the southern shore borders the dune system. The lake is surrounded by a narrow shore belt of Schoenoplectus californicus and Typha dominguensis.
Temporary interdunal lakes Diverse temporary fresh water bodies of varying size occur between the inactive (fossil) dunes
447
Figure 1. (a) Location of the study area. (b) General overview of the coastal strip at Monte Hermoso and Balneario Sauce Grande, showing the location of the sampling sites: La Olla 1 and Laguna del Sauce Grande (asterisk) and temporary interdunal lakes (black circle, numbers I–IV); site number IV is Laguna Caliba (Fontana and Ballent 2004). Monte Hermoso I (diamond) another palaeoecological site mentioned in the text (Zavala et al. 1992; Bayo´n and Politis 1996). (c) Zonation of the active dunes with the location of the surface samples and pollen traps. The zones are based on the interpretation of air photographs (scale 1:20,000) and phytosociological studies of the vegetation (Fontana 2005).
located in the surrounding area of the Balneario Sauce Grande, about 3 km inland from the present coastline (Figure 1). They occupy shallow depressions which are filled with water after rainfall. Some lakes are isolated basins, while others are connected to each other at times of high water level. Most of these lakes dry out during the summer season. Four lakes were sampled (Figure 1, sites I–IV).
Methods Sampling procedure A 35-cm long sequence from La Olla 1 site was sampled in 1997. Blocks of sediment were collected
and packed in plastic bags. The sediment sequence was described and subsampled in the laboratory. Samples were taken continuously on the basis of distinct lithostratigraphic features. Sample thickness varies between 0.5 and 1.5 cm. Loss-on-ignition (LOI), calcareous microfossil (ostracods and foraminifers) and pollen analyses were carried out in this sequence. Every second sample was analysed for LOI using 0.5 g of dry sediment. Microfossil and pollen content were analysed in samples of 10 and 5 g dry sediment respectively, taken at predetermined intervals (their position and thickness are indicated in Figures 3–5). Depths were measured from the top of the sequence. Sediment cores from Laguna del Sauce Grande were collected from two boats roped together and secured in the southeast part of the lake, at 3857¢ S
448 and 6122¢ W, in January 2001 (Figure 1). During coring the water depth was 155 cm, the water temperature 16.5C and the conductivity 4.1 mS cm 1. Duplicate sets of overlapping cores, from sites about 1 m apart, were recovered using a 1 m long Russian sampler, 5 cm in diameter (Jowsey 1966). Core segments were wrapped in plastic film and aluminium foil and stored in plastic half pipes in a cold room at 4C prior to analyses. The uppermost part of the sequence, containing the sediment–water interface, was recovered with a Willner gravity sampler (http://www.ebc.uu.se/ limno/workshop/sedimentcorer.html). This core was sectioned at contiguous 1 cm intervals on site, and stored in plastic bags. Thus, a complete and undisturbed sediment sequence of 289 cm length was obtained. The overlapping core segments were described in detail in the laboratory. Colours were determined using Munsell soil colour charts. The sediment cores were analysed for magnetic susceptibility, LOI, pollen, ostracods and plant macrofossil remains, including mostly oospores of charophytes and seeds. Magnetic susceptibility measurements were made initially on all core segments and provide, together with the lithological descriptions, a means of correlation. All the other analyses were carried out on the same series of cores. Volumetric samples of 0.5 cm3 were taken at contiguous 1 cm intervals for determination of water content, dry density and loss on ignition and at 8 cm intervals for pollen analysis. Ostracods and plant macro fossil remains were analysed in samples of 8–12 g wet sediment taken at 4 cm intervals. All samples had a thickness of 1 cm. Depths were measured from the sediment–water interface. Sediment surface samples from the temporary lakes were collected during the summer season, January 2001. At the time of sample collection the lakes were dry. Samples were collected from the sediment accumulation area of the dry lakes. Ostracod and pollen analyses were carried out on each sample, using 20 and 2 – 3 g sediment respectively.
Sediment analysis Magnetic susceptibility was measured using a Bartington Instruments meter with a MS2E1
surface scanning sensor (Bartington Ltd., UK). The sensor is connected to an automatic core logging system, TAMISCAN-TS1 (http://www.geol.lu.se). Measurements were made at 2 mm intervals, to obtain a continuous record. A three-term running mean was applied to smooth the data. Water content and dry density were estimated by oven-drying the samples for 24 h at 105C. Organic matter and carbonate content of the sediment was then estimated by LOI. Samples were heated in a muffle furnace for 5 h, first at 500C, and then at 950C. Results are expressed as percentages of weight loss of the sediment in each step related to the dry weight of the samples before combustion (Heiri et al. 2001).
Calcareous microfossils and accompanying biota Samples were first sieved through 180 and 63 lm mesh without chemical treatment. Charophyte oospores, seeds, megaspores and Cladocera ephippia were recovered from the 180 lm sieve. The remaining material retained on the sieves was treated with hot dilute hydrogen peroxide, sieved through 180, 150 and 63 lm mesh and dried. Ostracods, foraminifera and gastropods were recovered from these fractions. Ostracod nomenclature follows Martens and Behen (1994), Gutentag and Benson (1962), Whatley et al. (1997) and Fontana and Ballent (2004).
Pollen analysis Samples for pollen analysis were prepared in accordance with standard methods described by Bennett and Willis (2001). Lycopodium tablets (Stockmarr 1971, 1972) were added to enable calculation of pollen concentration and accumulation rates. A minimum of 300 pollen grains and spores of terrestrial vascular plants were counted. Pollen and spores of aquatic plants, Bryophyta and introduced taxa were excluded from the total pollen sum. Pollen grains and spores were identified with reference to Stix (1960), Heusser (1971), Markgraf and D’Antoni (1978), Ha¨ssel de Mene´ndez (1989, 1990), Moore et al. (1991), Prieto and Quattrocchio (1993), Tellerı´ a and Daners (2003) and the
449 reference collection held at the Department of Earth Sciences, Uppsala University. Palynological richness was determined by rarefaction analysis (Birks and Line 1992), with a standard pollen sum of 300 (E(T300)). Plant nomenclature follows Ha¨ssel de Mene´ndez (1989, 1990) for Bryophyta, and Zuloaga et al. (1994) and Zuloaga and Morrone (1996, 1999) for vascular plant taxa. The term ‘seed’ used for Ruppia cf. maritima in this paper refers to the fruiting structure of the plant. Sediment, microfossil and pollen zones were defined numerically using binary and optimal splitting techniques (Bennett 1996). Additionally, modern pollen data from pollen traps (Fontana 2003) and surface samples (Fontana 2005; Figure 11) are considered here to allow comparison with fossil pollen assemblages from La Olla 1 and Laguna del Sauce Grande sequences. Principal component analysis (PCA) was applied on the combined data set of fossil and modern pollen spectra to assist in the interpretation of the fossil records. The modern pollen data considered in the PCA analysis include mean annual percentage values from two pollen traps placed in the study area and pollen percentage data from 25 surface samples collected in different vegetation units of the active dunes at Monte Hermoso. Additional information is presented and discussed in Fontana (2003, 2004, 2005).
