The paleoclimatic message from the polymodal grain-size distribution of late Pleistocene-early Holocene Pampean loess (Argentina)

Aeolian Research 42 (2020) 100563

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The paleoclimatic message from the polymodal grain-size distribution of late Pleistocene-early Holocene Pampean loess (Argentina) ⁎

T

⁎

Gabriela Torrea,b, , Diego M. Gaieroa,b, , Nicolás J. Cosentinoa,b, Renata Coppoa,b a

Universidad Nacional de Córdoba, Facultad de Ciencias Exactas, Físicas y Naturales. Av. Vélez Sarsfield 299, X5000JJC Córdoba, Argentina Consejo Nacional de Investigaciones Científicas y Tecnológicas (CONICET), Centro de Investigaciones en Ciencias de la Tierra (CICTERRA), Av. Vélez Sarsfield 1611, Edificio CICTERRA, X5016GCA, Ciudad Universitaria, Córdoba, Argentina b

A B S T R A C T

Wind-blown dust deposits are considered one of the most important terrestrial archives for past climate change studies. In the Southern Hemisphere, the Pampean loess is the most extensive paleo-dust record, whose origin is still a matter of debate. In this paper, grain-size was carried out at three high-resolution loess profiles deposited during the late Pleistocene-early Holocene and for present-day dust collected on the Pampean plain. Based on comparing loess records with in situ presentday dust, this work aims to provide constraints on the climatic conditions that allowed deposition of the Argentinean loess mantle. Unmixing methods allow differentiating three grain-size subpopulations/end members in the Pampean loess which are comparable to end members found in present-day dust. The provenance and transport conditions observed for present-day dust were used to infer that the fine silt components of Pampean loess were transported by high-level air stream associated with the deflation of the Puna-Altiplano Plateau. On the other side, the coarse-silt sediments were carried by dust storms associated with high transport energy events taking place in proximal dust sources. In the central Pampas, the increased abundance of coarse-silt sediments during the Antarctic Cold Reversal (ACR) may indicate the existence of an increased frequency of dust storm episodes. During the beginning of the Holocene, the rise in fine-silt loess could be linked to the activation of distant sources associated with lake desiccation in the Puna-Altiplano Plateau.

1. Introduction Emission, transport and deposition of mineral aerosols (i.e., dust) are intimately coupled to the climatic conditions prevailing in any given region (Arimoto, 2001). Dust extraction from the atmosphere through dry and wet deposition is the main process forming coarse-tofine-silt-dominated loess deposits. These aeolian records constitute an important archive of Quaternary climate changes (Pye, 1995), providing one of the most complete terrestrial records of glacial-interglacial cycles. Southern South America (SSA) is home to one of the biggest loess deposits of the world: the Pampean loess, whose origin is still a matter of debate (e.g., Torre et al., 2019a). Provenance studies have been conducted on Pampean loess sequences using mineralogical (e.g., Iriondo and Krohling, 1996; Kröhling et al., 2010), chemical and isotopic (Gallet et al., 1998; Smith et al., 2003) proxies. Based on lowresolution chronological control, or none at all, these studies have concluded that source areas contributing dust to the Pampean loess has have experienced little temporal variability. Recently, through higher temporal resolution, it has been suggested that the Pampean loess provenance is mainly associated with arid-semiarid regions in SSA (Fig. 1) (Torre et al., 2019a,b). These arid terrains intersect two

important zonal wind belts: the Southern Westerly Winds (SWW) and the Subtropical Jet Stream (SJS). These wind systems have the capacity of sweeping the north-to-south-oriented arid/semi-arid lands located on the eastern slope of the Andes and extra-Andean areas, depositing dust in downwind environments such as the Pampas in central Argentina (proximal), the South Atlantic Ocean (middle) or in the Antarctic continent (distal) (Gili et al., 2017). The proximal deposits developed an extensive loess mantle (~106 km2), which is the thickest and most extensive of the Southern Hemisphere. The Pampean loess provides a unique long-term terrestrial archive for reconstructing the history of climate parameters that control dust deposition in the continental interior close to source areas of aeolian sediments (i.e., mainly regional wind and precipitation patterns). Several physicochemical properties of dust can be used to trace its source areas and to characterize the mechanism by which this material was transported and deposited. In this sense, the grain-size distribution (GSD) of loess deposits is an excellent proxy for the reconstruction of past wind activity and atmospheric circulation patterns of a region (e.g., Folk, 1966; Machalett et al., 2008; McCave et al., 1995; Pye, 1995; Sun, 2002; Vandenberghe, 2013). The GSD of aeolian deposits is mainly influenced by (1) the GSD of dust sources, (2) the distance from the source, and (3) the frequency of rain events (e.g., Vandenberghe,

⁎ Corresponding authors at: Universidad Nacional de Córdoba, Facultad de Ciencias Exactas, Físicas y Naturales. Av. Vélez Sarsfield 299, X5000JJC Córdoba, Argentina. E-mail addresses: [email protected] (G. Torre), [email protected] (D.M. Gaiero).

https://doi.org/10.1016/j.aeolia.2019.100563 Received 27 July 2019; Received in revised form 10 December 2019; Accepted 16 December 2019 1875-9637/ © 2019 Elsevier B.V. All rights reserved.

