Changes in Seasonal Precipitation Recorded by Speleothem Rock Magnetism

Abstract:

Magnetic material incorporated within speleothems can provide useful information about changes in the paleoenvironment. Speleothems dated using high-precision U-Th radiometric techniques constitute a unique resource for environmental magnetism. Rock magnetic measurements were carried out on a speleothem collected from Buckeye Creek Cave, West Virginia, USA. The studied sample spans an interval from 58 to >123 ka. Using a previously published age model and stable isotope data [Springer et al., 2014], we compare rock magnetic measurements made on the speleothem to stable-isotope, paleoenvironmental proxies. Variations in the oxygen isotope record follow precession-scale forcings, where relatively enriched δ¹⁸O values arise from an increase in summer precipitation. Alternating-field demagnetization of the saturation magnetization (SIRM) of representative samples reveals that there is no major variation in the coercivity spectra, suggesting that the composition and grain size of the magnetic carriers in the speleothem remain uniform across the studied interval. We use S-ratio measurements to investigate changes in the proportion of the speleothem’s magnetization carried by the magnetically ‘hard’ minerals such as goethite and any ‘soft’ component, such as magnetite. Between 123 and 58 ka, the contribution of the soft component (IRM_soft) to the SIRM varies between a maximum of 5.56 x 10^-7 Am²kg^-1 and a minimum of 8.25 x 10^-8 Am^2­ kg^-1 accounting for between 66% and 100% of the total SIRM. The ‘soft’ component record shows three peaks at 71.5 ka, 96.9 ka and 118.1 ka. These peaks present an intriguing correlation with intervals of enriched δ¹⁸O. The peaks in IRM­_soft­ appear to lag the most enriched δ¹⁸O values by approximately 1.5 kyr. We attribute this correlation between IRM_soft and increased summer rainfall to pedogenic processes, whereby environmentally-driven variations in soil magnetite production are recorded by the speleothem.