effect in the aquatic samples within the sedimentary unit. The shift between the aquatic and terrestrial samples was estimated as 783 ± 55 14C yr by applying a polynomial (two-term) age-depth model (Bennett 1994) to the aquatic radiocarbon dates of the sequence (Figure 2a). Thus, an age for the point at 22.60 cm depth could be calculated and then subtracted from the terrestrial radiocarbon age at that depth. This offset was applied during the calibration of the samples at La Olla 1 site. In addition, two dates were considered as outliers with prior probabilities of 50 and 100% respectively (Table 1). These probabilities were assigned to the radiocarbon dates during the BCal calibration procedure. Five radiocarbon dates were carried out on shells of Heleobia parchappii (d’Orbigny) to establish the chronology of the Laguna del Sauce Grande core (Table 1). In addition, a 14C age determination was obtained on shells of living specimens of Heleobia parchappii to evaluate the reservoir effect of the samples. A polynomial line-fitting (three-term) age-depth model (Bennett 1994) was applied to reconstruct the chronology in calendar years (Figure 2b). Shells of the modern material gave an absolute modern 14C age (Table 1), so no correction for old carbon was needed. Ages are given as calendar years before present (cal. BP) where ‘present’ is defined as AD 1950. Diagrams, rarefaction, accumulation rates, zonation, PCA and age-depth models were carried out using psimpoll (Bennett 2003).
Chronology Chronological control of the sediment records was obtained by accelerator mass spectrometry (AMS) radiocarbon dating at the Tandem Laboratory, Uppsala University (Ua) and at the INSTAAR Laboratory, University of Colorado (NSRL). Radiocarbon dates were calibrated against the IntCal98 calibration curve (Stuiver et al. 1998) using the BCal online system (http://bcal.shef.ac.uk). An offset of 23 ± 414 C yr, due to natural 19th century Southern Hemispheric D14C differences, was applied during the calibration process (Stuiver and Braziunas 1998; Stuiver et al. 1998). The chronology of La Olla 1 site was based on five radiocarbon dates of macroscopic remains of terrestrial and submerged aquatic plants (Table 1). The variability of 14C ages suggests a reservoir
Results La Olla 1 site Radiometric age determination and subsequent calibration of terrestrial and aquatic macrofossils from La Olla 1 (Table 1) suggest that the sequence was deposited over a time span of ca. 260 years from 7890 to 7630 cal. BP. The sediments (Figure 3) consist of silty sand with distinct sand layers. On the basis of the exture, the profile can be divided into two parts: the lower half shows clear laminations while the upper part is massive with occasional undulations. Desiccation cracks occur in the lowermost 2.5 cm. The organic matter content of the samples is low throughout the record (3 – 12%). The carbonate
450 Table 1. Radiocarbon dates from La Olla 1 and Laguna del Sauce Grande. Sample depth (cm)
Material analysed
La Olla 1 0 – 1.1 Ruppia cf. maritima seeds 5.5 – 6.2 Ruppia cf. maritima seeds 8.3 – 8.9 Ruppia cf. maritima seeds 22.1 – 23.1 Terrestrial macro remains 33.7 – 34.4 Ruppia cf. maritima seeds Laguna del Sauce Grande Shells of living H. parchappi 66 – 67 H. parchappi shells 123 – 124 H. parchappi shells 170.5 H. parchappi shells 221 H. parchappi shells 264.5 H. parchappi shells
Uncalibrated age (14C yr BP)
Calibrated age weighted average/cal. yr BP (2r interval)
DR
Laboratory no.
7580 ± 60 7750 ± 60** 7635 ± 75 7040 ± 55*** 7920 ± 90
7630 7695 7740 7815 7890
806 ± 59 806 ± 59 806 ± 59 23 ± 4 806 ± 59
NSRL-11044 NSRL-11045 Ua-16106 NSRL-11046 NSRL-11047
105.4 ± 0.4 pM* 550 ± 40 1275 ± 50 1740 ± 35 2450 ± 65 2600 ± 35
565 (506 – 644) 1170 (1059 – 1284) 1625 (1536 – 1711) 2490 (2346 – 2707) 2670 (2496 – 2774)
(7509 – 7754) (7590 – 7795) (7635 – 7843) (7685 – 7941) (7719 – 8045)
23 ± 4 23 ± 4 23 ± 4 23 ± 4 23 ± 4
Ua-20985 Ua-20865 Ua-20866 Ua-20982 Ua-20983 Ua-20984
*Absolute modern % (pM). **50 % outlier probability. ***100 % outlier probability. D R Offset applied during the calibration process.
Figure 2. Age-depth model: (a) La Olla 1, fitting a two-term polynomial line by singular value decomposition (using uncalibrated ages) to estimated the offset between the aquatic samples and the terrestrial sample. (b) Laguna del Sauce Grande, fitting a three-term polynomial line by singular value decomposition (using calibrated ages). Top sample was assigned to an age of 50 ± 50.
451
Figure 3. Distribution and abundance of calcareous microfossils (ostracods and foraminifera) and macro remains (seeds) from La Olla 1, including chronology, lithology and loss on ignition. Zones are defined using optimal splitting by information content after applying square-root transformation on the combined microfossil and macro remain data set. Black bars indicates position and thickness of the samples analysed.
content of the sediments is higher at the bottom of the sequence (7 – 17%) and decreases towards the top (2 – 4%).
