Se comparan ostrácodos modernos y cuaternarios de dos cuencas lacustres, la Laguna Cari-laufquen (41 °S) y el Lago Cardiel (49 °S) localizadas en Patagonia. Análisis taxonómicos, cualitativos junto con estudios isotópicos y químicos de la fauna actual indican que las asociaciones de ostrácodos pueden ser reconocidas a partir de la conductividad. De esta manera, tres asociaciones de ostrácodos pueden distinguirse: 1) manantiales, ojos de agua o arroyuelos caracterizados por una baja conductividad (p.ej., 1.015 µs cm−1); 2) lagos y lagunas permanentes representados por una conductividad media (e.g. 1.625 µs cm−1) y 3) ambientes lacustres efímeros en donde la conductividad es mayor (por ej., 16480 µs cm−1). Estas asociaciones de ostrácodos fueron reconocidas en secuencias antiguas como ser en afloramientos de líneas costa de la Laguna Cari-Laufquen o en testigos sedimentarios del Lago Cardiel. La dominancia de Limnocythere rionegroensis Cusminsky y Whatley en las secciones de la Laguna Cari-Laufquen sugieren la presencia de un lago salino y turbio durante el Pleistoceno tardío-Holoceno temprano el cual estaría relacionado con un incremento de las precipitaciones en esas latitudes. Por otro parte, en el Lago Cardiel durante los últimos 16.000 años calibrados AP se observaron variaciones tanto en la abundancia como en la asociación de ostrácodos. Dichos cambios se han debido a las variaciones hidrológicas sustanciales acaecidas durante el Pleistoceno tardío-Holoceno medio en donde hubo una disminución de la conductividad como consecuencia de los cambios en la relación precipitación/evaporación (por ej., un incremento de la relación P/E indicaría una menor conductividad). Estos dos ejemplos muestran la utilidad de los ostrácodos como indicadores para la interpretación de cambios palaeoclimáticos y palaeoambientales en Patagonia.
Modern lakes and lacustrine sediments are ideal sites for the study of environmental change (Ariztegui et al., 2008). Lakes respond quickly to environmental change and their sediments provide a wealth of multiproxy information (Schwalb, 2003) that can be used for the reconstruction of past climate patterns. Non-marine ostracods (microscopic crustaceans often exhibiting a calcareous shell of calcium carbonate) are benthic organisms found in almost every aquatic environment (Horne et al., 2002; Frenzel and Boomer, 2005 and Laprida and Ballent, 2008). Their shells, which are used for taxonomic identification, function as archives for information on the geochemistry of trace elements, as well as carbon and oxygen isotope ratios, all of which can inform on environmental conditions at the time of deposition.
Temporal changes in species composition and trace element ratios can be used for estimating past climate variations. Hence, ostracods recovered from lake sediments provide information on the physical and chemical properties (e.g. salinity, solute composition, temperature, and flow conditions) of the environment in which they flourished, which then allow inferences on lake level changes (Palacios-Fest et al., 1994; Belis et al., 1999; Schwalb, 2003; Mezquita et al., 2005) and hence climate. Although there are a substantial number of studies that deal with ostracods from Argentina, only a small part of this work is devoted to Quaternary non-marine deposits. Daday (1902) was the first to describe a number of non-marine taxa from Patagonia, whereas Ramirez (1967) analyzed the soft and calcareous parts of the ostracod fauna from Lake Chascomús in Buenos Aires province. Other studies have examined systematic, palaeoecological, and palaeoenvironmental issues from outcrops and sediment cores from central and northern provinces in Argentina (Chaco, Entre Ríos, and Buenos Aires; Zabert, 1981; Zabert & Herbst, 1986; Bertels & Martínez, 1990; Ferrero, 1996; Laprida, 2006; Laprida & Valero-Garcés, 2009) but not from Patagonia. More recently, ostracod studies in Patagonia focused on the analysis of Quaternary outcrops and sediment cores from the Cari-Laufquen area (41°S) by identifying new species and inferring their palaeoecological and palaeoclimatological signals (Cusminsky & Whatley 1996, 2008; Whatley & Cusminsky 1995, 1999, 2000).
