Geomorphological and paleogeographical implications of the phosphatized crusts in the são pedro and são paulo archipelago, north atlantic

Abstract

In the São Pedro and São Paulo Archipelago, deposition of bird excrements has been responsible for rock phosphatization, generating phosphatized crusts. This study describes these crusts and establishes their geomorphological and paleogeografi cal implications, based on methodologies such as fi eld observation, description and sampling, as well as, laboratorial analyses (geochemical, mineralogical and microscopic). The crusts were defi ned as features of excrement accumulation, with high P content and mineralized surfaces over homogeneous rocks (milonitizaded peridotites). The hydrophobic behavior of these crusts protects the underlying rock against the accelerated advance of chemical weathering and, consequently, the role of erosion processes, developing steeper and more convex slopes. In contrast, at sites in which the formation of the crust has not been verifi ed, the rock is more weathered and the slopes more concave. The presence of these crusts also indicates an arid paleoenvironment, where guano accumulation was possible, differently from presentday climate conditions.