HR: 10:20h
AN: OS32B-01    [Abstracts]
TI: Influence of Black Shale Weathering on Riverine Osmium Isotopes: Examples from the Orinoco
AU: * Chen, C
EM: Cynthia.Chen@dartmouth.edu
AF: Dartmouth College, Department of Earth Sciences, Hanover, NH 03755 United States
AU: Landis, J D
EM: jlandis@dartmouth.edu
AF: Dartmouth College, Department of Earth Sciences, Hanover, NH 03755 United States
AU: Donoghue, J C
EM: jdonoghue@dartmouth.edu
AF: Dartmouth College, Department of Earth Sciences, Hanover, NH 03755 United States
AU: Bostick, B C
EM: bbostick@dartmouth.edu
AF: Dartmouth College, Department of Earth Sciences, Hanover, NH 03755 United States
AU: Sharma, M
EM: mukul.sharma@dartmouth.edu
AF: Dartmouth College, Department of Earth Sciences, Hanover, NH 03755 United States
AB: Weathering of organic rich shales in the Andes has often been suggested as a possible cause of increasing ¹⁸⁷Os/¹⁸⁸Os in seawater during the Cenozoic. We present data suggesting that uplift and weathering of the Andes did not play a crucial role in the increase of ¹⁸⁷Os/¹⁸⁸Os during the Cenozoic. The Orinoco River is ideally situated for studying the influence of Andean shale weathering on the Os budget of the oceans. The left bank tributaries of the Orinoco are primarily sourced in the Northern Andes while the right bank tributaries are draining the Precambrian Guyana Shield. The left bank tributaries display elevated Se (Yee et al, 1987) and Re (Colodner et al, 1993) contents indicating contributions from intense black shale weathering. River water samples along the Orinoco and its major tributaries were collected in December 2004 (intermediate stage) and analyzed at the Radiogenic Isotope Laboratory at Dartmouth College. Os contents range from 18 to 327 fmol/kg and were low for all samples not derived from black shale-bearing regions. The ¹⁸⁷Os/¹⁸⁸Os range from 0.39 to 1.19 (1σ errors < 0.005), the lowest values measured in tributaries originating in the shield. Rio Portuguesa, sourced directly within Andean shales, has elevated Os (327 fmol/kg) but is not highly radiogenic (¹⁸⁷Os/¹⁸⁸Os = 1.144); furthermore, its flow rate is too low to appreciably influence the Orinoco river effluent Os concentrations. At Ciudad Bolivar, well downstream of the confluence of the tributaries draining the Andes, the ¹⁸⁷Os/¹⁸⁸Os is 0.999 with a concentration of 33 fmol/kg. The lack of a highly radiogenic signal indicates that black shale weathering of the Andes cannot be directly responsible for the rise of seawater isotopic composition during the Cenozoic. The global weighted average Os concentration of river water is 47 fmol/kg and ¹⁸⁷Os/¹⁸⁸Os = 1.38 (Peucker-Ehrebrink, 2002) and straddles around a canonical ¹⁸⁷Os/¹⁸⁸Os ratio of 1.26 for the continental crust (Esser & Turekian, 1993). The data indicate that the Os isotopes in large rives are not influenced by erosion of specific lithologies.
DE: 1030 Geochemical cycles (0330)
DE: 1040 Radiogenic isotope geochemistry
DE: 1050 Marine geochemistry (4835, 4845, 4850)
DE: 1065 Major and trace element geochemistry
SC: Ocean Sciences [OS]
MN: Fall Meeting 2005