Secular variation from Mexican stalagmites: their potential and problems

As part of a feasibility study to see whether stalagmites could provide useful records of secular variation, nine oriented stalagmites were collected from the states of Chiapas and San Luis Potosí, Mexico. Of these, six have yielded measurable natural remanent magnetizations (NRMs) throughout their length. The cleaned magnetizations of one of these samples were shown unequivocally to have recorded the ambient field, and there is no reason to believe that this is not true of the primary magnetizations of other samples. The sequences of palaeofield directions, up the samples' length, have varying degrees of resolution and serial correlation, depending on the rate of field change as averaged by the stalagmite growth rate and the thickness of the measured specimens. Ages and growth rates were estimated by the U-Th method. The main problem in stalagmite palaeomagnetic analysis is a weak NRM, although this may be avoided by judicious choice of the sample. Samples which possess significant viscous components may be cleaned by alternating field or thermally if there is sufficient magnetic material. Unlike many sediments, stalagmites do not appear to suffer from depositional error problems. Dating problems may include low initial U-content, yielding ages with large errors, and the presence of allogenic 230Th in detritus, which causes older apparent ages. The dating limit is ∼ 350 ka. It is usually not possible to obtain long records comparable with those of most lake sediments, and there are aesthetic reasons for not spoiling caves adorned with stalagmites. The method is seen to be complementary to the use of sedimentary sequences to study palaeosecular variation. Studies of the rock magnetism of stalagmites are presented to suggest the mineral carriers of the magnetization and the origins of the natural remanence.