Late Cretaceous paleosols as paleoclimate proxies of high-latitude Southern Hemisphere: Mata Amarilla Formation, Patagonia, Argentina
Graphical abstract
Introduction
Paleosols can be useful and direct paleoclimatic proxies due to the fact that soil forms at Earth's surface, recording the atmospheric and climatic conditions during formation (e.g., Lal, 1999, Lal, 2004, Retallack, 2001, White et al., 2001, Sheldon et al., 2002, Nordt and Dreise, 2010). In particular, soil clay minerals can be correlated with climate factors such as temperature and water availability due to the fact that these factors strongly affect chemical weathering in the soil profile (see review in Sheldon and Tabor, 2009). In addition, there are key elemental ratios (Sheldon and Tabor, 2009) and weathering indices (Maynard, 1992, Sheldon et al., 2002, Nordt and Dreise, 2010) that serve as proxies for different pedogenic processes and for determining the weathering intensity in paleosols. Further, paleosol geochemistry can allow for the determination of quantitative climate parameters such as paleotemperature and paleoprecipitation (e.g., Sheldon and Tabor, 2009, Adams et al., 2011, Hyland et al., 2015, among others).
Mid-Cretaceous climate reconstructions indicate that greenhouse conditions prevailed, with a globally averaged mean annual temperature (MAT) of ~ 8 °C greater than present (Barron, 1983, Caldeira and Rampino, 1991, Frakes, 1999, Poulsen et al., 1999, Poulsen et al., 2001, Poulsen et al., 2007, Royer, 2010, Hay, 2011). The present pole-to-equator sea-level temperature difference is ~ 50 °C, whereas that of the mid-Cretaceous ranged from 30 °C to as little as 24 °C, implying a warm and more uniform climate (Barron, 1983, Frakes, 1999, Poulsen et al., 1999, Poulsen et al., 2001, Poulsen et al., 2007, Hay, 2011). The increased poleward heat transfer by H2O vapor may explain these reduced equator-to-pole temperature gradients (Ufnar et al., 2004, Hay, 2011). Likewise, an atmospheric pCO2 four times higher than present values may have increased average annual precipitation (MAP) globally by 25% (Barron et al., 1989, White et al., 2001, White et al., 2005, Royer, 2010, Ludvigson et al., 2015). Paleosol data from the North American Cretaceous Western Interior Basin and mass-balance modeling suggest that mid-Cretaceous precipitation rates exceeded modern rates at both mid- and high-latitudes; and also suggest amplification of the mid-Cretaceous atmospheric hydrologic cycle (White et al., 2001, Ufnar et al., 2004, Poulsen et al., 2007).
All global models of continental precipitation and climatic conditions of the mid-Cretaceous are based on paleofloral assemblages and paleosol data from the Northern Hemisphere, and are especially concentrated in the Western Interior Basin of the U.S.A. (e.g., Barron et al., 1989, Poulsen et al., 1999, Poulsen et al., 2001, Poulsen et al., 2007, White et al., 2001, White et al., 2005, Ufnar et al., 2002, Ufnar et al., 2004, Floegel and Wagner, 2006). In the Southern Hemisphere, all mid-Cretaceous reconstructions are based on fossil evidence, e.g., angiosperm and conifer floras, and palynology (Parrish et al., 1998, Iglesias et al., 2007, Pole and Philippe, 2010, Cantrill and Poole, 2012, Fletcher et al., 2014, Bowman, 2015).
Here we expand the evidence from South America by incorporating reconstructions based on paleosol data from Argentina. The focus is on the Mata Amarilla Formation in southern Patagonia, Argentina, because it contains a complete mid-Cretaceous high-latitude succession of paleosols. This succession provides an excellent opportunity to: i) characterize the main pedogenic processes using micromorphological and clay mineralogical analyses, and paleosol geochemistry; ii) constrain the paleoclimatic conditions, e.g., MAP and MAT, and seasonality; and, iii) make the first interhemispheric comparison of the mid-Cretaceous global climate based on paleosol data.
Section snippets
Geological background
The Mata Amarilla Formation (mid-Cretaceous) is a terrestrial succession in southern Argentinean Patagonia related to the foreland stage of the Austral/Magallanes Basin (Varela, 2015) (Fig. 1a). Through a sedimentological and sequence stratigraphic analysis, Varela (2015) divided the Mata Amarilla Formation into three informal sections (lower, middle and upper; Fig. 1b). The contact between the underlying Piedra Clavada Formation and the lower section of Mata Amarilla Formation has been
Methodology
Paleosols were identified in outcrop based on macroscopic pedofeatures such as structure, mottles, nodules, color, slickensides, cutans and rhizoliths (e.g., Retallack, 2001). Paleosol horizons, thickness, contact types, mean grain size, ped structure, type of nodules and evidence of bioturbation were described (e.g., Soil Survey Staff, 1975, Soil Survey Staff, 1998, Retallack, 2001). The paleosols (lithified material) were trenched to a depth of > 30 cm to avoid modern contamination (Fig. 2a).
The paleosols of the Mata Amarilla Formation
The Mata Amarilla paleosols are characterized macromorphologically by well-structured horizons, gley-colors, slickensides, angular blocky to wedge shaped peds, cutans, mottles and rhizoliths. These paleosols were classified following Soil Taxonomy (Soil Survey Staff, 1975, Soil Survey Staff, 1998, Retallack, 1993) as Vertisols (60%), Histosols (25%), Inceptisols (12%) and vertic Alfisols (3%), listed in order of abundance. The lower section comprised the development of Histosol and Vertisols in
Pedogenic processes
The micromorphological analyses together with the clay mineralogical and geochemical analyses suggest that the main pedogenic processes that occurred in the Mata Amarilla paleosols were: vertization, hydromorphism (gleization), argilluviation or lessivage (clay illuviation), and bioturbation. All of these processes took place under moderate hydrolysis and a wide range of weathering conditions.
The presence of vertic microfeatures such as cross-striated b-fabrics, b-axes of grains oriented with
Conclusions
The main pedogenic processes recognized in the Mata Amarilla Formation paleosols are vertization, hydromorphism (gleization), argilluviation (clay illuviation) and bioturbation. These processes occurred under elevated chemical weathering rates resulting in moderate losses of exchangeable bases typical of temperate–warm and humid climates with marked seasonality, all acting on fine volcaniclastic parent material in a wetland environment. The preponderance of smectite is related to the alteration
Acknowledgments
The authors deeply thanks to the Editor J. Knight, G. Basilici and two anonymous referees for their comments and corrections on the first version of this manuscript. This research was funded by the Agencia Nacional de Promoción Científica y Tecnológica (PICT 2012-0828 awarded to A.N. Varela) and Subsidio Jóvenes Investigadores de la UNLP 2013 (La Plata University Exp. Cod.100 N°19333/2/13 awarded to A.N. Varela). The authors would like to thank P. García, A. Iglesias for their assistance in the
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