Margarita Caballero

Environmental Characteristics of Lake Tecocomulco, northern basin of Mexico, for the last 50,000 years

Paleoenvironmental studies have documented the late Pleistocene to Holocene evolution of the lakes in the central and southern parts of the basin of Mexico (Texcoco and Chalco). No information was available, however, for the lakes in the north-eastern part of this basin. The north-eastern and the central and southern areas represent, at present, different environmental conditions: an important gradient exists between the dry north and the moister south. To investigate the late Pleistocene to Holocene characteristics of the north-eastern lakes in the basin of Mexico two parallel cores (TA and TB) were drilled at the SE shore of Lake Tecocomulco. Stratigraphy, magnetic properties, granulometry, diatom and pollen analyses performed on these sediments indicate that the lake experienced a series of changes between ca. > 42,000 yr BP and present. Chronological control is given by five radiocarbon determinations. The base of the record is represented by a thick, rhyolitic air-fall tephra that could be older than ca. 50,000 yr BP. After this Plininan event, and until ca. 42,000 yr BP, Lake Tecocomulco was a moderately deep, freshwater lake surrounded by extended pine forests that suggest the presence of cooler and moister conditions than present. Between ca. 42,000 and 37,000 yr BP, the lake became shallower but with important fluctuations and pollen suggests slightly warmer conditions. Between ca. 37,000 and 30,000 yr BP the lake experienced two relatively deep phases separated by a dry interval. A second Plinian eruption, represented in the sequence by a dacitic an air-fall tephra layer dated at 31,000 yr BP, occurred in the area by the end of this dry episode. Between ca. 30,000 and 25,7000 yr BP Tecocomulco was a fresh to slightly alkaline lake with a trend towards lower level. After ca. 25,700 yr BP very low lake levels are inferred, and after ca. 16,000 yr BP the data indicate the presence of a very dry environment that was persistent until the middle Holocene. After 3,500 yr BP lacustrine conditions were re-established and the vegetation cover shows a change towards higher percentages of herbaceous taxa.

Sta. Cruz Atizapán: a 22-ka lake level record and climatic implications for the late Holocene human occupation in the Upper Lerma Basin, Central Mexico

Abstract The Upper Lerma is a high altitude basin with three water bodies linked by the Lerma River. This basin has a long archaeological history, characterised by the establishment of settlements within the lacustrine ecosystem itself (man-made islands) during the late Classic to Epiclassic (AD 550–900), which were abandoned by the end of the Epiclassic. The Upper Lerma is an ideal site to study climatic and environmental conditions during the period of human occupation, as well as during the last full-glacial/interglacial cycle. Two sediment cores (STCRZ: 9.54 m and Almoloya del Rio: 5.12 m) were recovered from the highest lake in the system (Chignahuapan). Ten radiocarbon dates provide chronologies for these sequences in which the Tres Cruces Tephra (c. 8500 yr BP) and the Upper Toluca Pumice (c. 11 600 yr BP) serve as stratigraphic markers. Magnetic properties, loss on ignition, and diatom analyses were used to infer lake level fluctuations during the last c. 22 000 yr BP. The Late Pleistocene environment was characterised by a freshwater lake. High sediment input and variable lake levels are recorded during the Last Glacial Maximum (c. 19 000–16 000 yr BP), while slightly higher water levels and reduced sediment input are recorded during the Late Glacial (c. 16 000–11 000 yr BP). A short episode of shallow conditions is inferred by c. 12 400 yr BP. Holocene lake levels were generally shallower, and three episodes of very shallow, slightly alkaline waters are identified. The first dates to the early Holocene (c. 11 000–7000 yr BP). The second is centred at c. 4600/4500 yr BP. The third occurred between c. 2000 (?) and 800 yr BP (c. 200 BC–AD 1100, calibrated ages) with very shallow water after c. 1400 yr BP (AD 550, calibrated age). Lake level increased after c. 800 yr BP. These three shallow water events are also recorded at other sites in Central Mexico indicating regional climatic trends rather than local events. A deeper water phase occurred between 7000 and 6400/6200 yr BP. The last shallow water phase correlates with the Classic and Epiclassic periods (AD 200–900), and shallowest conditions occurred in the late Classic to Epiclassic (c. AD 550–900), when the construction of man-made islands reached a peak. An increase in lake level after c. 800 yr BP (AD 1100 calibrated age) may have led to the abandonment of this life strategy.

