Rock-magnetic signature of precipitation and extreme runoff events in south-eastern Patagonia since 51,200 cal BP from the sediments of Laguna Potrok Aike
Introduction
Millennial-scale climate change recorded in Antarctica ice cores during the Last Glacial period led changes in Greenland (e.g., EPICA community members, 2006) and the last climatic transition was probably triggered in the Southern Hemisphere by the action of changing westerly winds (Anderson et al., 2009) or sea-ice cover (Knorr and Lohmann, 2003) on ocean circulation. Despite its importance in the global climate system, the pre-Holocene Antarctic millennial-scale climate variability remains less documented than its Greenland counterpart (e.g., Dansgaard/Oeschger events). This limitation is partly due to the scarcity of pre-Holocene high-resolution records in the Southern Hemisphere largely dominated by the open ocean. Nevertheless, in recent years a growing number of high-resolution paleoclimate records from the Southern Hemisphere emerged and revealed in-phase climate patterns with Antarctica, including marine sediment cores from offshore Australia and New Zealand (e.g., Pahnke, 2003, Barrows et al., 2007), southern South America (e.g. Kaiser et al., 2007, Caniupán et al., 2011, Weber et al., 2012) and in the path of the Antarctic circum-polar current (Mazaud et al., 2007, Pugh et al., 2009). Continental records from Australia (Williams et al., 2009), southern Africa (Gasse et al., 2008) and southern South America (Kilian and Lamy, 2012) also document Antarctic-like climate changes during the Last Glacial, however generally at low temporal resolution and often discontinuously.
The Potrok Aike maar lake Sediment Archive Drilling prOject (PASADO) in the framework of the International Continental Scientific Drilling Program (ICDP) recovered a high-resolution sedimentary archive reaching back to the Last Glacial in southern South America (Zolitschka et al., 2009, Zolitschka et al., 2013 and papers therein). Previous records from this region are mostly limited to the Holocene and the Lateglacial (cf. Kilian and Lamy, 2012). Located in a region identified as the source area for Glacial dust deposited in Antarctica during the Last Glacial periods (Basile et al., 1997), the new continental record from Laguna Potrok Aike provides a unique opportunity to document past climate changes in south-eastern Patagonia since the Last Glacial period for comparison with the Antarctic climate record.
Careful macroscopic sedimentological study of the long PASADO sedimentary sequence revealed that mass movement deposits (MMD) including ball and pillow structures, normally graded beds, structureless sands and fine gravel layers constitute about half of the record (Kliem et al., 2013a, Kliem et al., 2013b). Recent geochemical, mineralogical and elemental studies indicate rare diagenetic remobilization linked to oxic conditions and only sparse organic and carbonate inputs (Hahn et al., 2014, Nuttin et al., 2013). Despite a relatively homogeneous clastic composition throughout the sedimentary sequence, Jouve et al. (submitted for publication) documented identical geochemical signatures for different types of microfacies and as a result, Jouve et al., 2013, Jouve et al., 2014) and Hahn et al. (submitted for publication) raised caution against the use of some elemental ratios (e.g., Fe/Mn, Fe/Ti, Mn/Ti) to infer paleoclimatic changes on the complete sequence. In addition, the presence of micropumice (Jouve et al., 2013) and fine sands to coarse silt layers disseminated throughout the record (Kliem et al., 2013a, Kliem et al., 2013b) highlight the difficulty for readily identifying some MMD and for interpreting paleoclimatic signals.