Calcareous microfossils and accompanying biota The microfauna recovered (Figure 3) consists of abundant ostracods and foraminifers although with low diversity. Cyprideis salebrosa dominates the ostracod assemblages. It occurs in most of the samples by complete populations of adults of both sexes and large number of juveniles of A-1 to A-6 instar stages. None of the specimens recorded show variable nodding. Leptocythere cf. darwini and Semicytherura cf. clandestina are present in a few samples. Their population age structure comprises adults and a large number of juveniles. The remaining ostracod species, Sarscypridopsis aculeata and Heterocypris incongruens, are represented by a few valves. Foraminifera are abundant only in a few samples. In addition, abundant seeds of Ruppia cf. maritima, an aquatic submerged macrophyte, were also found.
Calcareous microfossils occur almost exclusively in the lower half of the sequence, whereas only a few specimens were recovered in the upper half. In contrast, seeds of Ruppia cf. maritima become abundant towards the top of this sequence. The sequence has been divided into four zones: LOm-1 – 4. The ostracod assemblage of the lowermost zone, LOm-1 (35 – 28.5 cm, 7890 – 7850 cal. BP), is dominated by Cyprideis salebrosa. Few specimens of Heterocypris incongruens and Leptocythere cf. darwini were also recorded. Ruppia cf. maritima is only present in the lowest most sample. Zone LOm-2 (28.5 – 21 cm, 7850 – 7800 cal. BP) presents the highest concentration and diversity of fossils. Cyprideis salebrosa is the most abundant ostracod species, whereas Leptocythere cf. darwini and Semicytherura cf. clandestina are less common. Sarscypridopsis aculeata and Heterocypris incongruens are present. Quinqueloculina dominates the foraminiferal assemblage, together with Elphidium and Ammonia species. The subsequent zone, LOm-3 (21-17 cm, 7800 – 7780 cal. BP), lacks foraminifera but contains a
Figure 4. Variation of pollen and spores frequencies from La Olla 1, including chronology and palynological richness (E(T300)). Zones are defined using optimal splitting by information content, including all taxa within the pollen sum above 1%. Outline curves represent an exaggeration of 10 · for minor taxa. Black bars indicates position and thickness of the samples analysed. Notice different scales.
452
453
Figure 5. Variation in selected pollen and spores concentration from La Olla 1, including chronology. Zones are defined using optimal splitting by information content, including all taxa within the pollen sum above 1%. Outline curves represent an exaggeration of 10· for minor taxa. Black bars indicates position and thickness of the samples analysed. Notice different scales.
few specimens of Cyprideis salebrosa together with scarce Leptocythere cf. darwini, Semicytherura cf. clandestina and Sarscypridopsis aculeata. Few Ruppia cf. maritima seeds were also recovered in this zone. Zone LOm-4 (17 – 0 cm, 7780 – 7630 cal. BP) is characterized by numerous Ruppia cf. maritima seeds. Cyperaceae seeds occur in low concentration. Scarce specimens of Cyprideis salebrosa represent the micro fauna of this zone.
Pollen analysis Pollen percentage and pollen concentration diagrams for La Olla 1 site are plotted in Figures 4 and 5, respectively. The whole record is dominated by high pollen frequencies of Chenopodiaceae, reaching 80%, and Poaceae with a maximum of 40%. Pollen of Asteraceae and Ephedra also occur in all samples with values up to 10 and 5% respectively. Extraregional pollen of Nothofagus, Myrtaceae and Alnus acuminata is found in low quantities
throughout the record. Palynological richness fluctuates between about 8 and 17 taxa. The sequence can be divided into two pollen zones, LOp-1 – 2. The lowest zone, LOp-1 (35 – 17 cm, 7890 – 7630 cal. BP), is characterized by Cressa truxillensis and Limonium brasiliensis, which reach their highest percentages in the sequence, together with Cyperaceae and Typha. Pollen frequencies of Chenopodiaceae range between 40 and 60% and Poaceae between 25 and 40%. Small quantities of Apiaceae, Phacelia, Phyla, Erodium and Oenothera pollen occur in this zone. Total pollen and spore concentrations are low and variable (7 – 27 · 103 grains g 1 of dry sediment). The subsequent zone, LOp-2 (17 – 0 cm, 7780 – 7630 cal. BP), is characterized by pollen of Brassicaceae, which reach highest frequencies in the sequence, and small amounts of Polygonaceae. Chenopodiaceae pollen frequencies are higher than in LOp1, at about 55 – 80%, while Poaceae percentages are lower, at about 15 – 40%. Pollen of the aquatic plant Ruppia cf. maritima is abundant and increased to a maxi-
454 mum of about 80% at the top of the sequence, although maximum pollen concentration values are recorded at the beginning of the zone. Pollen concentration is higher in LOp-2 and reaches its maximum at the beginning of zone (55 · 103 grains g 1 of dry sediment).
Palaeoenvironmental interpretations The microfossil assemblages are associated with a shallow-water and marginal-marine environment such as a littoral lagoon. Low species diversity suggests a stressed environment. The presence of articulate ostracod carapaces and complete populations with large numbers of adults and juvenile instars well back into the ontogeny indicates that the sediments were deposited during low-energy conditions.
The sequence begins with a brackish, shallowwater phase, LOm-1. The dominant species Cyprideis salebrosa inhabits at present shallow coastal lagoons with salinities of 0 – 29& (Ornellas and Wu¨rdig 1983). Thus, a mesohaline environment with large salinity fluctuations can be inferred for the earliest phase of the sequence. This is followed by a period of rising salinity in zone LOm-2. The high abundance of foraminifera, particularly Quinqueloculina, suggests a marine connection. The lack of foraminifera in LOm-3 indicates a decrease in salinity and the ostracod assemblages indicate similar conditions to the beginning of the sequence (zone LOm-1). The scarce number of ostracods recorded during the time interval represented by zone LOm-4 indicates unfavourable conditions for the microfauna. In contrast, high abundance of Ruppia cf. maritima may signify a rise in water level.
Figure 6. Sediment description, magnetic susceptibility, dry density, water content and loss on ignition from Laguna del Sauce Grande, plotted against depth. Zones are defined using binary splitting by information content after applying square-root transformation on the dataset (excluding magnetic susceptibility).