The Patagonian Lake Drilling Project (PATO-PALATRA) was a multi-disciplinary, international collaborative initiative, with the objective of reconstructing latitudinal palaeoenvironmental changes during the Late Quaternary using two closed basins in Argentina: Laguna Cari-Laufquen in northern Patagonia (41°S) and Lago Cardiel in southern Patagonia (49°S) (Fig. 1A) (Ariztegui et al., 2001, 2008; Gilli et al., 2001, 2005a, b; Markgraf et al., 2003; Beres et al., 2008). Modern limnological data were collected as part of this collaborative effort from a wide range of lacustrine environments in the surrounding regions in both basins (Schwalb et al., 2002; Cusminsky et al., 2005). In the present study, we summarize our findings focusing on the relationship between the abundance, diversity, and distribution of ostracods with climatically-induced environmental changes.
At present, the closed-basin (endorheic) Laguna Cari-Laufquen (41°S, 69°W) encompasses two water bodies: Cari-Laufquen Grande at 800 m a.s.l and Cari-Laufquen Chica at 820 m a.s.l. The mean annual precipitation in the area is approximately 160 mm, primarily occurring between May and August with prevailing westerly winds; mean annual temperature is 4 °C. (Coria, 1979). Laguna Cari-Laufquen Chica is a permanent water body, with pH 8.7 (Schwalb et al., 2002) and a sodium bicarbonate concentration of 230 p.p.m. (Galloway, Markgraf & Bradbury, 1988). Occasionally, excess water overflows through the Maquinchao River into Cari-Laufquen Grande, which is an ephemeral body of brackish water with pH 8.8 (Schwalb et al., 2002) and solute concentration of 4000 p.p.m. (Galloway et al., 1988) (Fig. 1B). The Pleistocene sequence of the Cari-Laufquen lake system can be seen on outcrops along the shore of the present lakes. Two high stages have been recognized in shorelines at 55 and 35 m above modern lake level. These have been radiocarbon dated at 19 000 years BP and between 14 000 years BP and 10 000–8000 years BP, respectively (Galloway et al., 1988; Bradbury et al., 2001). The two lakes merged during those high stands forming a larger palaeolake (Ariztegui et al., 2008). Lago Cardiel (49°S, 71°W) is also a closed (endorheic) lake basin situated on the Patagonian Plateau (Argentina) at 276 m a.s.l. (Fig. 1C). This heart-shaped lake has a diameter of 20 km covering an area of 370 km2. The maximum modern water depth is approximately 76 m with a catchment area of approximately 4500 km2. Río Cardiel is the principal perennial inflowing river. The mean annual precipitation varies from approximately 500 mm in the mountain area west and north-west of the lake to 160 mm near the lake. The mean annual temperature is approximately 8 °C, and strong westerly winds prevail during summer months (December to March) (Gilli et al., 2001, 2005a, b; Markgraf et al., 2003; Ariztegui et al., 2008, 2010). Galloway et al. (1988) and Stine & Stine (1990) recognized and dated several palaeoshorelines indicative of former lake level high stands. A deeply dissected highstand at +75 m a.s.l. was beyond radiocarbon dating. A highstand at +55 m above the actual lake level was dated to 9780 14C years BP (approximately 10 800 calibrated years BP). An intermediate highstand at +21 m a.s.l was dated at 5130 14C years BP and four minor lake level fluctuations between +3 and +10 m were dated, respectively, between 4530 and 3070 14C years BP, approximately 2000, 1450 and 800 14C years BP (Markgraf et al., 2003). Seismic surveys of Lago Cardiel allowed the classification of the sedimentary succession into six major seismic sequences that correspond to different lake levels. Thus, these geophysical data show very low lake levels and even complete evaporation during glacial times before the dramatic increase in water level that characterizes the beginning of the present interglacial (Holocene). It further shows that the lake level during the Holocene has never been lower than today (Gilli et al., 2001, 2005a, b; Beres et al., 2008).