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Fig. 1. Map of southern South America (SSA) showing the potential sources and sinks of dust. The area within the dashed black line corresponds to the loess belt sector of the Pampean region (e.g., Zárate and Tripaldi, 2012), where the loess profile sites (white circles) of Lozada (Lz), Tortugas (Tr), and Gorina (Gn) are located. The red triangle indicates the position of the Marcos Juárez dust monitoring station. Full black lines encircle modern dust source areas (Prospero et al., 2002). These source areas are PAP: Puna-Altiplano Plateau, S-CWA: southern central-western Argentina. The wind systems that intersect SSA are also represented; SWW: low altitude southern westerly winds and STJ: high altitude subtropical jet stream. Annual precipitation in SSA is based on Hijmans et al. (2005).

2. Materials and methods

2013). The GSD of aeolian sediments provides a valuable tool to discriminate between different aeolian processes of transport and deposition, transport distance and origin. Here, we provide a detailed grain-size analysis for three late Pleistocene to early Holocene loess profiles exposed across the Pampean plain in SSA. The main purpose of this contribution is to discuss how paleoclimate information can be extracted from GSDs of loess samples. For that objective, loess grain-size data are compared to GSDs of present-day dust obtained from a monitoring station located at the core of the Pampas, which serves to discuss past atmospheric transport of dust based on modern processes. This combined GSD dataset (i.e., from loess samples and present-day dust) allows to discriminate different dust components in loess and to relate them to possible sediment transport processes and dust supply patterns.

2.1. Loess sampling and regional setting Full details on loess sampling techniques and the regional setting of the studied sites can be found in Torre et al. (2019a,b). Briefly, the Pampean plain is located to the east of the Andes and is limited to the west by the Sierras de Córdoba piedmont and by the Atlantic Ocean to the east (Fig. 1). We selected three 1-m loess vertical profiles previously dated by luminescence methods (Torre et al., 2019a) and distributed within the core of the Pampean loess plain (Zárate and Tripaldi, 2012) (Fig. 1). Loess sampling was performed using thin-walled plastic cylindrical corers driven into the face of the sections at an average resolution of ~2.3 cm according to the external diameter of the plastic cylinders. 2

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The westernmost loess profile within the “loess belt” selected for this study is the Lozada (Lz) site (~31°39′S; 64°08′W) (Fig. 1). This site is located to the east of the Sierras de Córdoba, in a gentle slope (< 3–5%) dissected by rivers and streams that drain the mountainous system. The 9.3-m vertical section is exposed along an abandoned road excavation on an extensive plain. A 1-meter section was sampled in detail from 2.8 to 1.8 m below the surface. Numerical ages obtained by optically stimulated luminescence (OSL) methods indicate that this loess section was deposited during the transition from the last deglaciation to the early Holocene (i.e., ~16.3 ka to 8.9 ka BP) (Torre et al., 2019a). The second profile is located at Tortugas (~32°43′S; 61°48′W), 250 km to the SE from Lozada and constituting one of the main Pampean loess type sites in the region (Fig. 1). At this site, we sampled a 1-meter section between 2.2 and 1.2 m depth below the surface, deposited during the last deglaciation (i.e., 19.5 ka to 12.6 ka BP) (Torre et al., 2019a). Finally, the easternmost loess profile is exposed at the Gorina quarry (~34°54′S; 58°01′W) (Fig. 1). This area is known as the ‘Pampa Ondulada’, its undulating topography resulting from erosion by tributary streams of the Rio Paraná and the Rio de la Plata (Zárate et al., 2009). The Gorina profile shows a more complex regional stratigraphic sequence than previously recognized (Tonni et al., 1999), most notably in terms of alternating discrete loess and paleosol units (Zárate et al., 2002). The 1-meter sampled section (e.g., between 1.6 and 0.6 m below the surface) was deposited during mid-MIS 3 and the early Last Glacial Maximum (i.e., 47 ka to 22.1 ka BP) (Torre et al., 2019a). According to Kemp et al. (2004, 2006) and Zárate et al. (2009) the 1-m loess profiles studied in this contribution show little post-depositional alterations. At the Lozada site, field observations indicate varying concentrations of continuous and discontinuous sub-horizontal laminations at approximately 2.0 to 2.9 m depth below the ground. Thin sections indicate that these microstructures comprise well-defined sorted layers of fine sand, coarse silt, fine silt and clay. These laminations were subject to fragmentation and disruption, likely by either erosion, desiccation or bioturbation processes. For the Tortugas site, this unit was deposited under a relatively stable land surface, allowing the development of an argillic soil (‘Hypsithermal Soil’). Here, micromorphological analyses indicate rare and/or no detection of fragments of sorted layers. In the case of the 1-m Gorina loess sequence no erosional surfaces were observed.

