Rock-magnetic proxies of environmental changes since 51.2 ka cal BP from Laguna Potrok Aike, southern Patagonia

A. LISÉ-PRONOVOST; G. ST-ONGE; C. S. G. GOGORZA; T. HABERZETTL; B. ZOLITSCHKA; THE PASADO SCIENCE TEAM

Bremen

Workshop; 4th International PASADO workshop; 2012

GEOPOLAR - University of Bremen

Laguna Potrok Aike in southern Patagonia is a key site for paleoenvironmental reconstruction because it is located on one of the only landmasses in the pathway of the Southern Hemisphere Westerly winds (SWW) and within the main dust source area for the Southern Ocean and Antarctica during the last glacial period. Here we present a high-resolution rock-magnetic study of the long sedimentary sequence recovered as part of the PASADO-ICDP project in order to interpret the magnetic susceptibility record and to develop a proxy of wind intensity and environmental changes since 51.2 ka cal BP.The rock-magnetic data were acquired at the Institut des sciences de la mer de Rimouski (ISMER) using 1) a Bartington magnetic susceptibility point sensor mounted on a multi-sensor core logger on u-channels, 2) Bartington systems for high temperature and frequency-dependant magnetic susceptibility measurements on discrete samples, 3) a 2G Enterprises u-channel cryogenic magnetometer and 4) a Princeton Measurement Corp. alternating gradient force magnetometer for discrete samples. The high-resolution rock-magnetic results clearly indicate that magnetite is the dominant magnetic mineral deposited and preserved in the sediments of Laguna Potrok Aike since 51.2 ka cal BP. In addition to an overall homogeneous magnetic mineral assemblage optimal for paleomagnetic reconstructions (Gogorza et al., 2012; Lisé-Pronovost et al., in press), the variable amount of coarser magnetite grains and the preservation of smaller magnetite grains in specific intervals are useful paleoenvironmental indicators. In particular, the median destructive field of the isothermal remanent magnetisation (MDFIRM) is a measure of the coercivity of the coarse (>10 µm) magnetite grains (Bibliography) and is interpreted as a wind intensity proxy over Southern Patagonia, with coarser grains deposited during times of lower lake level and stronger winds (stronger wind intensity during cold and semi-arid conditions??). In addition, sharp changes in the magnetic grain size indicators sensitive to the smallest grain sizes (Mr/Ms and MDFARM) reveal the preservation of distinctively finer magnetic grains at the end of each warm millennial-scale event since 51.2 ka cal BP. This result suggests rapid climate change at the end of the Late Glacial warming, the Antarctic warm events 1 and 2, and at 50 ka cal BP. In addition, the high-resolution rock-magnetic results indicate that the magnetic susceptibility record at Laguna Potrok Aike is controlled by two factors: the flux of ferrimagnetic grains to the lake floor and changes in the input of coarse (>10 µm) magnetite grains. We demonstrate that the input of coarse magnetite grains is controlled by the wind intensity, while the flux is most likely related to the availability of exposed erodible land in Southern Patagonia. This double control on the magnetic susceptibility signal from within the dust source area of Southern Patagonia results in both similarities and differences when compared to distal dust records such as marine sediment cores from the Southern Ocean and ice cores in Antarctica. References:Gogorza, C.S.G., Irurzun, M.A., Sinito, A.M., Lisé-Pronovost, A., St-Onge, G., Haberzettl, T., Ohlendorf, C., Kastner, S., Zolitschka, B., 2012. High-resolution paleomagnetic records from Laguna Potrok Aike (Patagonia, Argentina) for the last 16,000 years. Geochemistry, Geophysics, Geosystems 13, Q12Z37.Lisé-Pronovost, , A., St-Onge, G., Gogorza, C., Haberzettl, T., Preda, M., Francus, P. Zolitschka, B. and the PASADO science team, in press. High-resolution paleomagnetic secular variation and relative paleointensity since the Late Pleistocene in Southern South America. Quaternary.