Lead isotopes trace interhemispheric transport of water masses and pollution to the tropical western North Atlantic Ocean

The use of leaded gasoline significantly impacted North Atlantic seawater during the 20th century, leaving a legacy of pollution that remains traceable despite the global ban reached in 2021. This study presents new Pb concentration and isotope data from four depth profiles in the western tropical North Atlantic along Leg 2 of GEOTRACES section GA02 from 31.7°N to 1.2°N. The data capture atmospheric inputs and water masses from both hemispheres, revealing latitudinal variability in Pb isotope signatures in the upper part of the water column. In surface waters, low 206 Pb/ 207 Pb ratios (1.166–1.173) indicate South American and North African anthropogenic sources at the southern stations, and higher 206 Pb/ 207 Pb ratios (1.174–1.179) reflect anthropogenic North American and natural North African mineral dust inputs at the northern stations. A comparison of North Atlantic surface water data for 1981–1999 and 2010–2014 reveals distinct shifts in Pb concentrations and isotope ratios, confirming the effectiveness of Pb isotopes as tracers for identifying and tracking Pb sources over time in the North Atlantic. The Pb concentrations and isotopic signatures of central water masses (formed 4–15 years ago relative to sampling time) and intermediate water masses (formed 20–50 years ago) are consistent with the expected atmospheric Pb sources at the time and location of their formation, emphasizing the influence of both spatial and temporal variations in atmospheric Pb deposition. However, in the intermediate layer, where northern- and southern-sourced water masses mix, a deviation of the Pb concentration from the expected water mass mixing trend suggests removal of some dissolved Pb by particles. North Atlantic Deep Water exhibits persistent anthropogenic Pb signals from historical US and European emissions (20–30 years ago), while bottom waters within a nepheloid layer show evidence of natural Pb release from sediments. Overall, these findings suggest that Pb isotope ratios predominantly act as conservative tracers of water masses in the strongly advective western tropical North Atlantic Ocean.