Erosional History of the Himalayan and Burman Ranges During the Last Two Glacial-Interglacial Cycles J
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Reprinted from EARTH AND PLANETARY I SCIENCE LETTERS Earth and Planetary Science Letters 171 (1999) 647-660 Erosional history of the Himalayan and Burman ranges during the last two glacial-interglacial cycles J. C. Colin a,*, L. Turpin b, Bertaux ', A. Desprairies a, C. Kissel a Laboratoire de Géochimie des Roches Sédimentaires, Université Paris-Sud, 91405 Orsay Cedex, France Laboratoire des Sciences du Climat et de E 'Environnement,Laboratoire mixte CNRS-CEA, Avenue de la Terrasse, 91 198 Gif-sur-Yvette Cedex, France Centre ORSTOM, 72 route d'Aulnay, 93143 Bondy Cedex, France Received 14 July 1998; accepted 13 July 1999 EPSL ELSEVIER Earth and Planetary Science Letters 171 (1999) 647-660 1 www.elsevier.com/locate/epsl Erosional history of the Himalayan and Burman ranges during the last two glacial-interglacial cycles C.‘Colin ai*, L. Turpin b, J. Bertaux ‘, A. Desprairies a, C. Kissel Lnboratoire de Géochimie des Roches Sédimentaires, Université Paris-Siid, 91405 Orsay Cedex, France Laboratoire des Sciences du Climat et de I’Environnenient, Laboratoire mixte CNRS-CEA, Avenue de Ia Terrasse, 91 198 Gif-sur-Yvette Cedex, France Centre ORSTOM, 72 route d’Aiilnay, 93143 Bondy Cedex, France Received 14 July 1998; accepted 13 July 1999 Abstract The results of a clay mineralogy study combined with major element geochemistry, strontium, neodymium and oxygen isotopes, and 14C AMs stratigraphy are reported for deep-sea gravity cores located in the Bay of Bengal (MD77-180) and the Andaman Sea (MD77-169). &Nd(O) and s7Sr/s6Sr from Holocene and last glacial maximum (LGM) sediments of fifteen other cores have also been investigated to identify sediment sources and to estimate oceanic sedimentary transport. The data show the contribution of three sources: (1) Ganges/Brahmaputra rivêrs; (2) Irrawaddy River; and (3) sediment derived from the western part of the Indo-Burman ranges. The dispersion of the detrital material issuing from these sources has been constrained by the geographic distribution of Nd and Sr isotopic ratios. The LGM sediments are characterized by a significant increase of s7Sr/s6Sr, small changes in &Nd(O), and a general decrease of smectite/(illite + chlorite), which together imply a decrease in weathering intensity. The increased s7Sr/s6Sr are attributed to a decrease in .chemical weathering, which should release preferentially 87Srduring weathering processes. In the Andaman Sea sediments, smectite/(illite + chlorite) and kaolinitejquartz ratios combined with a chemical index of alteration (CIAk)indicate that the weathering intensity of the Irrawaddy River basin is mainly controlled by the summer monsoon rainfall intensity. The wet summer monsoons increase vegetation cover in the plains and favor soil development by the production of pedogenic clays (smectite and kaolinite). O 1999 Elsevier Science B.V. All rights reserved. Keywords: monsoons; deep-sea sedimentation; erosion; Burman ranges; Himalayas 1. Introduction summer monsoon rainfall represent an important fac- tor driving weathering and erosion of the Himalaya. The Himalaya has one of the world’s highest Sediments of the Bay of Bengal and Andaman Sea, physical and chemical erosion rates [1,2] and thus which are fed by the Ganges/Brahmaputra and Ir- constitute an interesting region to establish a re- rawaddy rivers, thus provide an exceptional record lationship between erosion and climate. Over ge- of the variability of the intensity of erosion in the Hi- ological time scales, changes in the strength of the malayan and Burman ranges. This in turn is related to paleoclimatic and paleoenvironmental changes af- *Corresponding author. Tel.: $33 1 6915 6745; Fax: +33 1 fecting SW Asia. 69 15 4882; E-mail: [email protected] Previous studies, based on cores retrieved from 0012-821X/99/$ - see front matter O 1999 Elsevier Science B.V. All rights reserved. PII: SOO 12-821X(99)00184-3 647-660 648 C. Colin et al. /Earth aiid Plaiietaiy Scieiice Letters 171 (1999) 7 80 90 3 10 30"N. 20. 10. Mfi77-169 @ , RC12-3390 MD77-1900 An aman ,Tertiary and Cretaceou,& I UBasalttraps [III] sedimentary formations Indo-Burman Ranyes Paleozoic and 0- ODP Leg 116 OQuaternary Precambrian formations 1 Paleozoic and I hi ma lava Triassic formations Fig. 1. Schematic geological map of the Indian subcontinent and location of the studied cores. The two cores investigated in detail in this study are underlined. The seventeen other cores are geographically distributed to ensure a good coverage of the Bay of Bengal and the Andaman Sea. the distal portion of the Bengal Fan (ODP Leg climatic cycles (last 280 kyr). This shorter time scale 116, Fig. l), allowed the reconstruction of the is independent of major tectonic activity and permits erosional history of the Himalayan-Tibetan