Whole-rock geochemistry and Sr-Nd isotopic composition of the pre-rift sequence of the Camamu Basin, northeastern Brazil

Whole-rock geochemistry, combined with Sr-Nd isotopic composition of pelitic sedimentary rocks, have been considered to be useful parameters to estimate not only their provenance but also to make inferences about their depositional environment as well as the weathering processes they have been through. The basal sedimentary units of the basins of the northeastern Brazilian continental margin, particularly those of the pre-rift sequence, have been subject of interest of studies based on chemical and isotopic data, since they lack fossil content to establish their age and, therefore, stratigraphic correlations are difficult. The major and trace element contents as well as Sr-Nd isotopic compositions of whole-rock shale samples from five outcrops attributed to the pre-rift supersequence of the Camamu Basin were analyzed with the purpose of characterizing and obtaining further information that would allow a better correlation between the sites studied. The geochemical data suggest that the rocks exposed in the studied outcrops are part of the same sedimentary unit and that they might be correlated to the Capianga Member of the Alianca Formation of the Reconcavo Basin, exposed to the north of the Camamu Basin. The chemical index of alteration (CIA) suggests conditions associated with a humid tropical/subtropical climate at the time of deposition. Nd isotopic compositions indicate provenance from the Paleoproterozoic rocks of the Sao Francisco craton. The results presented here, therefore, show that the combined use of chemical and isotopic analyses may be of great interest to characterize and correlate lithologically homogeneous clastic sedimentary sequences. (C) 2012 Elsevier Ltd. All rights reserved.

The role of asymmetries in rock-paper-scissors biodiversity models.

The maintenance of biodiversity is a long standing puzzle in ecology. It is a classical result that if the interactions of the species in an ecosystem are chosen in a random way, then complex ecosystems can't sustain themselves, meaning that the structure of the interactions between the species must be a central component on the preservation of biodiversity and on the stability of ecosystems. The rock-paper-scissors model is one of the paradigmatic models that study how biodiversity is maintained. In this model 3 species dominate each other in a cyclic way (mimicking a trophic cycle), that is, rock dominates scissors, that dominates paper, that dominates rock. In the original version of this model, this dominance obeys a 'Z IND 3' symmetry, in the sense that the strength of dominance is always the same. In this work, we break this symmetry, studying the effects of the addition of an asymmetry parameter. In the usual model, in a two dimensional lattice, the species distribute themselves according to spiral patterns, that can be explained by the complex Landau-Guinzburg equation. With the addition of asymmetry, new spatial patterns appear during the transient and the system either ends in a state with spirals, similar to the ones of the original model, or in a state where unstable spatial patterns dominate or in a state where only one species survives (and biodiversity is lost).