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Constraining the controls on the incongruent release of hafnium: implications for long-term chemical weathering rates and long-term CO2

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Chemical weathering and atmospheric CO2

Marine scientists investigated the processes that control the release of hafnium (Hf) isotopes from the continents. The results will provide insights into the nature and rate of past weathering of the continents and its environmental impact.

Climate Change and Environment

Chemical weathering of silicate rocks provides a sink for atmospheric carbon dioxide (CO2) and helps to regulate Earth's climate over geological timescales. Increased temperature and a more active hydrological cycle during warm periods result in increased levels of chemical weathering and effective drawdown of CO2 from the atmosphere. A number of studies have indicated that precipitation and temperature are important factors for affecting chemical weathering rates. However, there is also evidence that it is the intensity of physical weathering that exerts the greatest influence. During the Quaternary Period the retreat of continental ice sheets may have resulted in chemical weathering rates that are higher than in an ice-free world. The oscillation between glacial and interglacial periods during the Quaternary may therefore have acted to maintain low atmospheric CO2. This represents a stable mode of operation of the climate system at low temperatures. The aim of the EU-funded HAFNIUMWEATHERING project was to gain an understanding of the release of the element Hf during chemical weathering. This enabled scientists to interpret the presence of Hf in marine sediments, which offer records of the composition of seawater in the past. These records can provide valuable information regarding past rates of chemical weathering and CO2 drawdown and of long-term climate change. Researchers studied three different hypotheses for explaining the incongruent nature of Hf weathering. The traditional explanation is that it is related to resistance by zircons (a common mineral that occurs as a small crystal) to weathering. However, it has been suggested that this is reduced during times of intense chemical weathering. Therefore, Hf isotopes can be used to reconstruct the intensity of physical weathering in the past. More recently, it has been posited that the preferential release of Hf by Lutetium and Hf silicate controls the incongruence of Hf weathering. This infers rapid evolution of the released Hf isotopes with soil age as they are easily weathered and become depleted. Finally, there is evidence that carbonate weathering is also important in determining the level of Hf in the ocean. Results from the HAFNIUMWEATHERING project will allow a more robust interpretation of seawater Hf isotope records. This will provide a better understanding of the nature and rate of past weathering and its effect on atmospheric CO2.

Keywords

Chemical weathering, hafnium, atmospheric carbon dioxide, physical weathering, Quaternary, marine sediments, climate change, zircons, carbonate weathering, seawater, Hf isotope

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