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


An understanding of the controls on incongruent release of hafnium (Hf) during chemical weathering provides the key for the interpretation of past Hf seawater isotopic compositions, which are recorded in authigenic marine sediments. These records, in turn, integrate the weathering signal from the continents, containing valuable information about rates of chemical weathering in the past, with links to atmospheric CO2 drawdown and long-term climate change. To date, three different hypotheses have been proposed to explain the incongruent weathering of Hf. Traditionally, it has been related to the resistance of zircons to weathering. Their retention of unradiogenic Hf, however, has been suggested to be reduced during times of intense physical weathering, indicating that Hf isotopes allow the reconstruction of the intensity of physical weathering in the past. Recently, it was suggested that the preferential release of radiogenic Hf by high lutetium/Hf silicate accessory phases controls the incongruency of Hf weathering. This implies fast temporal evolution of the released Hf isotopic compositions with soil age as these phases are easily weathered and become depleted. Lastly, there is evidence that carbonate weathering is also significant for the oceanic Hf budget. The Hf isotopic evolution of seawater in the past could thus reflect the changes in the relative weathering contributions of unradiogenic silicates and radiogenic carbonates. The research seeks to obtain a detailed understanding of the processes that control the release of Hf isotopes from the continents by studying soil chronosequences and stream waters that drain variable lithologies under variable climatic conditions. The new observations will allow a more robust interpretation of seawater Hf isotope records, so that the information they contain on the character and rate of past weathering and its inferred effect on atmospheric CO2 can be better exploited.

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