Late Pleistocene alluvial fan evolution along the coastal Atacama Desert (N Chile)

Journal Pre-proof Late Pleistocene alluvial fan evolution along the coastal Atacama Desert (N Chile)

Melanie Bartz, Janek Walk, Steven Binnie, Dominik Brill, Georg Stauch, Frank Lehmkuhl, Dirk Hoffmeister, Helmut Brückner PII:

S0921-8181(19)30576-4

DOI:

https://doi.org/10.1016/j.gloplacha.2019.103091

Reference:

GLOBAL 103091

To appear in:

Global and Planetary Change

Received date:

30 May 2019

Revised date:

15 November 2019

Accepted date:

26 November 2019

Please cite this article as: M. Bartz, J. Walk, S. Binnie, et al., Late Pleistocene alluvial fan evolution along the coastal Atacama Desert (N Chile), Global and Planetary Change(2019), https://doi.org/10.1016/j.gloplacha.2019.103091

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© 2019 Published by Elsevier.

Journal Pre-proof Late Pleistocene alluvial fan evolution along the coastal Atacama Desert (N Chile)

Melanie Bartz1* , Janek Walk2 , Steven Binnie3 , Dominik Brill1 , Georg Stauch2 , Frank Lehmkuhl2 , Dirk Hoffmeister1 , Helmut Brückner1

Institute of Geography, University of Cologne, 50923, Cologne, Germany

2

Department of Geography, RWTH Aachen University, 52056, Aachen, Germany

3

Institute of Geology, University of Cologne, 50923, Cologne, Germany

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*corresponding author: [email protected], +492214707719

Journal Pre-proof Abstract Due to their sensitivity to both tectonic activity and climatic variations, coastal alluvial fans (CAF) along the western flank of the Coastal Cordillera in the Atacama Desert (northern Chile) are important geo-archives for unravelling Quaternary environmental change. Our study focuses on terrestrial and marine deposits of five CAF complexes between 20° and 25°S along the coastal zone of the Atacama to identify phases of alluvial fan activity during the Late Quaternary. Based on a combination of luminescence dating and

10

Be cosmogenic

nuclide exposure dating as well as existing chronological data in the area, insights into climatic variations along the hyper-arid coast are presented for the Late Pleistocene derived

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from CAF morphodynamics.

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Activity of alluvial fans could be documented during time spans 95-80 ka, 60-45 ka, 35-20 ka, as well as the Holocene. Numerical dating of marine terrace deposits gives insights into the

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tectonic uplift of the Coastal Plain in northern Chile during the Late Quaternary period, for which estimated uplift rates between ~0.06 and ~0.57 m/ka have been derived. While tectonic

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activity induces base-level changes, long-term tectonic activity rather indirectly controls changes from the Pacific Ocean.

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alluvial fan activity. We suggest that alluvial fan activity is mainly controlled by atmospheric

Based on our observations, CAF in the hyper-arid Atacama Desert serve as suitable geousefulness

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archives for reconstructing climate changes during the Quaternary. In particular, the of alluvial fan systems in a water-limited

environment is important for

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understanding the palaeoenvironmental evolution in a coastal desert.

Keywords Atacama Desert, luminescence dating, cosmogenic nuclide dating, alluvial fans, marine terraces, palaeoclimate, tectonic uplift

Journal Pre-proof 1 Introduction An alluvial fan is a distinct terrestrial landform that is part of an erosional-depositional system, where fan deposits radiate downslope from mountainous areas (cf., Bull, 1977, 1968). Due to its sensitivity to both climatic changes and tectonic activity, an alluvial fan may serve as an important record to understand the environmental evolution (Harvey et al., 2005; Mather et al., 2017). Although this fluvial system may develop in almost all climatic settings, it has been most systematically studied in drier parts of the world (e.g., Blair and McPherson, 2009, 1994; Clarke, 1994; de Haas et al., 2014; Hartley et al., 2005; Harvey et al., 2005; Porat et al., 1997; Sohn et al., 2007). This is mainly due to the good preservation of landforms provoked

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by arid to hyper-arid climate conditions in desert environments (e.g., Dunai et al., 2005;

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Evenstar et al., 2017; Houston, 2006).

The hyper-arid Atacama Desert in northern Chile represents an excellent setting for studying

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these landforms since an extensive record of Quaternary alluvial fans has been preserved along both flanks of the Coastal Cordillera in the western margin of this desert (Hartley et al.,

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2005; Walk et al., this issue). Due to the orographic situation of the Coastal Cordillera as a barrier to precipitation between the Pacific Ocean and the Central Depression, contrasting

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evolution between the active coastal alluvial fans (CAF) and the less active ones located in the eastern Coastal Cordillera has been observed (Hartley et al., 2005). As for the recent

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development of the CAF, a relationship has been established between El Niño Southern Oscillation (ENSO)-related events and morphodynamic activity (Bozkurt et al., 2016; Vargas

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et al., 2006, 2000). Although ENSO reflects the primary cause of climate variability in this region, it is still unclear whether fluvial erosion and sediment transport are mainly controlled

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by ENSO (Bozkurt et al., 2016; Morera et al., 2017). Thus, the research question is still open as to whether climate presents the main trigger for alluvial fan activity during the Late Quaternary in the Atacama Desert (de Haas et al., 2014; Hartley et al., 2005; Harvey et al., 1999, 2005; Vargas et al., 2000, 2006; Walk et al., this issue). Understanding environmental evolution is dependent on a robust chronology. So far, CAF in northern Chile have mainly been studied with focus on their geomorphological characteristics (e.g., distribution, drainage basin characteristics or surface morphology, and depositional processes) (de Haas et al., 2014; Hartley et al., 2005; Walk et al., 2019), while constraining the timing of morphodynamic activity in CAF beyond the upper dating limit of radiocarbon techniques (e.g., Ortlieb, 1995; Vargas et al., 2006, 2000) has received only little attention. Recently, Bartz et al. (in review) showed for the first time the potential of post infraredinfrared stimulated luminescence (pIR-IRSL) dating of K-feldspars in combination with electron spin resonance (ESR) dating of quartz on three CAF and underlying marine terraces along the northern coast of Chile between 21° and 23°S. As part of the dating study of Bartz

Journal Pre-proof et al. (in review), we extend the study area along the coastal zone of northern Chile from 20° to 25°S to get further insights into the evolution of alluvial fans in a hyper-arid coastal setting where the limiting factor for CAF development is precipitation. We focus in detail on five CAF, where sampling has been carried out on terrestrial, marine and aeolian deposits, since the three different compartments can be indicative for tectonic and climatic variations in the study area due to coastal uplift and wetter/drier climate conditions, respectively. By doing so, we aim to (i) map geomorphological and sedimentary characteristics of the studied alluvial fans interacting with marine terraces; (ii) establish a chronostratigraphical framework of the studied CAF by using a combination of luminescence dating and terrestrial cosmogenic nuclide (10 Be) dating; and (iii) evaluate the role of climate for changes in sedimentary and

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geomorphic sequences on Quaternary fans along the north coast of Chile. In particular, we try to test the potential of using coastal alluvial fans in a hyper-arid environment as a

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palaeoclimate archive in Quaternary time scales. In addition, we give insights into coastal

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uplift in the tectonically active area and the potential to affect CAF dynamics.

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2 Study Area 2.1 Regional climatic setting

The coastal zone of northern Chile is part of the Atacama Desert (Fig. 1), where hyper-arid

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climate conditions are evident for at least the last 25 Ma (Dunai et al., 2005). This is primarily

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caused by the prevention of precipitation due to (i) upwelling cold water of the Peru-Chile (Humboldt) current; (ii) the zonal location between 15° and 30°S in the sub-tropical high-

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pressure belt (Hadley circulation); (iii) the rain-shadow effect that is exhibited by the central Andes; and (iv) the large distance from the Amazonia-Atlantic moisture source (e.g., Garreaud, 2011; Houston, 2006; Houston and Hartley, 2003; Insel et al., 2010; Takahashi and Battisti, 2007).

