Quantifying basalt denudation rates and their contribution to the global carbon dioxide budget using cosmogenic Mn-53
Final Report Abstract
Chemical weathering of silicate rocks at the Earth’s surface consumes atmospheric carbon dioxide and thus has the potential to influence global climate. In particular, the weathering of basalt rocks could be an important sink for carbon dioxide. Our understanding of what controls the chemical weathering rates of rocks has been limited by our inability to measure rates over the timescales that are relevant to land-forming processes. During the last two decades this situation has improved due to the development of terrestrial cosmogenic nuclide techniques, able to quantify denudation and thus extract rates of chemical weathering over millennial timescales and longer. Basaltic terrains have received little attention in this work, however, due in part to a lack of suitable target elements for cosmogenic nuclide measurements. Recent progress has shown that the cosmogenic nuclide Mn-53 can be measured in iron oxides by accelerator mass spectrometry (AMS), which means that it has the potential to be applied to measure denudation rates of basalts. This project attempted to develop the chemistry required to prepare iron oxide samples from basalts in the form required for AMS measurements. The preparation chemistry had to be able to deal with sample specifics that pertain to erosion rate samples, including the removal of secondary iron oxides from the primary iron oxides, larger masses and thus larger cation loads during separation and purification of the Mn, plus separation of the Mn from isobaric Cr. Iterative protocols were developed that dealt with these issues. In particular, the use of techniques developed for the soil sciences proved useful in removing secondary iron oxides and a novel chelating resin approach was able to improve the separation of Mn from Cr with less loss of Mn. Low Mn-55 yields in the samples are desirable to maximise the likelihood of successful Mn-53 measurements by AMS. However, the Mn-55 yields of magnetite (an iron oxide) may have a lower limit that restricts the use of the technique in rapidly denuding settings.
Publications
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The first (53Mn/55Mn) isotopic ratio measurements at the Cologne FN-Tandem Accelerator. Nuclear Instruments and Methods in Physics Research Section B: Beam Interactions with Materials and Atoms, 437, 87-92.
Schiffer, M.; Spanier, R.; Müller-Gatermann, C.; Herb, S.; Feuerstein, C.; Hackenberg, G.; Marock, M.; Heinze, S.; Stolz, A.; Dewald, A. & Binnie, S.
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Method developments for accelerator mass spectrometry at CologneAMS, 53Mn/3He burial dating and ultra-small 14CO2 samples. Global and Planetary Change, 184, 103053.
Schiffer, Markus; Stolz, Alexander; López, Damián Alejandro; Spanier, Richard; Herb, Susan; Müller-Gatermann, Claus; Heinze, Stefan; Binnie, Steven; Melchert, Jan; Kivel, Niko; Schumann, Dorothea; Rethemeyer, Janet; Dunai, Tibor & Dewald, Alfred
