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

Final Report Summary - HAFNIUMWEATHERING (Constraining the controls on the incongruent release of hafnium: implications for long-term chemical weathering rates and long-term CO2)

1.1 Introduction
Chemical weathering of silicate rocks is a sink of atmospheric CO2 and is involved in regulating Earth’s climate on long geological time scales. In a simple model of a climate stabilizing feedback between warming and weathering, increased temperatures and an intensified hydrological cycle during warm periods will lead to elevated chemical weathering rates and effective CO2 drawdown [1]. Although, a variety of studies have shown that precipitation and temperature are important parameters affecting chemical weathering rates, there is also evidence that the intensity of physical weathering possibly exerts the largest control [2]. Intense physical weathering due to glacial activity or pronounced topography creates fresh rock substrate, which is weathered at much faster rates than long exposed soils. A large impact of physical on chemical weathering rates potentially implies that during the Quaternary the waxing and vanishing of continental ice sheets may actually have led to chemical weathering rates which are, in spite of the cold temperatures, higher than in an ice free world. The oscillations between glacial and interglacial periods during the Quaternary may thus act to maintain low atmospheric CO2, representing a stable mode of operation of the climate system at low temperatures [3].
An unambiguous characterization of physical weathering rates in the past, which could help to clarify their impact on climate, is difficult. The evolution of the seawater isotopic composition of radiogenic isotopes, such as lead and strontium, has previously been used and has provided some evidence for a link between physical weathering rates and climate [4,5]. In this project, we have characterized the behavior of hafnium (Hf) isotopes during weathering. Hafnium isotopes have been proposed to give at least a qualitative indication for physical weathering rates in the past [6,7]. The hypothesis is that physical grinding of rocks will lead to a breakdown of zircons, which in turn will increase the release of unradiogenic Hf isotopes into the hydrosphere. Accordingly, the released Hf should be isotopically similar to the weathered parent material and the weathering should be relatively congruent in Hf (see section 1.2). We have tested this hypothesis through a comprehensive study of the riverine Hf isotope composition in a Greenland river, just as it emerges from underneath the ice-sheet, in relation to the rocks and sediments from the catchment. A second hypothesis is that the Hf isotope composition released from soils could be a function of soil age, and hence weathering rates, as is observed for lead [4,8]. We have evaluated this hypothesis through a study of a soil chronosequence from Glen Feshie (Scotland), which documents the effect of Hf release through weathering on the Hf isotope composition of the residual soil. The findings of both studies and their implications are discussed below (section 1.3 1.4).

1.2 Hafnium and neodymium isotopes
Hafnium and neodymium (Nd) isotope compositions are positively correlated in rocks reflecting similar geochemical properties of the corresponding isotope systems (lutetium (Lu)-Hf and samarium (Sm)-Nd, respectively). The resulting linear relationship is referred to as the terrestrial array [9]. Seawater isotope compositions are, however, shifted towards more radiogenic Hf for a given Nd isotope composition relative to the terrestrial array. The corresponding relationship in seawater is referred to as the “seawater array” and is thought to result primarily from continental weathering processes [10,11]. Variable Lu/Hf ratios in rock forming minerals lead to different Hf isotope compositions. Preferential weathering of radiogenic minerals causes incongruent weathering effects for Hf [7, 10, 11], which contrast with the congruent weathering observed for Nd isotopes. The objective of this project was to constrain processes which affect the congruency of Hf weathering. Such an understanding represents the basis for the application of Hf isotopes as a tracer of weathering processes. For convenience, Hf and Nd isotope compositions (176Hf/177Hf and 143Nd/144Nd) are reported in epsilon notation (ε, εNd) as relative deviations from the bulk earth multiplied by 10,000.