The present climate along the Pacific coast of the Atacama Desert experiences very low precipitation rates with <5 mm/a (Cereceda et al., 2008; Garreaud et al., 2010; Houston and Hartley, 2003; Vargas et al., 2006). However, coastal fog, locally termed Camanchaca, that originates from subsiding warm air formed by the interaction of the SE Pacific anticyclone and the cold Peru-Chile current penetrates the Coastal Cordillera up to elevations of 800-1000 m a.s.l. (above sea level) (Aravena et al., 1989; Marchant et al., 2007). Although coastal fog does not produce rain, it brings moisture and associated salts into alluvial fan catchments, which can lead to chemical weathering of near-coast rocks and sediments (de Haas et al., 2014; Goudie et al., 2002). Since ~2 Ma, sea surface temperatures (SST) in the eastern Pacific declined significantly, while in turn the present ENSO climate system was established, meaning that the latest period

Journal Pre-proof of aridity has been prolonged and intervened with the onset of the ENSO-like climate system (Amundson et al., 2012). The temperature regime along the South American coast is largely influenced by the ENSO and SST anomalies of the adjacent ocean (Schulz et al., 2011), while the magnitude of ENSO-related SST anomalies increases northward to the tropics (Montecinos and Aceituno, 2003). Infrequently occurring precipitation events are likely related to warm ENSO cycles that may trigger alluvial fan activity (Bozkurt et al., 2016; Vargas et al., 2006, 2000). For instance, an extraordinarily strong precipitation event occurred in March 2015, when ~24 mm rainfall over one day was observed in the Antofagasta region (Bozkurt et al., 2016). The coastal Atacama bears witness of this event, which is evidenced at several locations north of Antofagasta by reactivated alluvial fans and correlated deposits

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(Bozkurt et al., 2016).

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[Figure 1, 1 column image]

2.2 Regional Late Quaternary climate evolution

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Palaeoclimate records in the hyper-arid Atacama Desert covering the Late Quaternary period are rare and often discontinuous. Off the coast of N Chile, marine sediments give insights into climatic variations of the last ~400 ka, reconstructed from SST at 17°S (Calvo et al., 2001).

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Maximal SST values are found during interglacial periods, which are well correlated with the

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orbital parameter of eccentricity (Calvo et al., 2001). Based on their marine record off the coast of the southern Atacama (27°S) covering the last ~120 ka, Stuut and Lamy (2004)

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documented drier climate conditions that follow a pronounced variability on the orbital precession band (Lamy et al., 2000; Stuut and Lamy, 2004). While latitudinal shifts of atmospheric and oceanic frontal zones enhance precipitation, long-term changes in ENSO also affect the precipitation pattern in northern Chile, which in turn is in agreement with lower SST and weakened Walker circulation during glacial periods (e.g., Last Glacial Maximum; LGM) (Stuut and Lamy, 2004). Lacustrine deposits in the Altiplano show evidence of climate fluctuations in the Late Quaternary. Wetter climate conditions are documented during glacial periods in the Bolivian and Peruvian Altiplano (e.g., Baker et al., 2001; Baker et al., 2001; Baker and Fritz, 2015; Fritz et al., 2012, 2007; Wirrmann and Mourguiart, 1995). In particular, Fritz et al. (2012) stated that the marine isotope stage (MIS) 5e is the driest period at Lake Titicaca. Climate trends during the last tens of thousands of years are also evident in the Chilean Altiplano. At Salar de Atacama (23°S), wetter phases are documented from ~76-61, ~53-15, ~27-17 ka and as short wet periods during the Holocene (Bobst et al., 2001), which coincide with enhanced aridity during interglacial periods. The vegetation history reconstructed from fossil rodent

Journal Pre-proof middens from the Calama and Salar de Atacama basins (22°-25°S) gives information on climatic conditions during the last ~40 ka (Latorre et al., 2002). While conditions from ~40 to ~22 ka are interpreted as drier periods, more humid conditions are evident between ~15 and ~10 ka (Latorre et al., 2002). In the latter time range, lake level highstands (Geyh et al., 1999) and discharge in wetlands in the Chilean Altiplano (Betancourt et al., 2000; Rech et al., 2002) are also reported. However, little is known about climate variability in the central and western Atacama Desert. Recently, Ritter et al. (2019) documented more humid climate phases during MIS 7 and MIS 5 from a sedimentary record in the core of the Atacama (21.5°S). The authors also stated that

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glacial periods are contrarily characterized by drier conditions. Obviously, wetter phases in

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the core of the Atacama are synchronous to marine sediment records, while being rather asynchronous to wet phases in the Altiplano (Ritter et al., 2019). The authors interpreted such

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humid periods as resulting from El Niño-like conditions that are directly reflected in enhanced erosion and sediment transport. In contrast, fluvial archives connected to the drainage of the

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Peruvian Andes do not clearly show that changes in ENSO are responsible for fluvial erosion

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and sediment transport (Morera et al., 2017).

2.3 Structural and geomorphological setting

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The seismogenic junction between the Nazca and the South American plates leads to a

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Pliocene-Quaternary tectonic extension at the active Andean forearc in northern Chile and southern Peru (e.g., García-Pérez et al., 2018; González et al., 2003; Hartley and Jolley, 1995;

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von Huene and Ranero, 2003). Consequently, directly linked to plate coupling at the subduction zone, crustal deformations are evident in the westernmost subaerial part of the South American continental crust, the Coastal Cordillera (e.g., Hartley et al., 2000; Mather and Hartley, 2006; von Huene and Ranero, 2003). Here, the Atacama Fault Zone (AFZ) extends >1000 km between Iquique (20°S) and La Serena (30°S), where large-scale structures are expressed as mostly N-S- to NE-SW-striking normal faults as well as sinistral strike-slip faults, and reverse faults (Lavenu et al., 2000). Our study area (20°-25°S) comprises two main geomorphological regions (Fig. 1): the Coastal Cordillera and the Coastal Plain. The Coastal Cordillera is an eroded Mesozoic volcanic arc, mostly ~1000-2000 m high and on average ~30 km wide (García-Pérez et al., 2018; González et al., 2003; Scheuber and Gonzalez, 1999). The western margin of the Coastal Cordillera of northern Chile is distinguished by the Coastal Cliff. Tolorza et al. (2009) reported an age of <2.8 Ma of the Coastal Cliff based on a cross-cutting relationship with tephra deposits within the Alto Hospicio near Iquique (Fig. 1). Various interpretations exist for the development of the Coastal Cliff, from an eroded Miocene fault scarp (Paskoff, 1978), or an active fault with

Journal Pre-proof subsequent uplifting (Armijo and Thiele, 1990) to marine erosion due to sea-level changes during the Early Cenozoic (Mortimer and Saric, 1972) and erosion due to extensive large landsliding (Mather et al., 2014). The Coastal Plain is characterized by a recent extension up to 3 km between the modern coastline and the Coastal Cliff in the study area, encompassing alluvial fan sediments, which interact with uplifted marine terraces (Hsu et al., 1989; Leonard and Wehmiller, 1991; Marquardt et al., 2004; Ortlieb et al., 1996; Radtke, 1989; Ratusny and Radtke, 1988; Victor et al., 2011). The cliff foot was estimated by Regard et al. (2010) to ~110 m a.s.l.; it is chronologically constrained by extrapolation of re-appraised uplift rates to the marine isotope stage (MIS) 11 (~400 ka; cf., Regard et al., 2010). The published Quaternary uplift rates are between 0.1 and 0.3 m/ka in the area from 18° to 25°S (Hsu et al.,

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1989; Leonard and Wehmiller, 1991; Ortlieb et al., 1996; Victor et al., 2011), they should be taken with caution, since coastal uplift has most likely not been spatially and temporally

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constant during the Quaternary (e.g., Binnie et al., 2016; González-Alfaro et al., 2018;

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Saillard et al., 2009).

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2.4 Coastal alluvial fans (CAF) between 20° and 25°S

Hundreds of coastal alluvial fans (CAF) occur at the slope break between the Coastal Cliff and the Coastal Plain along the western margin of the Coastal Cordillera, where fan toes are

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truncated at the coastline (Fig. 1; Walk et al., this issue). Due to the unique tectonic and climatic situation of the Mejillones Peninsula (Binnie et al., 2016; Melnick, 2016; Walk et al.,

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this issue), we do not consider alluvial fans from the peninsula or the Coastal Plain to the east

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of it but only along the N-S trending coastal zone. These include five CAF between ~20° and ~25°S, namely Río Seco (SEC, Fig. 2), Guanillos (GUA, Fig. 3), and Caleta El Fierro (VIR, “Virgen del Camino”, Fig. 4) north of the Mejillones Peninsula as well as Coloso (COL, Fig. 5) and Botíja (BOT, Fig. 6) south of the Mejillones Peninsula.

