Climate Change
How mountain building and tropical shelves are connected to climate change
Weathering on Orbital Timescales
Chemical weathering of silicate minerals is known to be in effective mechanism for removing carbon dioxide from the atmosphere and thus influencing global climate, because of the role that this molecule plays as a greenhouse gas. The Sunda Shelf of SE Asia is the worlds largest continental shelf which was exposed during the last ice age (~20 ka) when sea levels were more than 100 m lower. At this time large volumes of sediment were exposed to the atmosphere and available for chemical weathering. Ground water flowing through this exposed shelf region would have moreover altered sediment buried in the subsurface. The resulting consumption of CO2 would have reduced atmospheric levels and intensified the glaciation that caused the sealevel fall in the first place. Conversely when the Earth warmed, and the shelf is flooded, this supply of chemical weathering was cut off and wetlands, representing one of the biggest natural sources of greenhouse gas methane on Earth was established around the Gulf of Thailand. I plan a scientific drilling expedition in this region to test the hypothesis that the Sunda Shelf amplifies global climate variation and represents an important feedback on the orbitally modulated climate cycles.
Weathering on Million Year Timescales
The idea that mountain building, especially in Asia, might be important in increasing erosion, and in turn, increasing the amount of chemical weathering of silicate minerals is not a new one. However, volumetric estimates of chemical weathering fluxes, based on new scientific ocean drilling, and associated seismic surveys of the large sedimentary basins around Asia, now indicate that chemical weathering related to the uplift of the Himalayas and Tibetan plateau has actually reduced since the middle Miocene, the opposite of what would be predicted if this mountain building was causing the long-term cooling since that time (Clift and Jonell, 2021).
A recent compilation now indicates that the burial of organic carbon in the large submarine fans of the Indian Ocean is also not important in the sequestration of carbon from the atmosphere, contrary to previous work. Instead, I now focus on assessing whether the impact of mountain building in tropical Southeast Asia has been the critical control. It has been argued that the composition of the rocks in this region dominated by silica-poor mafic igneous rocks makes them more effective at consuming CO2 than average upper continental rocks, such as found in the Himalayas. My Ph.D. student Yifan Du and I, together with colleagues at the University of Bremen and the University of Oldenburg in Germany, have analysed sediment from the Gulf of Papua and show an increase in chemical weathering, especially during the last 6 million years. Since New Guinea has only emerged as an island starting around 12 Ma chemical weathering of this edifice cannot have affected global climate prior to that time, but may be important in intensifying the northern hemisphere glaciation. Our estimates indicate that New Guinea may be responsible for as much chemical weathering flux as all the major rivers draining the Himalayas and Tibet combined.
REFERENCES
Clift, P.D., Jonell, T.N., 2021. Himalayan-Tibetan Erosion is not the Cause of Neogene Global Cooling. Geophysical Research Letters, 48(8): e2020GL087742. doi:10.1029/2020GL087742.