2.3. Chemical treatment Present-day dust samples were not chemically treated for grain-size analyses. Instead, loess samples (an average of ~45 for each loess section) were subjected to chemical treatment to separate the particles that could be chemically or physically bonded together due to postdepositional processes to obtain a solution with good dispersion. Carbonates were removed with 1 N hydrochloric acid (HCl) overnight and then washed several times with ultrapure water. Organic matter was oxidized using 3% hydrogen peroxide (H2O2) and then samples were washed again with ultrapure water. To complete the task of dispersing the clays, a small portion of each sample was left overnight in a solution of sodium hexametaphosphate (HMP) at 5x10-3 kg/L. Additionally, both loess and present-day dust samples were dispersed through ultrasonic incorporated in the particle analyzer during 20 to 40 s.

2.4. Grain-size analyses and processing of data Grain-size measurements of present-day dust (Cosentino et al., 2019) and loess (Table S1) samples was performed by laser diffraction analysis using a Horiba LA-950 particle analyzer. The precision (reproducibility) of the laser diffractometer was tested using glass mixtures (NIST Traceable polydisperse particle standard PS202/3‐30 μm and PS215/10–100 μm, Whitehouse ScientificW). For both runs (PS202, n = 6 and PS215, n = 5) the median (D50) was found within the 3% certified nominal value, and the D10 and D90 percentiles were within 5% of the ratings for standards. To unmix grain-size distribution data from the three loess profiles into subpopulations, we performed parametric end-member analysis (EMA) with components with general Weibull distributions. To overcome the non-uniqueness of solution associated to these methods (Weltje and Prins, 2007) we used an algorithm that performs nonparametric end-member analysis to determine a first guess for the parametric end-members (Paterson and Heslop, 2015). In order to analyze spatial variability of grain size along the sampled loess transect, we performed parametric EMA for each loess profile separately. Also, we compare these results with parametric EMA performed on the present-day dust sample set (Cosentino et al., 2019).

2.5. Chemical analyses 2.2. Present-day dust sampling Five loess samples were also analyzed for major elements by inductively coupled plasma with optical emission spectroscopy (ICP-OES) at the Lamont-Doherty Earth Observatory laboratories. Samples were dissolved by alkaline fusion method (Li2B4O7, 1050 °C, with HNO3 dissolution). To aid to the interpretation of the major element geochemistry, we calculated the chemical index of alteration (CIA), which is described in terms of the molar proportions of the oxides of aluminum (Al), calcium (Ca), sodium (Na) and potassium (K) by: CIA = (Al2O3/(Al2O3 + CaO*+Na2O + K2O)) × 100 (e.g., McLennan, 1993; Nesbitt and Young, 1982) (Table 1). In this formula, CaO* represents the Ca in the silicate fraction only, which is obtained by adjusting for some other Ca-bearing minerals such as apatite and carbonates. As carbonates were previously removed from the samples, we applied here the correction for apatite, using the concentrations of P2O5 as it was proposed by McLennan (1993). CIA estimates the extent of weathering of continental rocks and sediments, where higher values indicate greater alteration, and has been shown to be a very useful interpretation of loessic sediments (e.g., Campodonico et al., 2019; Jahn et al., 2001).

In order to compare grain-size characteristics of past dust deposits with those of present-day dust, atmospheric dust samples were collected in a monitoring station located ~30 km to the west of the Tortugas loess profile (Fig. 1). A full description of the sampling methodology can be found in Cosentino et al. (2019). Briefly, we employed an inverted pyramid (CP) receptacle designed to measure the deposition rate of atmospheric particles (Cosentino et al., 2019; Gaiero et al., 2013, 2004; Gili et al., 2017; Goossens and Rajot, 2008; Orange et al., 1990; Skonieczny et al., 2011). The CP collector was attached to a pole at 5-m height from the ground to avoid the collection of local material transported by saltation. The dust monitoring station ran automatically for about 10–12 days, after which an operator collected the samples using a vacuum pump to filter (through a dry or wet medium) and retain particles on a pre-weighed 0.45 μm membrane. The station has been operative since 2004. Selection of samples for grain size characterization (n = 45, between 07/2004 and 09/2017) was based on the analysis of contrasting meteorological conditions detected during the different sampling periods, as for example, high/low average wind speed, periods with/ without rainfall and periods where dust storms were observed/lacking (e.g., Cosentino et al., 2019). 3

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Table 1 Age, major element composition and chemical index of alteration (CIA) for loess samples from the Gorina section. Sample

Age (Ka)

SiO2 (%)

Al2O3 (%)

Fe2O3 (%)

MnO (%)

MgO (%)

CaO (%)

Na2O (%)

K2O (%)

TiO2 (%)

P2O5 (%)