455 The pollen assemblages are characteristic of halophytic associations. Several psammophytic taxa characteristic of the present dune vegetation like Ephedra, Phacelia, Calycera crassifolia and Oenothera are present in the pollen record. Xerophytic woodland taxa such Condalia microphylla and Geoffroea decorticans are also present. Water-level variation of a relatively small water body can be inferred from the fluctuation of taxa associated to the shore of the paleolagoon like Cressa truxillensis and Limonium brasiliensis, together with Cyperaceae and Typha. All these taxa are abundant in the lower zone of the sequence, LOp-1. The conditions in the littoral lagoon seem to have changed towards the top of the core. Although there is an abrupt decline in the presence of Cressa truxillensis and Limonium brasiliensis, the aquatic Ruppia cf. maritima increases at the beginning of LOp-2. These changes are possibly related to an extension of the water body as interpreted from the high abundance of Ruppia cf. maritima seeds.
Laguna del Sauce Grande The age model based on the five calibrated radiocarbon dates provides a time control for Laguna del Sauce Grande core expanding the last ca. 3000 cal. BP (Figure 2b).
Sediment analysis The 289-cm long sediment core from Laguna del Sauce Grande is characterized by frequent colour changes. Colour shifts are abrupt at the bottom and diffuse towards the top. Distinct layers of black sand are frequent in the lower part of the core and layers of Heleobia parchappii shells occur throughout the core. On the basis of sediment colour and texture it is possible to distinguish seven different sediment units (Figure 6). The zonation of the parameters of dry density, water content and loss on ignition (LOI) at 500 and 950C yielded six zones (LSGs-1 – 6). Zone LSGs-1 (289 – 264.5 cm; 3050 – 2750 cal. BP) is characterized by highest values of dry density and lowest LOI at 500C. Magnetic susceptibility of the core shows high peaks in this zone, which corresponds to distinct layers of black sand
in a matrix of sandy light grey clays and silts. The major sand layers are also reflected by the peaks of dry density and in the trough of the records of water content and LOI. However the highest values for dry density were measured in the lowermost samples, which did not return the highest values for magnetic susceptibility. Magnetic susceptibility and dry density decrease at the beginning of LSGs-2 (264.5 – 220.5 cm; 2750 – 2230 cal. BP). Values of LOI at 950C show a peak in the lowermost three samples of the zone. The subsequent decline coincides with an abrupt change in colour, which marks the beginning of the second sediment unit characterized by grey clays and silts with clay lenses. Distinct sand layers are scarce in the lower half of the zone but occur more frequently in the upper half. LOI at 500C shows a relatively stable curve with values between 4 and 8%. A distinct black band coinciding with a trough in LOI at 950C marks the beginning of zone LSGs-3 (220.5 – 117.5 cm; 2230 – 1095 cal. BP). The band was also used to delimit the beginning of the third sediment unit consisting mainly of dark grey clays and silts with frequent sand layers. A distinct sand layer, which was however finer than those in LSGs-1, corresponds to the distinct peak in magnetic susceptibility at 207 cm. Peaks in magnetic susceptibility are decreasing upwards in the zone. The sediment water content increases gradually towards the top of the core. LOI at 500 and 950C is variable in this zone. The percentage weight loss at 950C reaches a maximum of 33% and values for LOI at 500C increase to 14% in this zone. The occurrence of reddish brown bands delimits the fourth sediment unit and the change to homogeneous brownish grey sediments marks the beginning of the fifth sediment unit. Zone LSGs-4 (117.5 – 50.5 cm; 1095 – 415 cal. BP) comprises the upper part of the fifth sediment unit with low and often negative magnetic susceptibility values. LOI values are relatively stable, between 10 and 14% for the loss at 500C and 24 – 29% for the loss at 950C. The beginning of zone LSGs-5 (50.5 – 28.5 cm; 415 – 100 cal. BP) is marked by a decline in LOI at 950C to values of less than 15%. This decline corresponds to an abrupt change in sediment colour to black, which marks the sixth sediment unit, consisting of soft clays.
456
Figure 7. Distribution and abundance of ostracods and accompanying biota from Laguna del Sauce Grande, plotted against calibrated radiocarbon ages (cal. BP). Zones are defined using optimal splitting by information content after applying square-root transformation on the dataset (excluding Limnocythere cf. staplini).
Zone SGs-6 (28.5 – 0 cm; 100–( 55) cal. BP) is characterized by a raise in LOI at 950C to values above 15% as well as a raise in LOI at 500C, which reach a maximum of 22% in the uppermost sample. The sediment is again harder and changes colour from black to dark grey marking the seventh sediment unit.
Calcareous microfossils and accompanying biota Figure 7 summarizes the stratigraphical distribution of the taxa recovered. Ostracods and charophytes occur throughout the entire core in high abundance. Shells of Heleobia parchappii, Daphnia resting eggs and numerous seeds (mostly Typha) and megaspores were frequently recovered. The ostracod assemblages are characterized by low diversity. Limnocythere cf. staplini is the dominant species, occurring in all the samples with complete population of adults and large number of juveniles. Sarscypridopsis aculeata,
Cyprideis salebrosa, Cyprideis multidentata and Heterocypris incongruens are represented by only a few specimens. The remaining ostracod species are represented by a few valves. Charophytes are dominated by Nitella hyalina (De Candolle) C. Agardh. Statistical zonation of the ostracod and macrofossil records from Laguna del Sauce Grande returned six significant zones. The lowermost zone, LSGm-1 (289 – 195 cm; 3050 – 1940 cal. BP), is characterized by high abundance of Nitella hyalina and Sarscypridopsis aculeata, occurring mainly in the lower half of the zone. Highest values of Limnocythere cf. staplini are also recorded. Chara and Typha are present in low abundance. Zone LSGm-2 (195 – 99 cm; 1940 – 900 cal. BP) presents the highest diversity of taxa. The ostracod assemblages are characterized by Cyprideis salebrosa and Cyprideis multidentata. Variable nodding was not observed in any of the Cyprideis species. However, one or two lateral spines were frequently
457 Table 2. Scarce pollen types at Laguna del Sauce Grande, recorded as 1 grain at each level.
observed in the posterior half of valves of different juvenile stages of Cyprideis salebrosa. Specimens with and without spines occur together in the same sample. Spines were recorded in either one or both valves of a carapace. Charophytes are abundant. Numerous seeds (mostly Typha) and megaspores of Azolla occur. In the subsequent zone LSGm-3 (99 – 75 cm; 900 – 655 cal. BP) numerous resting eggs of Daphnia
were recorded together with oospores of Nitella hyalina and seeds of Typha. Limnocythere cf. staplini decrease with respect to the previous zone. Zone LSGm-4 (75 – 35 cm; 655 – 265 cal. BP) is characterized by low diversity of fossils. Nitella hyalina together with Heleobia parchappii and Limnocythere cf. staplini are the main taxa recorded. Limnocythere cf. staplini decreases drastically towards the upper part of this zone. The
458
Figure 8. Variation in frequencies of selected pollen and spore taxa from Laguna del Sauce Grande, plotted against calibrated radiocarbon ages (cal. BP) and including palynological richness (E(T300)). Zones are defined using optimal splitting by information content, including all taxa within the pollen sum above 1%. Outline curves represent an exaggeration of 10· for minor taxa.
