The effect of the Himalayan Orogeny on the Cenozoic climate (J.Standing 2014)
- 1. 1 The effect of the Himalayan Orogeny on the Cenozoic climate Introduction The Himalayan Orogeny began around 50Ma and is an excellent example of a continental collision event, it has resulted in the closing of the Neotethys and the northward movement of India, ending with the collision of India with Eurasia along three sutra zones; the Indus, Shyok, and Tsangpo, Figure 1 shows a palaeographic reconstruction of this event, whilst Figure 2 shows the path of the Indian subcontinent and its current position (S. Chatterjee et al, 2013). Figure 1: Palaeographic reconstruction showing the position Figure 2: Diagram showing the movement of India and other continents around 50Ma when India made its and its post collision position and the Himalayan initial collision with Asia, with the 3 sutras also labelled mountain range (N. Harris, 2000). (S. Chatterjee et al, 2013). As the orogeny has continued this has led to faulting and thickening of the continental crust along the collision zone creating the Himalayan mountain range which spans an arc 2500km long, the continued thrusting of the Indian plate beneath the Eurasian continent combined with the shortening of the Eurasian southern crust, has created the raised Tibetan plateau which is approximately 5000m above sea level (Molner et al, 1993, and S. Chatterjee et al 2013). During the Himalayan Orogeny, proxy data from sedimentary archives show that the Earth’s climate system experienced continuous change. At the start of the Cenozoic the Earth was a warm, ice-free, “Greenhouse” climate (J. Zachos et al, 2001), with hypothermal episodes including; the Paleocene Eocene Thermal Maximum and Mid-Eocene Climatic Optimum (M. Kuppusamy & P. Ghosh, 2012, and J. Hansen et al, 2013). After the onset of the Himalayan Orogeny temperatures began to fall and the “Icehouse” climate system prevailed with permanent polar ice caps and fluctuating continental ice sheets (J. Zachos et al, 2001), Figure 3 gives a graphical interpretation of the changing temperature over the past 65 Ma.
- 2. 2 Figure 3: Graph showing the surface temperature estimate for the past 65 Ma (J. Hansen et al, 2013) Central to the decreasing temperatures and changing climate system is the decrease at the start of the orogeny 50Ma, from 1000ppm to 170ppm in atmospheric CO2 levels (J. Hansen et al, 2013) as illustrated in Figure 4. Figure 4: Graph showing estimates CO2 levels for the Cenozoic (M. Kuppusamy & P. Ghosh, 2012). It has been proposed that the tectonic uplift of the Himalayas, the lowering of atmospheric CO2 and the subsequent change from a Greenhouse to Icehouse climate was all linked via chemical weathering (M. Raymo and W. Ruddiman, 1998), it is the aim of this review to explore this theory and also examine if other processes may have also influenced the Cenozoic climate. Effects of the Himalayan Orogeny on the regional climate The uplift of the Himalayas and the Tibetan Plateau formed a climatic divide between India and Central Asia (S. Chatterjee et al 2013). This created a barrier to atmospheric flow that generated perturbations in Northern Hemisphere circulation, which are largely responsible for the mid-latitude aridity of Eastern Asia. The Himalayas also produced a rain shadow on the leeward slope, which would have further increased the aridity of both the Tibetan Plateau and Eastern Asia. The rain shadow is a combination of a lack of precipitation; air passing over the orographic obstruction precipitates, and a drying effect; the evaporative potential of the air increases as it descends on the leeward side, this process is illustrated in Figure 5 (W. Hay, 1995). The rain shadow effect and the perturbations in atmospheric flow also affected the albedo of the region and energy balance, with wider implications for the global climate (S. Chatterjee et al 2013).