com- us to establish a relationship between past changes in plex since the Early Miocene (last 20 Ma) [3-lo]. the intensity of the Indian monsoon rainfall [ 11-14] Variations in the erosion rate of the Himalaya- and the intensity of chemical and physical weather- Tibet complex over such time scales have been ing in the Himalayan and Burman ranges. However, shown to depend on several factors, including past changes in the Indian summer monsoon inten- paleoclimatic/paleoenvironmentalchanges and tec- sity have also impacted the sea surface circulation tonic activity. in the Bay of Bengal and the Andaman Sea [11,12]. This paper reports on a high-resolution study of This may affect oceanic sedimentary transport and clay mineralogy and major element geochemistry of thus modify the source, grain size, and composition two deep-sea gravity cores from the Bay of Bengal of sediment at a given site. (MD77-180) and Andaman Sea (MD77-169). These To determine the sediment sources and assess were analyzed to reconstruct the erosional history of the effects of oceanic transport in the sedimentary the Himalayan and Burman ranges over the last two record, we have also investigated Nd and Sr isotopes c I71 647-660 C. Coliii et 01. / Errrtli arid Planetary Scierice Letters (I999) 649 in several Holocene and LGM sediments distributed lene syringe with a 4 mm @ millex@ filter. Nd throughout the Bay of Bengal and Andaman Sea. was isolated by reverse-phase chromatography on Clay mineralogy, major element compositions and HDEHP-coated Teflon powder. Isotopic measure- spectral analysis of the two deep-sea cores recov- ments were performed using static multicollection ered from the Bay of Bengal and Andaman Sea on a Finnigan MAT-262. Repeated analysis of NIST permit us to establish the impact of climatic and sea SRM987 during the study gave a mean 87Sr/86Srof level changes on the weathering and erosion of the 0.710237 & 0.000007 (2 o). The mean 133Nd/144Nd Himalayan and Burman ranges. for a Johnson Matthey internal lab standard from A. Michard was 0.51 1093 f 0.000005, which corre- sponds to a value of 0.5 11845 for La Jolla. 2. Materials and methods Clay mineralogy determinations were made by standard X-ray diffraction [ 151 on the carbonate- 2.1. cores kxcitiori and sampling free, t2 pm size fraction. Samples were split in deionized water and decarbonated in 0.2 N HC1. The The deep-sea gravity cores investigated here are integrity of the clay fraction after acid leaching was plotted in Fig. 1, and their characteristics are given controlled by observations on a transmission elec- in Table 1. Two cores from the MD77 cruise were tron microscopy and X-ray diffraction patterns of studied in detail: MD77-169 and MD77-180. Core bulk samples. The t2 pm clay fraction was isolated MD77-169 was taken on the Sewell Seamount, cen- by gravity settling [ 151. X-ray diffractograms were tral Andaman Sea, in order to avoid slope deposits made on a INEL CPS 120 X-ray diffractometer from such as turbidites or slumping. The lithology is ho- 2.49" to 32.5" using a cobalt Ka, radiation. Three 1 mogeneous, dominated by terrigenous muddy clay tests were performed on the oriented mounts: (1) I and nannofossil carbonate ooze. Core MD77-180 untreated; (2) glycolated (12 h in ethylene-gycol); t was taken near the continental slope (about 100 and (3) heated at 540" for 2 h. X-ray diffraction iden- miles off the coast) (Fig. 1) and is characterized by tified: illite, ch!orite, kaolinite, smectite and complex intercalated clay and silty clay layers. These two mixed-layers. These mixed-layer clays were mainly cores were sampled at 10- to 20-cm intervals for the assigned to randomly mixed illite-smectite species mineralogical and geochemical investigations. Fif- with predominantly smectite layers. These will be teen other cores, selected to cover the Bay of Bengal referred to as 'smectites' in the text. The semiquanti- and Andaman Sea (Fig. 1 and Table l), were sam- tative composition of the clay fraction was obtained pled only at the top and the Last Glacial Maximum either by measuring the height of basal reflections (LGM) in order to establish the spatial distribution on XRD diagrams or by measurement of peak areas. of Sr and Nd isotopes during these two periods. The basal reflection heights were weighted by empir- .I ically estimated factors [15]: the smectite 17 Å peak i 2.2. Methods height is multiplied by 1.5, the kaolinite 3.6 Å peak Y by 0.7 and both the illite 10 Å peak and chlorite 3.55 Sr and Nd isotopic measurements were performed Åpeak by 1. 1 on the carbonate free fraction. Samples were de- Kaolinite, quartz, and carbonate content of the J carbonated by leaching with 20% acetic acid in an bulk fraction were determined by Fourier Transform r ultrasonic bath, then rinsed 5 times and centrifuged Infra-Red (FTIR) spectroscopy [ 161. The samples I to eliminate traces of the carbonate solution.