Tracing anthropogenic aerosol trace metal sources in the North Atlantic Ocean using Pb, Zn and Ni isotopes

Abstract

Atmospheric deposition of trace metals of natural or anthropogenic origin is an important input of micronutrients to the surface ocean. However, understanding its direct impact on oceanic element cycles is challenging due to scarce data, coupled to diverse aerosol sources and variable solubilities. Here, we present a dataset that combines Ni, Zn and Pb isotopes for samples from the Moroccan and Senegalese coasts and in the high latitude North Atlantic Ocean. We combine the new with published data for other circum-North Atlantic sources to assess the processes that determine the isotope signatures in different types of aerosols. We then use open marine aerosol data to investigate the impact of these signatures in the open ocean. Isotope analyses were conducted on bulk aerosols (TSP), on their ultra-high-purity water leachates, and on rainwaters. Aerosols characterized by crustal elemental abundances have isotope compositions similar to Saharan mineral dust. Mixing with anthropogenic aerosols from Europe/North Africa results in lower ²⁰⁶Pb/²⁰⁷Pb and ²⁰⁸Pb/²⁰⁷Pb values for the Eastern North Atlantic region. Higher ²⁰⁶Pb/²⁰⁷Pb at a given ²⁰⁸Pb/²⁰⁷Pb, observed near the Canadian margin and occasionally at the Senegalese coast, points to anthropogenic inputs from North America. Based on trends in the aerosol data (e.g., δ⁶⁶Zn_JMC-Lyon versus ²⁰⁶Pb/²⁰⁷Pb, δ⁶⁰Ni_SRM986 versus Ni/V), we identify several anthropogenic sources of Zn and Ni. The δ⁶⁶Zn_JMC-Lyon of low-temperature pollution (e.g., non-exhaust traffic emission) appears to be around −0.1‰ to 0.2‰, while leachate δ⁶⁶Zn_JMC-Lyon as low as −0.21‰ indicates contributions from high-temperature combustion or smelting processes. Among aerosols with good correlations between Ni and V, δ⁶⁰Ni_SRM986 > 0.40‰ traces Ni contributions from oil combustion. Other Ni-enriched sources, possibly originating from laterite or sulfide, show relatively low δ⁶⁰Ni_SRM986 (as low as −0.85‰) and low V/Ni. Generally, aerosol sources for Zn are consistent throughout the North Atlantic, while Ni can be highly heterogenous. Combining the new data with literature elemental data, ratios of soluble Zn/Pb in anthropogenic aerosols are 1–100 times surface ocean ratios, suggesting that the low δ⁶⁶Zn_JMC-Lyon observed in anthropogenic aerosol can be key in controlling the upper ocean Zn isotope composition. These aerosols have, however, much less significance for surface ocean Ni.