MINERAL MAGNETIC PARAMETERS PROVIDE NEW EVIDENCE ON THE CLIMATE-DRIVER OF THE TRIASSIC LATEMAR CARBONATE CYCLES

Mineral magnetic data collected from the Latemar succession provides new information about the cyclostratigraphy of the Middle Triassic Latemar platform, Dolomites, Italy. The more than 600 meter-scale shallow-marine carbonate cycles of the Latemar were originally interpreted as the products of astronomically driven, 20-kiloyear sea-level oscillations. Radioisotope dating of ash-beds within the platform subsequently indicated that the individual cycles must have formed much faster, in 2- to 4-kiloyears, or even less. This inter-calibration discrepancy constitutes the so-called “Latemar controversy.” Magnetic susceptibility, anhysteretic remanent magnetization (ARM), and saturation isothermal remanent magnetization (SIRM) of 200 samples from a 40-m transect of the Latemar succession at Forcellone reveal a coherent signal indicating magnetic mineral concentration variations in tune with a depth index derived from facies cyclicity. The ARM, which is a measure of the concentration of fine-grained magnetite, shows spectral peaks at 500 cm and 110 cm, while the facies depth index shows periodicities at 500 cm and 118 cm. Periodicities observed in the magnetic susceptibility (800 cm; 120 cm) and SIRM (822 cm; 153 cm; 119 cm) are in reasonable agreement with the ARM data. Susceptibility and SIRM are also measures of magnetic mineral concentration, but measure either all magnetic minerals or all the remanent magnetic minerals, respectively. Modeling of IRM acquisition data and an average S ratio of 0.92 indicate that magnetite is the dominant magnetic mineral in these rocks, with minor amounts of a high coercivity mineral. Since magnetite is typically a primary, detrital magnetic mineral in marine sedimentary rocks, ARM is probably the best parameter to measure variations in environmental processes. The ARM susceptibility (ratio of ARM/DC field) of 0.941 kA/m indicates that the magnetite is submicron in size and could mean that it is derived from airborne dust. The correspondence between variations in sea level, as indicated by the facies depth index, and aeolian input, as indicated by sub-micron magnetite, suggests that two independent global climate proxies are beating at the same frequencies in the Latemar carbonate cycles.