BayCEER Workshop 2008

Palaeo-environmental research: Past climatic cyclicity revealed by rock magnetism - an example from Saharan dust trapped on Lanzarote (Canary Islands)

Palaeo-environmental reconstructions rely mostly on the analysis of sedimentary archives. Reading the environmental information stored in these archives and direct dating of the sediments provide the essential data for the reconstruction of environmental change with time.Magnetic properties of sediments and soils are widely used to characterise the palaeo-environment and to date indirectly sedimentary sequences. Quaternary dust deposits like loess/palaeosol sequences provide valuable environmental 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 physical-chemical alteration of the sediment. These alterations result in the enhancement and transformation of magnetic minerals. On Lanzarote, the sedimentary infill of valleys -dammed by younger volcanic material- largely consists of Saharan dust but contains also volcanic slope debris of the local catchments. Direct age controlled is provided by luminescence dating. Rock magnetic data vary regular with depth/time, following an almost sinusoidal cyclic pattern which probably reflects 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 palaeo-climatic history of Lanzarote, rather than the palaeo-environment of the source area of the Saharan dust. Direct field observations as well as sedimentological, pedological and geochemical data corroborate this interpretation.