Relationship Among Modern Pollen Spectra, Vegetation And Sedimentary Environment From The Drainage Of The Red River, Vietnam

Pollen,vegetable distributing and sedimentary environment are analyzed to discuss the relationship among the three factors.The results of pollen,particle size, magnetic parameters,the soil moisture content,the soil organic matter,charcoal and vegetation from 76 surface sediment samples along the Red River,Vietnam,are showed that pollen spectra are infected by the vegetation and sedimentary environment.Pollen spectra of the Fansipan Mountains are characterized by the highest abundances of the subtropical evergreen and temperature deciduous taxa.Herb and. shrub taxa are lowest.The primeval vegetation is the subtropical evergreen broad-leaf forest and temperate broad-leaf forest.The surface sediments are classed to clay-silt, and have smaller average particle size.SIRM,S_-100 andχhave high values,whereasχfd%,χARM/χare in low values.The soil moisture content,the soil organic matter and charcoal all show high values.Pollen spectra are controlled by the vegetation, associated with a vertical vegetation zone shift in this mountain area.The sedimentary environment affects the vegetation,and then has an influence on pollen spectra.Along the valley of the Fansipan Mountains,pollen assemblages are characterized by a sharp increasing of the herb and the shrub taxa.The subtropical evergreen taxa and temperature deciduous taxa decrease to very low contents. Tropical arboreal taxa have higher abundant.This area is covered by grass,shrub and tropic trees.The surface sediments are also classed to clay-silt,however,the average particle size become smaller.SIRM andχdecrease,whereasχfd%,χARM/χand S_-100 increase sharply.The soil moisture content,the soil organic matter and charcoal all decrease to low values.The pollen results obviously demonstrate the valley of the Fansipan Mountains is the transition area,indicated the sediments environment is reflected climate impacts.Pollen spectra in the floodplain and delta of the Red River are characterized by higher contents of herb and shrub taxa.Temperate deciduous and subtropical evergreen arbor elements are fewer.The tropical types increase considerably.The vegetation is swamp forest.The main sediments types are sand,silt-sand or clay-silt. The sudden increase ofχ,SIRM,χfd%,χARM/χand S_-100 indicated this area contain highest values of magnetic minerals,and the magnetic grain sizes are SD.The soil moisture content,the soil organic matter and charcoal are in the low values.The sudden increase in herbs dominated by the Poaceae and Cyperaceae,along with the decrease in arboreal taxa,reflects human impacts on the original vegetation.Human impacts and agricultural cultivation are activity,indicated by a significant decline in arboreal pollen and an increasing in herb pollen.Pollen of Pinus and Temperate deciduous taxa suggests that fluvial transport is the primary transport mechanism in the drainage area.Pollen spectra associate with the affection of human impacts and fluvial transportion.Pollen spectra of the subaqueous delta are similar to the floodplain and delta, characterized by a higher content of herb and shrub types,absent of temperate deciduous and subtropical evergreen arbor elements.The main sediments types are clay-silt.SIRM andχdecrease to lowest values,whereasχfd%,χARM/χand S_-100 increase to highest values.The soil moisture content have highest precent.The soil organic matter is higher,whereas charcoal is in the low values.These are suggested that fluvial transport is the primary transport mechanism,at the same time,sea current also plays an important role.Canonical correspondence analysis(CCA) show that pollen of Quercus, Castanopsis/Lithocarpus,Hamamelidaceae,Fagus,Castanea,Ilex,Salix,Betula, Alnus represent the mountain area with high aititude and cooler temperature; Trachycarpus,Lauraceae,Elaeocarpus,piper indicate the plain with low altitude, warmer and weter climate.Poaceae,Cyperaceae,Compositae,Chenopodium suggest the plain with low altitude,warmer and drier climate.Ten parameters of sediments environment are summarized to grade the Mountain facies,the fluvial facies and the subaqueous delta facies.