decline in the abundance of Limnocythere cf. staplini coincides with a drop of carbonate content in the sediment (Figure 6). Samples with low abundance of this species show poor calcification of the valves. Abundant seeds of Chenopodium together with Rumex maritimus and Daphnia are the characteristic taxa of zone LSGm-5 (35 – 19 cm; 265 – 20 cal. BP). The uppermost zone LSGm-6 (19 – 0 cm; 20– ( 55) cal. BP) is dominated by ostracods, mainly Cyprideis salebrosa, Heterocypris incongruens and Limnocythere cf. staplini. The gastropod Heleobia parchappii is also abundant. Seeds of Typha and Juncus occur.
Pollen analysis The results of the pollen analyses are presented as percentage (Figure 8) and accumulation rate (Figure 9) pollen diagrams. Scarce pollen types are
displayed in Table 2. Pollen results are presented against time, calendar years before present (cal. BP) rather than sample depth. Throughout the core pollen spectra are characterized by Poaceae, Chenopodiaceae and Cyperaceae, together with Asteraceae, Typha and Apiaceae. Long distance pollen of Nothofagus is recorded in all samples in small quantities. Palynological richness varies between 10 and 20. The sequence can be divided into five pollen zones, LSGp-1 – 5. The lowest zone, LSGp-1 (289 – 269 cm; 3050 – 2805 cal. BP), is characterized by high pollen frequencies of Chenopodiaceae (reaching 50%), and low values of Cyperaceae (about 10%). Pollen of Asteraceae (up to 10%) and Ephedra (up to 5%), together with Chenopodiaceae, reach their highest values in this zone. Poaceae pollen varies between 30 and 45%. Pollen of Typha, Plantago, Discaria americana and spores of Bryophyta (mostly Phaeoceros tenuis and Phaeoceros bulbiculosus) are recorded with less than 5%. Among the scarcer
459
Figure 9. Variation in accumulation rates of selected pollen and spore taxa and Pediastrum algae from Laguna del Sauce Grande, plotted against calibrated radiocarbon ages (cal. BP). Zones are defined using optimal splitting by information content, including all taxa within the pollen sum above 1%. Outline curves represent an exaggeration of 10· for minor taxa. Notice different scales.
pollen types, Phacelia, Rosaceae and Condalia microphylla occur. Pollen accumulation rates values are the lowest ever recorded in the sequence (1 – 2.5 · 103 grains cm 2 yr 1). The pollen assemblages of the following zone, LSGp-2 (269 – 213 cm; 2805 – 2145 cal. BP), are dominated by Poaceae, ranging between 35 and 55%, and Cyperaceae, reaching 35%. Chenopodiaceae pollen occurs at about 10 – 25%. Ambrosia and Schinus are recorded at low frequencies. Pollen of Lycium chilense and spores of Phaeoceros bulbiculosus are frequent among the scarcer taxa. Total pollen accumulation rates range between 2 and 4.5 · 103 grains cm 2 yr 1. Pediastrum accumulation rates reach highest values in this zone (up to 14 · 103 colo-
nies cm 2 yr 1) with two peaks near the lower and upper zone boundary. Zone LSGp-3 (213 – 141 cm; 2145 – 1345 cal. BP) is characterized by pollen of Chenopodiaceae, Cyperaceae and Typha which reaches highest values in this zone (up to 10%) at 185 cm. Poaceae pollen values are the lowest recorded (30 – 40%). Asteraceae pollen percentages are high in the lower part of the zone. Phacelia, Rumex, Zygophyllaceae and Phaeoceros tenuis are frequently recorded in low percentages. Pollen accumulation rates increased towards the upper boundary of this zone reaching highest values (up to 6 · 103 grains cm 2 yr 1). Poaceae pollen frequencies, with highest values of 30 – 55%, dominate the subsequent zone, LSGp-4
460
Figure 10. Surface samples from temporary fresh-water bodies in the surrounding area of the Balneario Sauce Grande. (a) Absolute abundance of ostracods and accompanying biota. (b) Pollen and spores frequencies, including paynological richness. Sample numbers refer to sites in Figure 1. Site IV corresponds to Laguna Caliba (Fontana and Ballent 2005).
(141 – 7.5 cm; 1345 – 5 cal. BP). Cyperaceae pollen shows a peak of 35% at 265 cm. Chenopodiaceae pollen frequencies vary from 10 to 20%. Pollen of Typha, Apiaceae, Plantago, Discaria americana and Rumex together with spores of Phaeoceros tenuis, occur at low frequencies. Limonium brasiliensis, Prosopis, and extraregional pollen of Alnus acuminata and Podocarpus are the most frequent among the scarce taxa. Total pollen varies between 2 and 4 · 103 grains cm 2 yr 1. The zone LSGp-5 (7.5 – 0 cm; 5–( 55) cal. BP), represented by the two topmost samples of the core, is characterized by the occurrence of introduced taxa (mostly Pinus and Eucalyptus) and high pollen frequencies of Chenopodiaceae and Asteracae. Ambrosia and Brassicaceae reach maximum values. Spores of the aquatic fern Azolla occur in both samples. Total
pollen accumulation rate 5 · 103 grains cm 2 yr 1.