CIA

Gn4 Gn13 Gn18 Gn36 Gn41

45.50 42.70 42.00 27.20 22.10

70.13 68.19 70.52 65.61 70.27

15.65 16.80 15.75 17.47 16.78

4.97 5.47 5.02 6.27 5.61

0.05 0.08 0.05 0.08 0.04

1.23 1.49 1.17 1.75 1.31

1.53 2.15 1.62 3.59 1.42

2.23 2.44 2.29 1.70 1.99

2.20 2.75 2.20 2.39 2.27

0.84 0.82 0.88 0.89 0.92

0.03 0.18 0.06 0.06 0.03

64.0 60.9 64.2 59.6 67.0

Fig. 2. Statistics for the unmixing of grain-size data for loess samples from (a) Lozada, (b) Tortugas and (c) Gorina.

the fit to the measured data of a set of models with increasing number of EMs. To quantify this fit, we run the model in order to calculate the coefficient of determination (R2) and angular deviation. Results are consistent with a polymodal GSD for all loess sections. For the Lozada and Tortugas sections, 95% of the dataset variance is accounted for by a model with at least two EMs (Fig. 2a, b), while at least three EMs are required to account for 95% of the Gorina dataset variance (Fig. 2c). The angular deviations of the Lozada and Tortugas two-EM models is 5.1° and 10.6°, respectively (Fig. 2a, b); while that of Gorina’s three-EM

3. Results and discussion 3.1. Pampean loess end-member analysis Naturally deposited sediments are composed of mixtures of materials (subpopulations) from different sources and/or transported by different mechanisms (Weltje, 1997 and references therein). To evaluate the number of subpopulations or end-members (EMs) based on the grain-size distributions (GSDs) of Pampean loess samples, we quantify 4

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Fig. 3. Calculated grain-size distribution curves for loess samples collected at (a) Lozada, (b) Tortugas and (c) Gorina. It is shown examples of the total fit of the calculated EMs for three randomly selected loess samples: (g) shows samples Lz 0, from Lozada; (h) shows sample Tr 10, from Tortugas and, (i) shows sample Gn 9 from Gorina.

to the presence of an additional small peak in the fraction between 2 and 10 μm. The origin of this fine dust is not always interpreted as of pedogenic origin and is frequently associated to an allochthonous contribution (Guo et al., 2002; Lu et al., 2001; Sun et al., 2008; Vandenberghe et al., 2018, 2004). A very fine dust component, comparable to Gorina’s EM1 is found in the Chinese Loess Plateau and in the European loess records. This is interpreted as the supply of a continuous atmospheric dust background (e.g., Kovács, 2008; Sun et al., 2004; Vandenberghe et al., 2004; Stuut et al., 2009) rather than to the sporadic contributions linked to dust storm events (Zhang et al., 1994; Zhang and Carmichael, 1999). Atmospheric transport studies indicate that these very fine sediments are transported through long distances by jet streams and/or westerlies in the middle to high troposphere (i.e., 3–8 km high) (Pye and Zhou, 1989; Zhang et al., 1997). For example, sediments deflated from the deserts of western China are transported to the north Pacific, North America and Greenland (e.g., Ruth et al., 2003; Unruh, 2001; Zdanowicz et al., 2006). Field inspections at the Gorina site do not allow to rule out that this grain-size subpopulation is produced by post-depositional processes,

model is 9.6° (Fig. 2c). More evidence for the reliability of these results is provided by a comparison of the GSDs of the original data (Fig. 3a-c) with that of the fitted curves (Fig. 3g-i), which reveals a high degree of similarity. At the Lozada site, two components are observed: a fine component (EM2) with a modal size of 19.82 μm, and a coarse component (EM 3) with a modal size of 51.45 μm (Fig. 3d). Similarly, for the Tortugas profile a fine component (EM2) shows a modal size of 14.39 μm, and a coarse component (EM3) shows a modal size of 52.46 μm (Fig. 3e). The finest grain-size population (EM1) obtained for the easternmost Gorina section has a modal size of 3.48 μm. The other two grain-size subpopulations (EMs) recorded at Gorina show modal sizes of 9.85 μm (EM2) and 33.85 μm (EM3) (Fig. 3f). 3.1.1. Environmental significance of the EM1 subpopulation This fine component is not always present at the Gorina section and it is not observed on the grain size frequency distribution curves as an individual subpopulation. However, as also observed by Vandenberghe (2013), when looking at the fine-grained limb of the Gorina frequency distribution curves there is often a characteristic asymmetry associated 5

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Fig. 4. (a) Chemical Index of Alteration (CIA) calculated for samples from the Gorina loess record. (b) Temporal variation of the contribution of EM1. (c) Natural γ radiation determined in lake sediments from the Uyuni basin (Baker et al., 2001).

periods they fall to 0–20% (Fig. 4b, c). We speculate that this is the case because of increased exposure of fine deep-water lake sediments during more arid periods. This mechanism of sediment recharge during wet periods and availability during dry periods may be a significant control on dust emissions over millennia as well as on shorter time scales (e.g., Marx et al., 2018).

