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Abstract: Opal phytolith, geochemical and sedimentological data from two cores taken in Peña Lagoon, a small water body on the Atlantic shore of Uruguay, are presented in this paper. The choice of this specific sampling location aimed to obtain high resolution records to infer late Holocene environmental and climatic changes in southeastern Uruguay, in order to improve previous models for early and middle Holocene. The combined record of two cores extends to 2500 14C BP, with a sedimentation rate ranging between 0.6 and 2 mm y−1. A multiproxy approach allowed the identification of changes in dynamics, particularly organic matter and sediment input variability from the watershed. With this information, differences in the amount and extent of environmental signals were evaluated and a comprehensive climate change model was generated, especially for the last 1500 years. Three main climatic events were distinguished. The oldest, which lasted until 700 AD, was characterized by temperate and humid conditions. The second extended until 1200 AD and warmer and wetter conditions were inferred, but also a colder and drier pulse was observed in between. This second event was assigned to the Warm Medieval Period (WMP). During the third event, assigned to the Little Ice Age (LIA), which extended until the present, a high variability in climate was observed, but also three dry/cold phases reaching their maxima by 1300, 1600 and 1900 AD were inferred. [Copyright &y& Elsevier]

A multiproxy analysis of diatoms, chrysophyte cysts, opal phytoliths, and palynomorphs was undertaken in Negra Lagoon, to decipher Holocene changes in paleobotanical proxies associated with sea-level changes and paleolimnological conditions. Before the Holocene transgression (7000 yr BP), a terrestrial system was inferred as no aquatic palynomorphs or biogenic silica remains were recorded. During the sea-level maximum (5200 yr BP), marine/brackish conditions were established as indicated by diatoms, Chenopodiaceae, and a high content of cysts of Peridinioideae. The catchment consisted of grasslands and wetlands as indicated by the opal phytolith data. The Holocene transgression was followed by a sea-level decrease, which led to the onset of brackish/freshwater conditions as inferred from the co-dominance of freshwater and marine/brackish diatoms. This is also supported not only by the concomitant increase in non-siliceous freshwater microalgae and emergent macrophytes, but also by the presence of Myriophyllum. As sea level continued to decrease during late Holocene, a freshwater system was observed because of the complete separation from the ocean. Chrysophyte cysts were consistently recorded and brackish diatoms exhibited reduced abundances. The observed limnological changes are consistent with the alternative states hypothesis of Scheffer’s model (). That is, the clear water phases were explained because of high macrophyte abundances, low phytoplankton frequencies, high cyst to diatom ratios, and increase in frequencies of benthic diatom species. Conversely, turbid phases were likely to occur when low cyst to diatom ratios together with increase in planktonic diatoms and decrease in macrophytes values were detected. This highlights the importance of this approach to detect long-term changes in shallow lake alternative states. [ABSTRACT FROM AUTHOR]

Abstract: This paper presents a phytolith analysis of selected native plants and modern surface soils from southeastern Uruguay. A modern phytolith reference collection was established based on 60 Poaceae species, 22 non-Poaceae monocotyledonous species, 17 species of herbaceous dicotyledons, 9 woody dicotyledonous species, and 2 species of fern. Nine modern surface soil samples were analyzed from the most representative vegetation units of the region, including wetlands, wet prairies, upland prairies, riparian forest, and palm forest. Of the 50 non-Poaceae plant species analyzed, 25 contribute diagnostic phytoliths at different taxonomic levels corresponding to all the major ecological zones of southeastern Uruguay. Patterns of phytolith production and morphology were concordant with those observed in related taxa studied from other regions of the world. The modern soil analysis revealed significant patterns that differentiate a number of specific habitats, showing that distinct vegetational units may be discriminated by the phytolith signatures they produce. These results reinforce the utility of using phytoliths as significant indicators for vegetation units dominated both by monocotyledonous and dicotyledonous plant species, and demonstrate the potential of phytolith analysis for paleoecological and archeological reconstruction in this region. [Copyright &y& Elsevier]

