Importance of vertical geochemical processes in controlling the oceanic profiles of dissolved rare earth elements in the northeastern Indian Ocean |
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| Institution: | 1. U.S. Geological Survey, PCMSC, 400 Natural Bridges Dr., Santa Cruz, CA 95060, USA;2. Earth and Planetary Sciences, University of California Santa Cruz, 1156 High Street, Santa Cruz, CA 95064, USA;3. Council for Scientific & Industrial Research, National Institution of Oceanography, Dona Paula, Goa 403004, India;4. Earth, Atmospheric & Planetary Sciences, MIT, 77 Massachusetts Ave., Cambridge, MA 02139, USA;5. Earth & Atmospheric Sciences, University of Houston, 312 Building SR-1, Houston, TX 77204, USA;1. Department of Earth, Atmospheric and Planetary Sciences, Massachusetts Institute of Technology, Cambridge, MA, United States;2. Lamont-Doherty Earth Observatory of Columbia University, Palisades, NY, United States;3. Department of Earth, Ocean, and Atmospheric Science, Florida State University, Tallahassee, FL, United States;1. Deep-Sea Multidisciplinary Research Center & Laboratory for Marine Mineral Resources, Qingdao National Laboratory for Marine Science and Technology, Qingdao, 266237, China;2. Center of Deep Sea Research, Institute of Oceanology, Chinese Academy of Sciences, Qingdao, 266071, China;3. Center for Ocean Mega-Science, Chinese Academy of Sciences, Qingdao, 266071, China;4. University of Chinese Academy of Sciences, Beijing, 100049, China;5. Key Lab of Submarine Geosciences and Prospecting Techniques, Ministry of Education, Ocean University of China, Qingdao, 266100, China;1. Department of Earth and Environmental Sciences, Tulane University, New Orleans, LA 70118, United States;2. Department of Geology and Geophysics, University of Hawaii at Manoa, Honolulu, HI 96822, United States;3. United States Geological Survey, Pacific Coastal and Marine Science Center, Santa Cruz, CA 95060, United States;4. International Atomic Energy Agency, 98000, Monaco;5. Earth and Environmental Sciences Division, Los Alamos National Laboratory, Los Alamos, NM 87545, United States;6. Department of Ocean, Earth, and Atmospheric Sciences, Old Dominion University, Norfolk, VA 23529, United States |
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| Abstract: | Vertical profiles of dissolved rare earth elements (REEs) were obtained in the Bay of Bengal and the Andaman Sea. The REE concentrations at various depths in the Bay of Bengal are the highest in the Indian Ocean. This is attributable ultimately to the large outflow of the Ganges–Brahmaputra and Irrawaddy rivers, but the dissolved REE flux to surface waters alone cannot explain the large and near-constant REE enrichment throughout the entire water column. The underlying fan sediments serve as not a source but a sink for dissolved REE(III)s. Absence of excess 228Ra in the deep waters suggests that lateral input of dissolved REEs from slope sediments is also small in these regions. Partial (<0.3%) dissolution of detrital particles, which are carried by the rivers and lateral surface currents and subsequently settle through the water column, appears to be a predominant source for the dissolved REEs. Vertical profiles showing an almost linear increase with depth are common features for the light and middle REEs everywhere, but their concentration levels are variable from basin to basin and from element to element. This suggests that their oceanic distributions respond quickly to the variation of particle flux and its REE composition through reversible exchange equilibrium with suspended and sinking particles much like the case for Th. The relative importance of the vertical geochemical processes of reversible scavenging over the horizontal basin-scale ocean circulation with passive regeneration like nutrients decreases systematically from the light to the heavy REEs. Using a model, the mean oceanic residence times of REEs in the Bay of Bengal are estimated to range from 37 years for Ce to 140–1510 years for the strictly trivalent REEs. In the deep water of the Andaman Sea, isolated from the Bay of Bengal by the Andaman–Nicobar Ridge (maximum sill depth of ～1800 m), the REE concentrations are almost uniform presumably due to rapid vertical mixing. The REE(III) concentrations are similar to that of ～1250 m depth water in the Bay of Bengal, consistent with other oceanographic properties. However, the REE composition of the deep water appears to be altered slightly by preferential scavenging of the light REE(III) at the bottom boundary of the basin. |
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