Loess and soil magnetic properties are easily correlated to climate and environmental variations, but our quantitative knowledge of the exact climatic and environmental parameters controlling this correlation is limited. Controlled pedogenesis experiments, through our interdisciplinary project, will allow us to define the appropriate transfer functions.
The global hypotheses that we want to test are: (1) the intensity of specific magnetic parameters in modern soils are in general higher than those of the parent material (due to increased concentrations in magnetite, Fe3O4, and maghemite, Fe2O3), and (2) these properties provide an empirical measure of specific climatic and environmental parameters, in particular precipitations. These hypotheses are reasonable because based on numerous publications since the 80’s. The novelty of our project resides in the fact that we wish to establish formal and quantitative links between (a) mineral physics and chemistry, biochemistry and (b) magnetic properties of modern sols from different regions of the world. We need to study the minerals that are present in these environments, and synthesize iron oxides and oxyhydroxides. In particular, we shall investigate the possibility of the presence of a biogenic magnetite formed by iron reductive bacteria from iron oxyhydroxides. We will focus on iron oxides of a few nanometers, because (1) chemical alteration starts when minerals are nano-sized, and (2) one of these minerals, ferrihydrite, exists only as nanophase.
The scientific objectives of our project require that we emphasis the techniques used for (1) analysis of natural samples of modern soils developed over loess and (2)synthesis and analysis of iron oxides nanoparticles. We propose perform oxidation-reduction experiments on synthetic and natural samples, with controls on environment, mineral surfaces, and type of reaction. Our collaborative team has access to all the necessary equipment for the characterization of initial and final products, as well as the measurement of experiments characteristics (i.e., charge transfer, reaction kinetics, etc…). Synthesis of iron oxides and oxyhydroxides will be achieved using protocols used at the University of Minnesota and at IMPMC. Natural samples will be taken from various sites in the North America and Europe. For the reduction experiments, several methods will be used. In particular, we will compare results obtained from biotic and abiotic experiments. The techniques that we will use to characterize the samples are XRD, HRTEM, EELS, EXAFS, XMCD, Mossbauer spectroscopy, and low temperature magnetometry.
During the first half of the project, we conducted a study of the magnetic properties of ferrihydrite, an important mineral in Earth sciences, biology, and technology. Its crystallographic structure remains a matter of debate, in particular the hypothetical presence or absence of tetrahedrally coordinated iron (III). Our new study present the first XMCD measurements acquired on a synthetic 6-line Fh sample, at the K and L2,3 edges. These results demonstrate the presence of 28% iron (III) in tetrahedral sites in the mineral structure, with antiferromagnetic coupling between octahedral and tetrahedral sites. We also made progress in the study of the magnetic properties of lepidocrocite. This iron oxyhydroxide is considered as antiferromagnetic with a weak ferromagnetic moment. Mossbauer spectra show a blocking temperature of 40K, while low-temperature magnetic measurements show a blocking temperature of 50K, which is not compatible. In theory, the Mossbauer unblocking temperature should be higher. We propose that the magnetic properties of lepidocrocite are dominated by ferrimagnetic impurities. We also achieved substantial progress in reduction/oxidation experiments on lepidocrocite and started the alteration of other minerals.
The main scientific objective of our project is to establish a quantitative link between (1) magnetic studies conducted on natural samples of modern soils formed over loess, their geochemical signatures, etc., and (2) the chemistry and physics of nanoparticles of synthetic iron oxides responsible for the field observations. This objective leads us to treat three themes: (1) develop our basic knowledge of the magnetic properties of these iron oxides, (2) distinguish the various processes biotic and abiotic present in soils and favoring the presence of iron oxides, and (3) from the variations of the magnetic properties during laboratory experiments, deduct which are the dominant variables in the formation process of the iron oxides observed in nature. A long term scientific prospective of this project is to quantify the various climatic and environmental variables based on the measurement of the magnetic properties of loess and paleosol sequences.
Guyodo,Y., et al. (2011), Low-Temperature Magnetic Properties of Environmentally Relevant Iron Oxyhydroxides and Their Alteration Products. Eos Trans. AGU, 92, Fall Meeting Suppl.,GP33A-1107.
Guyodo, Y., et al. (2012), X-ray magnetic circular dichroïsm provides strong evidence for tetrahedral iron in ferrihydrite, Geochem. Geophys. Geosyst., 13, Q06Z44, doi:10.1029/2012GC004182.
We propose an interdisciplinary and multi-institutional research initiative between US (U. of Minnesota, U. of California, LBL) and French (IMPMC, IPGP, SPEC) research teams that will create a new perspective and help develop creative physico-chemical and microbiological techniques to address some of the Grand Challenges faced in utilizing continental sediments and soils as faithful recorders of climatic and environmental history of the past. Fluctuating magnetic susceptibility of glacial loess and interglacial soil deposits in China, and a few places elsewhere, have been correlated very well with the marine isotope stages and ice core records while sub-Milankovitch scale changes in magnetic susceptibility have raised our hope of finding regional scale changes in paleoclimate that can provide important benchmarks for testing the proposed regional-scale numerical models of climate change over the continents. One major roadblock has been our lack of understanding which climate variable causes change in magnetism. Well-controlled pedogenic experiments with synthetic iron precursors, carried out over a number of temperatures, may yield the activation energy of nano-magnetite formation. One should then also be able to test whether time is an important factor in the development of magnetic topsoils. Such scientific goals require emphases on the techniques to be employed for (1) analyses of field samples and (2) syntheses and analyses of manufactured nanophase iron oxides. The programmatic theme of our research is bridging the ‘gap’ between selected high quality field observations of topsoil magnetism, particle size, geochemical signatures, etc. and the mineral physics and mineral chemistry of well-characterized synthetic nanoparticles of iron oxides (sensu lato) that are responsible for the field observations. In particular, we propose to run oxidation and reduction experiments on both natural and synthetic samples, with a control on initial environmental conditions, mineral surfaces, and types of reaction. Our consortium has access to all the tools and techniques that will be needed to ensure characterization of initial and final products, as well as experimental conditions and characteristics of reaction (e.g., study of charge transfer, kinetics of reaction, reactivity). Because of the importance of multiple types of necessary tools and ideas, we have built an international group of researchers with complementary expertise in chemistry (wet and dry), microbiology and biochemistry, sample characterization (XRD, HR-TEM, EELS), environmental magnetism, rock and mineral magnetism (low temperature, high temperature, low field, high field), Mossbauer spectroscopy, synchrotron radiation techniques (EXAFS, XMCD), and computer modeling.
Monsieur YOHAN GUYODO (CENTRE NATIONAL DE LA RECHERCHE SCIENTIFIQUE - Délégation Paris B) – Yohan.Guyodo@impmc.upmc.fr
The author of this summary is the project coordinator, who is responsible for the content of this summary. The ANR declines any responsibility as for its contents.
IMPMC CENTRE NATIONAL DE LA RECHERCHE SCIENTIFIQUE - Délégation Paris B
IPGP INSTITUT DE PHYSIQUE DU GLOBE DE PARIS (IPGP)
SPEC COMMISSARIAT A L'ENERGIE ATOMIQUE ET AUX ENERGIES ALTERNATIVES - Centre SACLAY
Help of the ANR 280,000 euros
Beginning and duration of the scientific project: - 36 Months