Using tephrochronology to date temperate ice: correlation between ice tephras on Livingston Island and eruptive units on Deception Island volcano (South Shetland Islands, Antarctica)

Tephra layers are interstratified in the ice caps of the South Shetland Islands. Although previously poorly investigated, they are potential targets for the application of tephrochronology and, hence, may provide temporal constraints on glaciological models for the region. Several tephra layers crop out in the coastal ice-cliffs and ablation ramps of Livingston Island. Using stratigraphical position, granulometry and bulk sample geochemistry, the tephra layers can be divided into three groups (TPH1, TPH2 and TPH3, from top to base). The source for all of the tephras is unequivocally identified as Deception Island, a large active volcano in Bransfield Strait, situated about 35 km south of Livingston Island. TPH1 (a single layer) is strongly correlated compositionally with tephra erupted in 1970 from centres close to Telefon Bay. This is the first time it has been possible to correlate a distal tephra with a pyroclastic unit in the source volcano in the Antarctic Peninsula region. TPH2 (usually two layers, sometimes only one) was probably erupted from a tuff cone centre within the Crater Lake cluster of vents. From historical accounts, it is deduced that the numerous co-eruptive Crater Lake vents were active prior to 1829 and, from their relatively fresh appearance, an eighteen-century age for the eruptions is possible. TPH3 comprises at least four discrete tephra layers with a much wider compositional range than either TPH1 or TPH2. It may have been erupted during successive months or years. Compositional comparisons of TPH3 with possible source vents on Deception Island are ambiguous, but there is a reasonably good similarity with tephras erupted at Wensleydale Beacon and/or Vapour Col. However, it is also conceivable that the source(s) for TPH3 are no longer preserved on Deception Island. The age of the TPH3 eruptions is unknown but it must be prior to 1829 and is unlikely to be more than a few centuries

Forma da paisagem como critério para otimização amostral de latossolos sob cultivo de cana-de-açúcar Landscape form as a criterion for sampling optimization of an oxisol under cultivation of sugar cane

O número de pontos amostrais é fundamental para estabelecer um programa de avaliação da variabilidade espacial dos atributos dos solos. O objetivo deste trabalho foi utilizar a forma da paisagem como critério auxiliar na otimização do esquema amostral na avaliação dos atributos químicos de latossolos, em áreas sob cultivo de cana-de-açúcar. Utilizou-se uma área contínua com duas pedoformas: côncava, que ocorre na posição mais elevada da área; e linear, constituída pelos segmentos escarpa, meia encosta e encosta inferior. Foi utilizado um espaçamento amostral regular de 50x50 m em uma malha de 300x3.000 m numa área total de 94 ha, com 421 pontos amostrados. Coletaram-se amostras de solo nas profundidades 0,0-0,2 m e 0,6-0,8 m, em cada ponto da malha, e determinaram-se as propriedades químicas do solo. Na pedoforma côncava, houve maior variabilidade espacial para os atributos químicos do solo. A aplicação do programa Sanos 0.1 na malha amostral (pedoforma côncava e pedoforma linear) revelou que a pedoforma côncava, em ambas as profundidades, apresenta maior variabilidade espacial dos atributos químicos do que a pedoforma linear.<br>The number of sampling points is essential to establish an evaluation program of the spatial variability of soil atribute. The objective of this work was to use the form of landscape as auxiliary criterion in the optimization of the outline sample for the estimate of chemical attributes of oxisol, in a area under sugarcane cultivation. It was possible to choose a continuous area with two landforms: concave, that occur in higher positions of the area, and linear, comprising steep, stocking leans and inferior leans. A 50x50 m spacing in a mesh of 300x3.000 m (total area 94 ha), with 421 sampling points were used. Soil sample in depths 0.0-0.2 m and 0.6-0.8 m were collected, in each point of the grid, in order to evaluate the soil chemical attributes. In the concave landform, larger space variability was observed for the chemical attributes of the soil. The application of the program Sanos 0.1 in the sampling grid (concave landform and linear landform) allowed to observe that the landform concave in both depths presents larger space variability of the chemical attributes than the linear landform