However, the mechanisms of remanence acquisition in speleothems are still poorly understood, and paleomagnetic results currently cannot be evaluated with respect to their origins. As continuous recorders in which lock-in time is subannual and material suffering from postdepositional alteration is readily identified and avoided, speleothems are an attractive alternative to materials like the lava flows and sediment cores more commonly used in paleomagnetic research. In recent years, improved magnetometer sensitivities have enabled the measurement of ever-smaller samples of stalagmites and flowstones, thereby refining the potential temporal resolution of speleothem paleomagnetic records. To overcome the difficulties presented by such weak magnetizations, researchers typically rely on large sample volumes (≥8 cm 3) that average geomagnetic field behavior on timescales of 100–4000 years per sample. In this regard, the challenges for paleomagnetic studies on “clean” speleothems (i.e., those devoid of flood material) are similar to those faced by studies on pelagic limestones with very little detrital input, where weak NRM intensities make the acquisition of progressive demagnetization data difficult. The equivalent magnetization for a 2 cm cube (8 cm 3) would range from 8 × 10 −12 to 8 × 10 −9 Am 2, which is close to the sensitivity limit for most SQUID-based cryogenic rock magnetometers (10 −12 to 10 −11 Am 2). The major obstacle for most paleomagnetic studies on speleothems is the low magnetic intensity displayed by stalagmites and flowstones, with natural remanent magnetization (NRM) intensities typically ranging from 10 −6 to 10 −3 Am −1. Perkins was the first to perform electron microscopy on magnetic extracts from speleothems, providing independent confirmation of magnetic results and giving unprecedented insight into the processes involved in remanence acquisition in stalagmites and flowstones.
#CAVE CREEK DRM NETWORKS SERIES#
This work sparked a series of paleomagnetic studies expanding on his theories over the next two decades. Throughout the 1980s, Latham used speleothems to construct secular variation curves that aligned well with data collected from sediment cores and archaeological material. measured the ancient geomagnetic field directions recorded by a group of flowstones and stalagmites and established that speleothems can hold stable magnetizations for thousands of years after their initial deposition in pre-existing caves.
Speleothems were first proposed as paleomagnetic recorders in the 1970s, when Latham et al. Speleothems, including stalagmites, stalactites, and flowstones, are secondary mineral deposits that form in caves and record valuable information about the environment in which they grow. Until the remanence of goethite can be shown to be paleomagnetically meaningful, we propose that paleomagnetic studies of speleothems include a 150☌ thermal demagnetization step to remove any goethite remanence. The magnetite and titanomagnetite, as well as their partially oxidized equivalents, likely hold a depositional remanent magnetization (DRM), whereas if goethite grains were precipitated in situ on the stalagmite drip surface, they are more likely to hold a chemical remanent magnetization (CRM) because of their small size and low saturation magnetization. Goethite was observed in three morphologies: isolated needles, needle aggregates, and polycrystalline aggregates of randomly oriented nanoscale grains. These Ti-rich grains display a range of morphologies from euhedral to well rounded, indicating that earlier morphological models for speleothem magnetization do not apply in all geologic settings. Titanomagnetite and exsolved intergrowths of Fe-Ti oxides indicate that a significant portion of the magnetic mineral assemblage is allochthonous and was transported into the caves. These grains are unlikely to have precipitated abiotically in the cave environment. Magnetite grains are likely detrital, showing evidence of transport and weathering, including plumose markings and etch pits. Magnetite and goethite are present in all samples, albeit in varying abundances and morphologies. We present tandem rock magnetic characterization of bulk samples and electron microscopy of magnetic extracts from five North American stalagmites. However, our understanding of the magnetic mineral assemblages within speleothems is not well developed.
Speleothems, especially stalagmites, hold great potential as recorders of the Earth's ancient magnetic field.
0 kommentar(er)
