On the Contributions of Environmental Magnetism for Exploring the Critical Zone: A Case Study in an Urban Environment of a Tropical Megacity

Environmental magnetism uses rock and mineral magnetic methods to study changes of magnetic minerals influenced by various environmental processes. The field is well-established and has significantly contributed to our understanding of past and present environmental changes on Earth, including those driven by land use. Magnetic methods are used to identify ferromagnetic minerals, which serve as tracers for anthropogenic pollutants. Besides being non-destructive, these methods are fast and cost-effective when compared to chemical analyses. The magnetic properties of contaminated soils can provide information about the transformations of the environment affected by the degradation of organic matter, enriching our knowledge of the spatial and temporal evolution of the pollutant and the impacted area. In this work we present the environmental magnetism study conducted in the São Paulo Critical Zone Observatory (CZO) seed site, an endeavor to understand anthropogenic effects on groundwater, soils, and vegetation in a tropical megacity that has experienced diverse urban transformations over time. The multidisciplinary team of São Paulo CZO’ scientists approached the following main questions: (1) How are soils, water and vegetation resources in a tropical megacity responding to natural and anthropogenic drivers? and (2) How can a critical zone observatory in an urban environment advance the understanding of the critical zone response to natural and anthropogenic drivers? We hypothesize that CZOs can more effectively identify and monitor biogeochemical processes in urban environments, where land is heavily degraded. This study allowed a better understanding of the architecture and dynamic of the urbanization impacted CZ, revealed by magnetic signatures that indicated Fe-bearing minerals transformations driven by changes in redox conditions. Our results also reveal striking differences between the analyzed soils that can be linked to anthropogenic activities. More specifically, magnetic properties identified one important soil interface, which show mineral phases and grain size transformations of Fe-bearing mineral at depth. Characterizing the architecture and dynamic processes of the subsurface in urbanized areas provides a comprehensive understanding of how human-induced changes impact the natural environment.