Marine sedimentary records of chemical weathering evolution in the western Himalaya since 17 Ma

Abstract

The Indus Fan derives sediment from the western Himalaya and Karakoram. Sediment from International Ocean Discovery Program drill sites in the eastern part of the fan coupled with data from an industrial well near the river mouth allow the weathering history of the region since ca. 16 Ma to be reconstructed. Clay minerals, bulk sediment geochemistry, and magnetic susceptibility were used to constrain degrees of chemical alteration. Diffuse reflectance spectroscopy was used to measure the abundance of moisture-sensitive minerals hematite and goethite. Indus Fan sediment is more weathered than Bengal Fan material, probably reflecting slow transport, despite the drier climate, which slows chemical weathering rates. Some chemical weathering proxies, such as K/Si or kaolinite/(illite + chlorite), show no temporal evolution, but illite crystallinity and the chemical index of alteration do have statistically measurable decreases over long time periods. Using these proxies, we suggest that sediment alteration was moderate and then increased from 13 to 11 Ma, remained high until 9 Ma, and then reduced from that time until 6 Ma in the context of reduced physical erosion during a time of increasing aridity as tracked by hematite/goethite values. The poorly defined reducing trend in weathering intensity is not clearly linked to global cooling and at least partly reflects regional climate change. Since 6 Ma, weathering has been weak but variable since a final reduction in alteration state after 3.5 Ma that correlates with the onset of Northern Hemispheric glaciation. Reduced or stable chemical weathering at a time of falling sedimentation rates is not consistent with models for Cenozoic global climate change that invoke greater Himalayan weathering fluxes drawing down atmospheric CO2 but are in accord with the idea of greater surface reactivity to weathering.

Himalayan‐Tibetan Erosion Is Not the Cause of Neogene Global Cooling

“…(2007), and Indus from Zhou et al. (2021). Sediment flux data for the Indus and Pearl rivers are from Clift (2006), with flux data for the Mekong from Wu et al.…”

“…Records of chemical weathering in the Arabian Sea are noisier than those from SE Asia (Zhou et al., 2021), reflecting the greater abundance of less weathered sandier sediment transferred to the deep sea rather than the more uniform hemipelagic deposits in the South China Sea (Figure 2f). Provenance studies indicate that sediment recovered at IODP Sites U1456 and U1457 are dominantly sourced from the Indus River and not from peninsular India (Clift et al., 2019).…”

Late Miocene unroofing of the Inner Lesser Himalaya recorded in the NW Himalaya foreland basin

“…(c) Carbon isotopes from palaeosols constrain vegetation and are from NW India (Vögeli, Najman, et al, 2017) and the Potwar Plateau, Pakistan (Quade et al, 1989). (d) Hematite data from the Arabian Sea are from Zhou et al (2021), and (e) Ar‐Ar muscovite data are from Najman et al (2009).…”

“…Because chemical weathering rates are generally considered to slow as moisture reduces and temperatures fall (Filippelli, 1997; West et al, 2005), weakening of the monsoon might be expected to cause less chemical weathering and slower erosion, although slower sediment transport would have the opposite effect. The same submarine fan sediments also show increasing amounts of hematite after 10 Ma (Zhou et al, 2021), which is also suggestive of drying environments, or at least increasing seasonality characterized by a prolonged dry season (Schwertmann, 1971).…”

“…In the Sub‐Himalayas of NW India and Pakistan, a Cenozoic marine to continental foreland basin sequence is exposed, which comprises sedimentary rocks shed from the orogen (Badgley & Tauxe, 1990; Johnson et al, 1985; Parkash et al, 1980; Ravikant et al, 2011; Sorkhabi & Arita, 1997). These foreland sediments represent an invaluable archive of the early development of the mountain belt (Burbank et al, 1996; Meigs et al, 1995; Najman, 2006) spanning important climatic and environmental transitions, especially around 7–8 Ma when the climate dried, oceanic upwelling increased and vegetation in the foreland shifted from being C3 to C4 dominated (Clift et al, 2020; Kroon et al, 1991; Quade et al, 1989; Zhou et al, 2021), as well as more recently identified older changes in wind and oceanography in the Arabian Sea starting around 11–13 Ma (Bialik et al, 2020; Gupta et al, 2015).…”