Paleo- and rock magnetic study in support of the Laguna Potrok Aike maar lake sediment archive drilling project

GOGORZA, C.S.G.; OLHENDORF,C.; ZOLITSCHKA, B

Viena

Workshop; 2nd International ICDP PASADO Workshop; 2010

PASADO GROUP - GEOPOLAR (University of Bremen)

Three composite sediment cores collected from the centre of Laguna Potrok Aike, which represent a total of approximately 150 m of sediment, will be used to obtain a continuous and well-dated record of paleomagnetic and paleoenvironmental changes for the last 40,000 to 50,000 years. The coring activities were carried out from August 31 to December 1, 2008 in the framework of the ICDP-project PASADO. In detail, the objectives for this study are: a) To extend the paleosecular variation record into the period of the last glaciation. Creer et al. (1983) carried out first paleomagnetic studies of sediments from Argentina. Gogorza et al. (1999; 2000a,b; 2002) presented paleomagnetic results of sediments from Lakes Escondido, Moreno and El Trébol of southwestern Argentina (41°S; 72°W; Irurzun et al., 2006) and established a regional paleosecular variation (PSV) type curve (Gogorza et al., 2000a). b) To reconstruct hydrological variations. Understanding the rock magnetic properties in a sedimentary sequence and the processes involved in formation, transport, and preservation of these magnetic minerals will lead to the identification of magnetic parameters with the best performance as environmental proxies.  c) To investigate sources (e.g. fluvial vs. aeolian) of magnetic minerals.  d) To determine post-depositional changes in magnetic minerals caused by diagenesis/authigenesis and by biological processes. Magnetic mineral assemblages (combined with geochemical data) in a relatively small lake are helpful to analyze diagenetic dissolution as a result of changing redox conditions associated with e.g. eutrophication (Anderson and Rippey, 1988). Methods  The following measurements will be carried out:  (1) intensity and directions (declination, inclination) of natural remanent magnetization using a 2G high-resolution cryogenic magnetometer with RF SQUID sensors. Stability of the magnetization will be analyzed by alternating-field (AF) demagnetization. The directions of the stable remanent magnetization will be determined by vector analysis of the demagnetization results; (2) low-field susceptibility (specific, X and volumetric, k), using a Kappabridge KLY-2 (Agico) with operating frequency of 920 Hz, and a magnetic induction of 0.4 mT;  (3) isothermal remanent magnetisation (IRM) acquisition in fields up to 0.7T using a 2G high-resolution  cryogenic magnetometer with RF SQUID sensors; (4) anhysteric remanent magnetisation (ARM) using a 2G high-resolution cryogenic magnetometer with RF SQUID sensors, in a peak 100 mT AF and 0.1 mT bias field and stepwise AF demagnetisation in nine steps up to 100 mT and kARM. For a set of pilot samples the temperature dependence of susceptibility, up to a maximum temperature of 700°C, with a furnace-equipped Kappabridge KLY-2 and the hysteresis properties (such as saturation magnetisation MS, saturation remanence MRS, coercitivity HC, coercitivity of remanence HCR and high-field susceptibility (kh)) using a PMC Alternating Gradient Field Magnetometer will be measured. The ratios ARM/k, SIRM/ARM and BCR (remanent coercitive field) and the Sratio will be calculated. Assuming a uniform ferromagnetic mineralogy dominated by magnetite, grain size variations can be deduced from the inter-parametric ratios kARM/k, kARM/MRS and MRS/k (Maher and Thompson, 1999; Peck et al., 1994). However, these ratios should be used as grain size indicators with caution due to influence of dia- or paramagnetic minerals and non-linear behaviour with concentration (Stockhausen and Zolitschka, 1999).