Intrusions of dust and iberulites in Granada basin (Southern Iberian Peninsula). Genesis and formation of atmospheric iberulites

Abstract

Fourteen samples of deposited atmospheric dust collected during desert dust intrusions over Granada in the summer of 2010 are studied here. During these atmospheric dust events the PM10 ranged from 25 μg m−3 to 200 μg m−3, surpassing occasionally the standard limit (50 μg m−3) established by the European Union Directive as a risk for the human health. The mineralogical composition of the dust samples is very heterogeneous, showing that the origin of collected particles is from north-northwest of Africa and local/regional soils. The analyzed dust samples contain between 1 and 9% of iberulites, polymineral spherical particles with diameter between 34 and 111 μm. New compositional results obtained by mapping chemical elements and mineral compositions of iberulites with VPSEM-EDEX technology allowed as to go further than previous studies and provide new insight on the iberulites genesis. The SEM-microstructure analysis of the iberulites and the compositional results obtained by VPSEM-EDEX technique showed that clay and sulphur components are important in determining their spherical shapes. The analysis also shows that iberulites present a typical vortex at one of the poles and an external covering by nano-clays in laminar clusters, a form of rind and a core internal with sizes less than 10 μm. On the other hand, the micromorphological analysis evidences that the bacteria and its polymeric exudates participate in the iberulite genesis, acting as aggregation agents and contributing to its protection and compensating its fragility. The role of bacteria and its polymeric exudates in the iberulite genesis has not been described previously, and it would explain the flux, transport and survival of tropospheric microorganisms over long distances. These new observations and finding led us to take into account the role of bacteria in iberulite genesis and to reconsider the previous hypothesis regarding the iberulite genesis proposed in previous works.