Dissolved Lead (Pb) Concentrations and Pb Isotope Ratios Along the East China Sea and Kuroshio Transect—Evidence for Isopycnal Transport and Particle Exchange

Abstract

Lead (Pb) isotopes are widely used in tracing processes that transport Pb within the environment due to the globally spread Pb pollution. In today’s ocean, Pb is mainly sourced from anthropogenic emissions and is removed by particle scavenging. However, recent efforts involving Pb isotopes have started to indicate that sediments at ocean boundary, either suspended or benthic, are a previously underestimated source and sink of Pb to the seawater dissolved pool. To assess the contributions of Pb from seawater‐sediment boundary, we measured the dissolved Pb concentrations and isotopic compositions in seawater from the outer East China Sea (ECS) and along the Kuroshio. Along the Kuroshio, depth profiles exhibit highly similar Pb concentrations across constant density planes, suggesting isopycnal transport. Pb isotope ratios in most of the samples show strong contributions of aerosol Pb, except for seawaters from the ECS and deep Okinawa Trough (>800 m), which show evidences of crustal Pb contribution. The Okinawa Trough deep water has overlapping Pb isotope ratios with ECS water, where the suspended particulate matter is abundant. Our data on Pb isotopes in the ECS and the Okinawa Trough deep water, together with data from previous studies at other continental margins suggest that the seawater‐sediment boundary along the continental shelves may be an important source and sink in marine Pb cycling.

Exchange of Pb from Indian to Atlantic Ocean is driven by Agulhas current and atmospheric Pb input from South Africa

“…Similarly, assuming salinity as a conservative property, the plots of salinity versus Pb concentration and Pb isotopic compositions were used to show the mixing between two intermediate water masses along the isopycnal of tropical Atlantic (GEOTRACES GA06) 13 . A recent study also used a similar approach to determine the mixing of dPb in the surface water masses between South China Sea and Kuroshio core 36 . Surface waters, as in this study region; however, are often influenced by air-sea heat exchange, evaporation, and precipitation impacting in situ properties such as temperature, salinity, and density.…”

“…Combined, all these factors have minimal quantitative impact on the dPb concentration in the study area (up to 0.02% of the measured dPb; see Supplementary Section ST1 ). Therefore, even though dPb is particle reactive and may behave non-conservatively in different reservoirs, it is possible to use dPb to develop the mixing diagram between two water masses (for example 7 , 13 , 35 , 36 ).…”

“…The equations used to draw a mixing diagram of dPb are based on the following equations: where f represents the fractional contribution and subscript em stands for the end-members. The isotopic mixing of Pb produces a nonlinear trend with salinity, while the mixing of dPb concentration forms a linear trend with salinity 13 , 36 . Therefore, 206 Pb/ 207 Pb vs. 1/dPb diagram produces a linear mixing trendline (Supplementary Fig.…”