Speleothem derived secular variation in Morocco for the past 6000 years

Recent work has highlighted the potential of speleothems as faithful recorders of the geomagnetic field. They acquire remanence almost instantly, are unaffected by post-depositional effects and can be dated with high accuracy. A ~6,000 yrs geomagnetic record was retrieved from the paleomagnetic analysis of three stalagmites (WIN1, WIN2 and WIN3) from the Wintimdouine cave in the Agadir region (Morocco). Chronological models developed from 54 U-Th ages of WIN2 and WIN3 speleothems indicate a slow growth of 0.04 mm/yrs on average, with marked interruptions from 1669 to 857 BP and from 3990 to 1942 BP for samples WIN2 and WIN3, respectively. For WIN1 sample only few U-Th ages were obtained given its high content of detrital ²³²Th, thus specimens from this speleothem were used to determine the magnetic carriers and as pilots to test the stability of the magnetic remanence. In contrast to most speleothems reported to date, in this case the magnetic remanence is carried by magnetite and hematite, and both present the same magnetic orientation as revealed from stepwise thermal and alternating field demagnetization. A small fraction of the remanence is carried by goethite, but this component is eliminated at low temperatures during the thermal treatment. Relative paleointensity was estimated from the low-coercivity magnetite fraction with the pseudo-Thellier method. The three records are internally consistent and the directional data generally agree in the last three millennia with the geomagnetic model ARCH3k.1, which is derived only from archeomagnetic data. Stalagmites with faster growth rates (WIN2 sample) are capable of recording the rapid variations of the geomagnetic field, including abrupt inclination variations between ~1320 and ~1340 CE reported in the archeomagnetic data. Our data provide a continuous geomagnetic record in high resolution for the last 6000 years, and improve the database for Northern Africa.