Project: Collaborative Research: US GEOTRACES PMT: Pb and Cr isotopes

NSF Award Abstract:
Most of the lead (Pb) in the ocean has been put there by human activities. These activities include high temperature industrial processes such as smelting, coal combustion, and incineration, as well as leaded gasoline consumption during the middle portion of the 20th century. Lead from these sources moves as fine particles around the world by the atmosphere's winds and eventually deposits on the surface ocean where it dissolves. Fortunately, it is possible to determine the different origins of these Pb sources (e.g. U.S., European, and Asian inputs) from its isotopic composition (isotopes are atoms of the same element with different numbers of neutrons in the nucleus). Lead has been shown to have different isotope ratios because the Pb has been extracted from mineral deposits from different geological periods. This project aims to determine how much of this Pb has moved into the deep North Pacific by ocean circulation or by attaching to sinking particles. This will be done by collecting seawater samples from the surface to the bottom of the ocean at about 30 stations between Tahiti and Alaska and analyzing these for their Pb concentration and Pb isotope ratios. This project will also determine variations in the chromium (Cr) isotope composition of Pacific seawater that are created when the lighter isotopes of Cr are selectively removed from the ocean in extremely low oxygen zones in the eastern tropical Pacific Ocean. This tool can help understand the ongoing evolution of decreasing oxygen in the ocean and past changes in the oxygen in the ocean established from geological archives such as sediments.

The first measurements of anthropogenic Pb in the ocean resulted in a profile in the North Pacific Ocean by Schaule and Patterson in 1981, and since then several labs have sparsely and erratically measured other profiles showing that the Pb in the Pacific Ocean is responding to regional changes in Pb fluxes - from the phasing out of leaded gasoline (mainly in Japan, Canada, Mexico, and the United States) to increasing amounts of coal combustion (mainly in China). The concentration of Pb decreased near Hawaii by a factor of two between 1981 and the present, and the 206Pb/207Pb isotopic composition of Pb decreased from ~1.20 (mainly U.S. Pb) to 1.165 (mainly Chinese coal Pb). The U.S. GEOTRACES Pacific Meridional Transect cruise will give us an unprecedented opportunity to obtain a detailed view of the penetration of anthropogenic Pb into the deep Pacific Ocean which can be used to determine the pathways that dissolved and particulate Pb take in arriving in the deep sea. This project will also determine the stable isotope composition of Cr, an element whose redox state is determined by the oxygen (O2) concentration in the ocean. In oxygen deficient zones (ODZs) where [O2] < 2 µmol/kg, Cr is reduced from chromate CrO4-2 (hexavalent chromium) to Cr3+ ion (trivalent chromium). The isotopically lighter atoms of Cr are preferentially reduced, and the reduced Cr3+ ion is "particle-reactive" (attaches to sinking particles) and removed from the water column. That leaves the residual Cr isotopically heavier, so we can detect this process by measurements of 53Cr/52Cr. This process occurs in conjunction with nitrogen reduction (denitrification) and can be used to trace the consequences as ODZ waters mix out into the oxic ocean.