1.3 Leverett River - Greenland
Due to the old age of the weathered crust (1.8 Ma) the Leverett River displays highly unradiogenic Nd isotope compositions ranging between εNd = -38 and -43. In spite the glacial grinding of the rocks in the catchment, the weathering of Hf is highly incongruent, yielding dissolved Hf isotope compositions between εHf = -18 and -9. These dissolved Hf isotope compositions contrast with the unradiogenic Hf isotope compositions of rocks and sediments in the catchment, which range between εHf = -60 and -47. Hence, the glacial grinding of the parent material does not seem to result in an effective release of Hf from unradiogenic zircons and the riverine isotope composition essentially plots on the seawater array.
The simplest interpretation of these observations would thus be that Hf isotopes do not respond to glacial activity as suggested previously [6,7]. This interpretation is complicated by the occurrence of some mafic lithologies in the catchment, which yield bulk Hf isotope compositions of up to +27. Minerals stemming from such lithologies are found as traces in most sediments collected from the catchment. The weathering of these minerals could potentially dominate the Hf budget of the river, and yet be relatively insignificant for the Nd budget. Since the Hf budget of felsic rocks is dominated by weathering-resistant zircons [e.g. 11], weathering of mafic minerals could possibly dominate the riverine Hf budget. The final interpretation of the study thus awaits the measurements of Hf isotopes in mineral separates of the different catchment rocks, which will complement the measurements of river waters, river suspension, catchment sediments and bulk rocks obtained so far.

1.4 Glen Feshie – soil chronosequence
The study explores the Hf isotope evolution of soils, which span a range of ages between 0.1 and 13 ka. Two digestions - pressurized bomb and hotplate - were performed on each sample to constrain the significance of zircons for the Hf isotope budget of the soils. Bomb digests show a relatively small range between εHf = -22.1 and -19.7. Hotplate digests, which exclude most Hf from zircons, are more radiogenic and also show a larger isotopic range (εHf = -20.1 to -16.0). The isotopic compositions of both digestion procedures are well correlated, which suggests that zircons attenuate the variability that is imparted by weathering of the zircon-free portion, and seem largely inert on the time scales of the chronosequence. Although all soil profiles consistently show a depletion of radiogenic Hf in their upper horizons, documenting the removal of radiogenic Hf, the evolution of the soils with time is not very systematic.
In summary, the data shows that Hf removed from soils is more radiogenic than the bulk, supporting the concept that continental weathering processes generate the seawater array. This preferential removal of radiogenic Hf mainly relates to the zircon-free portion of the soil, in which weathering imparts a variability of 4 εHf. The study will be completed by new leaching experiments, which were not successful so far, to possibly document systematic temporal variability in the Hf isotope composition of the more readily weathering soil fraction.

1.5 Seawater Hf and Nd isotope composition in the Pacific sector of the Southern Ocean
In a side project, we have investigated the potential exchange and/or release of Hf and Nd from the continental slope and shelf to the water column in the Southern Ocean. The data obtained during the study provides clear evidence for Hf and Nd release at the interface between oceans and continental margins. For Hf, this has so far not been documented unambiguously. The Hf released, however, has a relatively small effect on the seawater Hf isotope compositions, suggesting that sedimentary Hf fluxes are isotopically similar to seawater. Although, the significance for the Hf seawater budget of Hf fluxes from ocean margins is not constrained to date, the new finding could imply that seawater Hf isotope compositions are not governed by continental weathering fluxes only.

References:
[1] Walker et al. (1981) J. Geophys. Res. 86C. [2] West et al. (2005) Earth Planet. Sci. Lett. 235. [3] Vance et al. (2009) Nature 458. [4] Foster and Vance (2006) Nature 444. [5] Raymo and Ruddiman (1992) Nature 359. [6] Piotrowski et al. (2000) Earth Planet. Sci. Lett. 181. [7] van de Flierdt, T. et al. (2002) Earth Planet. Sci. Lett. [8] Harlavan et al. (1998) Geochim. Cosmochim. Acta 62. [9] Vervoort et al. (2011) Geochim. Cosmochim. Acta 75. [10] Bayon et al. (2006) Geology 34. [11] Rickli et al. (2013) Geochim. Cosmochim. Acta 101.