Late Pleistocene–Holocene sediment sequences and ostracod assemblages
Cari-Laufquen lake system (Fig. 2A) Cari-Laufquen Grande outcrop: (Fig. 2B). This 6 m thick section is located on the southern coast of Lago Cari-Laufquen Grande and was sampled in 1998 from the shore towards the surrounding dunes (41°10′26′′S, 69°28′41′′W). The outcrop comprises from the base upwards a sequence of ash and clay layers with ostracods, overlain by banded yellow clays and silt with chippy carbonate, clays, and a thick carbonate layer with ostracods, banded yellowish silty-clays and chocolate-coloured clays (Pineda et al., 2010). Two 14C ages were obtained for this sequence (Table 1). Ostracod assemblages (Fig. 3) are dominated by Limnocythere rionegroensis Cusminsky and Whatley, which are especially abundant in the lower levels. Eucypris virgata Cusminsky and Whatley and Eucypris fontana (Graf) are also present. The upper levels are characterized by an abrupt decrease in abundance of all species, including Limnocythere patagonica Cusminsky and Whatley. The population of each ostracod species, including Limnocythere rionegroensis along the outcrop, comprises adults and juveniles, and male and female individuals (Pineda et al., 2010).
Table 1. Radiocarbon and calibrated ages from the Cari-Laufquen Grande outcrop
14C age (years BP)
Calibrated age (calibrated years BP)
15 630 ± 110
16 960 ± 239
19 750 ± 128
21 692 ± 288
Maquinchao outcrop: (Fig. 2C). This outcrop is located in the lower valley of the Maquinchao River. The lithology of the section exhibits different facies (Whatley & Cusminsky, 1999). Facies A (5 m thick) comprises levels with gravel, sand, and mud (facies A1) overlain by lacustrine rhythmites and carbonate-rich levels (facies A2). The facies A1 represents environments of high energy, which were interrupted periodically by lacustrine episodes of deeper water (facies A2). Facies B1, composed of 6 m of laminated clays with silty marls and sporadic gypsum layers, represents deep-water lacustrine sedimentation. Facies C1 (1 m thick) comprises coarse-grained sediments with irregular stratifications and stromatolites, representing a regressive sequence of the palaeolake. A tephra layer (a distinctive level of pyroclastic material associated with a volcanic eruption) divides facies B1 from facies C1 (Fig. 2C). Three samples from this section have been radiocarbon dated and an additional sample was dated by thermoluminiscence (Table 2). Radiocarbon dating of two samples from the bottom and the top of the sequence (samples 2 and 3 in Table 2, respectively) show reversal values (i.e. the base older than the top). The latter suggests that radiocarbon dates do not reflect the true age of the sediments most probably as a result of the contribution of older carbonates from the watershed (i.e. ‘hard water effect’) and, thus, the present chronological model relies preferentially on the thermoluminiscence age (Whatley & Cusminsky, 1999, 2000). At the bottom (facies A1 and A2), the Maquinchao section (Fig. 4) is characterized by a low diversity and low abundance of ostracods; only four species were detected: L. rionegroensis, E. virgata, Candonopsis brasiliensis Sars and L. patagonica. In the immediately overlying levels (facies B1), the number of individuals of L. rionegroensis greatly increases along with the appearance of two extra species, E. fontana and E. virgata. The overlying level (facies C1), however, exhibits a dramatic drop in diversity (only two species detected: E. fontana and L. rionegroensis) and abundance, with the number of individuals of these two species greatly reduced. Both adults and juveniles of all species are present and their population structure does not suggest any evidence of transport or reworking. Limnocythere rionegroensis presents female and male individuals, especially in facies B1. For more details, see Whatley & Cusminsky (2000: fig. 6) and Whatley & Cusminsky (1999: table 1).