2.4.1 Río Seco (SEC) alluvial fan complex The SEC alluvial fan system debouches into the Pacific Ocean at 20.99°S and 70.15°W (Figs. 1 and 2). The source-area lithology mainly consists of volcanic and plutonic rocks associated with the La Negra formation and the intrusive complex of the Cerro Carrasco, respectively (Quezada et al., 2012; Vásquez and Sepúlveda, 2012). The terrain is dissected by a series of N-S trending faults predefining the extent and stream network of the catchment (Fig. 2a). The SEC alluvial fan complex is fed by one major catchment (~106 km²). With a drainage density of 2.8 km-1 , the catchment is characterized by a basin relief ratio, which relates the basins’ relief to its length (Schumm, 1956), of 0.14 and a mean slope of ~17° (Walk et al., this issue). At SEC, a single laterally non-confined alluvial fan spreads over the Coastal Plain with a

Journal Pre-proof mean mid-radial slope of 4° before encountering the shore after ~0.9 km (Fig. 2b). The recent cliff reaches heights of ~13 m. The main channel is incised from the apex to the toe and blocked at two locations by artificial dams (Fig. 2b). Four sedimentary sections have been investigated at the SEC site (S1-S4; Fig. 2b).

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2.4.2 Guanillos (GUA) alluvial fan complex The GUA alluvial fan system flows into the Pacific Ocean at 21.97°S and 70.18°W (Figs. 1

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and 3). The source-area lithology consists mainly of volcanic and plutonic rocks associated with the La Negra formation and the intrusive complex of Tocopilla (Mpodozis et al., 2015;

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Skarmeta and Marinovic, 1981; Walk et al., 2019) (Fig. 3a). The fan complex is fed from two adjacent catchments. The northern catchment is the much larger one (14.6 km²), featuring a

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drainage density of 2.7 km-1 , a relatively high basin relief ratio of 0.3, and is accordingly steep with a mean slope of 25° (Walk et al., this issue). The smaller southern catchment (1.9 km²)

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drains with a density of 3.2 km-1 just the flank of the Coastal Cordillera and, thus comprising an even higher basin relief ratio of 0.63 and a mean slope of 36° (Fig. 3a). The alluvial fan complex exhibits a mid-radial length of ~1.2 km and a mean slope of 9°. Encountering the

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shore, a cliff exceeding 50 m in height has developed due to marine erosion (Fig. 3b; Walk et

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al., 2019). We refer to Walk et al. (2019) for a detailed geomorphological description of the 3b).

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GUA site. Two sedimentary sections have been investigated at the GUA site (G1 and G2, Fig.

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2.4.3 Caleta El Fierro (VIR) alluvial fan complex The VIR alluvial fan system debouches in the Pacific Ocean at 22.64°S and 70.26°W in the bay locally named Caleta El Fierro adjacent to the site Caleta Tames investigated by Radtke (1989) and Hsu et al. (1989) (Figs. 1 and 4). The source-area lithology consists mainly of volcanic and plutonic rocks associated to the La Negra formation and the intrusive complex of Gatico (diorites, granodiorites and hornblende) (Mpodozis et al., 2015) (Fig. 4a). In total four drainage basins (altogether ~16 km²) feed the VIR alluvial fan complex. Catchment areas range between 2.2 and 5.7 km², while their basin relief ratios as well as mean slopes continuously decrease towards the south from 0.35 to 0.16 and from 30° to 16°, respectively. Drainage densities, however, vary inconsistently between 2 and 3.3 km-1 (Walk et al., this

Journal Pre-proof issue). The course of the main channels in the two northern catchments clearly follows NESW trending faults. In contrast, no faults are evident in the southern catchments (Fig. 4a). Two fans stretch radially over ~1.9 to ~2 km with mean slopes of 6° to 8° to a coastal cliff reaching a height of ~30 m a.s.l (Fig. 4b). Two sedimentary sections have been investigated at the VIR site (V1 and V2, Fig. 4b).

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2.4.4 Coloso (COL) alluvial fan complex

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At 23.76°S and 70.46°W, the COL alluvial fan system debouches in the bay of Antofagasta (Fig. 1). It is one of only few catchments in the Coastal Cordillera that are composed of

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(sub)metamorphic rocks – mainly orthogranulites and metadiorites – besides the typical plutonic lithology of intrusive complexes (González and Niemeyer, 2005) (Fig. 5a). The fan

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complex covers an area of ~0.2 km² and is fed by one major catchment from the south (~30 km²), a smaller adjacent catchment (<0.5 km²), and several colluvial cones emerging from the

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western flank of the Coastal Cordillera (Figs. 5a-b). According to the large differences in size, also the drainage density, basin relief ratio, and mean slope of the two southern catchments vary from 2.9 km-1 , 0.11, and 17° for the large catchment to 2 km-1 , 0.19, and 25° for the

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small one, respectively. The stream network is further influenced in both catchments by NW-

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SE-trending faults (Fig. 5a). The COL complex is short (0.3 km radius) and mean slopes from the alluvial fan apices to the 10-12 m high coastal cliff are relatively low (~4-6°).

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The fan surface is highly disturbed by settlements and construction sites (Fig. 5b). Besides the alluvial fan complex, three distinct levels of marine terraces are developed (Ortlieb, 1995; Radtke, 1989). The lower marine terrace, located at an elevation of up to ~6 m a.s.l., was attributed to the last interglacial transgression by Radtke (1989) based on ESR- and U/Thdated marine shells, and to MIS 5e by Ortlieb (1995) using amino acid racemization (AAR) dating. The upper (~70 m a.s.l.) and middle (~27 m a.s.l.) terraces yielded ambiguous dating results. Based on assumptions, a Pliocene to Early Pleistocene age, as well as a Middle Pleistocene age were assumed for the higher and middle terrace, respectively (Martinez and Niemeyer, 1982; Radtke, 1989). Three sedimentary sections have been investigated at the COL site (C1-C3, Fig. 5b). Sections C1 and C2 represent the sections investigated by Radtke (1989).

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Journal Pre-proof 2.4.5 Botíja (Quebrada de Izcuña) (BOT) alluvial fan The BOT alluvial fan system debouches from the Quebrada de Izcuña in the Pacific Ocean at 24.57°S and 70.56°W, located ~7 km south of Caleta Botíja (Figs. 1 and 6). The catchment measures ~196 km² in size and is mainly composed of the Izcuña quartzous diorite and the Paranal and Desplazado intrusive complexes (Álvarez et al., 2016; Domagala et al., 2016) (Fig. 6a). The lithology is cut by a series of N-S to NNW-SSE trending faults and the stream network shows a density of 2.4 km-1 . Although the Coastal Cordillera reaches maximal elevations in this area, the basin relief ratio of the catchment is with 0.12 comparatively low due to its large size. The catchment further features a moderate mean surface slope of 18°

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(Walk et al., this issue). On the coastal plain, a single, laterally non-confined alluvial fan

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radiates over ~1 km from the catchment outlet to the coast (Fig. 6b). Along the mid-radial profile, the BOT fan developed a mean slope of 4° before being eroded in a ~20 m high

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coastal cliff. The whole area is nowadays covered by cm-thick aeolian sands and very sparse Loma vegetation. Three sedimentary sections have been investigated at the BOT site (B1-B3,

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Fig. 6b)

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3 Field work and applied methodology

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3.1 Site selection

The heterogeneity of clast-rich alluvial fan deposits often hampered high-resolution sampling

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at most of the CAF sites along the coast of the Atacama Desert. Suitable sites had to have well preserved vertical sections (e.g., channel outcrops) with sandy parts for luminescence dating and quartz-bearing rocks at alluvial fan surfaces or marine formations for

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Be dating. Out of

the hundreds of CAF along the N-S trending coast (Walk et al., this issue), five CAF sites were systematically selected based on the following criteria: (i) availability of suitable material for luminescence and exposure dating in the just described sense; (ii) already existing chronological data at the CAF sites themselves or in the vicinity (e.g., independent age control given by ESR/U-Th dating of marine terrace deposits by Radtke, 1989); and (iii) the geographical position between 20° and 25°S to get insights into N-S variations.

3.2 Sampling strategy For luminescence dating, sandy parts in the CAF and the marine beach strata underneath the CAF were sampled by hammering steel tubes (25 cm long cylinders with a diameter of 5 cm) into the freshly cleaned vertical sections of the investigated profiles. The tubes were directly

Journal Pre-proof sealed with opaque tape and black bags to avoid exposure to light. The surrounding material was additionally collected for high-precision Germanium Gamma-Ray Spectrometry (HPGe) and water content evaluation. All luminescence samples were processed and measured at the Cologne Luminescence Laboratory (CLL, University of Cologne). Details of laboratory preparation and analytical procedures can be found in Bartz et al. (in review) and in Supplementary Information. For cosmogenic nuclide exposure dating, we collected surface bedrock samples from a wavecut platform at VIR (Fig. 4i-k) and exposed clasts from an alluvial fan surface at BOT, which represents one coherent fan generation (Fig. 6i-k). We chiselled the top few cm of the

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quartzous plutonic bedrock and the debris flow-transported exposed clasts, respectively (Table

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2). Large quartz-bearing boulders were preferred, but we also sampled some entire quartz cobbles at BOT (Fig. 6i-k). Only clasts slightly embedded in the matrix, located in

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topographically stable positions (i.e., not on slopes or slope edges), and without major signs of post-depositional erosion (spalling) were considered for sampling. In addition, we collected 10

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the recent channel sediment close to the catchment outlet in order to assess the

concentration accumulated prior to deposition (sample BOT10; Fig. 6b). The channel features

a heterogeneous petrographic composition reflecting the catchment

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sediment

lithology (Fig. 6a) and is composed of fine sands to coarse gravels. The topographic shielding was assessed in the field for each sample site. Details of laboratory preparation, measurement,

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Be concentrations can be found in Supplementary

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Information.

