High-resolution paleomagnetic secular variations and relative paleointensity since the Late Pleistocene in southern South America
Graphical abstract
Highlights
► A 51.2 ka paleomagnetic secular variation and relative paleointensity record from Patagonia. ► Identification of the Laschamp and possibly Mono Lake geomagnetic excursions. ► Sharp secular variations at ∼20 and 46 ka could be linked to important features of the field.
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.
Section snippets
Geological setting
Laguna Potrok Aike (51°58′S, 70°22′W) is a maar lake within the Pali Aike Volcanic Field in the province of Santa Cruz in southern Argentina. The Pali Aike volcanic field is a series of phreatomagmatic craters formed by back-arc volcanism since the Pliocene to the Pleistocene (Corbella, 2002; Zolitschka et al., 2006; Coronato et al., 2013). Laguna Potrok Aike is today a perennial lake in the Patagonian steppe, with a maximum water depth of 100 m and a maximum diameter of 3.5 km. The lake is fed
Coring and sampling
The PASADO-ICDP scientific drilling operations at Laguna Potrok Aike were completed during the austral spring 2008 (September to November). The PASADO science team recovered 533 m of azimuthally unoriented sediment cores from 2 sites using a piston coring system supported by a barge (GLAD800) and operated by the Consortium for Drilling, Observation and Sampling of the Earth's Continental Crust (DOSECC) (Zolitschka et al., 2009; Kliem et al., 2013). Site 2 was selected for high-resolution
Magnetic mineralogy
The isothermal remanent magnetisation (IRM) acquisition curves of the 43 pelagic sediment samples (Fig. 3A; position on Fig. 4) reach saturation below ca 200 mT, indicating a magnetic assemblage dominated by low coercivity minerals. The median destructive field of the natural remanent magnetisation (MDFNRM; Fig. 4) and the coercive force (Hc; not shown) vary around average values of 16 mT and 7.2 mT, respectively. Along with the typical shape of the hysteresis loop (Fig. 3C) (Tauxe et al., 1996
Paleomagnetic secular variations and geomagnetic excursions
Changes of the virtual geomagnetic pole by less than 40–45° are commonly referred to as geomagnetic secular variation and greater changes, also associated to low geomagnetic intensity, are considered as geomagnetic excursions (e.g., Merrill and McFadden, 1994; Laj and Channell, 2007; Roberts, 2008). In the last 52 ka cal BP, the Laschamp (40.7 ± 1 ka cal BP; Singer et al., 2009) and Mono Lake (32.4 ± 0.3 ka cal BP, Singer, 2007) geomagnetic excursions are the only two recognized geomagnetic
Conclusions
A new high-resolution rock-magnetic and paleomagnetic record was constructed from the long sedimentary archive of Laguna Potrok Aike (PASADO-ICDP) covering the last 51.2 ka cal BP. While the magnetic assemblage is dominated by PSD magnetite and remains stable from the last glacial period to the present interglacial, a marked decrease in the concentration of magnetic minerals at 17.3 ka cal BP is associated to the onset of the last deglaciation in Southern South America. A full vector
Acknowledgements
We thank F. Barletta, J. Labrie, D. Veres, M.-P. St-Onge and A. Leclerc for their help in the laboratory at ISMER. We are grateful to L.G. Collins, A. Hayashida, P. Macri, A. Mazaud, R. Muscheler, A. Nillson, J. Peck, J. Stoner and N. Teanby for kindly sharing their data. This research is supported by the International Continental Scientific Drilling Program (ICDP) in the framework of the “Potrok Aike Maar Lake Sediment Archive Drilling Project” (PASADO). Funding for drilling was provided by
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