Structural equation modeling of long-term controls on mercury and bromine accumulation in Pinheiro mire (Minas Gerais, Brazil)

Antonio Martínez Cortizas; Ingrid Horák-Terra; Marta Pérez-Rodríguez; Richard Bindler; Colin A Cooke; Malin Kylander

doi:10.1016/j.scitotenv.2020.143940

Structural equation modeling of long-term controls on mercury and bromine accumulation in Pinheiro mire (Minas Gerais, Brazil)

Sci Total Environ. 2021 Feb 25:757:143940. doi: 10.1016/j.scitotenv.2020.143940. Epub 2020 Dec 4.

Authors

Antonio Martínez Cortizas¹, Ingrid Horák-Terra², Marta Pérez-Rodríguez³, Richard Bindler⁴, Colin A Cooke⁵, Malin Kylander⁶

Affiliations

¹ Ecopast (GI-1553), Facultade de Bioloxía, Universidade de Santiago de Compostela, Santiago de Compostela, Spain. Electronic address: antonio.martinez.cortizas@usc.es.
² Instituto de Ciências Agrárias, Universidade Federal dos Vales do Jequitinhonha e Mucuri, Unaí, Brazil.
³ Institut für Geoökologie, Technische Universität Braunschweig, Braunschweig, Germany.
⁴ Department of Ecology and Environmental Science, University of Umea, Umea, Sweden.
⁵ Department of Earth and Atmospheric Sciences, University of Alberta, Edmonton, Canada.
⁶ Department of Geological Sciences, Stockholm University, Stockholm, Sweden.

PMID: 33321335
DOI: 10.1016/j.scitotenv.2020.143940

Abstract

The application of statistical modeling is still infrequent in mercury research in peat, despite the ongoing debate on the weight of the diverse factors (climate, peat decomposition, vegetation changes, etc.) that may affect mercury accumulation. One of the few exceptions is the Hg record of Pinheiro mire (souheast Brazil). Previous studies on this mire modeled mercury using principal components regression and partial least squares. These methods assume independence between factors, which is seldom the case in natural systems, thus hampering the identification of mediating effects and interactions. To overcome these limitations, in this reserach we use structural equation modeling (PLS-SEM) to model mercury and bromine peat records - bromine has been used in some investigations to normalize mercury accumuation. The mercury model explained 83% of the variance and suggested a complex control: increased peat decomposition, dust deposition and humid climates enhanced mercury accumulation, while increased mineral fluxes resulted in a decrease in mercury accumulation. The bromine model explained 90% of the variation in concentrations: increased dust deposition and peat decomposition promoted bromine accumulation, while time (i.e. peat age) promoted bromine depletion. Thus, although mercury and bromine are both organically bound elements with relevant atmospheric cycles the weights of the factors involved in their accumulation differed significantly. Our results suggest caution when using bromine to normalize mercury accumulation. PLS-SEM results indicate a large time dependence of peat decomposition, catchment mineral fluxes, long-term climate change, and atmospheric deposition; while atmospheric dust, mineral fluxes and peat decomposition showed high to moderate climate dependency. In particular, they also point to a relevant role of autogenic processes (i.e. the build up and expansion of the mire within the catchment), which controlled local mineral fluxes; an aspect that has seldom been considered.

Keywords: Bromine; Late Pleistocene-Holocene; Mercury; Structural equation modeling.