Geochemistry of Phosphate Rocks and Fertilizers: Implications for Origin and Environmental Impact

Abstract

Phosphate rocks (PRs) are used to manufacture phosphorus fertilizers, which can contain harmful trace elements that can be dispersed into the environment through fertilizer application, impacting soil and crop quality at intensively fertilized sites. Previous studies have highlighted the occurrence of toxic elements such as Cd, U, and Cr in PRs and P-fertilizers, yet without systematic analysis on their occurrence and impact. This dissertation aims to characterize the abundance and composition of trace elements and isotopes (Sr, U, Ca) of globally sourced PRs to reconstruct their depositional and diagenetic histories and develop isotopic tracers to track fertilizer-derived contamination in agricultural and environmental systems.The first two chapters of this dissertation examine the distribution of trace element concentrations and Sr, U, and Ca isotope systematics in global phosphate deposits spanning geological time. Phosphate rocks of marine sedimentary type were deposited during intervals of major oceanographic and environmental change under distinct depositional settings that governed nutrient accumulation (i.e., P) and trace element incorporation into the phosphate mineral (carbonate-fluorapatite). Through time, the dominant depositional environment of phosphogenesis shifted from largely peritidal systems in the Precambrian to upwelling-driven continental shelf environments in the Phanerozoic. These environmental differences exerted different controls on the geochemical signatures preserved in PRs. In addition to enrichment of redox-sensitive trace elements (i.e., V, Cr, U) in upwelling type deposits due to diagenetic incorporation under reducing porewater conditions, the isotope data provide three key geological insights. First, the strontium isotope ratio (87Sr/86Sr) of younger PRs (< 100 Ma) closely mimics coeval seawater values, whereas older deposits are systematically more radiogenic than coeval seawater, indicative of terrestrial controls on the diagenetic environment of older phosphate deposits. Second, the uranium isotope composition (δ238U) of PRs records the redox conditions of the depositional environment, which is influenced by global ocean redox conditions, demonstrating changes in surface ocean redox from the Precambrian to the Phanerozoic. Third, the calcium isotope composition (δ44Ca) of Permian to Proterozoic PRs indicates fluid-buffered diagenetic conditions and differential incorporation of Ca isotopes from the diagenetic fluids into the phosphate mineral resulting in a distinctive isotope signature of the phosphate phase relative to co-occurring carbonate phase in PRs. The latter three chapters of this dissertation investigate the geochemical continuity between PRs and phosphate fertilizer products and evaluate the application of isotope tracers to track fertilizer-derived trace elements in agricultural systems. During fertilizer manufacturing, toxic trace elements enriched in PRs (e.g., U, As, V, Cr, Cd, Mo) remain concentrated in fertilizer products, whereas the radioactive radium nuclide preferentially partitions into the phosphogypsum waste byproduct. In fertilizers derived from upwelling-type PRs, trace element concentrations scale with phosphorus content, reflecting inheritance from their sourced phosphate deposits. Uranium and strontium isotope analyses demonstrate that the isotopic signatures of PRs are largely preserved in phosphate fertilizer products. While the δ238U retains source-dependent characteristics, the incorporation of potassium components in NPK formulations shifts the 87Sr/86Sr of fertilizers toward more radiogenic values. These isotope tracers were subsequently applied to two long-term fertilization experiments: the Tidewater Research Station (TRS) in North Carolina, USA and the Static Fertilization Experiment (SFE) in Bad Lauchstädt, Germany. Although the δ238U signatures of soils remained unchanged following fertilizer application at the TRS, surface soils at both sites exhibited a shift in the 87Sr/86Sr ratio toward the 87Sr/86Sr signature of the applied phosphate fertilizer. The temporal dataset from the SFE further reveals cumulative enrichment of toxic trace elements, such as Cd and As, in soils under long-term fertilization, accompanied by measurable transfer of the trace elements to crops. Mineral and organic fertilization altered soil properties (e.g., pH, organic carbon), influencing trace element mobility and sequestration and consequently facilitating uptake into wheat. The persistence of the fertilizer-derived 87Sr/86Sr in wheat grain confirms the transfer of phosphate-derived strontium, and associated trace elements, through the soil-plant system. These results demonstrate the utility of isotope tracers for quantifying the pathways and impacts of fertilizer-derived trace element contamination in agricultural environments.