High-resolution paleomagnetic secular variations and relative paleointensity since the Late Pleistocene in southern South America

Abstract

Paleomagnetic inclination, declination and relative paleointensity were reconstructed from the sediments of Laguna Potrok Aike in the framework of the International Continental scientific Drilling Program (ICDP) Potrok Aike maar lake Sediment Archive Drilling prOject (PASADO). Here we present the u-channel-based full vector paleomagnetic field reconstruction since 51.2 ka cal BP. The relative paleointensity proxy (RPI) was built by normalising the natural remanent magnetisation with the anhysteretic remanent magnetisation using the average ratio at 4 demagnetisation steps part of the ChRM interval (NRM/ARM_10–40 mT). A grain size influence on the RPI was removed using a correction based on the linear relationship between the RPI and the median destructive field of the natural remanent magnetisation (MDF_NRM). The new record is compared with other lacustrine and marine records and stacks from the mid- to high-latitudes of the Southern Hemisphere, revealing consistent millennial-scale variability, the identification of the Laschamp and possibly the Mono Lake geomagnetic excursions, and a direction swing possibly associated to the Hilina Pali excursion at 20 ka cal BP. Nonetheless, a global-scale comparison with other high-resolution records located on the opposite side of the Earth and with various dipole field references hint at a different behaviour of the geomagnetic field around southern South America at 46 ka cal BP.

Introduction

Geological archives such as marine and lacustrine sediments are the only way to reconstruct the past millennial- to centennial-scale variability of the geomagnetic field beyond historical time; however the uneven distribution of the records on Earth does not allow addressing the possible global nature of its variability. The reason for this geographical bias is that high-resolution paleomagnetic records from the Southern Hemisphere are rare. As a consequence, global stacks are truly derived from a majority of records located in the Northern Hemisphere (e.g., GLOPIS-75, Laj et al., 2004; SINT-200, Guyodo and Valet, 1996) and geomagnetic field models lack calibration data from the Southern Hemisphere in order to better understand the core geodynamics (e.g., Korte et al., 2005; Roberts, 2008; Korte and Constable, 2011). A major limit to the collection of high-resolution records from the Southern Hemisphere is the scarcity of adequately high sedimentation rate basins and their accessibility. In particular, the mid-latitudes of the Southern Hemisphere are dominated by the open ocean realm with its typically low accumulation rates (e.g., Lund et al., 2006a) and south of 48°S, the South American continent is the only land mass beside islands in the Southern Ocean and the ice-covered Antarctica.

Despite technical and logistical difficulties and in order to better understand the geomagnetic field variability, a series of paleomagnetic records from the mid- to high-latitudes of the Southern Hemisphere (from 30°S to Antarctica) emerged in the last decade. The available high-resolution paleomagnetic records are often limited to the Holocene and deglacial period and include sediment drifts near Antarctica (Brachfeld et al., 2000; Willmott et al., 2006) and lacustrine sediments from Argentina (e.g., Gogorza et al., 2002, Gogorza et al., 2004, Gogorza et al., 2006; Irurzun et al., 2006). In addition, a very high-resolution record from offshore Chile covers the last 70 ka cal BP (Kaiser et al., 2005; Lund et al., 2006b). Other records from the Southern Hemisphere extending back to the last glacial period are however at lower temporal resolutions (e.g., Atlantic sector of the Southern Ocean, Stoner et al., 2002, Stoner et al., 2003; Indian sector of the southern Ocean, Mazaud et al., 2002; near Antarctica: Macrì et al., 2005, Macrì et al., 2010; Scotia Sea, Collins et al., 2012; Lake Pupuke in New Zealand, Nilsson et al., 2011).

In southern Argentina, the absence of a continental glacier over the Pali Aike Volcanic Field during the last glaciation (Zolitschka et al., 2006; Coronato et al., 2013) suggests a continuous accumulation of sediment in the maar lake Laguna Potrok Aike, and previous studies indicate high sedimentation rates of ca 100 cm/ka since 16 ka cal BP (Haberzettl et al., 2007). Therefore Laguna Potrok Aike is a key site for high-resolution paleomagnetic reconstruction in the mid-latitudes of the Southern Hemisphere. Previous paleomagnetic studies from the sediments of Laguna Potrok Aike include a series of short cores covering the last 0.7 ka cal BP (Gogorza et al., 2011) and the full vector paleomagnetic record for the last 16 ka cal BP was reconstructed as part of the South Argentinean Lake Sediment Archives and modelling (SALSA) project (Gogorza et al., 2012). A low-resolution rock-magnetic study from that lake recently revealed that no major change in the magnetic mineral assemblage occurred since the last glacial period (Recasens et al., 2011). Here we present a high-resolution rock-magnetic study of the lacustrine sediments and a continuous full-vector paleomagnetic record since 51.2 ka cal BP from a continental archive in the mid-latitudes of the Southern Hemisphere in order to document the variability of the geomagnetic field in an area of the world where observations are scarce.