Testing organic-rich sediments as recorders of the seawater Os isotope record

Author(s): Paquay, F.; Ravizza, G.
Author Affiliation(s): Primary:
University of Hawaii at Manoa, Department of Geology and Geophysics, Honolulu, HI, United States
Volume Title: AGU 2006 fall meeting
Source: Eos, Transactions, American Geophysical Union, 87( Fall Meeting Suppl.); American Geophysical Union 2006 fall meeting, San Francisco, CA, Dec. 11-15, 2006. Publisher: American Geophysical Union, Washington, DC, United States. ISSN: 0096-3941 CODEN: EOSTAJ
Note: In English
Summary: Organic-rich sediments of Eocene to Oligocene age, deposited under anoxic to moderately reducing conditions from the Ocean Drilling Program (ODP) Site 959 on the continental margin of the Cote d'Ivoire- Ghana were investigated in order to empirically test the fidelity of organic-rich marine sediments as recorders of past seawater Os isotope variations. Although initial 187Os/188Os ratios derived from Re-Os isochrons provide the best available constraints on the Mesozoic and Paleozoic portions of the marine Os isotope record, no systematic study comparing Re-Os analyses of organic-rich marine sediments to a well established portion of the Cenozoic marine Os isotope record has been made. Forty-two bulk sediment Os analyses from Site 959 reveal that Os concentrations range from 80 to 550 pg/g, indicating the dominance of hydrogenous Os. Measured 187Os/188Os ratios range from 0.48 to 1.40 and are larger than the 187Os/188Os of contemporaneous seawater. Variability among replicate analyses clearly indicates significant powder heterogeneity in some samples. A small subset of these samples has been subjected to an oxidative partial dissolution that allows analyses of Re and Os on the same sample splits, and calculation of initial 187Os/188Os ratios using an age model constrained by biostratigraphy. Comparison of leach analyses to bulk sediment analyses indicates that all the Re and in excess of 90% of the Os is liberated during leaching. In most cases the 187Os/188Os of leachable Os is indistinguishable from that of the bulk sediment. The resulting calculated initial 187Os/188Os ratios are in general agreement with the independently established Os isotope record of the Eocene-Oligocene transition. In particular, one sample analyzed records the pronounced minimum in the seawater 187Os/188Os record during the late Eocene. In total these results indicate that the Re-Os systematics of Cenozoic organic-rich sediments effectively record the 187Os/188Os of contemporaneous seawater, and that similar studies in the future can contribute to refining the Cenozoic marine Os isotope record and provide valuable chemostratigraphic constraints on sediment age models.
Year of Publication: 2006
Research Program: ODP Ocean Drilling Program
Key Words: 12 Stratigraphy, Historical Geology and Paleoecology; Africa; Atlantic Ocean; Carbon; Carbonates; Cenozoic; Coastal environment; Eocene; Equatorial Atlantic; Isotope ratios; Isotopes; Ivory Coast; Leg 159; Metals; North Atlantic; Nutrients; ODP Site 959; Ocean Drilling Program; Oligocene; Organic carbon; Os-188/Os-187; Osmium; Paleoenvironment; Paleogene; Platinum group; Sea water; Sediments; Stable isotopes; Tertiary; West Africa
Coordinates: N033740 N033740 W0024408 W0024408
Record ID: 2009021099
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