either chemical or physical. On the other hand, the Chemical Index of Alteration (CIA) (Nesbitt and Young, 1982) calculated for some of the Gorina loess samples (Table 1) indicates the existence of low to moderate degrees of weathering/pedogenesis (i.e., CIA between 60 and 67) (McLennan, 2001) (Fig. 4). Similar CIA values were reported for the Corralito loess sequence located ~20 km south of the Lozada section (Campodonico et al., 2019). Moreover, considering the five Gorina loess samples for which there is both chemical and grain-size data, there is no systematic correlation between EM1 contributions and CIA (Fig. 4a, b), suggesting that the very fine component found in this loess section is not derived from weathering or pedogenetic processes. We also discard a possible physical origin for EM1 given the lack of fluvial erosional or depositional surfaces in the 1-m loess sequence selected for this study (Zárate et al., 2002). Hence, we propose that the origin of the EM1 component is associated with a distant dust source. Several lines of evidence point to the Puna-Altiplano Plateau (PAP) as an important distant past dust source to the Pampas. Based on Rare Earth Element geochemical fingerprinting, the Puna region is identified as a relevant dust source to the Lozada site, the westernmost (i.e., most proximal) studied loess site in the Pampean region during the early Holocene (Torre et al., 2019b). Natural γ radiation from the Salar de Uyuni, a great saline lake located on the PAP (Fig. 1), serves as a good proxy to understand the paleo-climatic conditions of this region, as it reflects the conditions of the lake water body where an increase (decrease) in natural gamma radiation indicates a wetter (drier) phase (Torre et al., 2019a). During some periods (more evidently between 36 and 26 ka BP), lower natural-γ radiation values (i.e., drier periods) recorded in lake sediments from the Uyuni basin (Baker et al., 2001; Fritz et al., 2004) (Fig. 4c) correlate with a higher proportion of EM1 in Gorina, the easternmost (i.e., most distal) studied loess site (Fig. 4b). During dry periods between 50 and 20 ka BP at Uyuni lake, EM1 contributions to Gorina range between 20 and 45%, while during relatively wetter

3.1.2. Environmental significance of the EM2 subpopulation Although the three sites show EM2 subpopulations with a wide range of modal values (e.g., 19.82 μm at Lozada, 14.39 μm at Tortugas and 9.95 μm at Gorina), some lines of evidence discussed below and in Section 3.2.1 indicate that they could have a common source. EM2 is well represented in the three loess sections across the Pampean plain and show NW-to-SE decreasing modal values. It is classified into the group of aeolian sediments transported from long distances (Vandenberghe, 2013). Along with EM1, this group of aeolian sediments is classified as fine dust (Stuut et al., 2009). This fine sediment population can be dispersed over a wide altitudinal band and is mainly transported by upper-level flow and is deposited far from the source areas (Glaccum and Prospero, 1980; Pye, 1995; Pye and Tsoar, 1987; Windom, 1975). The main difference between EM1 and EM2 is that the former is transported at a higher altitude and longer distances, thus having a longer residence time in the atmosphere (Pye and Zhou, 1989; Zhang et al., 1997). This is consistent with EM1 only being defined for the easternmost sampled Gorina loess section, farthest away from potential dust sources (Fig. 1). The EM2 is one of the main sedimentary components of the studied Pampean loess sections. During the interval between late MIS 2 and the beginning of the Holocene, EM2 represents an average abundance of ~50% in Lozada (Fig. 5a). At Tortugas, this EM represents ~75% of the grain-size components for the late MIS 2 (Fig. 5b) and accounts for ~45% at Gorina during the interval from MIS 3 and the Last Glacial 6

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Fig. 5. Temporal variability of end member (EM) 2 at the Pampean loess records of (a) Lozada (b) Tortugas and (c) Gorina.

Tsoar, 1987). In the Pampean loess, the most proximal depositional center with respect to the main dust sources in SSA (Lozada, Fig. 1) has higher modal EM3 values compared to the most distal loess sequences of Tortugas and Gorina. For the Chinese Loess Plateau the supply of coarse silt sediments was maximized during glacial (cold) periods linked to more intense northwesterlies (Vandenberghe et al., 2004). A similar situation can be deduced from the proximal Pampean loess records (i.e., Lozada and Tortugas) (Fig. 6a, b), which show an increasing trend of the coarser component during the Antarctic Cold Reversal (ACR) period. At both sites, the higher abundances of coarser components may imply that during the ACR dust storms occurred with higher frequency and higher transport energy. Based on paleo-record interpretations, the southern central west of Argentina (S-CWA) (Fig. 1) has been singled out as an important dust source area during the ACR (Torre et al., 2019b; Tripaldi et al., 2011). Paleo-data point out to a possible equatorward SWW belt migration during the climatic reversion, which probably promotes the intensification of winds over northern Patagonia and S-CWA (Gili et al., 2017).