Tracing the effects of the Little Ice Age in the tropical lowlands of eastern Mesoamerica.

The causes of late-Holocene centennial to millennial scale climatic variability and the impact that such variability had on tropical ecosystems are still poorly understood. Here, we present a high-resolution, multiproxy record from lowland eastern Mesoamerica, studied to reconstruct climate and vegetation history during the last 2,000 years, in particular to evaluate the response of tropical vegetation to the cooling event of the Little Ice Age (LIA). Our data provide evidence that the densest tropical forest cover and the deepest lake of the last two millennia were coeval with the LIA, with two deep lake phases that follow the Spörer and Maunder minima in solar activity. The high tropical pollen accumulation rates limit LIAs winter cooling to a maximum of 2°C. Tropical vegetation expansion during the LIA is best explained by a reduction in the extent of the dry season as a consequence of increased meridional flow leading to higher winter precipitation. These results highlight the importance of seasonal responses to climatic variability, a factor that could be of relevance when evaluating the impact of recent climate change.

Developing diatom-based transfer functions for Central Mexican lakes

This paper is the first attempt to produce diatom-based transfer functions for the northern tropical Americas. A dataset of 53 modern diatom samples and associated hydrochemical variables from 31 sites in the volcanic highlands of central Mexico is presented. The relationship between diatom species distribution and water chemistry is explored using canonical correspondence analysis (CCA) and partial CCA. Variance partitioning indicates that ionic strength and ion type both account for significant and independent portions of this variation. Transfer functions are developed for electrical conductivity (r2 = 0.91) and alkalinity (as a percentage of total anions) (r2 = 0.90), reflecting ionic strength and ionic composition respectively. Prediction errors, estimated using jack-knifing, are low for the conductivity model, but the carbonate transfer function performs less well. This study highlights the potential for diatom-based quantitative palaeoenvironmental reconstructions in central Mexico. However, a number of key diatom species found in fossil material are not represented in the modern flora. Sampling of additional sites may resolve this, but it is thought that the lack of modern analogues may reflect the high degree of anthropogenic disturbance in many of the catchments. This highlights the problem of trying to reconstruct pre-disturbance environmental changes in highly modified ecosystems. One possible solution is to merge the central Mexican data with the African dataset, which includes sites of similar chemical composition, but which have not suffered the same degree of disturbance.

Climate, palaeohydrology and land use change in the Central Iberian Range over the last 1.6 kyr: The La Parra Lake record

A multiproxy study of a 7 m long sedimentary sequence from Lake La Parra (39°50.948′, 1°52′, 1014 m a.s.l.) supported by 11 14C AMS and 210Pb/137Cs dates provides a robust, high-resolution hydrological and environmental variability record for the last 1600 years of the Las Torcas sinkhole Complex in the Central Iberian Range. The succession of depositional environments in Lake La Parra sinkhole is controlled by both changes in the regional water table and by the balance between sedimentary input through ephemeral creeks and in-lake production of carbonates and organic matter. Although synergetic links with climate are likely, phases of increased sediment delivery to the lake at c. ad 500–700, c. ad 1000, ad 1450–1500, ad 1550–650 and since 1700 till recent times are driven primarily by human impact in the watershed. Prior to c. ad 300, the sinkhole was dry, then became a lake at the end of the Roman Period (ad 350) when the doline was flooded, and it has not dried out during the last 1600 years. Moderate lake levels with deposition of coarser clastic facies dominated up to the 12th century (ad 400–1200), and relatively higher levels with deposition of laminated facies during the 13th–15th centuries (ad 1200–1600). The pattern of palaeohydrological evolution at a centennial scale is roughly coherent with most Iberian lacustrine records; however, the ‘La Parra’ sequence indicates that increased humidity during Iberian–Roman times was restricted to southern Spain and the humid phases of the ‘Little Ice Age’ (‘LIA’) starting and ending earlier in the central Iberian Range compared with the Pyrenean Domain and southern Spain. This new sequence highlights the heterogeneity through space and time of the main dry and wet climatic periods at shorter scales, emphasizing the impact of latitudinal climate gradients on the Iberian Peninsula climate variability.