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and calculation of reagent blank corrected

3.3 Altimetric positioning and uplift determination Differential global positioning system (DGPS) with a TOPCON HiPer Pro positioning instrument in real-time kinematic mode (cm-accuracy in all three dimensions) and a laser distance metre (TruPulse 200 Rangefinder; accuracy of <0.5 m) were used in combination with satellite images (Esri, Digital Globe). The mean sea level (MSL) was taken as reference level to measure heights. In order to estimate the MSL, we used sea level data by “Sea level station monitoring facility” (http://www.ioc-sealevelmonitoring.org) for March 2018. Uplift rates derived from marine terraces and a wave-cut platform have been estimated by using their known age (existing chronological data and newly established chronological constraints) and their altimetric position that can be correlated to MIS (e.g., Pedoja et al., 2014). Shoreline angles of marine terraces are often covered by alluvial fan deposits and are difficult to explore beyond the recent marine cliff. Therefore, elevations of sample sites have been used for the uplift rate calculations with extrapolation to the elevation of the shoreline angle if possible. Shoreline angles have been either used from the literature or assumed where

Journal Pre-proof topographic evidence exist (e.g., BOT; Fig. 6b). The ancient sea level is taken from Dura et al. (2016), Pedoja et al. (2014), and Schellmann and Radtke (2003). Errors are derived from altimetric and chronological (at 1σ-convidence interval) uncertainties.

4 Results and interpretation 4.1 Sedimentology 4.1.1 SEC site Sections S1 and S3 (Figs. 2c and 7a) represent alluvial fan deposits of similar nature: they consist of hyperconcentrated flow deposits. Angular clasts mostly cover the whole gravel

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spectra and are randomly oriented similar to observations by Hartley et al. (2005). However, while S1 is dominated by matrix-rich (medium- to fine-sand) hyperconcentrated flow deposits

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(Fig. 2d), S3 shows mostly clast-rich hyperconcentrated flow deposits. Both sections are characterized by intercalated fine-laminated sand lenses (Figs. 2f-g) (e.g., Mather and Hartley,

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2005). Section S3 also shows few beds that are rather matrix-rich hyperconcentrated flows (Fig. 7a). Sections S2 and S4 document marine deposits (~10-20 m a.s.l.) underlying several

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metres-thick successions of clast-rich hyperconcentrated flow deposits (Figs. 2e, h). Greyish sands, rounded boulders and mollusc shells represent a beach facies in section S4 (~22 m a.s.l.), while brownish homogeneous fine-grained sediments with shell fragments underlying

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rounded gravel and boulder characterize the beach facies of section S2 (~10 m a.s.l.) (cf.,

4.1.2 GUA site

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[Figure 7, 2 column image]

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Hartley and Jolley, 1999) (Fig. 7a).

Section G1 is characterized by ~25 m-thick alluvial fan deposits where aeolian sand subdivides the fan phases (Figs. 3d-e and 7b). Such interlocking aeolian facies are comparable to modern dune formations on steep terrains south of Antofagasta (Ventra et al., 2017). Alluvial fan beds are characterized by debris-flow deposits showing ungraded, cm to dm-thick clasts in a sandy-pebbly matrix. Section G2 is characterized by two distinct units with sandy fluvial deposits that are intercalated by thin coarser layers in the lowermost part and overlying heterogeneous, clast-rich, several metres-thick debris-flow deposits (Fig. 3c). This alluvial fan section G2 is likely fed by palaeochannels from the southern GUA fan and a recent one from the neighbouring fan to the south (Walk et al., 2019) (Figs. 3b and 7b).

Journal Pre-proof 4.1.3 VIR site One section is located within the debris-flow deposits at the confluence of the two fans (V1, Fig. 7c). This section lays open due to anthropogenic works and presents sediments, which are characterized by heterogeneous debris-flow deposits with a sandy matrix, clasts ranging in size up to ~40 cm, and some rare dm-thick fine-laminated sand lenses (Fig. 4c-e). Section V2 (a and b, both can be horizontally correlated; Fig. 7c) comprises marine terrace sediments underlying clast-rich hyperconcentrated flow deposits close to the outlet of the distally incised channel at the coastal cliff (Fig. 4f-h). Bed thickness of the hyperconcentrated flow layers reaches up to 1 m (Fig. 4h). The marine deposits are represented by well-preserved cross-

f

stratified lenses of fine- to medium-grained sand, rich in shell fragments, and interpreted as

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upper shoreface deposits (~17 m a.s.l., section V2a, Fig. 4g). They are comparable to other marine formations at several locations between the Mejillones Peninsula and Gatico (Hartley

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and Jolley, 1999, 1995). The uppermost part of the marine sequence is characterized by ~1.5 m-thick homogeneous fine- to medium sands (~20 m a.s.l., sections V2a and V2b), which

e-

comprise many shell fragments, and overlies rounded cobbles and boulders (V2a); it is

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interpreted as palaeobeach facies similar to observations by Hartley and Jolley (1999) (Fig. 7).

4.1.4 COL site

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The lower unit of section C1 represents marine deposits characterized by rounded cobbles and

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boulders with molluscs and shell fragments (Fig. 7d), while at 6 m a.s.l. a dm-thick transgression horizon ends in a clear unconformity to debris-flow deposits consisting of

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angular gravel to boulder embedded in a sand-rich matrix (Fig. 5c). The terrestrial deposits are partly intercalated by sandy to gravely lenses compared to the matrix of the clast-rich debrisflow deposits. The upper metres of section C1 may be attributed to matrix-rich hyperconcentrated flow deposits (Fig. 7d). Section C2 is located ca. 90 m downstream of the mouth of Quebrada de Jorgillo (Figs. 5b, d) and shows in the upper section marine sands (mostly medium to coarse sand with a minor fine sand component) and rounded cobbles to large boulders associated to a palaeobeach facies (~27 m a.s.l.) (Ortlieb, 1995; Radtke, 1989). The lower unit is hidden by eroded hill slope material. Section C3 shows an alternation of matrix-rich hyperconcentrated flow deposits composed of angular gravels in a silty to sandy matrix, similar to the upper metres of section C1, and well-sorted, laminated aeolian sands (Fig. 5e). Such interlocking aeolian facies has also been observed by Ventra et al. (2017, 2013) close to Coloso.

4.1.5 BOT site

Journal Pre-proof Sections B1 and B2 represent sedimentary features, where debris-flow and matrix-rich hyperconcentrated flow deposits are intercalated by aeolian sediments overlying marine terrace sediments (Fig. 6c,d,g and 7e). The lower marine terrace at ~10 m a.s.l. is characterized by greyish well-sorted, laminated, medium sand that is overlain by rounded cobbles and boulders with shell fragments; the strata represent a former beach environment (Hartley and Jolley, 1999). A higher marine terrace has been found at an elevation of ~35 m a.s.l. (section B2), which is characterized by similar sedimentary features as in section B1. In comparison to the results of Radtke (1989), the beach facies in section B2 may be attributed to foreshore sediments. The inner shoreline angle can be expected at similar elevations due to relief changes of only few metres, as is visible on the upper alluvial fan surface (Fig. 6b). The

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f

marine terrace deposits are overlain by terrestrial flow sediments (transition zone; Fig. 7e) and the metres-thick, matrix-rich hyperconcentrated flow deposits that are intercalated by finer

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lenses. Section B3 shows an alternation of debris-flow deposits and aeolian sand (Fig. 6h). Generally, debris-flow deposits at BOT consist of poorly-sorted, angular gravels to larger

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clasts in a sandy-pebbly matrix. They are clast-rich with a sandy matrix and often intercalated

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by well-sorted aeolian sands.

