The Os isotopic composition of Himalayan river bedloads and bedrocks: importance of black shales
Introduction
The Os isotopic ratio of seawater has increased dramatically during the Cenozoic Era and particularly during the past 15 Myr [1], [2], reaching a current 187Os/188Os ratio of about 1.05 [3], [4]. This increase is of interest because variations in the 187Os/188Os ratio of seawater are related in part to the organic carbon cycle [1], [5], [6] and silicate weathering [1], [7]. The causes of this increase are not well known. Three factors control the osmium composition of seawater: (1) continental erosion, which provides radiogenic Os (187Os/188Os of average upper continental crust=1.2–1.8 [1], [8], [9]); (2) submarine alteration of oceanic basaltic crust and abyssal peridotites (187Os/188Os∼0.12–0.13 [10], [11], [12], [13], [14]); (3) alteration of cosmic dust (187Os/188Os∼0.12–0.13 [15], [16]). The Os isotopic ratio of the oceans is thus controlled by the relative contribution of continental erosion versus submarine alteration or cosmic dust input. While little is known about changes in the intensity of submarine weathering during the Cenozoic, cosmic dust input appears to have been roughly constant during this time period [7]. On the other hand, continental erosion rates were clearly increased during the latter part of this time period by the uplift of the Himalayan range. Thus several authors [1], [2] have proposed that the marked rise of the 187Os/188Os ratio during the past 15 Myr results at least in part from erosion provoked by the Himalayan collision, in analogy with similar models proposed to explain the seawater Sr isotopic record [17], [18].
Recent analyses of black shales from the LH [19] demonstrate that this lithology represents a potential source of radiogenic osmium during Himalayan weathering. A radiogenic signature has also been found in Ganga river water [9], in Ganga river sediment leachates from the Ganga [20] and in leachates of sediments accumulated in the Bengal Fan [21]. In this study, we characterize the Os isotopic composition of river bedloads sampled from major rivers and tributaries in Central Nepal and Bangladesh, as well as potential source rocks in the various formations of the Himalayan range. This allows us to identify the origin of radiogenic Os among the different Himalayan lithologies.
Section snippets
The Himalayan range and the Ganga–Brahmaputra river system
The High Himalayan (HH) range can be divided into three main units, separated by major faults [22], [23] (Fig. 1, Fig. 2). Moving downstream, i.e. from north to south, these are:
- 1.
the Tethyan Sedimentary Series (TSS), composed primarily of clastic sediments, carbonates, and variably metamorphosed clastic platform sediments of Cambrian to Eocene age. Most of the TSS sediments were deposited in a passive margin environment. Among the silicate rocks, 87Sr/86Sr≤0.720, while εNd varies from −16 to −12.
Analytical procedures
Each sediment sample was dried at about 60°C. Then all samples (sediment and rock) were disaggregated and ground in an agate mortar. Major and trace elements were measured by ICP–MS and ICP–AES (S.A.R.M.-C.R.P.G., Vandœuvre-lès-Nancy, France).
The samples were spiked for the determination of Re and Os concentrations by isotope dilution. Spikes of Os and Re were prepared in HCl and HNO3, respectively. All reagents were purified by distillation (except H2SO4 and ethanol). Concentrated HNO3 and CrVI
The TSS unit
In the TSS, 187Os/188Os varies between 0.60 and 1.97 and the Os concentration ranges from 8 to 196 parts per trillion (ppt) among the source rocks (Table 1). The TSS bedloads (Table 2) display a more homogenous Os signature (∼0.95) than the bedrocks. These values are comparable with the average upper crustal sediment values determined by Esser et al. [8]. The leachate of a bedload sample (NAG 33) contained about 50% of the total Os, with an isotopic composition notably less radiogenic than that
Discussion
Two trends are evident in the distribution of the Os isotopic ratios of river bedloads and bedrocks (Fig. 3). One of these, composed mainly of samples from the TSS formation, is defined by a variable Os concentration at roughly constant 187Os/188Os ratio, similar to that of the average erodable continental crust [8]. The second trend is towards markedly higher Os isotopic ratios at only slightly increasing Os concentration. This trend implies the addition of a highly radiogenic component.
Acknowledgements
We thank L. Brown, A. Gajurel, P. Le Fort and A. Pêcher for providing samples from various locations. We also thank C. Spatz for technical assistance and A. Galy for numerous discussions. The advice of J.-L. Birck and F. Capmas in setting up the chemical procedure is greatly appreciated. Reviews by Drs. R. Oxburgh and S. Krishnaswami were very constructive and helped to improve the manuscript considerably. This study was supported by CNRS Programs PROSE and PNSE. This is Contribution No. CRPG
References (48)
- et al.
The record of sea water 187Os/186Os variation through the Cenozoic
Earth Planet. Sci. Lett.
(1992) - et al.
The marine 187Os/186Os record of the past 80 million years
Earth Planet. Sci. Lett.
(1995) - et al.
The concentration and isotopic composition of osmium in the oceans
Geochim. Cosmochim. Acta
(1997) - et al.
A possible link between the seawater osmium isotope record and weathering of ancient sedimentary organic matter
Chem. Geol.
(1993) - et al.
Osmium isotopic variations in metalliferous sediments from the East Pacific Rise and the Bauer Basin
Geochim. Cosmochim. Acta
(1993) Accretion of extra-terrestrial matter during the last 80 million years and its effects on the marine osmium isotope record
Geochim. Cosmochim. Acta
(1996)- et al.
The osmium isotopic composition of the continental crust
Geochim. Cosmochim. Acta
(1993) - et al.
Osmium isotopes as petrogenetic and geological tracers
Earth Planet. Sci. Lett.
(1980) Osmium isotopic characteristics of mantle-derived rocks
Geochim. Cosmochim. Acta
(1991)- et al.
187Os/186Os ratios of mid-ocean ridge basalts and abyssal peridotites
Geochim. Cosmochim. Acta
(1994)
Os isotopic systematics of the MORB mantle: results from altered abyssal peridotites
Earth Planet. Sci. Lett.
Abundances of the elements: Meteoritic and solar
Geochim. Cosmochim. Acta
The strontium isotope budget of the modern ocean
Earth Planet. Sci. Lett.
Strontium isotopes and rubidium in the Ganga-Brahmaputra river system: Weathering in the Himalaya, fluxes to the Bay of Bengal and contributions to the evolution of oceanic 87Sr/86Sr
Earth Planet. Sci. Lett.
Re-Os isotope systematics in black shales from the Lesser Himalaya: their chronology and role in the 187Os/188Os evolution of seawater
Geochim. Cosmochim. Acta
The isotopic composition of leachable osmium from river sediments
Earth Planet. Sci. Lett.
Os isotopic compositions of leachates and bulk sediments from the Bengal Fan
Earth Planet. Sci. Lett.
The significance of Himalayan rivers for silicate weathering rates: evidence from the Bhote Kosi tributary
Chem. Geol.
Weathering processes in the Ganges-Brahmaputra basin and the riverine alkalinity budget
Chem. Geol.
Negative thermal ion mass spectrometry of osmium, rhenium, and iridium
Geochim. Cosmochim. Acta
Osmium isotope ratio determinations by negative thermal ionisation mass spectrometry
Mass Spectrom. Ion Process.
δ13C of organic carbon in the Bengal Fan: source evolution and transport of C3 and C4 plant carbon to marine sediments
Geochim. Cosmochim. Acta
Osmium in marine sediments
Geochim. Cosmochim. Acta
Precise Re-Os ages of organic-rich mudrocks and the Os isotope composition of Jurassic seawater
Earth Planet. Sci. Lett.
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