Maximum (Fig. 5c). EM2 is the main component of the loess deposits from central, southern and eastern Europe (Bokhorst et al., 2011; Varga et al., 2013) and the Chinese Loess Plateau, with increased proportions during interglacial periods (Vriend et al., 2011). Similarly, the Lozada record is characterized by a noticeable trend of increasing EM2 abundance during the early Holocene. Here, EM2 represents more than 90% of the loess deposit at ~9 ka BP (Fig. 5a) and suggests a relatively increasing supply of sediments from distal areas (i.e., ~2000 km away) during the beginning of this wetter phase. During this phase, a stock of fine sediments may have arguably accumulated at the surface of the Uyuni basin at the Puna-Altiplano Plateau, followed by the onset of a drier period with increased dust deflation and transportation over the Pampas (Torre et al., 2019a). 3.1.3. Environmental significance of the EM3 subpopulation The coarse silt subpopulations found in the loess records of the Pampean plain (i.e., EM3) are within the range of the medium-tocoarse-grained silt fraction (Vandenberghe, 2013), classified as large dust (Stuut et al., 2009). These medium-to-coarse silt aeolian components represent an average abundance of ~50% of the Lozada (Fig. 6a), ~26% of the Tortugas (Fig. 6b) and ~37% of the Gorina loess sections (Fig. 6c). This coarse EM is also a typical component of loess from the Chinese Loess Plateau (e.g., Zhang et al., 1994) and the Negev desert (e.g., Crouvi et al., 2008; Pye and Tsoar, 1987). The presence of EM3 in the loess records have their origin in nearby proximal sources, carried during dust storms, and are interpreted as transported in short-term, near-surface to low suspension clouds (Vandenberghe, 2013; Pye and

3.2. Comparison of textural compositions of present-day dust and Pampean loess Comparing GSDs of loess with those of present-day dust at the same location may aid in the interpretation of past environmental conditions affecting emission, transport and deposition of dust. Given that each modern dust sample collected at Marcos Juárez integrates an average of 12 days of dust activity, each sample contains dust that in general has suffered different transport pathways and comes from different sources. 7

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Fig. 6. Temporal variability of the end member (EM) 3 in the Pampean loess records of (a) Lozada, (b) Tortugas and (c) Gorina.

commonly transported by the high-level air stream. Background dust can be deposited in any of three ways: by rainfall wash-out (i.e., wet deposition) (Rea and Hovan, 1995), by attachment to larger grains (Pye and Tsoar, 1987), or by deposition as individual particles when wind speed decreases (i.e., dry deposition). Nevertheless, there are no clear climatic patterns that help to explain the deposition of type-B dust samples (Table 2). In most cases (n = 20), these samples were collected during periods of relatively abundant rainfalls (> 2 mm/day) and hence, likely dominated by wet deposition. However, some of these samples (n = 7) were collected during periods of low rainfall (< 0.4 mm/day), and dry deposition mechanisms could have dominated. Present-day type-B dust is equivalent to present-day dust EM2 and is comparable to the fine-silt EM2 subpopulation found in the loess records of the Pampean plain, including the nearby Tortugas loess section (Fig. 7b). This suggests that the presence of EM2 in the loess records could be explained by similar transport mechanisms linked to the fallout of fine aeolian sediments supplied from distant and high-altitude dust sources. In agreement with a major contribution from the PAP and consistent with an increasing distance from this area, the Pampean loess sequences display a clear decreasing exponential trend of the mean EM2 modal value (Fig. 9).

We thus perform grain-size analysis at two levels: (i) qualitative dust type classification at the sample level, and (ii) quantitative end-member analysis of the full sample set, more indicative of the processes acting on dust transport. Three EMs are defined for the set of 45 present-day dust samples chosen for grain-size analysis (Fig. 7a), two of which (EM2 and EM3) roughly coincide with EMs from the nearby Tortugas loess section (Fig. 7b). Based on data from Fig. 7a, we classified present-day dust into four main types (Table 2). Type-A dust has a unimodal distribution curve with a mode at ~6 µm, which coincides with present-day EM1 and dominates in only one sample. Type-B dust is also unimodal but with a coarser modal size (~12–17 µm), coincides with present-day dust EM2 and dominates in most samples (n = 34). Type-C dust is the coarsest of the unimodal dust types (modal size of ~51 µm), has the closest GSD to present-day dust EM3 and dominates in a single sample. Finally, Type-D dust shows a bimodal GSD with modes at ~13–20 µm and ~45–68 µm, roughly coinciding with present-day dust EM2 and EM3. 3.2.1. Environmental significance of present-day type-B dust Type-B present-day dust curves show mostly well-sorted unimodal GSDs (Fig. 7a), suggesting a dominant distant source (Cosentino et al., 2019; Gaiero et al., 2013; McTainsh et al., 1997). This is supported by two type-B present-day dust samples collected after a dust storm event detected in the PAP. Ground and satellite evidence indicates that dust from this region was transported through high-level westerly streams to the central Pampas (Fig. 8a) (Gaiero et al., 2013; Gili et al., 2017). A likely explanation for the deposition of type-B present-day dust is that this fine material is settled-down by a gentle, continuous dust depositional process which provides the background dust fall that is

3.2.2. Environmental significance of present-day type-A dust The very fine silt present-day type-A dust is equivalent to presentday dust EM1, is only dominant in one sample and cannot be visually discerned in other samples' GSD curves (Fig. 7a). It was deposited during a considerably wet period (~8 mm/day) (Table 2), and so wet deposition is the most likely mechanism of atmospheric extraction. Even if no analog grain-size component is identified in the nearby Tortugas loess section, it is in fact observed in the easternmost Gorina 8