4.2.1 pIR-IRSL dating results

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4.2. Dating results

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Sensitivity changes could be adequately corrected by using a test dose size of 80% of the expected natural dose. pIR225 signals showed adequate signal brightness and recuperation was

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<5% of the sensitivity-corrected natural signal (Fig. S1a). Measured dose to given dose-ratios are consistent with unity for most (8 of 11) of the samples (Fig. S1b), while samples C-L4790, C-L4792 and C-L4799 show rather lower dose recovery results. Based on observations by Bartz et al. (in review), this might be related to saturated grains in the multi-grain aliquots, which may tend to an underestimated average dose. Fading experiments showed g-values for the pIR225 signal between ~1.11 and 3.18 %/decade, which agree with the previous dating study by Bartz et al. (in review) and independent luminescence dating studies in the area (Trauerstein et al., 2014; del Río et al., 2019). Dose dispersions are characterized by low to moderate scatter with overdispersion (OD) values between ~14 and ~28% (Table 1). The aeolian samples are likely sufficiently bleached (e.g., Veit et al., 2015). Modern analogue samples from the littoral area at VIR confirmed also sufficient signal resetting for the marine samples (Bartz et al., in review). Based on the latter observations, palaeobeach deposits at COL and BOT may also be assumed to be sufficiently bleached. Debris-flow deposits are expected to have worse bleaching characteristics during transport compared to those of aeolian or littoral sediments (e.g., Clarke et al., 1994; Porat et

Journal Pre-proof al., 1997). However, based on the consistency between different hard-to-bleach trapped charge dating signals (K-feldspar pIR-IRSL and quartz ESR), Bartz et al. (in review) showed that even short and high energetic transport of alluvial fans may yield sufficient signal bleaching and, may, thus, give a reliable dose estimation. Thus, we consider the estimated doses as reliable for further age calculations using the central age model (Galbraith et al., 1999). Final dose rates and derived age estimates can be found in Table 1. Final ages of alluvial fan deposition cover the last ~100 ka (Fig. 8).

4.2.2 Cosmogenic nuclide (10 Be) results 10

Be/9 Be measurements lie between 4.3x10-15 and 3.9x10-13 . The reagent blank measured

alongside all samples except for BOT10 gave a

10

Be/9 Be ratio of 1.1x10-15 . The

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The

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f

[Table 1]

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of the reagent blank for BOT10 amounts to 3.0x10 -15 . The subtraction of reagent blank from the samples constituted ≤2.2% of the total 10

Be/9 Be ratio

Be measured in the

Be atoms measured in the

Be atoms measured in the VIR

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BOT samples and between 8.3% and 26.7% of the total

10

10

10

samples (Table 2). We report the blank-corrected concentrations for all samples, however, because of the low concentration and subsequent large subtraction of background

10

Be for

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VIR07, 08 and 11 (14.5% to 26.7%) these measurements are deemed less reliable. The blank

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corrections for the samples VIR09 and 12 (12% and 8.3%, respectively) are not considered so large as to discount these measurements and we derived ages for both these two samples.

10

Be

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concentrations range from 1.13 ± 0.29 x10 4 to 2.71 ± 0.38 x104 at/g and 7.76 ± 0.42 x104 to 2.67 ± 0.09 x105 at/g for VIR and BOT, respectively (Table 2). The recent channel sediment sample BOT10 shows a

10

Be concentration of 2.47 ± 0.12 x105 at/g.

Exposure ages are given in Table 2 with production rates scaled according to Lifton et al. (2014) (LSDn). For the exposure age calculation we used erosion rates ranging between 0 (zero erosion) and 3 m/Ma, based on a Monte Carlo simulation (Hidy’s routine) applied by Ritz et al. (2019) for a

10

Be depth profile at the Mejillones Peninsula (~23.2°S). The erosion

rates interpreted as minimum and maximum bedrock and clast erosion rates correspond to the best percent of fits out of 10,000 possible solutions. Moreover, exposure ages using a mean erosion rate of 2.56 m/Ma corresponding to the total average of modelled erosion rates after Ritz et al. (2019) are given. Similar to the results from Ritz et al. (2019), Martinod et al. (2016) assessed depth profile-based erosion rates close to Huasco at the southernmost extension of the Atacama Desert (~28.4°S) of 0 to 1 m/Ma. Values for both the ‘internal’ and ‘external’ uncertainties are provided in Table 2, where internal equates to the relative analytical uncertainties from the concentrations and external to the combined analytical and

Journal Pre-proof production rate uncertainty. Exposure ages based on the mean erosion rate are considered for further interpretation. The two ages derived for VIR09 and VIR11 of ~10 ka are in agreement within internal uncertainties (Table 2). The ages of the BOT samples range between 31±2 and 126±6 ka with three of the six samples giving the same age of ~33 ka within internal uncertainties (Fig. S2).

[Table 2]

4.2.3 Chronostratigraphical framework

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All ages estimated in this study – i.e., a combination of pIR-IRSL and cosmogenic nuclide (10 Be) ages – are in correct stratigraphical order (Fig. 7). The consistency between both dating consistent

10

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techniques, e.g., observed for sediments at BOT, is especially noteworthy: The three Be ages from the fan surface generation yield a mean exposure age of 32.5±1.1 ka

e-

(Table 2, Fig. S2). Cross-checking was rendered by a pIR-IRSL age of 27±3 ka of a sample collected close to the alluvial fan surface and the comparison shows a 2σ-consistency (Fig. 6). 10

Be exposure ages can be considered as most robust in

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Hence, the group of youngest

representing stabilization and subsequent abandonment of this alluvial fan generation. In incorporate

10

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contrast, the three older exposure ages at BOT (69±6 ka to 126±12 ka; Table 2) likely Be produced prior to exposure on the fan (Fig. S2). Using a generalised extreme

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Studentized deviate test (for details see Jones et al., 2019), the three older ages can also be statistically identified as outliers. 10

Be due to pre-exposure can constitute significantly to the total

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That

10

Be concentration in

some clasts, is also indicated by the concentration in the recent channel sediment (BOT10). In contrast to the majority of highly-connected small catchments draining the western Coastal Cordillera, the large catchment at BOT comprises vast segments which are prone to intermediate sediment storage (Walk et al., this issue), which in turn can lead to significant 10

Be inheritance. However, the inherited

10

Be concentration in BOT10 exceeds those of all

samples except for BOT11 (Table 2). A weakness of the approach used to estimate the influence of pre-exposure are the different clast sizes sampled for alluvial fan surface exposure ages (cobbles to large boulders) and from the recent channel sediment (fine sand to coarse gravel). The highly energetic debris flows transporting cobbles and boulders are more likely to flow throughout the catchment without interruptions (Blair and McPherson, 2009); in particular, because they mostly originate from the steep, proximal western flank of the Coastal Cordillera (Walk et al., this issue). In contrast, it can be argued that alluvial sands and gravels accumulate higher amounts of

10

Be due to their gradual erosion and intermittent transport in

Journal Pre-proof the shallow hinterlands (Brown et al., 1995; Carretier et al., 2015). Others have reported similar grain size bias in nuclide concentrations from catchments draining the western Central Andes (Aguilar et al., 2014; Carretier et al., 2015). Van Dongen et al. (2019) noted that the scatter in

10

Be concentrations in fluvial sediments from the Coastal Cordillera can result from

variable depths of excavation during mass wasting processes, which are common in the steep catchments in our study area (e.g., Mather et al., 2014). Thus, the

10

Be concentration of

BOT10 is not directly comparable to the larger fan surface samples, but supports the notion that the youngest clasts have the lowest inherited

10

Be component and provide the most

f

reliable depositional ages.

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[Figure 8, 1 column image]

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4.3 Geomorphological implications

The

studied

alluvial

fan

complexes

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4.3.1 Alluvial fan deposits

represent

mainly

clast-rich

debris-flow

and

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hyperconcentrated flow deposits with a varying proportion of sandy matrix (Hartley et al., 2005) (Fig. 7). Finer sand lenses may be the result of finer matrix material which was

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entrained from debris-flow deposits further upstream, or trapped in channels or topographic lows (de Haas et al., 2014), such as in sections S3 (Figs. 2f-g) or V1 (Fig. 4d-e). At the

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southern fan toe at GUA, metres-thick fine-grained fluvial/alluvial deposits represent a rather atypical toe facies (Fig. 3c). On all fans, salt weathering of clasts and a reddish colouring are

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evident, which can be related to the coastal fog Camanchaca (de Haas et al., 2014; Veit, 1996). Approximately the half of all CAF (with a catchment size >1 km²) along the study area feature major incision based on observation by Walk et al. (this issue). Of those, several are fully incised with a single channel cutting directly from the fan apex to the coast (Hartley et al., 2005; Walk et al., 2019). An exception is the VIR alluvial fan complex, where no incision is obvious; it is rather dominated by shallow washouts (maximum depth of 1 m), which spread over the alluvial fan apron (Hartley et al., 2005). The VIR fan displays distal incision at the fan toe, cutting into the deposits of the debris-flows and the underlying marine terrace. Here, a 10 m-high knickpoint has developed at the channel outlet (Fig. 4c). In this case, it might be assumed that base-level changes lead to landward head-cutting, but eustatic sea-level changes cannot be the reason for distal incision (Hartley et al., 2005; Harvey et al., 1999).

