Baseline data on selenium micronutrient concentration in parts of Southern Nigeria show provenance and paleodepositional controls

Abstract

This study evaluates the Se concentration and examines the geologic controls on Se concentration in Upper Cretaceous and Palaeogene mudrocks in parts of Southern Nigeria. Geochemical analysis [(major and trace elements, total organic and inorganic carbon (TOC and TIC)] and pH measurements were conducted on seventy-three mudrock outcrop samples from the estuarine Late Campanian to Mid-Maastrichtian Mamu Formation and ditch cuttings from water wells that penetrated the marine Palaeogene Imo Formation from the Anambra basin and Niger Delta basin in the Southern Benue Trough. Selenium concentration in the Palaeogene calcareous marine mudrock samples reaches up to 21 ppm, significantly exceeding data observed in the Upper Cretaceous estuarine mudrock samples. Covariation charts suggest that Se adsorption onto clay, Fe-minerals, and limited sorption by organic matter in the estuarine mudrock samples are compared to adsorption onto clays, calcite, and very limited sequestration by Fe-minerals observed in the calcareous marine mudrocks. Principal component analysis and redundancy analysis reveal a dominant influence of carbonate minerals on selenium sequestration. Additionally, the Palaeogene calcareous marine mudrock samples are less acidic (mean pH 5.92) than the Upper Cretaceous estuarine mudrocks with a mean pH of 5.02. Therefore, it is hypothesized that higher pH may slightly promote the bioavailability of Se, resulting in higher Se intake by plants in areas underlain by the Palaeogene calcareous mudrocks.