Boundary exchange completes the marine Pb cycle jigsaw

Abstract

Material fluxes at the land-ocean interface impact seawater composition and global cycling of elements. However, most attention has been focused on the fluvial dissolved fluxes. For elements like lead (Pb), whose fluvial particulate flux into the ocean is two orders of magnitude higher than the dissolved counterpart, the role of particulates in elemental cycling is potentially important but currently less appreciated. Using both chemical analyses on samples collected from around equatorial Southeast Asia and model simulations, we show that particulate-dissolved exchange is an important mechanism controlling the concentration and isotopic composition of dissolved Pb in the ocean. Our model indicates that Pb contributed from particulate-dissolved exchange at ocean boundaries is larger than, or at least comparable to, other major Pb sources to the seawater before the Anthropocene, when the anthropogenic Pb was absent. Our work highlights the importance of boundary exchange in understanding marine element cycling and weathering-climate feedback.

Keywords: Pb; Southeast Asia; boundary exchange; isotope; marine elemental cycling.

Fig. 1.

(A) Fluvial discharge of sediments to the global ocean replotted from ref. . Numbers are mean annual suspended sediment discharges (million tons/year). Colour shades represent the illustrative catchment regions with major rivers highlighted. (B) The seawater sampling stations in this study (yellow circles) and seawater stations in the literature are shown for comparison [red circles, green rectangle, blue triangle (26, 28, 29)]. The rivers relevant to the sampling sites are highlighted in blue. The surface ocean currents are marked with arrows. (C) Sampling stations in the Johor River estuary with salinity values (pss) marked next to the station locations; the estuary is highlighted with a square in panel B.

Fig. 2.

Geographical distribution of (A) concentration and (B) isotopes of dissolved Pb in seawater samples around the Malayan Peninsula. Major cities or rivers are also highlighted. Figure plotted through Ocean Data View (30).

Fig. 3.

Comparisons of Pb isotopes in seawater, salinity and SPM. (A) Triple-isotope plot of the seawater samples in this study in comparison with regional sources from three main references (28, 29, 31). The locations of the sites are the same as Fig. 1. The weighted-average salinity distribution was also indicated. (B) Scatter plot of the ²⁰⁶Pb/²⁰⁷Pb in Johor River estuary waters compared with SPM concentrations. Regression statistics are presented. (C) Scatter plot of dissolved ²⁰⁶Pb/²⁰⁷Pb ratios from this study (all samples) and identified endmembers against salinity. The calculated Pb isotope ratios based on conservative mixing between two endmembers (Johor River Stn1, salinity = 8 pss; and Malacca Strait Stn 2, salinity = 33 pss) are plotted with a blue curve. The uncertainties for Pb isotope measurements are within the size of each symbol. The orange dot denotes a sample treated as an outlier for Pb concentration probably due to poor filtering, but its isotope ratios may still be usable as the dissolved and particulate pools are in equilibrium.

Fig. 4.

Observation and modelling of (A) ²⁰⁶Pb/²⁰⁷Pb ratios and (B) concentrations of dissolved Pb compared with salinity. The locations of the sample sites are the same as Fig. 1. The conservative mixing model (blue line), the mixing plus particulate-dissolved exchange model (brown line), and the sensitivity test with random anthropogenic Pb additions (with Pb isotopic composition similar to regional aerosol) are also indicated in green squares. Uncertainties for Pb isotope measurements are within the size of the symbols.

Fig. 5.

Conceptual illustration of the estimate Pb flux from natural sources in the pre-Anthropocene ocean. In addition to fluvial particulates (this study), estimates of other natural sources include wind-borne dust particles (12), volcanic aerosols (11), hydrothermal systems (13, 14), and fluvial dissolved inputs (15), are also illustrated. The two Inset figures on the right illustrate the expected behavior and observation of particulate-dissolved Pb exchange in the present-day and pre-Anthropocene ocean.