10

Be exposure ages of 10±1 ka for a

wave-cut platform directly located underneath the channel outlet and ~7 m a.s.l., comparable to Holocene sediments at similar elevation at Caleta Michilla (Leonard and Wehmiller, 1991),

Journal Pre-proof clearly show evidence of coastal uplift since the early Holocene that suddenly affected the fan’s base-level. Secondary distal incision is also evident at GUA (Walk et al., 2019) (Fig. 3b), where gullies are cut through alluvial fan deposits. Based on the results in this study, alluvial fan deposits date back to the time spans 95-80 ka, 60-45 ka, 35-25 ka as well as ~14 ka (Fig. 8). In general, based on our chronological data, it seems that insignificant variations in alluvial morphodnamic activity are evident over N to S in Late Pleistocene time scales (Fig. 8).

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[Table 3]

4.3.2 Aeolian deposits

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Aeolian facies in the incised channels or at the coastal cliff consists of well-sorted, laminated fine- to medium sand at GUA, COL, and BOT. According to Walk et al. (this issue), an

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abundance of aeolian sediments is presently evident between 20° and 21°S as well as between 23.5 and 24.5°S. Overall, aeolian deposits date back to ~93-84 ka as well as ~50-27 ka (Fig.

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9). Similar aeolian deposition phases have been observed south and north of the study area in north-central Chile (27.9°S; Nash et al., 2018) and southern Peru (17.68°S; Londoño et al., 2012), respectively. In our study area, the interpretation of wind-blown sediments from local

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alluvial fans (Londoño et al., 2012) is rather unrealistic due to the mainly clast-rich fan

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deposits. Despite the frequent occurrence of coastal fogs, weathering rates of rocks are somewhat low (≤3 m; Ritz et al., 2019) and therefore not responsible for the primary sand

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production. Nash et al. (2018) suggest a predominantly marine sediment source for aeolian deposition, which might also be assumed in our study area in times of sea-level lowstands (Fig. 9).

[Figure 9, 1 column image]

4.3.3 Marine deposits Three different levels of marine terraces have been found at the Coastal Plain buried under alluvial fan

deposits;

they

were

also

compared

to

existing

geomorphological and

geochronological studies in the research area (Table 4). At SEC, deposits from the peak of the last interglacial transgression (MIS 5e) occur in sections S2 and S4 (Fig. 2e, h). This is in agreement with Ratusny and Radtke (1988) and Radtke (1989) from marine terraces close to Punta Patache and Iquique, respectively, in the

Journal Pre-proof north of the SEC site (Fig. 1). Although interpretations have to be taken with caution due to the close to saturation level of the luminescence signals (Bartz et al., in review), it seems that in section S4 (Fig. 2h), the beach facies corresponds to the MIS 5e-sea level highstand (cf., Hartley and Jolley, 1999). A MIS 5e stage can also be observed at the VIR site. The upper shoreface deposits are dated to 120±10 ka representing a MIS 5e-facies (Bartz et al., in review). According to Leonard and Wehmiller (1991), the MIS 5e-transgression peak with beach deposits may be expected at an elevation of ~40 m a.s.l. The beach facies at ~20 m a.s.l. at VIR has also been observed at Caleta Michilla at similar elevations. Leonard and Wehmiller (1991) dated these deposits to

f

the last interglacial sea-level highstand, which is consistent with the dating results by Bartz et

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al. (in review), who yielded consistent pIR-IRSL ages of ~90 ka. This can be interpreted as MIS 5c-palaeobeach (Figs. 2g-h and 7).

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Section C1 (Fig. 5c) represents a similar sequence as described by Radtke (1989) and Ortlieb (1995), where beach deposits (dm-thick rounded boulders with shells) have been found at an

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elevation of ~6 m a.s.l. (Fig. 5c). In contrast to Radtke (1989), who ESR-dated in situ mollusc shells from the beach facies to 103-112 ka (n=5, ±15%-uncertainty according to Radtke,

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1989), we sampled the overlying sandy horizon which likely represents the transition zone to terrestrial alluvial fan input (back beach environment with a mixture of shell fragments and

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angular gravel) (Fig. 7). pIR-IRSL dating yielded a deposition age at 98±13 ka (C-L4790), similar to observations by Radtke (1989). Section C2 represents the next higher marine terrace

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at ~27 m a.s.l. (Radtke, 1989). While Radtke (1989) presented ambiguous ESR/U-series dating results, new pIR-IRSL ages indicate a deposition at 139±18 ka (C-L4792). This clearly

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contradicts assumptions of at least MIS 9-deposits (cf., Ortlieb, 1995; Radtke, 1989). Given the fact that marine deposits at section C1 are likely representing the MIS5e sea-level highstand, one may assume that the next higher level at section C2 presents sediments from MIS 7. The reason for age underestimation with the pIR-IRSL technique is uncertain, but might be related to signal saturation (i.e., underestimated dose recovery ratio). At BOT, two marine terraces have been observed at elevations of ~10 and ~35 m a.s.l. (Fig. 6d, g). In comparison to marine terrace formations at similar elevations ca. 10 km north of BOT and at Las Losas ~60 km south of BOT (Radtke, 1989) (Table 4), the lower marine sediments are expected to have been deposited during the MIS 5 sea-level highstand. However, the sandy horizon was pIR-IRSL dated to 62±8 ka (C-L4796), which underestimates the expected age similar to sample C-L4792. A higher marine terrace exist at an elevation of ~35 m a.s.l., which is attributed to an older beach facies (Fig. 6g). In contrast to the marine facies at section B1, samples were taken above the palaeobeach horizon, likely in the transition zone to first alluvial fan input (Fig. 7). pIR-IRSL dating yielded ages up to

Journal Pre-proof 88±12 ka (C-L4799). However, based on the uncertainty of dating the marine samples at the BOT site, both terraces are not used for further interpretations.

[Table 4]

5 Discussion 5.1 Climatic impact on alluvial fan formation between 20° and 25°S According to our chronostratigraphical framework based on an unprecedented combination of 10

Be cosmogenic nuclide exposure dating as well as

f

luminescence dating of K-feldspars,

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existing chronological information in the area (cf., Vargas et al., 2006; Vásquez et al., 2018, Bartz et al, in review), the five alluvial fan complexes show episodic deposition during the

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last ~100 ka (Table 3; Fig. 10).

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[Figure 10, 2 column image]

Some parts of the alluvial fan complexes were accumulated during MIS 5 (Fig. 10). From a terrestrial record at similar latitude and ~10 km east of the coast, Ritter et al. (2019) reported

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wetter phases in the Coastal Cordillera during MIS 5. This is in agreement with higher sea

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surface temperatures (SST) from marine palaeoclimate records off the coast of Peru (17°S; Calvo et al., 2001) (Fig. 10). Warmer SST can be connected to a warmer East Equatorial

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Pacific cold tongue and a southward shift of the equatorial ITCZ system (Rincón-Martínez et al., 2010). Stuut and Lamy (2004) introduced a humidity index for northern Chile based on the flux of terrestrial dust recorded in marine sediments off the coast of the southern Atacama (27°S). They reported increased humidity in phases of northward-driven Southern Westerlies and precession maxima, which point to higher frequency of precipitation events (Lamy et al., 2000). This may have caused morphodynamic activity of the alluvial fans during MIS 5 in the study area (Fig. 10). As for MIS 4, no morphodynamic activity phase has been observed in our fan records, although marine records show climate fluctuations with wetter conditions during that time range (Stuut and Lamy, 2004) (Fig. 10). It is noteworthy that also Ritter et al. (2019) reported fluvial inactivity during MIS 4. Increased morphodynamic activity during MIS 3 is identified for all of the studied locations (Figs. 8 and 10). Although marine records show lower SST during MIS 3 (Calvo et al., 2001; Rincón-Martínez et al., 2010), short-term fluctuations with increased SST are evident between

Journal Pre-proof ~40 and ~60 ka (e.g., Calvo et al., 2001). This is in agreement with the precession maxima observed off the coast at 27°S suggesting moister conditions (Lamy et al., 2000; Stuut and Lamy, 2004). Thus, enhanced fluvial activity along the western side of the Coastal Cordillera resulted in massive alluvial fan deposition such as at GUA (section G1; Fig. 3). Fluvial activity between ~45 and ~60 ka has also been reported further inland (Ritter et al., 2019; Diederich et al., this issue). Similarly, MIS 2 is characterized by increased CAF activity (Fig. 10). Stuut and Lamy (2004) reported more humid conditions between 17 and 27 ka, explained by a weakening of the SE Pacific anticyclone and the equatorward shift of the Southern Westerlies. In addition, cut-off

f

lows from the Westerlies resulted in more humid conditions reaching far inland during MIS 2

oo

(Ammann et al., 2001). These lows may have transported moisture-bearing air into the coastal Atacama, whereby it activated the CAF during MIS 2. Ritter et al. (2019) also showed short-

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term wetter phases at the onset of MIS 2, which is comparable with fluvial activity in our records (Fig. 10).