Table 2. Chronological data for the Maquinchao outcrop
14C age (years BP)
Calibrated age (calibrated years BP)
16 520 ± 120
17 880 ± 318
Base of palaeolake (2)
15 220 ± 180
16 406 ± 291
Top of palaeolake (3)
33 200 ± 260
35 732 ± 691
TL (thermoluminiscense dating)
Lago Cardiel Core CAR-99-7P (Fig. 5) was retrieved with an ETH-Kullenberg coring system. The lithology of the core (11.11 m long) is described in detail by Gilli et al. (2005b) defining six lithological units. Basal unit 6 consists of silty clays with very fine laminations and plant remains identified as Ruppia sp. A tephra layer identified as originated from the Reclús volcano is found at 1055.9 cm depth. Unit 5 consists of black silty clays with sand lenses, gravel, and woody debris towards the top of the unit. Unit 4 is characterized by two meters of silty clay and sandy layers with pebbles and cross bedding at the top of the unit, suggesting a hiatus. Unit 3 consists of finely laminated silty clays with abundant light coloured carbonate layers, alternating with blackish layers. Units 2 and 1 are characterized by laminated silty clays. The chronology of core CAR 99-7P is based on a combination of tephrochronology (Markgraf et al., 2003; Stern, 2008) and selected AMS radiocarbon dates (Gilli et al., 2005a, b). All radiocarbon ages were calibrated (Table 3) using calib 6.0.0 with the IntCal09 and SHCal04 calibration set (Stuiver & Reimer, 1993; McCormac et al., 2004; Reimer et al., 2009). Five majors units have been recognized in core CAR 99-7P on the basis of ostracod stratigraphy. Unit 1 (15 680–15 306 calibrated years BP) displays a low number of ostracod individuals but a relatively high diversity comprising E. fontana, E. virgata, and L. rionegroensis var. 1 with few individuals of Eucypris sp. 2. Unit 2 (15 490–12 990 calibrated years BP) contains the highest ostracod abundance of the core and is represented primarily by L. rionegroensis var. 1 and few individuals of E. fontana. Unit 3 (12 990–12 572 calibrated years BP) shows an increase in ostracod diversity. Limnocythere rionegroensis var. 1 is replaced by L. rionegroensis var. 2; E. fontana is present in higher abundance, whereas L patagonica and Eucypris sp. 1 appear. Unit 4 (12 572–4100 calibrated years BP) contains primarily E. fontana and L. patagonica in equal abundance with few individuals of E. virgata. Unit 5 (younger than 4100 calibrated years BP) shows an increase in both number of individuals and diversity. E. fontana predominates, L. patagonica is less abundant and L. rionegroensis var. 2 is also present.
Table 3. Lago Cardiel chronology for core CAR-99-7P
USING MODERN OSTRACOD DISTRIBUTION AS A PALAEOENVIRONMENTAL INDICATOR
According to De Deckker & Forester (1988) and Mezquita et al. (1999, 2005), the distribution of modern lacustrine ostracods can be related to water chemistry. With the same rationale, we analyzed modern ostracod assemblages in 38 localities of Cari-Laufquen and Cardiel basins using multivariable statistics such as canonical correspondence analysis (CCA) (Viehberg, 2006). Two main factors were identified as major determinants of the variance in the modern ostracod distribution: (i) Ca : pH ratio and (ii) chloride concentration. The Ca : pH ratio is a valuable measure for the alkalinity of a lake and is closely related to calcite precipitation (Wetzel, 2001). Low values characterize waters with a weak buffer capacity (both low calcium concentrations and pH values). This habitat is preferred by species such as E. fontana, Potamocypris smaragdina (Vavra), and Kapcypridopsis megapodus Cusminsky and Whatley. By contrast, L. rionegroensis dominates in waters with higher buffer capacities. Chloride concentration covaries with conductivity. Low values occur in waters with constant freshwater supply that are preferred by Penthesilenula incae (Delachaux) and P. smaragdina. Together with Heterocypris incongruens (Ramdohr), E. fontana, Amphicypris nobilis Sars and Ilyocypris ramirezi Cusminsky and