T32B-06
Climatic variation along strike in the Himalayas since Mid-Miocene

Wednesday, 16 December 2015: 11:35
306 (Moscone South)
Natalie Vögeli1, Peter Van Der Beek2, Yani Najman3, Pascale Huyghe1 and Peter Wynn4, (1)University Joseph Fourier Grenoble, Grenboble, France, (2)University Joseph Fourier Grenoble, IsTerre, Grenboble, France, (3)University of Lancaster, Lancaster Environment Centre (LEC), Lancaster, United Kingdom, (4)Lancaster University, Lancaster, United Kingdom
Abstract:
The young Himalayan orogen is the perfect laboratory to study the interactions between tectonics, erosion and climate. The major force driving the evolution of this mountain belt is the India-Asia convergence. Whilst the Himalayas has a major influence on global and regional climate, it is suggested that the monsoonal climate plays a major role in the erosion and relief pattern. Understanding the past variations in monsoonal strength along strike is crucial to understand the role of climate in the evolution of the mountain belt.

The Neogene sedimentary foreland basin of the Himalaya contains a record of tectonics and paleoclimate since Miocene times, within the so called molassic Siwalik Group. Therefore several sedimentary sections within the foreland basin along strike in the Himalayan range have previously been dated and studied with respect to determining hinterland exhumation rates, provenance and paleoclimatology. Lateral variations in hinterland exhumation rates have been observed but climate change in the past, especially the strengthening of the Asian summer monsoon along strike of the range, is still debated.

In order to track vegetation as a marker of monsoon intensity and seasonality, we analyzed δ13C on soil carbonate and associated δ13C on bulk organic carbon from previously dated Siwalik Group sedimentary sections in the West and the East of the Himalayan foreland basin. Sedimentary records span from 20-1 Myr in the West and 13-1 Myr in the East.

The presence of soil carbonate in the West, but its absence in the East is a first indication of lateral climatic variation. δ13C on soil carbonate shows a shift from around -10 ‰ to -2 ‰ at 6 Ma in the West. This is confirmed by δ13C analyses on bulk organic carbon which shows a shift from around -23 ‰ to -19 ‰ at the same time. Such a shift in isotopic value is likely to be associated with a change in vegetation type from C3 to C4. By contrast δ13C on bulk organic carbon remains at around -23 ‰ in the East. Whether the lateral difference in change of vegetation is caused by relief building or climate remains to be discussed.

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