- 3. Figure 5: Diagram illustrating the rain shadow effect, precipitation occurs on the windward side and a rain shadow develops on the leeward slope (W. Hay, 1995). The continued convergence was paramount in the onset of the Asian Monsoon (23 Ma), which peaked around 10Ma (S. Chatterjee et al 2013). Seasonal changes in wind direction generate the monsoon, which is an important component of the global climate system, influencing both Africa and Asia (M. Kuppusamy & P. Ghosh, 2012). The Tibetan Plateau’s elevation and immense breadth drives a regionally intense circulation (M. Raymo and W. Ruddiman, 1998) by providing a heat source that opposes Hadley Circulation between the equator and temperate latitudes, this drives the opposite circulation characteristic necessary for the monsoon to exist, as illustrated in Figure 6 (P. Molnar et al, 1993). Figure 6: Diagram showing Classical Hadley circulation and the effect of the Tibetan Plateau on circulation over the Indian Subcontinent that generates the Indian summer monsoon (P. Molnar et al, 1993). 3
- 4. This intense rainfall is evident in regional relief changes; Tibet shows slow changes over the past 10 Ma whereas the Himalayan region has significant incisions in topography (1-3 km) (J.D. Champagnac, 2014). This increase in erosion and weathering rates had possible implications to atmospheric CO2 levels (M. Raymo and W. Ruddiman, 1998). Effects of the Himalayan Orogeny on global CO2 levels In the early Cenozoic CO2 concentrations were >1000ppm this decreased to 170ppm before Anthropogenic inputs started to raise levels again (D. Kent and G. Muttoni, 2008). Although part of the reason for the decline was related to decreasing volcanic emissions, this is insufficient to account for the massive reduction in atmospheric CO2, which was instrumental in shifting the Cenozoic climate from Greenhouse to Icehouse. (M. Kuppusamy & P. Ghosh, 2012). Precipitation on the windward slopes of the Himalayas, especially during the monsoon, generated increased weathering and erosion of the newly uplifted mountains, and exposed silicate minerals, creating reactions such as that illustrated in the Figure 7 (N. Harris, 2000). Figure 8: Equation showing the dissolution of feldspar (N. Harris, 2000). Continental weathering, especially silicate weathering, is an important regulator of atmospheric CO2 levels, drawing down CO2 and transporting carbon to marine sediments where it is lithified and placed in long term storage, as part of the feedback in the carbon cycle that is illustrated in Figure 8 (G. Hilley and S. Porder, 2008, A. Goudie and H. Viles, 2012). Figure 8: Diagram showing the linkages between weathering, tectonics, biology, geomorphology and the carbon cycle (A. Goudie & H. Viles, 2012). It is possible to identify the relationship between Himalayan uplift, weathering and the reduction of atmospheric CO2 using the proxy related to the strontium isotope record 87Sr/86Sr which is preserved in marine sediments. The seawater 87Sr/86Sr ratio reflects the balance between the input of radiogenic 4
- 5. material with a high 87Sr/86Sr that is derived from continental weathering, and non-radiogenic material low in 87Sr/86Sr derived from hydrothermal activity. Erosion of the Himalayan-Tibetan Orogeny has been linked to the increase in late-Cenozoic oceanic 87Sr/86Sr ratios, as sea-floor spreading was stable during this period hydrothermal influence is considered negligible. When cross-referenced with other proxy data such as; pollen data, that records a change circa 38.3 Ma to colder climate plant taxa, e.g. conifers, and alkenones, the only proxy for CO2, which show a dramatic post 37Ma decline in atmospheric CO2, the proxy data, displayed in Figure 9, indicates a direct link between the Himalayan Orogeny and reduced Cenozoic CO2 values (C. Garzione, 2008, and M. Raymo and W. Ruddiman, 1998). Figure 9: Cenozoic atmospheric CO2 and seawater Sr, also included is proxy data; red circles with error estimates are boron isotopes, yellow circles with isotopes are carbonate records, blue field shows alkenone records, the first appearance of conifers in Tibet is labelled as is the Eocene-Oligocene transition (EOT) 34Ma (C. Garzione, 2008) The effects of the Himalayan Orogeny on the global climate The correlation between Himalayan-Tibetan uplift, increased continental weathering rates and decreased atmospheric CO2 levels, inevitably this affected the global Cenozoic climate (P. Molnar and P. England, 1990). The manifestation of these three processes was a cooling of the climate that led to the onset of continental glaciation, beginning with Antarctic glaciation in the Oligocene, circa 34Ma (A. Goudie & H. Viles, 2012). Using proxy data from oxygen isotope records taken from deep-water sediments by the DSDP (Deep Sea Drilling Project), the cooling of the Cenozoic can clearly be observed as shown in Figure 10 (P. Molnar and P. England, 1990). 5
- 6. Figure 10: Graph showing the relationship between temperature and oxygen isotope record (J. Hansen et al, 2013) As temperatures continued to fall and glaciation spread to mountain ranges (including the Himalayas) albedo rates increased resulting in further heat loss and lowering temperatures further strengthening a positive feedback diagram highlighted in figure 11 (P. Molnar and P. England, 1990, and Kump et al, 2011). 6 Global Mean Temperature (+) Planetary Albedo Growth of continental ice sheets Figure 11: Feedback diagram showing the effects of albedo on temperature and glacial growth. The growing ice-sheets converted regions that had reduced albedo in the summer months, when snow cover had melted, to areas with high albedo rates throughout the year. Increased albedo lowered global temperatures, which encouraged the growth of continental and mountainous ice-sheets enabling them to spread to lower latitudes (Kump et al, 2011). During the Late-Cenozoic the Himalayan uplift intensified, as did the Indian Monsoon, the result was lower CO2 levels and lower temperatures, as the positive feedback, identified above, strengthened further. This led to the onset of Northern Hemisphere glaciation around 8-5 Ma, which fed into the positive feedback and helped to further influence the Cenozoic climate (K. Huntington et al, 2006). Had the positive feedback been allowed to continue without the counter balance of negative feedbacks in the form such as the subduction of pelagic carbonates, the climate would have continued to cool as the atmosphere was stripped of CO2 (M. Raymo and W. Ruddiman, 1998), this would have resulted in a Snowball Earth scenario now experienced since the Late-Proterozoic, circa 0.7 billion years ago (Kump et al, 2011). Other processes that may have influenced the Cenozoic climate Although the erosion of the Himalayas is an important factor in CO2 drawdown and Cenozoic cooling other mountain ranges would also have been subject to erosion during this period, the Andes and Alps as wells as mountain ranges in Africa and Asia all supplied large sedimentary deposits that filled local basins, figure 12 shows sedimentary rates from some of these regions and the Himalayas with all showing an increase in the mid-Cenozoic (S. Wan et al, 2009).
