Chemical Weathering in New Guinea since the Mid Miocene

Chemical weathering of silicate rocks is a well recognized method by which carbon dioxide is removed from the atmosphere and fixed as calcium carbonates in the sedimentary record. For many years the long term cooling of the Earth during the Cenozoic has been linked to uplift, erosion and weathering of the Himalayas and Tibetan Plateau, however following scientific ocean drilling of the submarine fans in the Asian marginal seas it now seems that this region could not be responsible for cooling, at least during the Neogene. Although other factors such as burial of organic carbon and the rates of degassing during seafloor spreading may also be important, erosion and weathering of other regions may also be important in controlling global CO2 concentrations. In this study we focus on the role of New Guinea, the large (>2500 km long) orogen formed as Australia collided with Indonesia since the Mid Miocene. New Guinea comprises slices of arc and ophiolite rocks that are susceptible to weathering, and is located in the tropics where warm, wet conditions favor rapid weathering. Rainfall exceeds >4 m annually in the island center. Analyses of sediment from Deep Sea Drilling Project Sites 210 and 287 in the Gulf of Papua now allow the weathering and erosion history of the island to be reconstructed. A trend to more continental erosion since 15 Ma reflects uplift and erosion of tectonics slices of the Australian plate. At the same time chemical weathering shows increasing intensity, especially since 5 Ma, as proxied by major element ratios (K/Rb, K/Al) and clay minerals. Greater proportions of kaolinite point to more tropical weathering since the Mid Miocene. Trends to more weathering contrast with Himalayan records that show the reverse, and suggest that New Guinea may be an important component in controlling global climate in the past 15 Ma.