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Although Holocene alluvial fan activity has not been observed in our study, Vargas et al. (2006, 2000) and Vásquez et al. (2018) reported debris-flow deposition in the studied coastal

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zone based on radiocarbon dating of shells in debris-flow deposits (Fig 10; Table 3). CAF activity increased since the onset of the modern El Niño at ca. 5 ka in northern Chile (Vargas

al

et al., 2006). Bozkurt et al., (2016) demonstrated that recent CAF activity was influenced by tropical Pacific.

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cut-off lows off the coast of northern Chile and positive SST anomalies over the eastern

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Additional important climatic phenomena are the persistent coastal fogs and marine stratus clouds along the coast of northern Chile. Variations of the position of the Southern Westerlies also influence the strength and frequency of the coastal fog (Lamy et al., 2000). The moisturebearing fog triggers pedogenesis and weathering rather than producing rainfall that would lead to surface run-off (de Haas et al., 2014; Veit, 1996). Hartley et al. (2005) already suggested that climate is the principal control for alluvial fan activity at the western flank of the Coastal Cordillera. Climate causes processes of aggradation, weathering, as well as erosion in the highly episodic fluvial systems and forms the alluvial fan surface morphology and texture (e.g., de Haas et al., 2014; Hartley et al., 2005; Walk et al., this issue). In respect to the Coastal Cordillera as an orographic barrier between the Central Depression and the Pacific Ocean, it can be suggested that tropical atmospheric air masses crossing the Andes and the Coastal Cordillera are of negligible influence (Fig. 1) (Houston, 2006). This is in agreement with Ritter et al. (2019) who showed an opposite trend of moisture availability in the Coastal Cordillera compared to that of palaeoclimate records from the Altiplano, such as Lake Titicaca or Salar de Uyuni (e.g., Fritz

Journal Pre-proof et al., 2007, 2004). Thus, based on our observations and in comparison with marine palaeoclimate records (Calvo et al., 2001; Lamy et al., 2000; Rincón-Martínez et al., 2010; Stuut and Lamy, 2004), alluvial fan dynamics along the western flank of the Coastal Cordillera seems to be influenced by an interplay between northward-driven austral Westerlies, ENSO-related positive SST anomalies, and variations in the strength and the position of the SE Pacific anticyclone.

5.2 Tectonic impact on alluvial fan morphodynamics Raised marine terraces have been shown to be a good approximation for tectonic uplift along

f

the coast of northern Chile (e.g., Binnie et al., 2016; Ortlieb et al., 1996; Regard et al., 2010).

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Based on the ages derived here, estimated mean uplift rates in our study area range between ~0.06 and ~0.57 m/ka (MIS 5e to Holocene), showing local differences, but insignificant

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changes from N to S (Table 4). The relative spatial variability of uplift is overall in agreement with Quaternary uplift rates modelled by Melnick (2016) that combine morphometric analysis

e-

with a numerical landscape evolution model. However, modelled absolute uplift rates for the

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entire Quaternary are generally somewhat lower ranging between ~0.05 and ~0.25 m/ka (Melnick, 2016).

Although the Mejillones Peninsula presents a unique tectonic situation (e.g., Mejillones fault,

al

block tectonics; Binnie et al., 2016) compared to that of the coastal strip north and south of

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the Peninsula, González-Alfaro et al. (2018) showed that uplift increased after ~44 ka based on radiocarbon data from MIS 3-marine terraces, and that uplift followed a more regional

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trend along the coast of northern Chile rather than a local process at the Mejillones Peninsula. Somewhat accelerated uplift rates have also been observed in our study from the MIS 5e onwards (Table 4). González-Alfaro et al. (2018) attribute accelerated uplift to changes in the subduction contact between the Nazca and South American plates in northern Chile and to the deep-moderate and large megathrust earthquakes that lead to co-seismic vertical displacement. The VIR and COL sites are located in the area around the Mejillones Peninsula (Fig. 1). At VIR, the fan morphology shows rather untrenched channels in the upper CAF, while incision is evident in the distal part of the CAF (Fig. 4d, f). In particular, it seems that eustatic sealevel fall is not the main reason for Late Pleistocene incision in the distal zones of the CAF (e.g., Harvey et al., 1999). The uplift of the early Holocene wave-cut platform (~7 m above present datum), exceeding the eustatic transgression (~3 m above present datum) by ~4 m (Dura et al., 2016), clearly shows the tectonic influence which creates sudden base-level changes. Compared to the long-term tectonic uplift history in the area, short-term tectonic uplift from megathrust earthquakes (Ruiz and Madariaga, 2018) seem responsible for the missing incision in the proximal CAF at VIR. Apparently, the hydromorphic state of the CAF

Journal Pre-proof has as yet not adapted to the Holocene climate- and tectonic-driven base-level change. This has also been shown at Caleta Michilla ~10 km south of VIR, where vertical deformation of Holocene marine sediments is associated with deep-moderate earthquakes (González-Alfaro et al., 2018). Co-seismic uplift of several decimetres has been observed in the south of the Mejillones Peninsula after the 1995 Antofagasta subduction earthquake as well (Ortlieb et al., 1996). However, at COL, co-seismic vertical deformation has not been observed in the CAF morphology. With respect to the long-term development of the CAF catchments, tectonic influence can be seen in the long-term tectonic activity within the Coastal Cordillera, which consequently

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affects catchment shapes and structures (Hartley et al., 2005; Walk et al., this issue). This is,

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in turn, reflected in different sediment connectivity from the catchment to the CAF. Due to the prominent decrease of relief energy towards the catchment hinterlands and the Pacific origin

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of storms, high sediment transport potential is strongly limited to the western flank of the Coastal Cordillera (Walk et al., this issue). In contrast to the rather indirect control on

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catchment geometry, it seems that tectonic activity is – same as the source-area lithology (cf. Hartley et al., 2005; Walk et al., this issue) – rather insignificant for CAF morphodynamics. It

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is mainly limited to tectonic-driven base-level changes that overlie eustatic base-level changes. Although the established chronostratigraphical framework gives new insights into

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the tectonic history in the area, a quantification of the tectonic influence on CAF

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6 Conclusion

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morphodynamics, however, remains so far difficult.

The successful combination of luminescence dating of K-feldspars and

10

Be cosmogenic

nuclide exposure dating renders the first chronostratigraphical framework for the alluvial fan complexes of coastal Chile between 20° and 25°S. The results provide insights into the landscape evolution during the last ~100 ka. Although the recent climate is hyper-arid, climatic fluctuations during the Quaternary have to be taken into account. Activity of alluvial fans can be documented during the time spans 9580 ka, 60-45 ka, 35-20 ka, as well as during the Holocene. These phases can be linked to marine palaeoclimate records off the coastal Atacama Desert. We suggest that alluvial fan activity is mainly controlled by atmospheric changes from the Pacific Ocean. Numerical dating of marine deposits yielded depositional ages of MIS 5e (~125 ka) as well as MIS 5c (~100 ka), while the wave-cut platform has been dated to the Early Holocene (~10 ka). These results can be considered in light of the tectonic uplift of the Coastal Plain in northern Chile during the Late Quaternary period, for which estimated uplift rates between ~0.06 and ~0.57 m/ka could be calculated. While Late Pleistocene coastal uplift influences

Journal Pre-proof base-level changes, it seems that long-term tectonic activity rather indirectly controls alluvial fan activity and only by governing drainage basin structure and shape and thus sedimentary coupling (cf., Walk et al., this issue). This work presents first insights into the alluvial fan evolution during the Late Pleistocene, which was so far rather underexplored due to missing chronological data. It is important to note that, based on our observations, CAF in the hyper-arid Atacama Desert serve as suitable geo-archives for reconstructing climate changes during the Late Quaternary. However, further research is needed to validate the interpretations of climatic control on the formation of the coastal alluvial fan complexes. This is particularly true for the interpretation of the alluvial fan

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activity along the N-S transect, which remains as yet difficult due to the still relatively small

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amount of absolute chronological data.