- 7. Figure 12: Examples of varying sedimentary rates for various locations, all of which show a distinct rise during the Mid-Cenozoic (S. Wan et al, 2009). The basaltic rocks of the Deccan Traps is another region that shows evidence of intense ancient chemical weathering during the climatically warm Early Eocene, when this region would have passed through the equatorial humid belt. Basalt consumes 5-10 times more atmospheric CO2 than granitic rocks so it is possible that significant CO2 removal and global cooling occurred prior to significant uplift of the Himalayas and Tibetan Plateau. The erosion of the orogeny would have continued the CO2 drawdown started by the Deccan Traps, which departed the equatorial humid belt during the continuing Indian-Eurasian convergence and became less effective in the removal of atmospheric CO2 (D. Kent and G. Muttoni, 2008). There is also the possibility that erosion and weathering had no part in influencing the Cenozoic climate. Studies show that only a minor drop in atmospheric CO2 accompanied increased sedimentation in the Quaternary. This example is used to hypothesise that the drop in CO2 values in the Cenozoic occurred prior to the increase in erosion rates, as shown in figure 4, and that the correlation is based on measurement errors, bias, and inadequate modelling (J. Willenburg and F.V. Blanckenburg, 2010). There are other climatic, biotic, and tectonic events that have been linked to changes in temperature, and oxygen and carbon isotope records during the Cenozoic, figure 13 identifies these events and their timeframe and shows how they match to the temperature and isotope variations (J. Zachos et al, 2001). 7
- 8. Figure 13: Diagram showing the correlation between Cenozoic temperature change, alterations in Oxygen and Carbon isotopes, climatic, tectonic and Biotic events, plus the timings of the Northern and Southern Hemisphere glaciations (J. Zachos et al, 2001). 8
- 9. Added to this there are also the Milankovich cycles to take into account, these have long since been linked to variations in continental ice-sheets, figure 14 shows the processes, their timescale and frequency over the past million years (J. Zachos et al, 2001). 9 Figure 14: Diagram showing the various Milankovich cycles (J. Zachos et al, 2001). How will the continuing Himalayan Orogeny respond to anthropogenic climate change? Rising CO2 levels due to anthropogenic burning of fossil fuels is likely to push the global temperature towards levels not seen since the Early Eocene (J. Hansen et al, 2013). A warmer climate will increase precipitation and enhance chemical weathering, as the highest mountain range on Earth, the Himalayas will inevitably play an important role in the carbon transfer process from the atmosphere, to the ocean and then into long term storage within carbonate sediments. Data compilation and modelling studies show that these weathering processes are responding to changes in the global climate (E. Beaulieu et al 2012). Current studies are working towards calculating CO2 consumption rates that the weathering of the Himalayas will generate. Rivers draining from the Himalayas and Tibetan Plateau, shown on the map in figure 15, give a range of values. Rivers from the arid Tibetan Plateau are not very radiogenic and their silicate contributions are low, in contrast the Indus and Brahmaputra, which drain from the syntaxes, have higher 87Sr/86Sr values. The Ganges, which drains the Himalayan front, is highly radiogenic and consumes twice as much CO2 than the Tibetan rivers (Y. Huh, 2014).