is
high,
reaching
Palaeoenvironmental interpretations The lake probably became established at ca. 3000 cal. BP. Its history begins with a temporary brackish water-phase, LSGm-1. The presence of Sarcypridopsis aculeata, a cosmopolitan ostracod species commonly found in temporary pools, suggests a salinity range of 0.5 – 15& (Ganning 1971; De Deckker 1981). During dry episodes sand was frequently blown out of the adjacent dunes and deposited as distinct layers. The scarce sand layers present in the following zone, LSGm-2, and the lack of Sarcypridopsis aculeata may suggest
461
Figure 11. Pollen percentage values of selected taxa from surface samples and pollen traps (Fontana 2003, 2005). Samples are plotted by vegetation type along a gradient from the shore inland. Pollen traps are arranged according to their location in the vegetation. Surface sample numbers refer to the sampling points on Figure 1. Trap A represents 3-yr average (1995/1998) and trap B 4-yr average (1995/1999).
more stable conditions. However, the presence of Cyprideis multidentata indicates that higher salinity values may have occurred during this time interval. Pinto and Ornellas (1965) found this species in a channel linking a coastal lagoon with the sea at a salinity range of 6.1 – 29.1&. The high abundance of seeds indicates the presence of a shore near to the sampling point. The next three zones (LSGm-3-5) are characterized by periods of water level fluctuation. Higher water levels are inferred for zone LSGm-4, while lower levels characterize zones LSGm-3 and LSGm-5. The change in the composition of the ostracod assemblages of zone LSGm-3, together with high abundance of Daphnia ephippia, indicate an abrupt shift in the environmental conditions. An environment comparable to the beginning of the sequence (LSGm-1) is suggested for this phase, although the
adjacent dunes had probably stabilized and sand layers were not deposited. Towards the top of LSGm-4 a sharp decline in Limnocythere cf. staplini together with a drop in the carbonate content of the sediments represent a major change of the environment to conditions that still prevail at present. The large number of seeds (mostly Chenopodium) recorded during the following phase, LSGm-5, indicates the location of the shoreline closer to the sampling point and the development of a halophytic plant community around the lake. Full modern conditions are only represented by the topmost samples of zone LSGm-6. Pollen spectra indicate a relatively stable vegetation composition once the lake was formed. Pollen assemblages reflect the present regional grassland vegetation with taxa characteristic of the surrounding dune communities such as Discaria
462
Figure 12. PCA plot of the taxa combined square-root transformed dataset including surface samples, pollen traps, lake sediment samples and fossil sequences. (a) Sample scores on the first and second axes. (b) Sample scores on the second and third axes.
americana, Ephedra, Phacelia, Rosaceae (e.g., Margyricarpus pinnatus) and Plantago (e.g., Plantago patagonica). Xerophytic woodland taxa like Zygophyllaceae (e.g., Larrea), Condalia microphylla and Prosopis are frequent in the pollen record in low quantities. Human settlement is indicated in the pollen spectra by the presence of introduced taxa in the uppermost samples.
Temporary interdunal lakes The interdunal lakes at Balneario Sauce Grande are characterized by a distinct biota (Figure 10). Ostracods occur in lakes where Ruppia cf. maritima is absent. The ostracod assemblage at lake II is dominated by Heterocypris incongruens and Sarscypridopsis aculeata together with Cypridopsis vidua and Eucypris virens.
463 The ostracod assemblage at lake IV (Laguna Caliba) is dominated by Amphicypris argentinensis, Limnocythere sp. and Neocypridopsis frigogena, together with Sarscypridopsis aculeata, Eucypris virens and Eucypris fontana. The accompanying biota comprise abundant resting eggs of Daphnia and a small number of seeds (Fontana and Ballent 2005). Seeds of Ruppia cf. maritima were recovered at lakes I and II together with abundant Daphnia ephippia. Only the sample from lake II contained charophyte oospores. Poaceae and Chenopodiaceae together with Asteraceae and Cyperaceae are the dominant taxa in the pollen spectra of the interdunal lakes. Poaceae pollen percentages fluctuate between 20 and 35% and Chenopodiaceae pollen varies between 20 and 50%. Asteraceae contributes between 10 and 25% to the total pollen assemblage and Cyperaceae accounts for 10 – 15%. Typha, Apiaceae, Brassicaceae and Plantago are some of the frequent pollen taxa present in small percentages.
Principal component analysis (PCA) The principal component analysis of the combined dataset shows three significant axes explaining 32, 18 and 12% of the variance. Sample scores on the first two axes are represented in Figure 12a and scores on the second and third axes are shown in Figure 12b. The first principal component axis reflects the amount of Chenopodiaceae pollen found in the samples. The second axis discriminates between samples with high Asteraceae and Discaria americana pollen frequencies and samples with high Cyperaceae and Poaceae, while the third axis separates pollen spectra with high Cyperaceae percentages and spectra with high Poaceae percentages. Fossil pollen assemblages from La Olla 1 and Laguna del Sauce Grande plot in discrete clusters indicating a small sample variance within sites but a large difference between sites. The surface samples from ephemeral lakes take a position intermediate between samples from both fossil records. Surface samples from different dune environments show all negative sample scores on the first PCA axis because of their low Chenopodiaceae pollen percentage values. However, these samples are differentiated on the second axis,
reflecting variation in the composition of dune vegetation (Fontana 2005). Pollen spectra from Laguna del Sauce Grande compare best with surface samples from back shore and slacks. However, semi-fixed and fixed dune vegetation cover large areas adjacent to the lake. Laguna del Sauce Grande is an extensive water body, receiving pollen from regional sources which may be the reason for the lack of correspondence between the pollen spectra from these vegetation types and the spectra from the lake. On the other hand, most Asteraceae species as well as Discaria americana, both important components of the semi-fixed and fixed dune zones, are insect pollinated taxa which produce pollen grains that are not transported over long distances and thus may be under-represented in the large basin. Fossil pollen assemblages from the La Olla 1 sediment record are distinctly different from all other modern samples by their high proportion of Chenopodiaceae pollen. However, if the sample scores on the second and third axes are compared it becomes clear that pollen assemblages from La Olla 1 share features with surface samples from mobile dunes, slacks and fixed dunes.
