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Phosphate rock bears both geologically and environmentally significant information. Rare earth elements and yttrium (i.e., REY) characteristics have been commonly used for reconstructing the redox conditions of depositional environments of and the effects of post-depositional diagenetic alteration on phosphate rock. In addition, phosphate rock is typically enriched in a range of trace elements such as uranium (U) and cadium (Cd) that can be dispersed as contaminants into the environment with phosphate mining and phosphate fertilizer application. Here we report the lead (Pb) isotope compositions combined with Pb and REY concentrations of both global sedimentary and igneous phosphate rocks, aiming to evalute the geological origin of phosphate rocks over time and the potential of using them for environmental tracing. Phopshate rocks samples analyzed in this study were sourced from major economic phosphate deposits in the world, including China, Southern Tethys (e.g., Morocco, Tunisia, Israel), the U.S., India, South Africa and Russia. Our results show a wide range of 208Pb/204Pb (35.70 to 60.58), 207Pb/204Pb (15.20 to 18.25), and 206Pb/204Pb (16.369 to 71.806) ratios in phosphate rocks, with sedimentary phosphate rocks being significantly more radiogenic than igneous rocks. The majority of the sedimentray phosphate rocks show a notable isotopic overprinting by non-radiogenic terrestrial Pb, except for those from Israel and Morocco that have the most radiogenic Pb isotope compositions. Correspondingly, phosphate rocks with more radiogenic Pb isotope ratios show relatively pristine seawater REY features, likely suggesting their preservation of the original oxic seawater conditions and/or minimal diagenetic alteration. In contrast, phosphate rocks with less radiogenic Pb isotope compositions show REY indications for more anoxic seawater redox conditions and/or greater diagenetic alteration. We further evaluate the potential utility of Pb isotopes for tracing the associated contamination with phosphate rock mining and fertilizer application in the environment. In most cases, the radiogenic Pb isotope composition of phosphate rocks and corresponding P-fertilizers is distinctive from both natural crustal Pb and major anthropogenic Pb sources (e.g., Pb ore deposits and pesticides), which provides a great advantage for applying Pb isotopes as environmental tracers for metal(loid) contamination from phosphate sources. The combination of Pb isotope ratios and REY proxies could further constrain the Pb source discrimination. Overall, this study provides new Pb isotopic and REY geochemical data on global phosphate rocks and fertilizers, which lays the groundwork for future regional and local studies on both their geological and environmental implications.
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Mapping the global distribution of lead and its isotopes in seawater with explainable machine learning
Abstract. Lead (Pb) and its isotopes are powerful tools for studying the pathways of Pb pollution from land to sea and, simultaneously, investigating biogeochemical processes in the ocean. However, the scarcity and sparsity of in situ measurements of Pb concentrations and isotope compositions do not allow for a comprehensive understanding of Pb pollution pathways and biogeochemical cycling on a global scale. Here, we present three machine learning models developed to map seawater Pb concentrations and isotope compositions, leveraging the global GEOTRACES dataset as well as historical data. The models use climatologies of oceanographic and atmospheric variables as features from which to predict Pb concentrations, 206Pb/207Pb, and 208Pb/207Pb. Using SHapley Additive exPlanations (SHAP), we found that seawater temperature, atmospheric dust, atmospheric black carbon, and salinity are the most important features for predicting Pb concentrations. Dissolved oxygen concentration, salinity, temperature, and atmospheric dust are the most important features for predicting 206Pb/207Pb, atmospheric black carbon and dust, seawater temperature, and surface chlorophyll a for 208Pb/207Pb. In line with observations, our model outputs show that (i) the surface Indian Ocean has the highest levels of pollution, (ii) pollution from previous decades is sinking in the North Atlantic and Pacific oceans, and (iii) waters characterised by highly anthropogenic Pb isotope fingerprints are spreading from the Southern Ocean throughout the Southern Hemisphere at intermediate depths. By analysing the uncertainty associated with our maps, we identified the Southern Ocean as the key area to prioritise in future sampling campaigns. Our datasets, models, and their outputs, in the forms of Pb concentrations, 206Pb/207Pb climatologies, and 208Pb/207Pb climatologies, are made freely available to the community by Olivelli et al. (2024a; https://doi.org/10.5281/zenodo.14261154) and Olivelli (2025; https://doi.org/10.5281/zenodo.15355008).
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- Award ID(s):
- 2140395
- PAR ID:
- 10648543
- Publisher / Repository:
- Copernicus
- Date Published:
- Journal Name:
- Earth System Science Data
- Volume:
- 17
- Issue:
- 7
- ISSN:
- 1866-3516
- Page Range / eLocation ID:
- 3679 to 3699
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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- Free Publicly Accessible Full Text
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Accepted Manuscript1.0
- Journal Article:
- https://doi.org/10.5194/essd-17-3679-2025
