Kais J. Mohamed

Discrimination of biogenic and detrital magnetite through a double Verwey transition temperature

Magnetite occurs widely in natural environments in both inorganic and biogenic forms. Discrimination of the origin of magnetite has important implications, from searching for past microbial activity to interpreting paleomagnetic and environmental magnetic records in a wide range of settings. In this study, we present rock magnetic and electron microscopic analyses of marine sediments from the continental margin of Oman. Low-temperature magnetic data reveal two distinct Verwey transition (Tv) temperatures that are associated with the presence of biogenic and inorganic magnetite. This interpretation is consistent with room temperature magnetic properties and is confirmed by electron microscopic analyses. Our study justifies the use of two distinct Tv temperatures as a diagnostic signature for discriminating inorganic and biogenic magnetite. Simple low-temperature magnetic measurements, therefore, provide a tool to recognize rapidly the origin of magnetite within natural samples. In addition, our analyses reveal progressive down-core dissolution of detrital and biogenic magnetite, but with preservation of significant amounts of fine-grained magnetite within sediments that have been subjected to severe diagenetic alteration. We demonstrate that preservation of magnetite in such environments is due to protection of fine-grained magnetite inclusions within silicate hosts. Our results, therefore, also provide new insights into diagenetic processes in marine sediments.

The influence of weather and climate on the reliability of magnetic properties of tree leaves as proxies for air pollution monitoring

Monthly monitoring of magnetic properties of Platanus hispanica tree leaves was used to assess atmospheric pollution in Madrid (Spain) and its suburban town of Pozuelo de Alarcón. Magnetic susceptibility, isothermal remanent magnetisation and metal concentrations were analysed to study the sources of atmospheric pollutants and their spatial and temporal evolution. In addition to urban dust, our results indicated that lithogenic dust and incorporation of trace metals in the leaf tissue also control the magnetic susceptibility of tree leaves. Global comparisons with cities of different climatic regimes suggest that air humidity is the key factor controlling the relative influence of pollutants, lithogenic dust and biological effects on the magnetic properties of tree leaves. Interaction of the atmosphere and tree leaves depends not only on local meteorology but also on climate. Climate, especially air humidity, and meteorology need to be considered when interpreting the magnetic properties of tree leaves as an atmospheric pollution tool.

Propiedades magnéticas de los sedimentos de tipo estuarino. El caso de las Rías Baixas.

Este trabajo establece la utilidad de la susceptibilidad magnetica como indicador de la influencia marina y evolucion diagenetica en sedimentos costeros de las Rias Baixas. Para ello se investigo la variabilidad geografica de la susceptibilidad en 200 muestras de sedimentos marinos superficiales en las Rias de Vigo y Pontevedra. Su variabilidad vertical se estudio en 80 muestras de tres testigos de entre 60 y 90 cm de longitud extraidos en la Ria de Pontevedra. La susceptibilidad de las muestras superficiales aumenta progresivamente hacia mar abierto, y a medida que nos alejamos de las zonas de mayor influencia continental, estando relacionada con la textura y composicion del sedimento, y por tanto, con su procedencia. Los testigos, mas homogeneos litologicamente, mostraron una disminucion muy importante de la susceptibilidad con la profundidad, estando esta asociada a la evolucion diagenetica del sedimento. Para estudiar mejor estos procesos, se determino la composicion elemental y mineralogica, y ademas se midieron otras propiedades magneticas de muestras representativas. Estos resultados se analizaron conjuntamente con los datos sedimentologicos disponibles, lo que permitio establecer que la variabilidad de 151 D. Rey et al. Propiedades magneticas de los sedimentos de tipo estuarino... la susceptibilidad dependia de tres factores principales: (a) procedencia y origen de los sedimentos, (b) regimen hidrodinamico de la zona y (c) presencia de particulas contaminantes discretas de origen antropogenico. Tambien puede concluirse que la evolucion de las fases magnetomineralogicas durante la diagenesis temprana esta controlada por el contenido en materia organica, lo que a su vez determina el potencial de oxidacion-reduccion.

