Award: OCE-1233749

The principal goal of the proposed research is to contribute to the international GEOTRACES investigation of spatial and temporal variations in key trace elements and isotopes (TEIs) in the oceans. The element lead (Pb) is one of the key trace elements that GEOTRACES requires from all of its expeditions. Most of the Pb in the ocean derives from human activities, with the largest contributor the past utilization of leaded gasoline with significant contributions from other high temperature industrial activities (smelting, coal combustion, incineration, etc…). However, natural ocean processes remove anthropogenic lead from the ocean on a time scale of decades to centuries, so as we reduce emissions we observe a decrease in oceanic lead, especially in the upper ocean, as has been amply documented in the North Atlantic Ocean (see Boyle et al., Oceanography, Jan. 2014, vol. 27, pages 69-74). In this project we specialized in the Pb distribution in the deepest water column of the eastern South Pacific Ocean (below 1000m); the upper 1000m was the subject of a parallel project by A.R. Flegal. In this zone of the ocean, Pb is added by deep sea ventilation and desorption from sinking particles that derive their Pb from regions influenced by gasoline and industrial emissions into the atmosphere. Because the deep waters of this region are hundreds of years old, advective sources are of lesser importance to the distribution of deep ocean Pb and desorption from sinking particles and scavenging onto sinking particles determine the balance. Particles sinking from the surface adsorb Pb onto their surfaces, but then as they sink to depths with lower Pb concentrations, they release Pb. This process is the principle source of Pb to the deep South Pacific. However, on the mid-ocean ridge crest at the East Pacific Rise, large amounts of iron and manganese are released by hydrothermal leaching of the basaltic seafloor. Although Pb is also leached from these rocks, when the hot fluids mix with cool oxic seawater, iron and manganese phases precipitate and remove all of the hydrothermal Pb as well as a large amount of the particle-released anthropogenic Pb. This is one of the principal findings of this project: that waters down current of the westward advecting EPR vent emissions have the lowest Pb concentrations ever observed in the ocean, down to only a few tenths of a picomole per kilogram of seawater (50 parts per quadrillion by weight). As these plume waters spread to the furthest westernmost region of this transect, they mix with higher-Pb ambient deep waters so the concentrations are higher – a few picomoles per kilogram – at the western end of this section. This data demonstrates one natural mechanism whereby anthropogenic contaminant Pb is cleaned from the ocean by natural processes. The same processes would have operated in the pre-anthropogenic ocean as well, keeping the lead concentration of the ocean very low despite the continual input of weathered continental Pb and volcanic Pb into the ocean. This process will operate on other oceanic trace metals and can help us better understand their distributions. Last Modified: 01/26/2016 Submitted by: Edward A Boyle