María Julia Orgeira

A Quantitative Model of Magnetic Enhancement in Loessic Soils

We present a quantitative model for the climatic dependence of magnetic enhancement in loessic soils. The model is based on the widely accepted hypothesis that ultrafine magnetite precipitates during alternating wetting and drying cycles in the soil micropores. The rate at which this occurs depends on the frequency of drying/wetting cycles, and on the average moisture of the soil. Both parameters are estimated using a statistical model for the soil water balance that depends on frequency and intensity of rainfall events and on water loss by evapotranspiration. Monthly climatic tables are used to calculate the average soil moisture and the rate of pedogenic magnetite production, which is proportional to a new parameter called magnetite enhancement proxy (MEP). Our model is tested by comparing MEP calculated for known present-day climates with the magnetic enhancement of modern soils. The magnetic enhancement factor, defined as the ratio between a given magnetic enhancement parameter and MEP, is expected to be a site-independent constant. We show that magnetic enhancement differences between soils from the Chinese Loess Plateau and from Midwestern U.S. are explained by our model, which yields similar magnetic enhancement factors for the two regions. Our model is also successful in predicting the mean annual rainfall threshold above which magnetic enhancement declines in a given type of climate.

Correlation between paleosol-soil magnetic signal and climate

A comparison of the index of potential water storage (PWS) with the magnetic properties of soils in Russia and of Argentina and China paleosols suggests the existence of one or two climatic thresholds that affect the formation, preservation or depletion of ferrimagnetic minerals. Soils characterized by a positive PWS are wetted during an important part of the year, creating an appropriate environment that favors the depletion of ferrimagnetic minerals due to mainly reductive dissolution. Such soils are characterized by a depletion of detrital ferrimagnetic minerals, as observed in Argentinean soils and paleosols. On the other hand, a negative PWS prevents highly reducing conditions in the soil, and the detrital ferrimagnetic minerals are preserved. The environmental conditions of these soils allow the formation of (superparamagnetic) pedogenic minerals, together with the preservation of lithogenic minerals. These conditions produce a net magnetic enhancement of the soil, as observed in Russia and China. The second threshold in PWS could be at a positive value. Above this second threshold, the water content of the soil during practically the whole year could allow the formation of (superparamagnetic) pedogenic minerals. This threshold could explain differences in the magnetic enhancement or depletion of Argentina soils characterized by a positive PWS.

Paleomagnetism, rock-magnetism and geochemical aspects of early Cretaceous basalts of the Paraná Magmatic Province, Misiones, Argentina

The basalts of the Posadas Formation were extruded during the huge continental volcanism that affected the Paraná Basin in the Lower Cretaceous. We have carried out a paleomagnetic and rock-magnetic study on samples collected along a basalt outcrop section in Misiones, Argentina and determined that rocks classified as tholeiitic basalts and andesi-basalts are characterized by a low to intermediate content of Ti. Paleomagnetic and rock-magnetic studies suggest that the main magnetic mineral is low-Ti titanomagnetite of superparamagnetic (SP) to single-domain (SD) sizes, and very low amounts of multi-domain (MD) particles. The stable magnetic remanence enabled us to define characteristic remanent magnetizations (ChRMs) with a maximum angular deviation (MAD) <5° in most cases; and in all the cases, a MAD <10°. The sequence has registered at least two polarity reversions, starting from a normal polarity at the base. The calculated virtual geomagnetic poles (VGPs) present an elongated distribution similar to other distributions of VGPs published for the Paraná Magmatic Province. The elongated distribution of the VGPs could be a real feature of the geomagnetic field at a time of frequent changes of polarity.

Rock magnetism in two loess–paleosol sequences in Córdoba, Argentina

Abstract This work presents new rock magnetic results along two loess–paleosol profiles in nearby locations in Argentina. The main objective of this study is to compare the magnetic signals and mineral content of two profiles and determine if the climate during Marine Isotope Stage 5 (MIS 5) and MIS 3 were similar to the present one. The two profiles are located in two different geomorphological settings, with effects on the water saturation characteristics and seasonality. Selected samples taken at these profiles were analyzed using laboratory procedures and environmental magnetism parameters to determine the climatic influence during the Late Pleistocene. Despite their proximity there are several differences between both profiles, such as their depth and geomorphological positions among others. The results of these analyses led to the following conclusions: climate conditions during MIS 5 were very similar to those of the present conditions. The hypothesis for this area suggests a slight increase in the magnetic signal associated with the generation of small amounts of magnetite and preservation of detrital magnetite and titano-magnetite. The results in this paper show a slight gain in the buried soils of Córdoba that would confirm the hypothesis.

The Influence of the Geomagnetic Field in Climate Changes

The present authors propose in this paper that a connection exists between the variations of the Earth’s magnetic field during polarity reversal and climate change. The mechanism by which the variations of the internal magnetic field could trigger climate changes would be produced by the influence of the internal magnetic field on Galactic Cosmic Rays (GCR) , since the geomagnetic field (GF) provides shielding to such radiation.