Neogene Himalayan weathering history and river 87Sr/86Sr; impact on the marine Sr record

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doi: 10.1016/0012-821X(96)00091-X
Author(s): Derry, Louis A.; France-Lanord, Christian
Author Affiliation(s): Primary:
Cornell University, Department of Geological Sciences, Ithaca, NY, United States
Other:
Centre de Recherches Pétrographiques et Géochimiques, Vandoeuvre-les-Nancy, France
Volume Title: Earth and Planetary Science Letters
Source: Earth and Planetary Science Letters, 142(1-2), p.59-74. Publisher: Elsevier, Amsterdam, Netherlands. ISSN: 0012-821X CODEN: EPSLA2
Note: In English. Cent. Rech. Pétrogr. et Géochim., Contrib. No. 1201. 50 refs.; illus., incl. 1 table
Summary: Clastic sediments in the Bengal Fan contain a Neogene history of erosion and weathering of the Himalaya. We present data on clay mineralogy, major element, stable and radiogenic isotope abundances from Lower Miocene-Pleistocene sediments from ODP Leg 116. Nd and Sr isotope data show that the Himalayan provenance for the eroded material has varied little since >17 Ma. However, from 7 to 1 Ma smectite replaces illite as the dominant clay, while sediment accumulation decreased, implying an interval of high chemical weathering intensity but lower physical erosion rates in the Ganges-Brahmaputra (GB) basin. O and H isotopes in clays are correlated with mineralogy and chemistry, and indicate that weathering took place in the paleo-Gangetic flood plain. The 87Sr/86Sr ratios of pedogenic clays (vermiculite, smectite) record the isotopic composition of Sr in the weathering environment, and can be used as a proxy for 87Sr/86Sr in the paleo-GB basin. The Sr data from pedogenic clays shows that river 87Sr/86Sr values were near 0.72 prior to 7 Ma, rose rapidly to ≥0.74 in the Pliocene, and returned to ≤0.72 in the middle Pleistocene. These are the first direct constraints available on the temporal variability of 87Sr/86Sr in a major river system. The high 87Sr/86Sr values resulted from intensified chemical weathering of radiogenic silicates and a shift in the carbonate-silicate weathering ratio. Modeling of the seawater Sr isotopic budget shows that the high river 87Sr/86Sr values require a ca. 50% decrease in the Sr flux from the GB system in the Pliocene. The relationship between weathering intensity, 87Sr/86Sr and Sr flux is similar to that observed in modern rivers, and implies that fluxes of other elements such as Ca, Na and Si were also reduced. Increased weathering intensity but reduced Sr flux appears to require a late Miocene-Pliocene decrease in Himalayan erosion rates, followed by a return to physically dominated and rapid erosion in the Pleistocene. In contrast to the view that increasing seawater 87Sr/86Sr results from increased erosion, Mio-Pliocene to mid-Pleistocene changes in the seawater Sr budget were the result of reduced erosion rates and Sr fluxes from the Himalaya. Abstract Copyright (1996) Elsevier, B.V.
Year of Publication: 1996
Research Program: ODP Ocean Drilling Program
Key Words: 02 Geochemistry; 07 Marine Geology and Oceanography; Alkaline earth metals; Asia; Bengal Fan; Cenozoic; Chemical weathering; Clastic sediments; Clay mineralogy; Erosion rates; Geochemical cycle; Geochemistry; Himalayas; Indian Ocean; Isotope ratios; Isotopes; Leg 116; Major elements; Marine sediments; Metals; Neogene; ODP Site 717; ODP Site 718; Ocean Drilling Program; Pleistocene; Provenance; Quaternary; Sedimentation; Sedimentation rates; Sediments; Sr-87/Sr-86; Stable isotopes; Stream transport; Strontium; Tertiary; Weathering
Coordinates: S005548 S005547 E0812325 E0812324
S010132 S010131 E0812404 E0812403
Record ID: 1996064253
Copyright Information: GeoRef, Copyright 2019 American Geosciences Institute. Reference includes data from CAPCAS, Elsevier Scientific Publishers, Amsterdam, Netherlands