JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 90, NO. D1, PAGES 2391-2408, FEBRUARY 20, 1985 ATMOSPHERIC TRACE ELEMENTS AT ENEWETAK ATOLL: 2. TRANSPORT TO THE OCEAN BY WET AND DRY DEPOSITION R. Arimoto, R. A. Duce, B. J. Ray, and C. K. Unni 1 Center for Atmospheric Chemistry Studies, Graduate School of Oceanography, University of Rhode Island, Kingston Abstract. The concentrations of trace ele- puts of mineral aerosol are most important for ments in precipitation and dry deposition are A1, Sc, and Th in suspended marine particles, but presented for samples collected at Enewetak they showed that atmospheric deposition also sup- Atoll (11 øN, 16• E) during SEAREX experiments in plies a significant fraction of the Fe, V, Mn, 1979. The concentrations of A1, Sc, Mn, Fe, Co, and Co associated with marine particulates. Min- and Th in rain are dominated by crustal material, eralogical studies have similarly shown that the and for these elements, wet deposition evidently atmospheric transport of weathered crustal mate- exceeds dry deposition. For most of these rial plays a significant role in the pelagic sed- elements the present rates of atmospheric deposi- imentary cycle [Rex and Goldberg, 1958; Griffin tion at Enewetak are similar to their mean rate and Goldberg, 1968; Windom, 1969; Glasby, 1971; of accumulation in sediments over the past Prospero, 1981; Uematsu et al., 1983]. 5-10,000 years, suggesting that the air-to-sea Sea salt aerosols are produced as bubbles exchange of particles is closely tied to the sed- burst at the oceanic surface, and this is the imentary cycle of the mid-Pacific. Noncrustal dominant source for elements such as Na, Mg, Ca, sources govern the concentrations of Pb, Zn, Cu, K, C1, and Br in the marine atmosphere. The Se, and Cd in wet and dry deposition samples. relative proportions of these elements in atmo- Analyses of dry deposition collected from a flat spheric particles and in rain from marine regions plastic plate indicate that the amount of mate- generally are similar to those in bulk sea rial recycled from the sea surface varies marked- water. Although the ocean is a major source of ly between samples, and even though these esti- atmospheric particulate material, the quantity of mates do not necessarily reflect the dry deposi- sea salt aerosols produced and redeposited in the tion to the ocean surface, the results suggest oceans each year is still quite uncertain that recycled sea spray often amounts to more [Blanchard, 1963; Lovett, 1978; Petrenchuk, 1980]. than 50% of the total dry deposition of the en- In remote marine regions, the atmospheric con- riched elements. Recycled sea spray also makes centrations of a third group of elements, includ- up a significant fraction of the total wet depo- ing Zn, Se, Ag, Cd, Sb, Cu, I, and Pb, are sub- sition of the enriched elements. The net deposi- stantially higher than those expected from the tion rates of elements such as Cu and Zn are dispersal of mineral aerosol or the injection of greater than or equal to their inputs from verti- bulk sea water into the atmosphere [Peirson et cal mixing, but the net deposition of Pb clearly al., 1974; Duce et al., 1976, 1983; Patterson et exceeds the input from upwelling. The current al., 1976; Rahn, 1976; Meinert and Winchester, net deposition rates of the enriched elements are 1977; Buat-Menard and Chesselet, 1979; Maenhaut also similar to their rates of removal to sedi- et al., 1981, 1983; Parrington et al., 1983]. ments. These results indicate that air-sea ex- Human activities have been implicated as the change processes may significantly affect the cause of the enrichments for some of these ele- chemistry of trace metals in the open ocean. ments, but the sources for many of these elements and the causes for their enrichments are not Introduction known; thus they are called the anomalously en- riched elements (AEEs). More is known about the Prevailing winds carry trace elements from sources, transport, and distribution of Pb than natural and anthropogenic continental sources to the other AEEs. Inputs of Pb to the atmosphere, remote marine environments. In the near-surface primarily from smelters and automobile exhaust, troposphere over the mid-Pacific Ocean, the dis- have increased the eolian Pb fluxes to the open persal of eolian aluminosilicate particles, com- oceans and shifted the oceanic distribution of Pb monly called mineral aerosol or dust, governs the away from the preindustrial steady state [Schaule concentrations of elements such as A1, Sc, Mn, and Patterson, 1981, 1983; Settle and Patterson, Fe, Co, Cs, Ba, Ce, Eu, Hf, Ta, and Th [Duce et 1982]. Recent data indicate that the atmospheric al., 1983; Parrington et al., 1983]. Moreover, transport of anthropogenic Sn has similarly en- the deposition of atmospheric particulate mate- riched this element in oceanic surface waters rial has been shown to influence the chemistry of [Byrd and Andreae, 1982]. The vertical distribu- trace elements in deep waters of the tropical tions of Pb and Sn in the oceans may be particu- North Atlantic [Buat-Menard and Chesselet, larly sensitive to anthropogenic perturbations 1979]. Those authors found that the eolian in- because their residence times are relatively short. Other elements, such as Cu, Ni, Zn, As, 1 Now at AnacondaMinerals Company,Research Se, Ag, and Cd exhibit vertical profiles in the Department, Tucson, Arizona. oceans similar to those of nutrients, suggesting that their concentrations are affected by bio- Copyright 1985 by the American Geophysical Union. logical activity [Spencer and Brewer, 1969; Bender and Gagner, 1976; Bruland 1980, 1983; Paper number 4D1325. Measures and Burton, 1980; Boyle et al., 1981; 0148-0227/85/004D-1325505.00 Campbell and Yeats, 1981]. 2391