Abstract: This article presents a combined pollen and phytolith record of a 1.70-m sediment core from the wetlands of India Muerta (33° 42′ S, 53° 57′ W) in the lowland Pampa (grasslands) of southeastern Uruguay. Six 14C dates and the pollen and phytolith content of the samples permitted the recognition of four distinct climatic periods between 14,850 14C yr B.P. and the present. The Late Pleistocene period (between ca. 14,810 and ca. 10,000 14C yr B.P.) was characterized by drier and cooler conditions indicated by the presence of a C3-dominated grassland. These conditions prevailed until the onset of the warmer and more humid climate of the Holocene around 9450 14C yr B.P. The early Holocene (between around 10,000 and 6620 14C yr B.P.) was characterized by the establishment of wetlands in the region as evidenced by the formation of black peat, the increase in wetland taxa, and the replacement of C3 Pooideae by C4 Panicoideae grasses. During the mid-Holocene, around 6620 14C yr B.P., began a period of environmental change characterized by drier climatic conditions, which resulted in the expansion of halophytic communities in the flat, low-lying areas of the wetlands of India Muerta. About 4020 14C yr B.P. a massive spike of Amaranthaceae/Chenopodiaceae coupled with a radical drop in wetland species indicates another major and more severe period of dryness. After ca. 4000 14C yr B.P., a decrease of halophytic species indicates the onset of more humid and stable climatic conditions, which characterized the late Holocene. The findings reported in this article substantially improve our knowledge of the late Glacial and Holocene climate and vegetation in the region. The data provide a detailed record of the timing and severity of mid-Holocene environmental changes in southeastern South America. Significantly, the mid-Holocene drying trend coincided with major organizational changes in settlement, subsistence, and technology of the pre-Hispanic populations in the region, which gave rise to early Formative societies. This study also represents the first combined pollen and phytolith record for southeastern South America reinforcing the utility of phytoliths as significant indicators of long-term grassland dynamics. [Copyright &y& Elsevier]

Paleolimnological data are presented on trophic development, climatic change and sea level variations in Negra Lagoon, a 142 km2 coastal lagoon in southern Uruguay. Using a sediment core that extended to the early Holocene, analyses of organic matter, carbonate, diatoms and opal phytoliths allowed us to track changes in trophic state and paleosalinity levels, which were closely related to Holocene sea level variation, temperature and humidity. Diatom Association Zones (DAZ) corresponding to transgressive events were dominated by marine/brackish taxa and relatively low organic matter values, while those DAZ corresponding to regressive events showed increases in brackish/freshwater diatoms and organic matter. Opal Phytoliths Association Zones (OPAZ) also were identified, temperature and humidity indices were calculated, and climatic trends were inferred. During the middle Holocene (i.e., ~5000–4000 yr BP), cool–dry climatic conditions were established. After ~4000 yr BP, a transition from cool to warm–humid conditions was observed, but fully warm humid conditions established only after ~1700 yr BP. Such climatic amelioration led to increases in trophic state. [ABSTRACT FROM AUTHOR]

The Butiazal de Tapes (30°31′39.86″S, 51°21′31.02″W) region is located in southern Brazil on the western shore of the Patos Lagoon and presents a remnant of a native palm grove ecosystem known as butiazal. It is characterized as savanna dominated by palm trees from the Butia genus, and spreads throughout southern Brazil, Uruguay and Argentina. Species from this genus are threatened by monoculture and livestock farming. In this region there are outcrops of Pleistocene coastal deposits which were exposed to the Holocene climate and to the influence of coastal dynamics. It is therefore an excellent location for palaeoclimatic and palaeoenvironmental reconstruction of the past 11,000 years. For such a reconstruction, and better understanding as to how this unique ecosystem was formed, tools such as bioindicators (phytoliths and palynomorphs), X-ray diffractometry analysis, total organic matter content determination, particle size analysis and carbon-14 AMS dating were used. Indications of the presence of Butia in the region from at least 11,125 to 10,751 yr cal BP were identified, along with the importance of the climate during its formation and expansion. Although the climate changes in the analysed interval were not expressive, it was possible to divide the profile into 4 zones according to plant cover and observe the influence of sea-level shifts during the Holocene on the evolution of the palaeoenvironment in the region, besides the evident human influence on vegetation evolution. • Variations in sea level can be detected through changes in vegetation evolution. • The ecosystem butiazal has been in the west shore of Patos Lagoon since at least 4154-3960 yr cal BP. • Butia odorata (Barb. Rodr.) Noblick responds well to phytolith analysis. • There are evidences of anthropic management in the evolution of "Butiazal de Tapes". [ABSTRACT FROM AUTHOR]

