AN ENVIRONMENTAL MAGNETIC RECORD OF PLEISTOCENE CLIMATE EVOLUTION FROM LOESS DEPOSITS OF THE NORTHERN PAMPAS OF BUENOS AIRES PROVINCE, ARGENTINA

The loess and loessoid deposits of the Argentine Pampas represent one of the largest Late Cenozoic sedimentary sequences in the Southern Hemisphere. The section for this study came from the Gorina Quarry, located 10 km northwest of La Plata in the Buenos Aires Province of Argentina. This location is influenced by warm, moist air of the South Atlantic Anticyclone (SAA) and winds of the sub-tropical pressure cells centered over northern Argentina making it particularly sensitive to changing wind strength and direction as well as to changes in the strength and position of the SAA.

Paleomagnetic measurements indicate that the 17-m section spans the last ~2 Myr based on the presence of four reversal boundaries; the Brunhes/Matuyama boundary at 9 m (0.781 Ma), the entire Jaramillo subchron between 10.5 and 10.8 m (0.988-1.072 Ma), and the top of the Olduvai subchron at 15 m (1.778 Ma). Spectral and wavelet analysis of magnetic susceptibility (K) and red reflectance intensity indicate the presence of all three Milankovitch periodicities, although their influences are variable throughout the record. A comparison of K (filtered at 41ka) to global marine oxygen isotope records (also filtered at 41ka) indicates that from 1.1 – 2 Ma, high K values were associated with periods of high ice volume. From 400 ka to 1 Ma, the phase relationship changes, having highest K values during periods of lowest ice volume. During the late Pleistocene and Holocene (400 ka – present), the spectral signal weakens but suggests that the high K values are associated with low ice volume, much like the mid-Pleistocene. The timing of these changes is nearly synchronous with the mid-Brunhes event (~430 ka) and the change from 41-ka year ice volume cycles to 100-ka year ice volume cycles (~1 Ma), both of which resulted in an increase in global ice volume. These increases in ice volume would have had significant impacts on the position of the SAA, the sub-tropical pressure cells of Argentina, and the position of the ITCZ, which in turn affect prevailing wind directions and strength as well as atmospheric moisture content.