The spatial distributions of the isotopes of thorium (Th) and protactinium (Pa) have previously been used to support the notion that there is enhanced "boundary scavenging" of particle reactive substances (i.e. their preferential removal from seawater) at ocean margins. However, recent evidence suggests that spatial variability in the chemical fractionation between Th and Pa (i.e. differential partitioning between the dissolved and particulate phases) may influence sedimentary 231Pa/230Th ratios as much as, or even more than, enhanced scavenging at margins. If true, then preferential removal of particle-reactive solutes at ocean margins may be less significant than previously believed.
Further research is needed to examine these two processes, and the proposed study by researchers at the Lamont Doherty Earth Observatory at Columbia University will address this problem and related objectives by analyzing samples collected at a location in the Subarctic North Pacific Ocean for the concentration and distribution of dissolved 230Th, 232Th and 231Pa together with analysis of their concentration in surface sediments. The samples have already been collected. Additional objectives of the proposed work include assessing the boundary sources and sinks for each isotope, the near bottom gradients and testing the hypothesis that enhanced scavenging near the sediment-water interface creates a significant sink at the base of the pelagic water column. Additionally, the potential regeneration and flux from settling labile biogenic particles, from the dissolution of lithogenic phases and/or mobilization of colloids will be evaluated in conjunction with an examination of the role of particulate phase composition in controlling chemical fractionation. The results will be used in modeling efforts aimed at examining the importance of the various processes outlined above. The proposed work will improve understanding of the enhancement of chemical scavenging and removal of particle-reactive substances at ocean margins.
The research will be beneficial to investigators in a variety of fields besides chemical oceanography, notably paleoceanography. The results will inform the debate concerning the applicability of sedimentary 231Pa/230Th ratios as a proxy for deep ocean circulation, and the validity and importance of various conclusions derived from prior research. The research will provide further information on aspects of sediment-water exchange of reactive substances. The results may also inform the research that will be conducted in the Pacific Ocean in the future as part of the GEOTRACES Program.
Broader Impacts: The scientific aspects of the research focus on improving the understanding the importance of boundary margin scavenging. Such knowledge is necessary for understanding the ocean biogeochemistry of many elements, not just the isotopes that are the focus of this study. The educational aspects of the proposal relate to graduate student education and related activities.
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.
