Entity

Time filter

Source Type

Puerto Ordaz, Venezuela

Montoya E.,Botanical Institute of Barcelona | Montoya E.,Autonomous University of Barcelona | Rull V.,Botanical Institute of Barcelona | Stansell N.D.,Ohio State University | And 6 more authors.
Journal of Quaternary Science | Year: 2011

The occurrence of the Younger Dryas cold reversal in northern South America midlands and lowlands remains controversial. We present a palaeoecological analysis of a Late Glacial lacustrine section from a midland lake (Lake Chonita, 4.6501°N, 61.0157°W, 884m elevation) located in the Venezuelan Gran Sabana, based on physical and biological proxies. The sediments were mostly barren from ∼15.3 to 12.7k cal a BP, probably due to poor preservation. A ligneous community with no clear modern analogues was dominant from 12.7 to 11.7k cal a BP (Younger Dryas chronozone). At present, similar shrublands are situated around 200m elevation above the lake, suggesting a cooling-driven downward shift in vegetation during that period. The interval from 11.7 to 10.6k cal a BP is marked by a dramatic replacement of the shrubland by savannas and a conspicuous increase in fire incidence. The intensification of local and regional fires at this interval could have played a role in the vegetation shift. A change to wetter, and probably warmer, conditions is deduced after 11.7k cal a BP, coinciding with the early Holocene warming. These results support the hypothesis of a mixed origin (climate and fire) of the Gran Sabana savannas, and highlight the climatic instability of the Neotropics during the Late Glacial. © 2011 John Wiley & Sons, Ltd. Source


Montoya E.,Botanical Institute of Barcelona CSIC ICUB | Montoya E.,Autonomous University of Barcelona | Rull V.,Botanical Institute of Barcelona CSIC ICUB | Stansell N.D.,Ohio State University | And 5 more authors.
Quaternary Research | Year: 2011

The southern Gran Sabana (SE Venezuela) holds a particular type of neotropical savanna characterized by the local occurrence of morichales (Mauritia palm swamps). , in a climate apparently more suitable for rain forests. We present a paleoecological analysis of the last millennia of Lake Chonita (4°39'N-61°0'W, 884. m elevation), based on biological and physico-chemical proxies. Savannas dominated the region during the last millennia, but a significant vegetation replacement occurred in recent times. The site was covered by a treeless savanna with nearby rainforests from 3640 to 2180. cal yr BP. Water levels were higher than today until about 2800. cal yr BP. Forests retreated since about 2180. cal yr BP onwards, likely influenced by a higher fire incidence that facilitated a dramatic expansion of morichales. The simultaneous appearance of charcoal particles and Mauritia pollen around 2000. cal yr BP supports the potential pyrophilous nature of this palm and the importance of fire for its recent expansion. The whole picture suggests human settlements similar to today - in which fire is an essential element - since around 2000. yr ago. Therefore, present-day southern Gran Sabana landscapes seem to have been the result of the synergy between biogeographical, climatic and anthropogenic factors, mostly fire. © 2011 University of Washington. Source


Laraque A.,French National Center for Scientific Research | Moquet J.-S.,French National Center for Scientific Research | Alkattan R.,French National Center for Scientific Research | Steiger J.,University Blaise Pascal | And 9 more authors.
Journal of South American Earth Sciences | Year: 2013

Seasonal variations of total dissolved fluxes of the lower Orinoco River were calculated taking into account four complete hydrological cycles during a five-year period (2005-2010). The modern concentrations of total dissolved solids (TDS) of the Orinoco surface waters were compared with data collected during the second half of the last century published in the literature. This comparison leads to the conclusion that chemical composition did not evolve significantly at least over the last thirty to forty years. Surface waters of the Orinoco at Ciudad Bolivar are between bicarbonated calcic and bicarbonated mixed. In comparison to mean values of concentrations of total dissolved solids (TDS) of world river surface waters (89.2 mg l-1), the Orinoco River at Ciudad Bolivar presents mainly low mineralized surface waters (2005-10: TDS 30 mg l-1). The TDS fluxes passing at this station in direction to the Atlantic Ocean between 2005 and 2010 were estimated at 30 × 106 t yr-1, i.e. 36 t km-2 yr-1. It was observed that the seasonal variations (dry season vs wet season) of total dissolved fluxes (TDS and dissolved organic carbon (DOC)) are mainly controlled by discharge variations. Two groups of elements have been defined from dilution curves and molar ratio diagrams. Ca2+, Mg2+, HCO3-, Cl- and Na+ mainly come from the same geographic and lithologic area, the Andes. K+ and SiO2 essentially come from the Llanos and the Guayana Shield. These findings are important for understanding fundamental geochemical processes within the Orinoco River basin, but also as a baseline study in the perspective of the development of numerous mining activities related with aluminum and steel industries; and the plans of the Venezuelan government to construct new fluvial ports on the lower Orinoco for the transport of hydrocarbons. © 2013 Elsevier Ltd. Source

Discover hidden collaborations