Evolution of Continental Environments and Chemical Weathering in the Western Himalayan Foreland Basin since 11 Ma
Abstract
The Asian monsoon is the dominant climatic phenomena in Southwest Asia and as the primary source of moisture is one of the major controls over the processes of chemical weathering, especially in the Himalayan foreland basin. The sedimentary records of the eastern Arabian Sea mostly reflect the evolving erosion and chemical weathering processes in the source mountains and foreland basin, with limited input from peninsular India. Analysis of the geochemistry of sediments from International Ocean Discovery Program (IODP) Sites U1456 and U1457 allow us to investigate how chemical weathering may relate to evolving environments and the intensity of the precipitation in the Indus catchment since 11 Ma. We employed X-ray diffraction methods to quantify clay mineral assemblages from the core samples of these two sites. kaolinite/(chlorite+illite) and illite crystallinity show a general long-term trend towards less chemical weathering from 10 Ma to 5 Ma. Meanwhile, the high-resolution hematite/goethite records were acquired from visible diffuse reflectance spectrophotometry from both Sites U1457 and U1456 show a general increase in hematite/goethite, This may represent a long-term drying of the climate and/or an increase in seasonality since 10 Ma, consistent with the long-term trend in carbon isotope values known from the Siwalik Group of the Himalayan foreland. In particular, there is an increase in the relative portion of hematite starting at 8.2 Ma with a subsequent decrease at 6.7 Ma, and a further notable increase after 5.7 Ma. Bulk sediment geochemistry allows us to calculate the Chemical Index of Alteration (CIA) as well other geochemical indices such as K/Al. Both these proxies indicate a strong decrease in chemical weathering intensity at 8.2 Ma, followed by a rapid increase in the degree of alteration after 7.8 Ma followed by a gradual decrease until 6.6 Ma. In general, drier/more seasonal conditions are associated with less chemical weathering over this critical transition. Sedimentary provenance does not drive the variations in weathering indices across most of the record. We therefore interpret the degree of chemical weathering in the floodplains of the Indus River as a record of changing monsoon precipitation (amount and seasonality) within the Indus catchment.
- Publication:
-
AGU Fall Meeting Abstracts
- Pub Date:
- December 2017
- Bibcode:
- 2017AGUFMOS54B..04Z
- Keywords:
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- 1051 Sedimentary geochemistry;
- GEOCHEMISTRY;
- 3022 Marine sediments: processes and transport;
- MARINE GEOLOGY AND GEOPHYSICS;
- 3036 Ocean drilling;
- MARINE GEOLOGY AND GEOPHYSICS;
- 8177 Tectonics and climatic interactions;
- TECTONOPHYSICS