The structure and species composition of the diatom communities in tropical volcanic lakes of eastern Mexico

Six nutrient-rich tropical volcanic lakes in eastern Mexico were investigated to document their characteristic diatom floras. Plankton and sedimentary diatom communities were compared to verify the representativity of the sediment assemblage by identifying the plankton species present in the sediment. Relationships between surface sediment diatom communities and 20 physico-chemical variables were evaluated using multivariate analysis. High phosphorus concentrations were measured in all the lakes sampled and chlorophyll a concentrations were found to be useful in describing lake trophic status which ranged from meso- to hypereutrophic. The results showed that the species composition and distribution of the diatom community in these lakes was most influenced by nutrient availability (P) and ionic composition. Canonical correspondence analysis showed that cation dominance explained a significant amount of the variation in the diatom community composition. It allowed the identification of three lake types: (1) Na+-dominated lakes where diatom communities had>50% Achnanthidium minutissimum (Grunow) Czarnecki; (2) Mg2+-dominated lakes, where the characteristic species were A. minutissimum (<30%) and Cymbella microcephala Grunow (<50%); and (3) one Ca2+-dominated lake, where Navicula arvensis Hustedt and Discostella pseudostelligera (Hustedt) Houk & Klee were the most abundant diatoms (>20%). The results suggest that some epiphytic diatoms (e.g., A. minutissimum, C. microcephala) can use other plankton algae as substrate; this is a strategy that allows them to remain within the euphotic zone in deep, eutrophic, turbid lakes. The lakes with the lowest nutrient levels had the best preserved terrestrial vegetation cover on the crater walls. This suggests that deforestation has been a factor that has accelerated the process of lake eutrophication during recent decades in this region.

TERTIARIUS HIDALGENSIS SP. NOV., A NEW DIATOM SPECIES FROM NEOGENE DEPOSITS IN CENTRAL MÉXICO

Se describe una nueva especie que fue estudiada con microscopio de luz y con microscopio electrónico de barrido, encontrada en sedimentos lacustres del Plioceno provenientes del centro de Mé;xico: Tertiaraus hidalgensis (Bacilariophyta: Stephanodiscaceae). Esta especie tiene alvéolos simples y complejos, formados por bandas hialinas engrosadas, una rimoportula elevada localizada dentro de una de las cámaras alveolares, 4 a 10 fultoportulae en la cara valvar rodeadas por 2 o por 2 y 3 poros stelitales, fultoportulae en el manto con 2 poros satelitales, localizadas al extremo de los costae gruesos interiores, sobre la pared. T. hidalgensis es una especie aparentemente extinta, de importancia bioestratigráfica, que extiende la distributeón geográfica del genéro hacia latitudes tropicalas y es una nueva evidencia de la gran diversidad que alcanzaron las especies de agua dulce de la familia Stephanodiscaceae durante el Neogeno. A new species, Tertiarius hidalgensis (Bacillariophyta: Stephanodiscaceae), studied with light (LM) and scanning electron microscopy (SEM), is described from Pliocene sediments in central Mexico. This species possesses simple and complex alveolae formed by thickened hyaline strips, a single raised rimoportula located within one of the alveolar chambers, 4 to 10 valve face fultoportulae bounded by 2 or by 2 and 3 satellite pores, and mantle fultoportulae with 2 satellite pores positioned above thick internal costae on the surrounding wall. T. hidalgensis is apparently an extinct species of biostratigraphic significance; it extends the geographic distribution of this genus into tropical latitudes and gives further evidence of the great diversity that the freshwater Stephanodiscaceae reached during the Neogene.