Distribution of neodymium isotopes and trace elements in the West Pacific: sources, sinks, and water mass signatures
Final Report Abstract
The West Pacific is an important area with respect to the northward flow of intermediate and bottom waters from the Southern Ocean, the southward flow of intermediate and deep waters from the North Pacific, and the eastward transport of surface and subsurface currents in the equatorial zonal current system. Any addition of trace elements or modification of trace element and isotope signatures occurring here through contact with the Asian continent and volcanic islands, will therefore be redistributed to the North, South, and East Pacific. Characterization of these sources and identification of the trace element transport is therefore crucial for our understanding of the biogeochemical characteristics and cycles in the Pacific Ocean. In this project, we studied the input and transport of trace elements in the West Pacific between South Korea and Fiji using dissolved rare earth elements (REE) and neodymium isotopes (143Nd/144Nd, expressed in εNd notation). The results provide a transect of these parameters at unprecedented vertical and spatial resolution for the Pacific and give insight into the sources of trace elements added to the Pacific Ocean as well as their transport by deep, intermediate, and surface to subsurface currents. Notably, REE are added to the West Pacific from South Korea, where elevated REE concentrations, very low Nd isotope ratios, and low salinity are consistent with input from local REE sources. In the tropical West Pacific, anomalously positive Nd isotopes and distinct REE patterns (positive Eu-anomalies) indicate input from volcanic islands, particularly Papua New Guinea, the Philippines, Fiji, and the Solomon Islands. Occurrence of these characteristic signatures in surface and subsurface currents flowing eastward, and contrasting open ocean Pacific values in westward flowing currents add unique detail to the tracing of this input towards the East Pacific. The major deep and intermediate water masses in the West Pacific show characteristic Nd isotope signatures, allowing the tracing of the pathways of these water masses away from their sources. Yet, for the first time, we show that these signatures cannot solely be explained by conservative water mass mixing but require significant modification of the Nd isotope composition along basaltic island margins and submarine ridges. Dissolved REE in shallow to abyssal water masses, on the other hand, are mostly unaffected by these alterations, in line with the concept of boundary exchange that modifies the Nd isotopic composition without substantially changing the Nd concentration of seawater. From subsurface to abyssal water depths, REE concentrations in the West Pacific are therefore dominated by later water mass transport. In summary, our new results fill an important gap in our understanding of the supply and redistribution of Nd isotopes and REE concentrations in the West Pacific. They provide new evidence for the dominant lateral transport of REE by water masses and the substantial modification of Nd isotopes along volcanic island margins and submarine ridges that reduces the value of Nd isotopes as conservative water tracer in this part of the ocean. In the tropical Pacific, the locally imprinted positive Nd isotope signatures and characteristic REE patterns provide unprecedented insight into the trace element supply to the tropical West Pacific and eastward transport in the zonal current system and lend support to the hypothesis that the tropical West Pacific is an important source of trace elements (including essential micronutrients) to the iron-limited tropical East Pacific.
Publications
- (2018) Rare earth element distributions in the West Pacific: Trace element sources and conservative vs. non-conservative behavior. Earth and Planetary Science Letters 486 166–177
Behrens, Melanie K.; Pahnke, Katharina; Paffrath, Ronja; Schnetger, Bernhard; Brumsack, Hans-Jürgen
(See online at https://doi.org/10.1016/j.epsl.2018.01.016) - (2018) Sources and processes affecting the distribution of dissolved Nd isotopes and concentrations in the West Pacific. Geochimica et Cosmochimica Acta 222 508–534
Behrens, Melanie K.; Pahnke, Katharina; Schnetger, Bernhard; Brumsack, Hans-Jürgen
(See online at https://doi.org/10.1016/j.gca.2017.11.008) - 2014. Neodymium Isotope Distribution in the West Pacific Between South Korea and Fiji, Ocean Science Conference, Honolulu, HI, USA, Abstract ID:13518
Behrens, M.K., Pahnke, K.
- 2014. Tracing water mass sources and element fluxes in the West Pacific. MPI Seminar, Max Planck Institute for Marine Microbiology, Bremen, Germany
Behrens, M.K., and Pahnke, K.
- 2015. Neodym-Isotope und Seltene Erden-Verteilungen im Westpazifik, Status seminar on ocean research with R/V Sonne, Feb. 2015, University of Bremen
Behrens, M.K., Pahnke, K.
- 2015. Trace element inputs to the upper West Pacific from Nd isotopes and rare earth elements, Abstract GC51F-1166, AGU Fall Meeting, San Francisco (Outstanding Student Paper Award)
Behrens, M.K., Pahnke, K., Schnetger, B., Brumsack, H.J.
- 2016. A detailed picture of trace element input and equatorial transport to the iron-limited tropical East Pacific from Nd isotopes and REE in the West Pacific. Dissertations Symposium in Chemical Oceanography (DISCO XXV), Honolulu, USA
Behrens, M.K., Pahnke, K., Schnetger, B., Brumsack, H.-J.
- 2016. Rapid and precise analysis of rare earth elements in small volumes of seawater - Method and intercomparison. Marine Chemistry 186, 110-120
Behrens, M.K., Muratli, J., Pradoux, C., Wu, Y., Böning, P., Brumsack, H.-J., Goldstein, S.L., Haley, B., Jeandel, C., Paffrath, R., Pena, L.D., Schnetger, B., Pahnke, K.
(See online at https://doi.org/10.1016/j.marchem.2016.08.006) - 2016. Rare earth element cycling in the West Pacific – method and application, Goldschmidt Conference, Yokohama, Japan
Behrens, M.K.