The phytolith assemblages of the El Palmar Formation (Late Pleistocene) are presented together with general stratigraphic and sedimentological data. The El Palmar Formation represents a channel system of the Uruguay river (South America) with longitudinal sandy gravel and gravelly sand bedforms, and associated floodplain deposits. The phytolith content of the unit was analyzed along the profiles on the right bank of the river in the middle reach (eastern Entre Ríos province, Argentina). Four outcropping profiles of the El Palmar Fm. were selected for detailed analyses: Santa Ana (Federación department), La Chola quarry, El Palmar National Park and Nueva Escocia (Colón department) localities. The herbaceous palaeoflora of the El Palmar Fm. is represented by mixed environments, such as meso-megathermic grassland with the presence of palm trees in a savanna ecosystem and micro-environments with aquatic plants and bamboo grasses that characterized a stratified forest in humid environments with high water availability. The reconstructed vegetation pattern is composed of a stratified gallery forest changing to temperate savanna, extended along the upper basin of the Uruguay river and occupying the river valley in the middle reach of the basin. The data contribute to the palaeobiodiversity knowledge of Quaternary fluvial sedimentary units of South America. In addition, the results give a better understanding of the evolutionary history of some vegetal communities, and especially the present palm trees of the El Palmar National Park reserve, as part of a relict flora that has inhabited the area since, at least, the Upper Pleistocene. The geological and micro-paleobotanical records of the El Palmar Fm. lead to the inference of general warm and humid conditions during most of the deposition of the formation, probably covering part of the last major interglacial period before the present (MIS 5). • Late-pleistocene phytoliths from uruguay river valley. • Phytoliths assemblages of the el palmar formation. • The sedimentological and palaeobotanical indicators of the el palmar fm. show general warm and humid conditions. [ABSTRACT FROM AUTHOR]

A paleolimnological study was undertaken for a sediment core from Laguna Formosa, a subtropical lake in the northern campos inland region of Uruguay. The record encompasses a Late Pleistocene and Late Holocene interval, interrupted by a hiatus. We analyzed (1) opal phytoliths and diatoms to infer the regional environment, climate, and changes in lake trophic state, and (2) total carbon and nitrogen (TC and TN) and stable isotope data (δ13C and δ15N) to infer organic matter sources and composition. Findings indicate that, in Late Pleistocene times, a cool, semi-arid or highly seasonal climate existed, with a dominance of C3 grasslands and riparian hydrophilous shrublands while the shallow freshwater environment was characterized by benthic diatoms and littoral macrophytes. Late Holocene times were initially characterized by a temperate and humid or sub-humid climate, and dominance of grasslands and bunch grasses, while Laguna Formosa was dominated by phytoplankton. The abundance and diversity of littoral hydrophilic vegetation, and riparian forest vegetation, increased progressively over time, as a result of increased rainfall, from ca. 1800 cal yr BP and the onset of a subtropical (temperate and humid) climate by ca. 1300 cal yr BP. Such inferences are in close agreement with an earlier palynological study of Laguna Formosa, and support the paleoclimatic models for the Late Pleistocene and Late Holocene of southeastern South America. • Evidence of grasslands and woody formations in the Late Pleistocene and Late Holocene • Different paleolimnological conditions in the Late Pleistocene and Late Holocene • Late Pleistocene under cool, semi-arid or highly seasonal climatic conditions • Late Holocene under temperate subtropical climatic conditions [ABSTRACT FROM AUTHOR]