Discussion La Olla 1 The composition of the ostracod assemblages recorded in La Olla 1 is comparable to that described from the locality Arroyo Jabalı´ , in the Atlantic littoral zone of Argentina, by Whatley et al. (1997). Although foraminifera species of Elphidium and Ammonia have been reported from coastal environments like lagoons and estuaries from Argentina and Brazil (e.g., Boltovskoy 1957; Madeira-Falcetta 1974), Quinqueloculina usually occurs in inner shelf habitats (Murray 1986; Alve and Murray 1994). The foraminifera association found in La Olla 1 is similar to those described by Woo et. al (1997) from a washover fan habitat in a barrier-lagoon system in Virginia, USA. The fossil pollen spectra from La Olla 1 can be compared to the present halophytic plant associations described from the estuary of Bahia Blanca (Verettoni 1961). Sediment surface samples taken from a channel of the estuary also correspond to the fossil spectra from La Olla 1 (Grill and
464 Guerstein 1995). The PCA analysis (Figure 12) shows that an environment similar to La Olla 1 does not exist at present in the coastal strip of Monte Hermoso. Still the PCA analysis indicates similarities and identifies the high amount of Chenopodiaceae pollen in the La Olla 1 record as the major difference from present day environments. Thus it seems likely that the sediments of La Olla 1 were deposited in an interdunal lagoon with close proximity to the sea. A good correspondence was observed between the upper zone of La Olla 1 (LOp-2) and the lower pollen samples from the nearby sequence Monte Hermoso I (Zavala et al. 1992; S.C. Grill, unpublished doctoral thesis). In both cases the samples were interpreted to represent periods of relative high water levels (S.C. Grill, unpublished doctoral thesis). The ostracod assemblages recorded at Monte Hermoso I differ in their species composition, although similar palaeoenvironments of interdunal coastal lagoons can be inferred. Different modern ostracod assemblages were recorded in temporary lakes at Balneario Sauce Grande (Figure 10a), indicating diverse aquatic environments in close neighbourhood. The decline of ostracods in the paleolagoon corresponds with an increase in the macrophyta Ruppia cf. maritima. Modern surface samples containing Ruppia cf. maritima seeds totally lack ostracod shells. Similar conditions are recorded in a fossil sequence of an interdunal lagoon recovered at Balneario Sauce Grande (Figure 1, site III; Fontana unpublished data). The 120 cm long sequence recovered presents high abundance of ostracods and charophytes throughout most of the record. However both decline drastically towards the top, when Ruppia cf. maritima seeds are being found. Several authors have observed a reduction of algal or invertebrate populations in aquatic environments where species of Potamogetonaceae (e.g., Potamogeton, Ruppia) were present (e.g., Hasler and Jones 1949; Fitzgerald 1969; Bramer 1979). Hootsmans and Blindow (1994) did not exclude allellopathic limitation of algal growth. In this respect, Cangiano et al. (2002) showed how bioactive diterpenes found in Ruppia maritima and Potamogeton natans exhibited high toxicity toward some aquatic organisms (e.g., Daphnia magna, Artemia salina).
The absence of ostracods in the upper part of La Olla 1 sequence may have been due to the release of bioactive substances by Ruppia cf. maritima. However, toxicity tests need to be performed in order to study the effects of allelopathic interaction between Ruppia and ostracod species. On the other hand, living ostracods have also been recorded in stands of Ruppia species in Western Australia (Geddes et al. 1981; Brock and Shiel 1983) and therefore alternative explanations must also be considered. It may be possible that abiotic factors like changes in ionic composition of the water were causing the decline in ostracods in the paleolagoon.
Laguna del Sauce Grande The development of the lake Laguna Sauce Grande was probably initiated at ca. 3000 cal BP, by the establishment of the river course in its present position (M.E. Quattrocchio, pers. commun.). Several authors have mentioned the existence of deposits probably associated to palaeocourses of the river Sauce Grande to the west of the present course (e.g., Frenguelli 1928; Vega et al. 1989; Bayo´n and Zavala 1997). The coastal dunes may have buried the pre-existent relief and forced the river to the east (Rabassa 1982). Zavala and Quattrocchio (2001) present stratigraphical evidence for the evolution of the river Sauce Grande and proposed a model for the development of the fluvial valleys of the southwest Buenos Aires Province during the Quaternary. The composition of the ostracod assemblages recorded in Laguna del Sauce Grande is, to some extent, comparable to those described from a stratigraphical section of the Queque´n Grande river estuary by Ferrero (1996). Good taxonomic correspondence may be seen between the assemblages from zone LSGm-2 of Laguna del Sauce Grande and the middle levels in the section of the Queque´n Grande river. Limnocythere cf. staplini dominates the entire record of Laguna del Sauce Grande. The dominance of Limnocythere staplini, a species described originally for Pleistocene deposits of Kansas (Gutentag and Benson 1962), could indicate high salinity conditions of a water body (Delorme 1971). In Argentina, Limnocythere staplini is
465 recorded from late Quaternary marginal marine environments in the east of Buenos Aires Province by Laprida (1998). In this record Laprida (1998) reports the presence of Limnocythere staplini together with seeds and charophytes, and suggests a fluvial influence and a reduced salinity of the marine environment. Smith (1991) showed that the dominance of Limnocythere staplini depends on the ionic composition of the water body. The species was almost exclusively found in saline prairie lakes with sulfate-dominated bicarbonatedepleted waters (Smith 1991). The decline of Limnocythere cf. staplini in Laguna Sauce Grande may indicate a decline in salinity. However, it may also be caused by a change in ionic composition of the water. Fossil pollen diagrams in the area were obtained by Borromei (unpublished doctoral thesis, 1995, 1998), Grill (unpublished doctoral thesis, 1995, 1997, 2003) and Prieto (1996) from outcrop sections along fluvial valleys. Differences in pollen transport, deposition and preservation between river flood plains and lakes make the comparison between these records and the pollen assemblages from Laguna del Sauce Grande complex. Considering these difficulties, a comparison of pollen records from the area is attempted for the last 3000 years. The pollen record from the middle valley of the river Sauce Grande (Borromei 1995), for the last ca. 2800 years (pollen zones SG-1 and SG-2), is dominated by pollen of Chenopodiaceae (up to 62%) together with Asteraceae (up to 27%). Poaceae reaches frequencies of only 5 – 27%. Brassicaceae and Apiaceae are important components of the pollen assemblages of zone SG-2, reaching up to 36 and 10% respectively. These pollen assemblages are interpreted by Borromei (1995) to represent psammophytic herbaceous (SG-2) and halophytic (SG-1) steppes. The pollen record for the last 2800 years in the upper valley of the river Sauce Grande (Borromei 1998) is subdivided into three pollen zones (BS-3, BS-2 and BS-1). They are characterized by pollen of Asteraceae, Poaceae, Chenopodiaceae, Apiaceae and Brassicaceae. Asteraceae pollen reaches frequencies between 17 and 37% without significant changes during the time interval considered. Pollen of Apiaceae reaches up to 35% in zone BS-3, while Poaceae dominates zone BS-2 with frequencies of 28 – 35%. Brassicaceae is an
important component of the pollen assemblages of zones BS-2 and BS-1 (varying between 11 and 22%) and Chenopodiaceae reaches its highest frequencies, 19 – 37%, in zone BS-1. The preceding pollen assemblages reflect psammophytic herbaceous (BS-3), gramineous (BS-2) and halophytic – psammophytic herbaceous (BS-1) steppes (Borromei 1998). Grill’s (1997) pollen sequence from Arroyo Naposta Grande also shows large differences in the composition of pollen assemblages during the late Holocene, with respect to Laguna del Sauce Grande pollen record. The sequence shows a succession of different plant communities for the last ca. 2000 years, beginning with a gramineous steppe with hydrophytic communities, to gramineous steppe and ending in a psammophitic herbaceous steppe (Grill 1997). The pollen record from the river Sauce Chico (Prieto 1996) reflects a more stable vegetation history for the late Holocene. It is characterized by pollen assemblages similar to the psammophytic and halophytic communities of the southern pampa (Prieto 1996). Most of the above records indicate significant vegetation changes during the last 3000 years. In contrast, the pollen record from Laguna del Sauce Grande shows only minor changes of the regional vegetation. However, changes in the local vegetation are recorded. These changes correspond to changes in ostracod, macrofossil as well as sedimentological records indicating changes of the water regime in Laguna del Sauce Grande. Work at more sites is needed to resolve the differences and clarify the extent to which the regional vegetation has changed in southern Buenos Aires Province.
