Award: OCE-1029211

This project was designed to study the processes that supply trace elements to the North Pacific Ocean as well as the processes that remove trace elements from the ocean. Trace elements are of interest for several reasons. For example, some trace elements, like iron, cobalt and zinc, are essential nutrients. Their availability influences the health and fertility of marine ecosystems. Other trace elements are measured in marine sediments as indicators of past changes in the ocean environment, including ocean circulation, seawater chemistry, biological productivity, and more. Still other trace elements, like lead and mercury, are contaminants introduced to the ocean by humans, and we want to understand their transport and fate in the ocean. The principal goal of the project was to determine the sensitivity of trace element removal processes in the North Pacific Ocean to spatial gradients in the abundance and composition of particulate material. Lithogenic particles are introduced to the ocean by continental erosion, transported both by rivers and via the atmosphere (dust). Biogenic particles are produced throughout the ocean, mainly by organisms living in surface waters. Adsorption to particles is the main process removing trace elements from the ocean. We sought to test the long-standing hypothesis that the rate of trace element removal would increase along a transect from particle-poor open-ocean waters toward particle-rich ocean margin regimes. Secondary objectives of this project were to quantify the supply of trace elements to the North Pacific Ocean by dust as well as the supply and removal of trace elements by exchange processes at the land-ocean boundary. Naturally occurring uranium and thorium isotopes provide valuable tracers to identify the processes that supply and remove trace elements in the ocean and, more importantly, to quantify the rates of those processes. We measured concentrations of 232Th, 230Th and 231Pa in seawater at seven stations in the North Pacific Ocean. We also measured concentrations of uranium, 232Th, 230Th, 231Pa, opal and calcium carbonate in surface sediments at about 30 sites in the North Pacific. We expanded our study by incorporating unpublished water column results for the northeast Pacific provided by colleagues at the University of British Columbia, and published data for surface sediments from throughout the Pacific Ocean. We discovered that boundary scavenging, the enhanced removal of dissolved trace elements from the ocean in particle-rich regimes near land, is much less effective than expected in the Subarctic North Pacific Ocean. It is so weak, in fact, that its influence on the water column distribution of dissolved 230Th was undetectable. Differences in particle abundance and particle composition across major the biogeographic province boundary associated with the Subarctic Front plays a far greater role in regulating the removal of trace elements from the ocean than previously anticipated. That is, the structure and composition of marine ecosystems is an important factor that regulates the removal of trace elements from the ocean. The pair of long-lived thorium isotopes (232Th and 230Th) were used to evaluate the supply of trace elements to the ocean by dissolution of lithogenic minerals, and this approach can be used to quantify the supply of micronutrients like iron from dust. Our estimates of dust supply obtained using paired Th isotopes are roughly consistent with fluxes estimated by global dust models, and substantially greater than dust fluxes to this region estimated using aluminum as a tracer of dust supply. An additional deep source of 232Th was revealed in deep waters, most likely dissolution of seafloor sediments, and offers a constraint on dissolved trace element supply due to boundary exchange. Last Modified: 11/18/2013 Submitted by: Robert F Anderson