TRACKING TRACE ELEMENT CONTAMINATION FROM PHOSPHATE ROCKS AND FERTILIZERS USING STRONTIUM ISOTOPES

Phosphorus is one of the primary nutrients essential for plant growth and thus its availability is critical for sustainable agricultural development and food security. Increased utilization of P-phosphate fertilizer over the last several decades has caused eutrophication of numerous water resources. Elevated levels of trace elements in phosphate rocks and P-fertilizers pose less recognized but significant potential risks to soil and water quality. Here we report trace element concentrations and the strontium isotope ratios data in global marine sedimentary phosphate rocks (n = 61) from major phosphate producing countries. The data show that young (<100 Ma) sedimentary phosphate ores from the Eastern U.S. and the Tethys Belt of Northern Africa and Middle East exhibit high concentrations of U, Cd, and Cr, while older phosphate ores from China and India show significantly lower levels. The Sr isotope data of the marine phosphate rocks from the Tethys Belt and the Eastern U.S. generally mimic the secular Sr isotope variations of contemptuous seawater, while older marine phosphate ores from China and India show large isotopic offsets toward more radiogenic ratios. Systematic analysis of several pairs of phosphate ores and P-fertilizers indicates selective enrichment of trace elements in P-fertilizer, including U, Cd and Cr, resulting in elevated levels of these toxic elements in major P-fertilizer utilized in the U.S. Yet, the Sr isotope ratios of the P-fertilizers mimic the isotope composition of their parent rocks. Given that 47 million metric tons of P-fertilizers are produced every year globally, we postulate large fluxes of trace elements are entering the environment through P-fertilizers utilization without adequate monitoring and evaluation of their impact. Given the lack of isotope fractionation of Sr during P-fertilizers production, we suggest that Sr isotope ratios can not only be used for reconstructing the geological conditions and sedimentary ages of phosphate rock deposition, but also as a proxy for detecting the mobilization of phosphate-derived trace elements in the environment.