Holocene sea level fluctuations The relative sea level curves reconstructed for the Atlantic coast of South America show some differences related to the timing of the maximum height in sea level as well as to the number of transgression/regression events (Isla 1989; Garcı´ aRodrı´ guez et al. 2004). Maximum height in sea level occurred in the study area approximately 6000 BP (Grill and Quattrocchio 1996). Therefore, littoral lagoons such as La Olla 1 occurred most likely before the transgression, during a time with
466 sea levels somewhat lower than at present. A marine influence at La Olla 1 at about 7800 cal. BP may indicate sea level fluctuations of smaller magnitude. After the maximum transgression sea levels fell steadily and the present coastal dune systems together with the modern interdunal lakes originated.
Conclusions Holocene vegetation changes and environmental conditions of La Olla 1 and Laguna del Sauce Grande have been inferred from changes of pollen, calcareous microfossils and plant macro fossil remains. The major changes of the palaeoenvironment are related to Holocene sea level fluctuations. La Olla 1 sequence was most likely part of a series of coastal lagoons. Its sediments were deposited over a time span of 260 years during the early-Holocene before 7600 cal. BP. The mean sea level was at that time lower than today. Microfossil assemblages suggest for the beginning of the sequence a brackish, shallow-water phase with large salinity fluctuations. Subsequently, a marine connection is recognised in the paleolagoon, probably representing the occurrence of sea level fluctuations of smaller magnitude. Towards the top of the sequence, plant macro fossil remains and pollen analysis indicate an extension of the water body. Pollen assemblages indicate regional dune vegetation communities, similar to those growing today on the dune systems. However, the halophytic associations surrounding the paleolagoon suggest a stronger marine influence than what is seen at the modern interdiunal lakes. The initial development of Laguna del Sauce Grande at about 3000 years ago coincides with the establishment of the River Sauce Grande in its present position. The microfossil assemblages suggest water level fluctuations and salinity changes throughout the sequence, which may have been caused by fluctuations in precipitation. Still the regional vegetation composition seems to have been relatively stable during the last 3000 years. Coastal environments are complex and dynamic ecosystems, which may have been changing rapidly and drastically throughout the Holocene due
to changes of environmental factors especially sea level fluctuations.
Acknowledgments I am greatly thankful to Leonor Maumus, Roberto Fontana, Thomas Giesecke and Cecilia Sobrero for their invaluable help during different field works. I thank Sara Ballent and Keith Bennett for their advice and continuous encouragement. Keith Bennett corrected the English. I am grateful to Viviana Oliva and Karina Pinilla for littological description of the sediments from La Olla 1 site. I also thank Mirta Quattrocchio who provided relevant information which improved the interpretation of the data. Special thanks are due to Guido Pastorino for solving many of my ‘zoological problems’. Vera Markgraf, Jonathan Holmes and John Smol are thanked for their valuable comments and suggestions on the manuscript. Cesar Uriarte kindly gave permission and supported the sampling of Laguna del Sauce Grande. AMS radiocarbon dates were funded by the INSTAAR Laboratory for AMS Radiocarbon Preparation and Research (NSRL), the Swedish Society for Anthropology and Geography (SSAG) and Uppsala University. References Alve E. and Murray J.W. 1994. Ecology and taphonomy of benthic Foraminifera in a temperate mesotidal inlet. J. Foram. Res. 24: 18 – 27. Angulo R.J., Giannini P.C.F., Suguio K. and Pessenda L.C.R. 1999. Relative sea-level changes in the last 5500 years in southern Brazil (Laguna-Imbituba region, Santa Catarina State) based on vermetid 14C ages. Mar. Geol. 159: 323 – 339. Bayo´n C. and Politis G. 1996. Estado actual de las investigaciones en el sitio Monte Hermoso 1 (Prov. de Buenos Aires). Arqueologı´ a 6: 83 – 115. Bayo´n C. and Zavala C. 1997. Coastal sites in southern Buenos Aires: a review of ‘Piedras Quebradas ’ . Quat. South Am. Antarct. Penin. 10: 229 – 253. Bennett K.D. 1994. Confidence intervals for age estimates and deposition times in late-Quaternary sediment sequences. The Holocene 4: 337 – 348. Bennett K.D. 1996. Determination of the number of zones in a biostratigraphical sequence. New Phytol. 132: 155 – 170. Bennett K.D. 2003. Psimpoll. Uppsala University, Sweden: Palaeobiology program, Software available from http:// www.kv.geo.uu.se/psimpoll.html. Bennett K.D. and Willis K.J. 2001. Pollen. In: Smol J.P., Birks H.J.B. and Last W.M. (eds), Tracking Environmental
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