Coherent deglacial changes in western Atlantic Ocean circulation

Abrupt climate changes in the past have been attributed to variations in Atlantic Meridional Overturning Circulation (AMOC) strength. However, the exact timing and magnitude of past AMOC shifts remain elusive, which continues to limit our understanding of the driving mechanisms of such climate variability. Here we show a consistent signal of the 231Pa/230Th proxy that reveals a spatially coherent picture of western Atlantic circulation changes over the last deglaciation, during abrupt millennial-scale climate transitions. At the onset of deglaciation, we observe an early slowdown of circulation in the western Atlantic from around 19 to 16.5 thousand years ago (ka), consistent with the timing of accelerated Eurasian ice melting. The subsequent weakened AMOC state persists for over a millennium (~16.5–15 ka), during which time there is substantial ice rafting from the Laurentide ice sheet. This timing indicates a role for melting ice in driving a two-step AMOC slowdown, with a positive feedback sustaining continued iceberg calving and climate change during Heinrich Stadial 1.The exact timing and magnitude of past changes in Atlantic Ocean circulation, and its relation to abrupt climate changes remains elusive. Here, the authors show a spatially coherent picture of western Atlantic circulation changes, which reveals a two-step AMOC slowdown at the beginning of the deglacial period.

Magnetic signature of the 22 June 1932 tsunami deposits (Jalisco, Mexican Pacific coast)

Recent studies have demonstrated that rock-magnetic analysis may provide additional information to distinguish and characterize extreme marine inundation events such as tsunamis. Rock-magnetic proxies reinforce and improve the environmental evidences supplied by other methods, adding some decisive clues for the interpretation of the origin and genesis of the sedimentary deposits. Here we report rock-magnetic, XRD and SEM microscopy results obtained in the Palo Verde estuary (Colima Pacific coast, Mexico) in order to enhance the tools for identification and reconstruction of two tsunami-induced deposits. The sedimentary sequence includes two sand units, a tsunami deposit (PV1) associated with the 22 June 1932 tsunami and a deeper sandy layer (PV2) related to a possible palaeotsunami that occurred around 1300 CE. Both sandy units are topped by finer grained units. n nMagnetic properties exhibit a significant correlation with the stratigraphy. High susceptibility (χ) and high saturation isothermal remanence (SIRM) values typical of high concentrations of (titano)magnetite are a distinctive feature of the most recent sandy tsunamigenic unit PV1 and the overlaying soil. The lower sandy tsunamigenic unit PV2 shows significantly lower χ and SIRM values, indicating lower concentration of (titano)magnetite in this unit and the overlaying clayey-silt unit. The latter also shows a higher coercivity component associated to (titano)hematite. Magnetic grain-size differences are also observed between PV1 and PV2 suggesting differences in hydraulic conditions at the time of deposition. The bulk mineralogical composition and sediment texture of these units also supports the hypothesis of different provenances for each tsunamigenic unit as inferred from magnetic properties.

A kinetic model to explain the grain size and organic matter content dependence of magnetic susceptibility in transitional marine environments: A case study in Ria de Muros (NW Iberia)

Magnetic minerals in marine sediments are sensitive indicators of processes such as provenance changes, climatic controls, pollution and postdepositional geochemical changes. Magnetic susceptibility is the bulk property of the sediments most commonly used to understand the magnetic characteristics of sediments. Before conclusions can be drawn from changes in this parameter, it is important to understand what factors and to what extent control changes in magnetic susceptibility. The magnetic susceptibility of surficial sediments in the Galician Rias Baixas, in NW Spain, has been shown to covary with sediment texture and organic matter content. Downcore, the magnetic properties of these sediments experience drastic changes as a result of strong dissolution caused by early diagenesis. In this paper, we further explore the relationship between these factors and formalise the observed covariations as the result of a simple second order kinetic model dependent on the content of organic matter in surficial sediments in the Ria de Muros. The reanalysis of previously reported data from the Rias de Vigo and Pontevedra confirmed the validity of this model, and suggested that further controls such as wave climate and water depth in the rates at which magnetic susceptibility changes are controlled by organic matter content.