Climatic Cyclicity Revealed by Rock Magnetism: An Example From Saharan Dust Trapped on Lanzarote (Canary Islands)

Hambach, U.¹, Suchodoletz, H. V.^1,2, Fuchs, M.¹, Zöller, L.¹, Faust, D.²

¹Department of Geomorphology, University of Bayreuth, Germany

²Institute of Geography, Technical University of Dresden, Germany

Magnetic properties of sediments and soils are nowadays widely used in Quaternary and palaeoclimatic research to characterise the palaeoenvironment and to date indirectly sedimentary sequences. Quaternary dust deposits like loess/palaeosol sequences provide valuable palaeoclimatic information from areas, where other archives are rare or even absent.Wind blown dust was deposited worldwide during the cold/dry periods of the recent geological past. It undergoes pedogenesis when more humid conditions predominate, which is reflected in physicalchemical alteration of the sediment. These alterations result in the enhancement and transformation of magnetic minerals. Especially the so called ferromagnetic minerals (s.l.) react in ambient laboratory magnetic fields orders of magnitude stronger than other iron-bearing minerals. Thus, already very tiny concentration changes of these minerals control the magnetic signal of a sediment or soil. Therefore, magnetic parameters as function of depth can serve as a proxy for palaeoclimatic variations allowing for a close match with all kinds of high resolution palaeoclimatic archives.

On Lanzarote (Canary Islands), sediment was trapped in Miocence to Pliocene valleys dammed by younger volcanic material. The sedimentary infill of these valleys largely consists of Saharan dust but contains also volcanic slope debris of the local catchments. The sections reveal a regular alternation of almost unweathered sediments with strongly rubified palaeosol-complexes. Direct age controlled is provided by luminescence dating giving an age range from recent to approx. 200 ka (von Suchodoletz et al. 2008, 2009 a, b).

The applied standard rock magnetic analyses provide both concentration dependent parameters such as initial magnetic susceptibility (κ) and isothermal remanent magnetisation (IRM), and grain size and mineral specific parameters such as frequency dependent susceptibility (κfd) and interparametric ratios, e.g. IRM/κ. κ roughly reflects the bulk amount of magnetic minerals (remanence and non-remanence carrying minerals) in the sediment, whereas IRM reflects only the amount of remanence carriers. κfd and IRM/κ do not depend on concentration changes at all but provide important information on the composition and grain size distribution of magnetic mineral assemblages..

The measurements yielded high κ- and IRM-values in loess-like yellowish/ silty layers, whereas lower values are found in reddish/clayey strata. Absolute values of κ (IRM) vary between 1,000 and 8,900·10−6 SI-units (4–97 A/m). These extreme values of κ and IRM – at least one order of magnitude higher than those typical for loess or loessic soils in mid-latitude settings – indicate that the signal is strongly dominated by ferrimagnetic minerals of detrital origin. κfd, indicating so called superparamagnetic (SP) particles, shows a generally anticorrelated pattern to initial magnetic susceptibility and IRM, with highest values in reddish/clayey layers and lowest in yellowish/silty beds. Although the courses of IRM and κ look almost identical, their ratio (IRM/κ) reveals clear differences. This is expressed by variations on the same wave length as κfd, but showing an opposite trend. Low IRM/κ values in the reddish/clayey layers point to the dominance of non-remanence carrying minerals in the magnetic assemblage. Overall, the depth-dependent variability of initial magnetic susceptibility (κ) and IRM with depth is much greater than that of κfd. κ and IRM show abrupt changes and a wide range of values which are at least partially independent of lithology and/or pedostratigraphy. IRM/κ and κfd, however, exhibit a more or less regular relationship to depth.

The extent of pedogenetic processes can be estimated from environmental magnetic properties. κfd as well as IRM/κ ratio reflects the proportion of very small SP particles newly formed during pedogenesis or, alternatively, produced by weathering controlled maghemitization of local volcanic or aeolian/detrital magnetite derived from Africa. We generally observe that κfd and clay content show similar distribution patterns, while initial κ, IRM and IRM/κ are generally anticorrelated. Whereas κ ,IRM and IRM/κ are increased in the pedogenic unaltered intervals of the sections and significantly decreased in the pedohorizons, κfd exhibits an opposite trend, showing higher values in the pedohorizons and indicating an increased contribution of SP particles here. Interestingly, the relationship of IRM/κ to depth is almost perfectly anti-correlated to κfd. This can be explained by the greater decrease in remanence in the pedohorizons as compared to the weaker decay of κ. This observation also points to the relatively increased contribution of SP particles. We propose that reduced values of κ and IRM in reddish/clayey layers reflect the transformation of ferrimagnetic magnetite/maghemite to less susceptible para- and/or antiferromagnetic phases during rubefication. These environmental magnetic results show that increasing clay contents are paralleled by rubefication processes that are also indicated by the reddish colour of the pedohorizons.

The almost regular cyclic changes of grain size dependend magnetic parameters with depth probably reflect semi-continuous high frequency and low amplitude depositional events accompanied by diagenetic/pedogenetic processes. The latter seem to control the magnetic signal. Thus, rock magnetism record on-site humidity changes and hence the palaeoclimatic history of Lanzarote, rather than the palaeoclimate of the source area of the Saharan dust. Direct field observations as well as sedimentological, pedological and geochemical data corroborate this interpretation.

Corresponding author: Ulrich Hambach| FALI MAIL!!!