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Fig. 7. Grain-size distribution curves of (a) present-day dust samples and present-day end members (EM) calculated for the Marcos Juárez sample set, and (b) a selection of loess EMs, present-day dust EMs and present-day dust samples.

loess section (Fig. 7b). Interestingly, EM1 is not visually discernible as a clear individual subpopulation in any loess sample, but rather as a characteristic asymmetric left limb, as is the case with all but one present-day dust sample. This coincidence may be indicative of a common source, transport process and/or atmospheric extraction mechanism for the EM1 subpopulation of both present-day dust and Gorina loess samples. The PAP has not only been a dust supplier to the Pampas in the past (Torre et al., 2019a,b), but also in the present, as evidenced by satellite (Gaiero et al., 2013), isotopic (Gili et al., 2017) and meteorological and dust mass flow time series analysis (Cosentino et al., 2019). The fact that modern dust collected at Marcos Juárez has an EM1 contribution of 7.5% in average and of less than 18% seems to be at odds with the

Table 2 Dust types of present-day dust samples, mean precipitation and mean wind velocity during the collecting periods. Dust type

Number of samples

Mean daily precipitations and range (mm)

Mean wind velocity and range (m/s)

A B C D

1 34 1 7

8 2.8 (0.1–10.1) 0 4.8 (0–12.5)

2.8 3.6 (2.1–6.2) 3.3 3.5 (1.9–5.1)

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Fig. 8. Satellite images (https://worldview.earthdata.nasa.gov) showing dust activity; (a) July 21th, 2009 (Aqua 17:40 UTC) dust plume originated in the PunaAltiplano region showing dust transported to the Pampean Region (Gaiero et al., 2013); (b) April 23rd 2009, dust entrained from the SCWA, and (c) August 29th 2009 dust emission from the north shore of the Mar Chiquita Lake. MJ indicate the position of the dust monitoring station located at Marcos Juárez.

(Bucher and Stein, 2016). Almost coinciding with the MODIS overpass on 29th August 2009, the dust plume was also detected by the Cloud‐Aerosol Lidar with Orthogonal Polarization (CALIOP) instrument on the CALIPSO satellite (Winker et al., 2009). The CALIOP instrument supports particle dispersal simulations indicating a vertical distribution of the dust plume near Marcos Juárez at ~500 masl. The strong similarity between present-day type-C dust obtained at Marcos Juárez and EM3 from loess records at Tortugas and Lozada (Fig. 7b) suggests that the coarse-silt subpopulation detected at the loess sequences is the product of episodic suspension of dust supplied from proximal sources (~300 km). EM3 is finer and less abundant at the easternmost Gorina loess record (Fig. 6). This probably suggests that the same proximal sources contributing dust to Lozada and Tortugas also transported a finer EM3 component to the more distant Gorina site. 3.2.4. Environmental significance of present-day type-D dust Type-D present-day dust is characterized by a bimodal distribution whose components roughly coincide with present-day EM2 and EM3 (Fig. 7a). No particular meteorological pattern of rainfall or wind strength characterizes deposition of this present-day dust type, given that both dry and wet, as well as calm to high wind intensity conditions are observed. As mentioned before, given that each present-day dust sample at the Marcos Juárez station integrates an average of 12 days of dust activity, and that typical dust events associated to type-B and type-C present-day dust last less than 48 h (Cosentino et al., 2019; Gaiero et al., 2013), type-D dust is probably the result of integrating type-B and type-C dusts over an individual sampling period.

Fig. 9. Decreasing trend of the average of EM2 against the mean distances from the Puna-Altiplano Plateau. The black points represent; 1) Lozada, 2) Tortugas and, 3) Gorina. The distances from the Puna-Altiplano are calculated as a mean distance between the northern and southern extremes from these areas to each loess deposits. The exponential decay fitting curve has R2 = 0.9926.

present-day being a drier period in the southern Altiplano than the 20–50 ka BP period (Fig. 4, see discussion in Section 3.1.1). However, this may be reconciled by the fact that the surface of the Uyuni basin has been a salt pan for the last 6–7 ka (Fritz et al., 2004), during which a lack of fine sediment delivery to the basin has arguably resulted in a present-day dearth of EM1-like materials.

