Low-temperature oxidation of magnetite in rock weathering - a humidity proxy?
Geophysics
Final Report Abstract
Previous work indicated that low-temperature oxidation (LTO) of magnetite is a potential magnetic proxy of paleo-humidity. To analyze the LTO effect, we performed a systematic study of magnetite alteration along a weathering gradient of fresh rock (FR) and different grain size fractions of weathered pebbles on top of it. Samples were collected at 9 sites of the Deccan basalts in India across mean annual precipitation (MAP) of ~500-4000 mm, and 3 sites of the Emeishan basalts in SW-China to compare alteration effects at same MAP (~1000 mm) but ~12-15 °C different mean annual temperature (MAT). The 12 sites were pre-selected from 59 sites, based on uniform rock magnetic properties of FR, with magnetite as the exclusive ferrimagnetic mineral. Results revealed LTO effects expressed by magnetic grain size fining and thermomagnetic behavior. However, statistically analyzed bulk magnetic properties showed no consistent trend with MAP. On the other hand, we clearly revealed an effect of magnetite alteration with humidity in a laboratory experiment, athough the degree of alteration was found to be non-linear with humdity. In this controlled experiment we used a well-defined magnetite sample. In contrast, the original FR samples already contain non-stoichiometric magnetite with different grain size and structure, and obviously the rockspecific variability outweighs the humidity-dependent magnetite alteration. Despite this variability, the Emeishan results showed a systematic alteration effect, including LTO and magnetite-to-hematite transformation, with surprisingly stronger alteration than in Deccan basalts at sites with similar MAP and much higher MAT. The lower chemical reaction speed at lower MAT may be counterbalanced by freezing and thawing cycles in winters leading to formation of micro-/nano-cracks, opening pathways for humidity and oxygen into the rock interior and into magnetite-bearing grains. The overall results demonstrate that humidity-dependent magnetite alteration during rock weathering is hidden behind the magnetic variability of fresh rocks, and can be ignored as a source of magnetic properties in the paleoclimate record of sediment sinks, at least for basaltic rocks in the catchment. In contrast, a temperature-dependent influence is possible. With no basalt in the catchment, such as in our second field study in the relatively small Heqing Basin (SE Tibetan Plateau), there may be a higher possibility that rock weathering induced magnetite alteration (in Heqing Basin magnetite is released from limestones) is imprinted in the magnetic properties of sediments in a sink (such as lakes). However, our results from Heqing Basin suggest that dicriminating magnetic properties from bedrock-derived detrital magnetite and magnetite in soil is problematic, because the typical superparamagnetic magnetite from soil can loose its frequency dependence by disintegration of particle aggregates.
Publications
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2020: Humidity related magnetite alteration in an experimental setup. Geophysical Journal International, 224(1): 69–85
Zhang Q., Appel E., Stanjek H., Byrne J.M., Berthold C., Sorwat J., Rösler W., Seemann T.
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2020: Nano-magnetite aggregates in red soil on low magnetic bedrock, their change during source-sink transfer and implications for paleoclimate studies. Journal of Geophysical Research Solid Earth, 125(10): e2020JB020588
Zhang Q., Appel E., Hu S.Y., Pennington R.S. Meyer J, Neumann U., Burchard M., Allstädt F., Wang L.S., Koutsodendris A.
