The lower Cambrian selenium-enriched black rock series in Southwest China: Occurrence modes and surface environmental implications

Selenium (Se) is commonly enriched in Lower Cambrian black shales, which are highly susceptible to weathering and may release Se into the surficial environment. However, variations in Se content, enrichment mechanisms, and occurrence modes across contrasting depositional settings remain poorly constrained, as do their environmental implications. This study presents a comparative geochemical analysis of two Lower Cambrian black shale formations in South China: the Niutitang Formation (shallow marine shelf) and the Lujiaping Formation (shallow to semi-deep marine). The Lujiaping Formation exhibits significantly higher Se concentrations (0.15–27.5 mg/kg; mean 5.37 mg/kg) and enrichment factors (Se_EF = 21.39) than the Niutitang Formation (0.14–7.37 mg/kg; mean 1.02 mg/kg; Se_EF = 1.10). Sequential extraction results show that Se in the Niutitang Formation mainly occurs in Fe-Mn oxide-bound (36 %), organic-bound (32 %), and residual (31 %) forms, while Se in the Lujiaping Formation is predominantly organic-bound (66 %). Risk assessment based on the Risk Assessment Code (RAC) indicates that all Niutitang samples pose low risk (1 % ≤ RAC < 10 %), whereas 13 % of Lujiaping samples fall within the medium-risk category (10 % ≤ RAC < 30 %). The Ratio of Secondary to Primary phases (RSP) reveals high pollution potential (RSP ≥ 3) in all Lujiaping samples and in 50 % of Niutitang samples. In the Niutitang Formation, Se is chiefly derived from weathered terrigenous detritus and deposited under anoxic conditions, whereas in the Lujiaping Formation, Se originates mainly from hydrothermal fluids, with deposition influenced by both anoxia and total organic carbon (TOC). These findings provide new insights into Se mobility and enrichment in early Paleozoic marine environments and offer a scientific basis for evaluating environmental risks and the sustainable use of Se-rich soils derived from weathered black shales.