4. Final remarks High-temporal resolution grain-size analysis of loess samples from a NW to SE transect in the Pampean plain reveals that loess deposited during the late Pleistocene-early Holocene has multimodal grain-size distributions consistent with the existence of multiple source areas and transport pathways. Unmixing of GSDs shows that the proximal loess depositional areas show two main grain-size subpopulations, while the record shows three different subpopulations. Evidence supports that the very fine silt-clayey component found at the most distal loess profile is not the product of weathering or pedogenetic process, but instead could be associated with a distant dust source. Also, fine dust subpopulations constitute the main component of all the loess profiles, whose contribution increased during the beginning of the Holocene, probably associated with the activation of distal sources located in the elevated Puna-Altiplano region. During the Antarctic Cold Reversal period (ACR; 14.3–11.9 ka) the proximal Pampean loess sections show an increasing

3.2.3. Environmental significance of present-day type-C dust Type-C is the coarsest of the modern unimodal dust types (~51 µm), and is observed in a single sample (Fig. 7a). Deposition of this sample took place under null rainfall (Table 2), so that dry deposition is the dominant mechanism. During the period of collection of this sample (20 to 30/08/09), the National Meteorological Service at Marcos Juárez reported 3 days of reduced visibility caused by dust in suspension associated with strong northerly winds. According to satellite images, for example from 29th August (MODIS image, Fig. 8c.), dust reaching Marcos Juárez was entrained from the shoreline of the Mar Chiquita lake, distant ~250–300 km N from this city. For this date, it is estimated that dust activity at the shore of the lake started at ~10 UTC and was detected at Marcos Juárez approximately four hours later. Forward particle trajectory modeling indicates that the dust plume was transported near the surface at an altitude of about 300 m from the ground 10

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trend of coarser aeolian components, probably explained by stronger low troposphere winds associated to an equatorward displacement of the westerlies belt during a cold phase, or due to the activation of closer source areas such as the Mar Chiquita shallow lake. Sampling of present-day dust flowing through a monitoring station located in the central Pampas constitutes a reliable modern analog for understanding past dust transport and deposition processes in the region. We detected three main subpopulations/end members that explain the grain-size distributions of present-day dust during different weather conditions. The comparison of present-day end members with those from the loess records suggests that the presence of fine and very fine-silt dust at the Pampean loess could be linked to the fallout of aeolian sediments supplied from distant and high-altitude dust sources (i.e., Puna-Altiplano Plateau). However, data did not permit to distinguish any dominant dry or wet deposition mechanism for explaining the accretion of this fine sediment fraction. Present-day dust data suggest that the coarse-silt subpopulation observed in the loess profiles are linked to episodic suspension of dust supplied from proximal sources (~300 km). The finer character of this coarse-silt subpopulation and its lower abundance in the most distal loess record suggests a similar origin but a greater distance of transport.

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CRediT authorship contribution statement Gabriela Torre: Conceptualization, Software, Methodology, Formal analysis, Investigation, Writing - original draft, Visualization. Diego M. Gaiero: Resources, Writing - review & editing, Supervision, Project administration, Funding acquisition, Conceptualization. Nicolás J. Cosentino: Software, Validation, Investigation, Writing - review & editing, Visualization. Renata Coppo: Validation, Investigation, Writing - review & editing. Acknowledgements We thank the Instituto Nacional de Tecnología Agropecuaria (INTA) for continuous support with the Marcos Juárez dust monitoring program. Financial support was provided by awards from Agencia Nacional de Promoción Científica y Tecnológica (PICT-2012-0525 and PICT-2017-2705) and from SECyT-Universidad Nacional de Córdoba, to DMG. It was also partly supported by the ECOS-MINCyT and CONICETCNRS projects. We thank Prof. J. Vandenberghe and two anonymous reviewers for their insightful comments, from which our manuscript has greatly benefited. We acknowledge S. Goldstein, Y. Kiro and L. Bolge for their contribution to the chemical analyses. Appendix A. Supplementary data Supplementary data to this article can be found online at https:// doi.org/10.1016/j.aeolia.2019.100563. References Arimoto, R., 2001. Eolian dust and climate: relationships to sources, tropospheric chemistry, transport and deposition. Earth Sci. Rev. 54, 29–42. https://doi.org/10. 1016/S0012-8252(01)00040-X. Baker, P.A., Seltzer, G.O., Fritz, S.C., Dunbar, R.B., Grove, M.J., Tapia, P.M., Cross, S.L., Rowe, H.D., Broda, J.P., 2001. The History of South American Tropical Precipitation for the Past 25,000 Years. Science 291 (5504), 640–643. https://doi.org/10.1126/ science.291.5504.640. Bokhorst, M.P., Vandenberghe, J., Sümegi, P., Łanczont, M., Gerasimenko, N.P., Matviishina, Z.N., Marković, S.B., Frechen, M., 2011. Atmospheric circulation patterns in central and eastern Europe during the Weichselian Pleniglacial inferred from loess grain-size records. Quat. Int. 234, 62–74. Bucher, E.H., Stein, A.F., 2016. Large salt dust storms follow a 30-year rainfall cycle in the Mar Chiquita Lake (Córdoba, Argentina). PLoS One 11 (6), e0156672. Campodonico, V.A., Rouzaut, S., Pasquini, A.I., 2019. Geochemistry of a Late Quaternary loess-paleosol sequence in central Argentina: implications for weathering, sedimentary recycling and provenance. Geoderma 351, 235–249. https://doi.org/10.1016/j. geoderma.2019.04.024. Cosentino, N.J., Gaiero, D.M., Torre, G., Pasquini, A., Coppo, R., Arce, J., Vélez, G., 2019.

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