Palynofacies, environments, and climate changes in the Magdalena River Basin

Abstract

Four environments (swamp, shallow lake, alluvial flood plain, and lagoon) from the Lower Magdalena River Basin were studied for palynofacies quantitative characterization. Each environment has been described based on four criteria: palynomorph assemblage, organic matter concentration, organic matter palynological composition, and organic particle morphology. Shallow lakes palynological assemblages are dominated by composite and grass pollen. The POM (particulate organic matter) morphology is characterized by a maximum at {Phi} 5 class (silt), and it has a sphericity histogram with bimodal distribution (peaks at 0.1 and 0.5/0.6). From a composition point of view, POM is mainly opaque amorphous materials. POM concentration is usually lower than 0.1%. Swamp environments palynological assemblages are dominated by grass pollen with a slightly smaller amount of composite pollen. The POM is dominated by finely dispersed amorphous and indeterminate "other" types (organo-mineral gel ?), depending on the oxidation degree. The swamp concentration of organic matter a few centimeters below the water-sediment interface varies between 0.1% and 0.3%. Below that, organic concentration is usually lower than 0.1%. Lagoon assemblages are rich in species and specimens, but assemblages are highly variable. Main components are either finely dispersed amorphous or plant cuticular/epidermal or amorphous homogeneous and heterogeneous or fungal remains. Peat lithology is rich in mangrove pollen, while clay assemblages are dominated by composites, grass, and water plants together with Botryococcus algal remains. Lagoon sediments are the richest in POM concentration, with values between 0.13% and 1% (excluding peats). Regarding particle size and shape, in this environment, they show a trend to decrease in grain size from {Phi} 1 to {Phi} 2 class (sand) dominated assemblages to {Phi} 5 to {Phi} 6 class (silt) dominated assemblages from base to top. Elongated shapes are abundant, with 30% to 50% of particles in the tabloid to elongated tabloid classes. Alluvial - fluvial flood basin samples are often barren in palynomorphs and organic matter. Occasionally present grass pollen and fungal remains. The POM, when present, is mainly of organo-mineral gel type and has a bimodal grain size distribution, with a minor peak at {Phi} 7 class (v.f.silt) and a major peak at {Phi} 4 to {Phi} 2 class (c. silt to f. sand). Significant changes in quantitative palynofacies occur within the top few meters of the cores, representing the last 1000 yr of sedimentation in the area. These changes are related to shifts in climate, from colder to warmer conditions or from dry to wet periods, most probably linked with E.N.S.O. A short dry and cold period related to the "Little Ice Age" was identified in the Ayapel and Cienaga de El Medio cores.