High-resolution paleomagnetic record from the Late Pleistocene sediments of the maar lake Laguna Potrok Aike, southern South America

A. LISÉ-PRONOVOST; G. ST-ONGE; C. S. G. GOGORZA; T. HABERZETTL

Montreal

Workshop; 3rd International PASADO workshop; 2011

PASADO Canadá

High-resolution paleomagnetic record from the Late Pleistocene sediments of the maar lake Laguna Potrok Aike, southern South America Lisé-Pronovost1,2, G. St-Onge1,2,3, C. Gogorza4, Torsten Haberzettl5 1 Canada Research Chair in Marine Geology, Institut des sciences de la mer de Rimouski (ISMER), Université du Québec à Rimouski (UQAR), Rimouski, Québec, Canada,2 GEOTOP research center, Québec, Canada3 College of Oceanic and Atmospheric Sciences (COAS), Oregon State University, Corvallis, Oregon, USA4 Instituto de Física Arroyo Seco (IFAS), Universidad Nacional del Centro de la Provincia de Buenos Aires (UNICEN), Tandil, Buenos Aires, Argentina5 Institute of Geography, Friedrich-Schiller-University Jena(agathe.lisepronovost@uqar.qc.ca) High-resolution paleomagnetic records are scarce in the Southern Hemisphere relative to the Northern Hemisphere. The uneven distribution of the records around the globe limits our understanding of what governs the rapid changes in the Earth’s magnetic field. In particular, more high-resolution records from the Southern Hemisphere is crucial to assess the possible global nature of the centennial- to millennial-scale variability. Because of its position at the southern tip of South America (52°N, 70°W) and its high sedimentation rates (>1 m/kyr), Laguna Potrok Aike is a key site for a high-resolution paleomagnetic study. In the austral spring 2008, the Potrok Aike maar lake Sediment Archive Drilling prOject (PASADO) science team drilled the maar lake Laguna Potrok Aike in the framework of the International Continental scientific Drilling Program (ICDP). Here we present the full paleomagnetic reconstruction (inclination, declination and relative paleointensity) from the sediments of the maar lake Laguna Potrok Aike and we compare the results of the last 25 000 cal BP with the closest available records from marine and lake sediments, as well as with geomagnetic model outputs. The natural remanent magnetization (NRM) was first measured at 1 cm intervals with step-wise alternating field demagnetization using a 2G u-channel cryogenic magnetometer at the sedimentary paleomagnetism laboratory of the Institut des sciences de la mer de Rimouski (ISMER). The paleomagnetic directions were calculated using principal component analysis (PCA).The anhysteretis and isothermal remanent magnetizations (ARM and IRM) were then induced and measured using the same instrument in order to derive paleointensity proxies and to inform on the magnetic grain assemblage. A detailed rock magnetic study was also conducted in order to precisely identify the magnetic mineralogy and grain size. The hysteresis properties were measured on all core catcher samples from site 2 hole A (5022-2A-CC) and at ca. 50 cm intervals on the composite profile site 2 (5022-2CP) using an alternating gradient force magnetometer. In addition, the temperature-dependant magnetic susceptibility was measured from 50 to 700 °C on all 5022-2A-CC using a Bartington system, the frequency-dependent magnetic susceptibility was measured on cube samples at ca. 200 cm intervals on the 5022-2CP and the volumetric magnetic susceptibility was measured on the u-channels using a point sensor mounted on a multi-sensor core logger (MSCL). The magnetic assemblage of the normally deposited sediment is dominated by low coercivity mineral such as magnetite in the pseudo-single domain state, which is optimal for a paleomagnetic study. Furthermore, reworked material such as tephra, sand and intervals with abundant vegetal remains display distinct rock-magnetic properties and are thus readily identifiable throughout the record. The new high-resolution paleomagetic inclination, declination and relative paleointensity record from Laguna Potrok Aike since 25 000 cal BP is compared with the closest available records, including lacustrine records from further north in Argentina (lakes El Trébol and Escondido) and marine records from offshore Chile (ODP-1233), the Antarctic Peninsula (Palmer Deep and West Bansfeld Basin) and the South Atlantic (SAPIS stack). The millennial- to centennial-scale variability is generally comparable between all records. Even though age offsets of several hundreds of years are observed, sharp and large amplitude paleomagnetic features can easily be correlated between the high-resolution records. Noteworthy is a 1000 yr-long period of low inclination recorded at ca.18 000 cal BP and observed in all the other southern Hemisphere records. Interestingly, a similar geomagnetic feature is observed in Hawaiian lavas and Arctic marine sediments between 18 and 22 cal BP, hinting at the possible global nature of the event. Altogether, the paleomagnetic results of Laguna Potrok Aike reveal a genuine Late Pleistocene geomagnetic record and indicate that high-resolution magnetostratigraphy could be useful to constrain the chronology of 5022-2CP.