Magnetic properties of the sediment appear especially suited to overcome these difficulties and build continuous paleoclimate proxies from the sediments of Laguna Potrok Aike. Magnetic properties have the advantage of targeting only magnetic mineral (primarily iron oxides, oxyhydroxides and sulfides) and they are not necessarily biased by dilution effects such as the presence of rhyolitic micropumice in Laguna Potrok Aike (Jouve et al., 2013, Wastegård et al., 2013). Mineralogy and grain size-dependent magnetic properties are not influenced by concentration changes, and concentration-dependent parameters can be normalized to avoid such influences (Verosub and Roberts, 1995, Dekkers, 1997, Maher and Thompson, 1999, Evans and Heller, 2003). Previous rock-magnetic measurements from the sediments of Laguna Potrok Aike indicate a magnetic assemblage dominated by magnetite (Gogorza et al., 2011, Gogorza et al., 2012, Recasens et al., 2012, Lisé-Pronovost et al., 2013, Lisé-Pronovost et al., 2014). But evidence for other iron oxides such as maghemite and/or hematite (Gogorza et al., 2012), as well as iron sulfides (Jouve et al., 2013, Vuillemin et al., 2013) were reported. The low-resolution analyses of the PASADO core-catcher samples recently uncovered the potential to use rock-magnetism to identify MMD (Recasens et al., 2012). Here we present a high-resolution rock-magnetic study of the complete sedimentary sequence from Laguna Potrok Aike in order to investigate changes in the magnetic assemblage associated with MMDs and discuss their paleoclimatic implications.
Section snippets
Geological setting
Laguna Potrok Aike (51°58′S, 70°23′W; 113 m a.s.l.) is the only maar lake in the Pali Aike volcanic field in southern Argentina (Fig. 1A), with a maximum diameter of 3.5 km and water depth of 100 m. Located on the lee-side of the Andean cordillera in south-eastern Patagonia, the region is today influenced by the strong, dry and persistent Southern Hemisphere Westerly Winds (SWW) (e.g., Garreaud et al., 2013). As a result, the annual precipitation is low (ca 200 mm/yr; Mayr et al., 2007,
Field work
A total of 533 m of sediment were retrieved in 2008 at two sites in the deep basin of Lake Laguna Potrok Aike (position on Fig. 1) in the framework of the ICDP. Cores were collected using the GLAD800 drilling platform operated by the consortium for Drilling, Observation and Sampling of the Earth's Continental Crust (DOSECC). Site 2 was selected for multi-proxy high-resolution analyses by the PASADO science team because of higher core recovery (98.8%) and lower apparent sand content (Zolitschka
Discrete samples
The rock-magnetic analysis of cube samples reveals that 1) the degree of anisotropy is weak (less than 1.15), which indicates that the magnetic assemblage is not dominated by minerals with strong magnetocrystalline or shape anisotropy, and 2) the frequency-dependant magnetic susceptibility is generally lower than 2%, which indicates that there is no detectable superparamagnetic (SP) contribution (Dearing, 1999) in the general magnetic assemblage. Similar frequency-dependant results were
Gyroremanent magnetization (GRM) and magnetic mineralogies
While the GRM acquisition during AF demagnetization of the NRM (such as in facies 1) is frequently used as diagnostic of iron sulfides in sediments (Snowball, 1997, Hu et al., 1998, Hu et al., 2002, Stephenson and Snowball, 2001, Roberts et al., 2011), to our knowledge the acquisition of GRM during AF demagnetization of the IRM (such as facies 2) was not previously reported. We interpret the acquisition of GRM in the facies 2 as indicative of field-dependant anisotropy in hematite and/or
Conclusions
The high-resolution rock-magnetic study of the sediments deposited in Laguna Potrok Aike since 51,200 cal BP indicates the presence of PSD magnetite with isolated intervals of iron sulfides (facies 1) and oxidized pedogenic particles (facies 2) representing a total of ca 5% of the sedimentary sequence. The two distinct types of magnetic signatures are associated with MMDs in Laguna Potrok Aike and display contrasting rock-magnetic signatures compared to the rest of the record dominated by PSD
Acknowledgments
We thank J. Labrie, F. Barletta, M.-P. St-Onge, A. Leclerc and D. Veres for their help in the laboratory at ISMER, J.J. Scagliotti and C. Ohlendorf for cube sampling at the University of Bremen, and M. Jackson for his help with anisotropy measurements at the Institute of Rock Magnetism of the University of Minnesota in Minneapolis. We thank Q. Simon, S. Brachfeld, A. Chauvin and an anonymous reviewer for comments on an earlier version of this manuscript. This research is supported by the
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