Collaborative Research: Factors Controlling Chemical Weathering in Regions of Very High Physical Weathering Rates

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0309564DibbOver the past decade, a debate has taken place concerning the ultimate controls on chemical weathering at Earth's surface. Some have argued that the rate of physical weathering, not climate, is the major control on chemical weathering. Physical weathering is highest in areas of rapid tectonic uplift. Rapidly uplifting tectonic regimes with high frequency rainfall events are the dominant feature of many high-standing oceanic islands in the SW Pacific/Australasian region. High-standing islands (HSIs) there produce at least 33% of the sediment entering the marine environment annually. Because of this, HSIs have some of the highest physical weathering rates known on Earth. Research in our previous grant (EAR 0096285) determined chemical weathering rates in some New Zealand watersheds. Comparison of newly determined chemical weathering rates to the previously determined physical weathering rates showed that, although the ratio of chemical to physical weathering is low, the absolute rates of chemical weathering are some of the highest ever observed. Preliminary observations from Taiwan indicate that chemical weathering rates there are also very high. The current project is a return to New Zealand and Taiwan to collect water, suspended sediment and soil and sediment samples in order to conduct the following work: 1. Use a suite of radionuclides (7Be, 137Cs, 210Pb) to determine residence times of sediments in the soils and floodplains of two previously investigated watersheds in New Zealand; 2. Use a full suite of major, minor and trace element analyses to evaluate the physical and chemical weathering rates in New Zealand and Taiwan watersheds whose primary lithology is volcanic rocks; 3. Use data from activities 1 and 2 and the results of our previous research to develop a quantitative framework to evaluate the relationship between physical and chemical weathering rates, especially in these regions of very high physical weathering. At the locations where we collect soil and streambed sediment profiles for 7Be, 137Cs and 210Pb, we shall collect additional samples for future analysis of uranium and thorium series nuclides. These samples will be archived until time and funds permit, we shall analyze these samples using the ICP-MS at OSU's MARC analytical center. The data will be used for calculation of watershed residence times. We plan these additional activities in response to the reviewer comment that the original choice of nuclides had half-lives too short to be useful in calculating the likely watershed residence times. Using the uranium and thorium data, we can use the approaches of Plater et al. (1994), Vigier et al. (2001), Moreira-Nordemann (1980) and Marques et al. (2003) to calculate erosion rates and timescales, and watershed residence times.Broader impacts of the work will include training provided to graduate students and the building of international partnerships with New Zealand and Taiwanese scientists. The PIs plan to establish a website where all the data from the proposed research and from EAR 0096285 will be placed so that it will be accessible by the broad scientific community. This research has important societal relevance in contributing to the overall understanding of CO2 concentration in the atmosphere. Because the chemical weathering of silicate minerals on the surface of Earth is thought to be the major control on the atmospheric CO2 concentration, this research has important implications for understanding the mechanisms of greenhouse gas regulation.