Whatley, they are characteristic of springs, seeps, and streams (Schwalb et al., 2002) with low and rather stable ionic concentrations (Cusminsky et al., 2005). Limnocythere rionegroensis is the dominant species in waters with the relatively highest ionic concentration such as ephemeral ponds and lakes (Schwalb et al., 2002), whereas I. ramirezi, E. fontana, and E. virgata live in waters with the most variable ionic concentrations, especially Na+, Cl- and SO4−2 (Cusminsky et al., 2005). Permanent ponds and lakes are characterized by the presence of L. patagonica Cusminsky and Whatley, Limnocythere sp., Eucypris labyrinthica Cusminsky and Whatley, and E. fontana (Schwalb et al., 2002). The Pleistocene–Holocene sequences presented here show ostracod associations that resemble the range of studied present-day lacustrine conditions. Limnocythere rionegroensis is present in great abundance in recent sediments, as well as in the Late Pleistocene sections of the Lago Cari-Laufquen system. Whatley & Cusminsky (1999, 2000) and Cusminsky et al. (2005) considered L. rionegroensis as a possible indicator of slightly saline waters and dry climatic conditions. Morphologically, this species resembles Limnocythere bradburyi Forester, recorded from the western USA and Mexico (Forester, 1985), and also Limnocythere sappaensis Staplin collected from several lakes on the Chilean Altiplano (Schwalb, Burns & Kelts, 1999) that inhabit a somewhat similar environment to those of our study sites.
For the Cari Laufquen lake system, the Cari-Laufquen Grande section contains a high number of individuals especially of L. rionegroensis. This is particularly so in the lower levels that are allocated to lacustrine saline environments. The presence of both male and female individuals suggest sexual reproduction what in ostracods is generally associated with environmental instability (Schwalb, 2003) or an increase of the solute concentration (Löffler, 1990). Parthenogenesis reproduction is in general characteristic of stable environments.
The Maquinchao section comprises three types of ostracod assemblages which can be related to the three lithological units described in the outcrop. At the base of the succession, the ostracod assemblage, represented by very few individuals, coincides with facies A1 and A2, which is interpreted as a relatively shallow-water and high energy lake. This interval is followed by a period of quieter-water conditions corresponding to facies B1. It represents a lacustrine environment with deeper-water and lower energy comprising high ostracod abundance especially of the species L. rionegroensis. The abrupt change in ostracod associations that follows this unit might be related to increases in volcanic ash at the boundary between Facies B1 and C1, which probably caused an increase of turbidity with diminution of the oxygen and light and/or by a major change on the dominant climatic conditions (Whatley & Cusminsky, 1999).
According to Bradbury et al. (2001), Laguna Cari-Laufquen shows several high stand levels during the Late Pleistocene and Early Holocene. The largest and most persistent lake occurred between 18 400 and 13 000 radiocarbon years BP. (i.e. Last Glacial Maximum and Late Glacial, respectively); a second high stand at +35 m above present lake level centered on 10 000 14C years BP, which is characterized by an unproductive saline and turbid lacustrine system (Bradbury et al., 2001). These authors suggest that presumably the Last Glacial Maximum (18 000 radiocarbon years BP), Late Glacial (approximately 13 000 radiocarbon years BP) and Early Holocene (10 000 radiocarbon years BP) highstands at Cari-Laufquen document higher precipitation at this latitude. The sources of the precipitation that generated these high lake levels are still unclear. A south-easterly origin for this moisture has been suggested that would be related to the increasing activity of mobile polar highs or to cold air outbreaks from Antarctica and southernmost South America.
