The Late Neogene 87 Sr/ 86 Sr Record of Lowland Himalayan Rivers
Tóm tắt
Fossil shells and paleosol carbonate from ancestral Himalayan river deposits provide a 87 Sr/ 86 Sr record of lowland Himalayan river water during the late Neogene. Reconstructed 87 Sr/ 86 Sr river values increased sharply in the late Miocene, probably marking the beginning of exhumation of high- 87 Sr/ 86 Sr metalimestones, more in the central than in the western Himalayas. These results imply that the marine 87 Sr/ 86 Sr record may not be a proxy for silicate weathering or consumption of atmospheric CO 2 resulting from that weathering.
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Tài liệu tham khảo
Carbonates were selectively dissolved for Sr analysis with doubly distilled 1 M acetic acid which has been shown to leach Sr from silicates negligibly [
]. 87 Sr/ 86 Sr analyses were performed on a VG 354 at the University of Arizona. Sixty-nine NBS-987 standards during this study yielded a mean of 0.710218 ± 11 (2σ). All light stable isotope analyses on carbonates are reported relative to PDB with the use of standard notation where δ 13 C(PDB) or δ 18 O(PDB) = [( R sample / R standard ) − 1] × 1000 and R = 13 C/ 12 C sample / 13 C/ 12 C standard or 18 O/ 16 O sample / 18 O/ 16 O standard respectively. We used the single-vessel fractionation factor between CO 2 and CaCO 3 of P. K Swart S. J. Burns and J. J. Leder [ Chem. Geol. 86 89 (1991)]; no correction was made for the presence of dolomite grains in some of the detrital samples.
D. W. Burbank R. A. Beck T. J. Mulder in Tectonics of Asia A. Yin and T. M. Harrison Eds. (Cambridge Univ. Press Cambridge in press) pp.149–188.
Petrographic evidence from the Panir section suggests that the provenance is distant Himalayan metamorphic rocks and more local Eocene Kirthar limestone. The low 87 Sr/ 86 Sr ratios displayed by soil carbonate in this section may result from weathering input by Eocene limestone into the ancestral middle Indus River system. Evidence that the lower two-thirds of this section represents the ancestral Indus River are that paleocurrent directions are generally southward and that phyllite and metaquartzite grains with accessory garnet indicate a Himalayan source.
P. G. DeCelles et al. Geol. Soc. Am. Bull. in press.
J. Quade Geol. Soc. Am. Abstr. Prog. 25 (6) 175 (1993).
The 87 Sr/ 86 Sr ratios of 10 detrital silicate separates from 0.7460 to 0.8692 (at time of burial) are poorly correlated with and generally much higher than that of soil carbonate (0.7224 to 0.7356) from the same stratigraphic levels. A more rigorous test of the 87 Sr/ 86 Sr weathering contribution would be to analyze mineral separates as weathering of different minerals proceeds at different rates. The abundance of detrital carbonate in these samples (2 to 13%) and its much greater solubility as compared with any silicate mineral make a significant contribution from silicates unlikely.
Bickle M. J., Chapman H. J., Wickman S. M., Peters M. T., Contrib. Mineral. Petrol. 121, 400 (1995);
J. W. Valley in Stable Isotopes at High Temperature Geological Processes J. W. Valley H. P. Taylor J. R. O'Neil Eds. vol. 16 of Reviews in Mineralogy (Mineralogical Society of America Washington DC 1986) pp. 445–489.
We thank J. Cater R. Adams and D. Schelling for discussions samples and unpublished sedimentological data from the Panir section; LASMO Oil Pakistan Ltd. and the Director General of Petroleum Concessions for provision of samples and permission to publish data from the Panir section; the Department of Soil Conservation in Babar Mahal Kathmandu; and C. France-Lanord and L. Derry for discussions and access to unpublished data. The early stages of this research were supported and encouraged by M. McCulloch and A. Chivas at Australian National University. This project was primarily funded by NSF grant EAR-9418207.