HAFNIUMWEATHERING

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

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 Obiettivo del progetto (Objective)

'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.'

Introduzione (Teaser)

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.

Descrizione progetto (Article)

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.