For the Lago Cardiel (Core CAR 99-7P), Mezquita et al. (2005) and Viehberg (2006) successfully developed calibration datasets to be applied in palaeoenvironmental research for the study of the responses of modern species assemblages to environmental factors. Using these calibration data sets, they used weighted averaging (WA) regressions to reconstruct past habitat conditions from fossils assemblages that share species with the modern data. Similarly, we used the modern ostracod data sets and applied these techniques (CCA and WA) to derive an ostracod-based transfer function for the reconstruction of the palaeoenvironmental history of Lago Cardiel (Fig. 5). The lower part of the core before 13 000 calibrated years BP is dominated by L. rionegroensis var. 1, suggesting high ionic concentrations. This interpretation is consistent with seismic, sedimentological, and limnogeological studies of Lago Cardiel (Gilli et al., 2001, 2005a, b; Markgraf et al., 2003; Ariztegui et al., 2008, 2010). These studies point towards a positive hydrological balance (i.e. abundant precipitation) at the Late Pleistocene–Holocene transition. Before 13 000 calibrated years BP, the lake level was substantially lower and the lake probably even dried out for short time periods (Gilli et al., 2005b). Between 13 000 and 12 100 calibrated year BP, L. rionegroensis var. 1 is replaced by L. rionegroensis var. 2 and E. fontana. Limnocythere patagonica is present suggesting a decrease of the ionic concentration and thus less evaporation. Between 12 100 and 4100 calibrated years BP, L. rionegroensis var. 2 is absent, which could reflect the lowest ionic concentrations in the core, suggesting a permanent, deep lake. For the Early Holocene, Gilli et al. (2005b) suggested an increase in moisture showing a positive balance and increasing lake level up to 55 m above modern levels. After 4100 calibrated years BP, the ostracod association is dominated by E. fontana with L. patagonica and L. rionegroensis var. 2. This represents the present-day ostracod association in this lake, suggesting that the lake level decreased after the Early Holocene highstand, bringing the ionic concentration up to present-day values. The accumulation of sediments in the middle of the basins after 6800 calibrated years BP suggests an increase of the southern westerlies strength at latitude 49°S (Gilli et al., 2005b; Ariztegui et al., 2008). The combined data indicate a sharp change in the Lago Cardiel hydrological balance from a dry Late Pleistocene to substantially more humid conditions for the Early and Middle Holocene. The Late Holocene is in turn characterized by a decreasing trend in the lake level, suggesting diminishing moisture availability in the area.
The study of modern lacustrine ostracods in the Cari-Laufquen and Cardiel basins in Patagonia shows distinctive associations that are responding to dominant environmental parameters among which conductivity is the most important. These modern associations were recognized at different intervals within the Late Pleistocene and Holocene sections. In Cari-Laufquen, the predominance of L. rionegroensis during the Late Pleistocene and Early Holocene suggest a saline and turbid lake, whereas, in Lago Cardiel, changes in ostracod abundance and association along with results of transfer function analyses show a decrease of conductivity from the Late Pleistocene to the Middle Holocene. These results are coherent with previous studies using independent proxies. Hence, all these observations show the usefulness of lacustrine ostracods as a proxy for changing hydrological conditions. These data indicate the hydrological budgets that differ between latitudes during the Late Glacial–Holocene transition. Although wet conditions characterized the Cari-Laufquen area (northern Patagonia) during this time, the Lago Cardiel region was dry and became dramatically humid during the Early Holocene. Our studies provide information that can be used to improve our understanding of palaeoclimate in Patagonia.
The core from Lago Cardiel was retrieved by projects funded by the USA and Swiss National Science Foundations (US National Science Foundation grants NSF-EAR-9709145, NSF-ATM-008267, and NSF-ATM-0081279 to Vera Markgraf and Kerry Kelts; and the Swiss National Science Foundation grant No. 21-5086297 to the ETH Limnogeology Group). G. C. thanks CONICET (Project PIP 00819) and Universidad Nacional del Comahue (project B001) for support of this work and SECYT-DAAD project AO731 for a travel grant to Gabriela Cusminsky and Antje Schwalb, as well as their team members. A.S. thanks the German Climate Research Program (DEKLIM E: PROSIMUL III, FKZ 01 LD 003) the DAAD (PROALAR D/07/09565). We thank Jorge Rabassa, Eduardo Tonni, Alfredo Carlini, and Daniel Ruzzante for participation in the Symposium of Palaeogeography and Palaeoclimatology of Patagonia Biodiversity, held in La Plata Museum in Argentina, in May 2009.