- 10. Figure 15: Location map showing the major rivers that drain the Himalayas and Tibetan Plateau (W. Huh, 2014) The warming climate will also lead to further retreat of the Himalayan glaciers, this will have a two fold effect, first the glacial melt waters will enhance the erosional effect and sediment transportation ability of the rivers that drain the Himalayas, secondly the glacier retreat will expose more of the region to chemical weathering, therefore enhancing CO2 drawdown (D. Burbank, et al 2012). With increased precipitation, especially with a potentially more intense monsoon, and weathering, the CO2 consumption by the Himalayan region will inevitably increase. Published CO2 drawdown rates suggest that 20% of the global CO2 drawdown currently occurs within the Himalayan Orogeny (G. Hilley and S. Porder, 2008), this supports the hypothesis that the Himalayas influenced the Cenozoic climate and suggests that it will continue to regulate against anthropogenic climate change. Conclusion The research explored indicates that the Himalayan Orogeny had a dramatic effect on the regional and global climate of the Cenozoic. In the Indian sub-continent and Southeast Asia the orogeny disrupted atmospheric flow, and influenced the albedo of the region, but the greatest influence was the initiation of the Indian Monsoon, this increased the erosion of the newly uplifted mountain range. The general consensus in the scientific community is that chemical weathering of the Himalayas played a significant role in reducing atmospheric CO2 levels, leading to the cooling trend of the Late-Cenozoic. However, it is evident that the Himalayan orogeny was one of numerous events influencing the Cenozoic climate. 10
- 11. To understand how the Himalayas truly influenced the Cenozoic climate requires further research on other geomorphic sites, this would provide comparable datasets and resolve the many uncertainties and controversies in understanding weathering – carbon cycle feedbacks (A. Goudie and H. Viles, 2012). When trying to predict the Himalayan response to anthropogenic climate change, requires using climate models, which can be problematic. Climate models interpret proxy records and have been useful in understanding Cenozoic climate change, however they are simplified representations of the real world. Climatic model results need to be interpreted with caution and cross-referenced with other models and physical experiments in order to establish accurate future predictions (D. Lunt et al 2014). Added to this it needs to be remembered that the past is not always the key to the future, climate sensitivity to atmospheric CO2 depends on various processes (Y. Godderis et al, 2012), and as shown these are in a constant state of fluctuation. The Cenozoic saw the last major climatic shift, from greenhouse to icehouse, with the Himalayan Orogeny playing a major part in that process, as shown it continues to assist in regulating the current climate and will continue to do so as long as the orogeny continues, but it should not be relied upon to counteract the anthropogenic climate change that is driven by the burning of fossil fuels. With conventional oil and gas reserves becoming depleted, humanity has a choice; move towards carbon-free energy and allow the planet to heal itself, or exploit unconventional fuels (J. Hansen et al, 2013) and hope that natural processes combined with CO2 sequestration techniques will be enough to halt the change back to the greenhouse climate that Earth last experienced 50Ma. References BEAULIEU, E., GODDÉRIS, Y., DONNADIEU, Y., LABAT, D. and ROELANDT, C., 2012. High sensitivity of the continental-weathering carbon dioxide sink to future climate change. Nature Climate Change, 2(5), pp. 346-349. BURBANK, D.W., BOOKHAGEN, B., GABET, E.J. and PUTKONEN, J., 2012. Modern climate and erosion in the Himalaya. Comptes Rendus Geoscience, 344(11), pp. 610-626. CHAMPAGNAC, J., VALLA, P.G. and HERMAN, F., 2014. Late-Cenozoic relief evolution under evolving climate: A review. Tectonophysics, 614, pp. 44-65. CHATTERJEE, S., GOSWAMI, A. and SCOTESE, C.R., 2013. The longest voyage: tectonic, magmatic, and paleoclimatic evolution of the Indian plate during its northward flight from Gondwana to Asia. Gondwana Research, 23(1), pp. 238-267. GARZIONE, C.N., 2008. Surface uplift of Tibet and Cenozoic global cooling. Geology, 36(12), pp. 1003-1004. GODDÉRIS, Y., DONNADIEU, Y., LEFEBVRE, V., LE HIR, G. and NARDIN, E., 2012. Tectonic control of continental weathering, atmospheric CO2, and climate over Phanerozoic times. Comptes Rendus Geoscience, 344(11), pp. 652-662. GOUDIE, A.S. and VILES, H.A., 2012. Weathering and the global carbon cycle: geomorphological perspectives. Earth-Science Reviews, 113(1), pp. 59-71. HANSEN, J., SATO, M., RUSSELL, G. and KHARECHA, P., 2013. Climate sensitivity, sea level and atmospheric carbon dioxide. Philosophical transactions.Series A, Mathematical, physical, and engineering sciences, 371(2001), pp. 20120294. 11
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