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Acknowledgements

This project is affiliated to the CRC 1211 ‘Earth – Evolution at the Dry Limit’ (subproject

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C2 ‘Transport and deposition: Deciphering the evolution of the alluvial fans between 21°S

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and 25°S – the interplay between climatic and tectonic control’), funded by the Deutsche Forschungsgemeinschaft (DFG, German Research Foundation) (Grant-No.: 268236062–SFB

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1211). E. Campos from the Universidad Católica del Norte in Antofagasta is gratefully acknowledged for helping to organize of the field campaigns. We are grateful to Elena Voronina for laboratory assistance. TanDEM-X WorldDEM™ data was provided by a DLR

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Science grant, 2017. We further thank C. Mpodozis for providing a regional geological map for the area between 22.5 and 23°S. The authors would like to thank V. Regard and an

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anonymous reviewer for their constructive comments, which helped to improve the quality of the manuscript.

Competing interests

The authors declare no competing interests.

Data availability All data generated or analysed during this study are included in this published article. However, the TanDEM-X WorldDEM™ cannot be shared due to legal limitations.

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Figure captions

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Wirrmann, D., Mourguiart, P., 1995. Late Quaternary spatio-temporal limnological variations in the altiplano of bolivia and peru. Quat. Res. 43, 344–354. https://doi.org/10.1006/qres.1995.1040

Fig. 1: Study area in northern Chile (modified after Walk et al., this issue) including main units. Relief in m a.s.l. based on the SRTM 1’’ Global (USGS, 2014), bathymetry in m b.s.l. based on the GEBCO 2014 Grid (BODC, 2014), mean annual precipitation (MAP) according to Fick and Hijmans (2017). The five studied coastal alluvial fan complexes are highlighted, namely Río Seco (SEC), Guanillos (GUA), Caleta El Fierro (“Virgen del Camino”, VIR), Coloso (COL), and Botíja (BOT). Fig. 2: Coastal alluvial fan Río Seco (SEC). a) Regional geological map including structural information (after Quezada et al., 2012; Vásquez and Sepúlveda, 2012), elevation, and hillshade based on TanDEM-X WorldDEM™; b) DigitalGlobe satellite image of the SEC alluvial fan (Esri, 2018) showing the main geomorphological features; c-h) Imagery of the studied sections at SEC (S1-S4) and age results following Bartz et al. (in review). Ages are derived from pIR-IRSL dating (yellow circles). Note that ages in italics are interpreted as possible maximum ages due to overestimation of the luminescence signals (cf., Bartz et al., in review). Fig. 3: Coastal alluvial fan complex Guanillos (GUA). a) Regional geological map including structural information (after Mpodozis et al., 2015; Skarmeta and Marinovic, 1981; Walk et al., 2019), elevation, and hillshade based on TanDEM-X WorldDEM™; b) DigitalGlobe satellite image of the GUA alluvial fan (Esri, 2017) showing the main geomorphological features c-e) Imagery of the studied sections at GUA (G1-G2) and age results following Bartz et al. (in review). Ages are derived from pIR-IRSL dating (yellow circles). Note that ages in italics are interpreted as possible maximum ages due to overestimation of the luminescence signals (cf., Bartz et al., in review).

Journal Pre-proof Fig. 4: Coastal alluvial fan complex Caleta El Fierro (“Virgen del Camino”, VIR). a) Regional geological map including structural information (after Mpodozis et al., 2015), elevation, and hillshade based on TanDEM-X WorldDEM™; b) DigitalGlobe satellite image of the VIR alluvial fan (Esri, 2016) showing the main geomorphological features; c-k) Imagery of the studied sections at VIR (V1V2) and age results following Bartz et al. (in review) derived from pIR-IRSL dating (yellow circles). Ages in green are derived from 10 Be cosmogenic nuclide dating (see Table 2). Fig. 5: Coastal alluvial fan complex Coloso (COL). a) Regional geological map including structural information (after González and Niemeyer, 2005), elevation, and hillshade based on TanDEM-X WorldDEM™; b) DigitalGlobe satellite image of the COL alluvial fan (Esri, 2018) showing the main geomorphological features; c-e) Imagery of the studied sections at COL (C1-C3). Ages are derived from pIR-IRSL dating (yellow circles).

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Fig. 6: Coastal alluvial fan complex Botija (BOT) (Quebrada de Izcuña): a) Regional geological map including structural information (after Álvarez et al., 2016; Domagala et al., 2016), elevation, and hillshade based on TanDEM-X WorldDEM™; b) DigitalGlobe satellite image of the BOT alluvial fan (Esri, 2017) showing the main geomorphological features; c-k) Imagery of the studied sections at BOT (B1-B3). Age results illustrated in yellow are derived from pIR-IRSL dating. Ages in green are derived from 10 Be cosmogenic nuclide dating (see Table 2).

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Fig. 7: Sedimentary logs of investigated sections along the coastal zone of northern Chile. df = debris flow, mrhf = matrix-rich hyperconcentrated flow, crhf = clast-rich hyperconcentrated flow, a.s.l. = above sea level. Note that luminescence ages in italics are interpreted as possible maximum ages due to overestimation of the luminescence signals (cf., Bartz et al., in review).

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Fig. 8: Compilation of age results (pIR-IRSL dating as filled circles and 10 Be cosmogenic nuclide dating as unfilled circles) of alluvial fans along the coastal zone in northern Chile. Note that chronological data from GUA and BOT are derived from intercalated aeolian deposits.

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Fig. 9: Age estimates of aeolian accumulation in the study area based on pIR-IRSL dating in comparison to studies in southern Peru (Londoño et al., 2012) and north-central Chile (Nash et al., 2018), and the sea-level curve based on scaled benthic isotopes (V19-30) after Cutler (2003) and modified after Siddall et al. (2007).

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Fig. 10: Correlation between terrestrial and marine palaeoenvironmental records. (A) coastal alluvial fan (CAF) activity illustrated as kernel density estimation (KDE) plot; (B) clay pan record in the Coastal Cordillera (Ritter et al., 2019); (C) marine record from off the coast of the southern Atacama Desert (Stuut and Lamy, 2004); (D) marine record off the Peruvian coast (Calvo et al., 2001). Morphodynamic activity phases are dated by pIR-IRSL (see also Bartz et al., in review) and 10 Be cosmogenic nuclide dating. Radiocarbon age results from the GUA site (Vargas et al., 2006) and close to the Río Loa mouth (Vásquez et al., 2018) are included to complement all existing age results on alluvial fans in the study area. Marine isotope stages (MIS) are implemented following Lisiecki and Raymo (2005).

Journal Pre-proof Table 1: Luminescence data of the alluvial fan, marine, and aeolian samples. Radioelement concentrations are derived from high-resolution γ-spectrometry analysis. The software DRAC v1.2 (Durcan et al., 2015) was used for dose rate and age calculation using the conversion factors of Guérin et al. (2011) and the alpha and beta attenuation factors of Bell (1980) and Guérin et al. (2012), respectively. nm /na = number of aliquots measured/accepted, OD = overdispersion, CAM = central age model (Galbraith et al., 1999), D e = equivalent dose. (1) the pIR225 signal has been corrected for anomalous fading when g2day s-values ≥1%/decade. Fading-corrected ages are considered for further interpretations. Errors are presented with 1σ-confidence interval. *fading rate has been used from sample C-L4793.

Section

Sample ID

C1

C-L4790

C1

C-L4791

C2 C3

238

Deposit

U (ppm)

232

K (%)

Total dose rate (Gy/ka)

n pIR225 n m/n a

ODpIR225 (%)

Fading rate (%/decade)

40

Th (ppm)

CAM De (Gy)

Fadinguncorrected age (ka)

Fadingcorrected (1) age (ka)

131.24±7.64

77.9±7.3

97.7±13.2

f o

0.58±0.04

1.74±0.15

0.71±0.01

1.68±0.12

47/41

18.6±2.2

2.48±0.95

0.50±0.04

1.42±0.10

0.64±0.01

2.06±0.12

35/34

25.1±3.1

1.59±0.77

100.62±6.62

48.9±4.3

56.3±4.3

C-L4792

marine alluvial fan marine

0.59±0.04

1.17±0.10

0.92±0.02

1.78±0.13

31/30

27.6±3.7

1.74±0.86

211.50±15.17

118.6±11.9

139.0±18.1

C-L4793

aeolian

0.63±0.04

1.32±0.10

0.60±0.01

1.53±0.10

22/16

17.04±3.4

3.18±1.14

105.69±7.12

69.1±6.4

92.5±16.4

C3

C-L4794

aeolian

0.59±0.04

1.08±0.09

0.61±0.01

1.51±0.10

24/22

25.2±4.0

3.18±1.14*

95.18±7.08

63.0±6.2

83.8±10.9

C3

C-L4795

aeolian

0.52±0.03

0.94±0.13

0.56±0.01

1.43±0.10

24/24

25.2±3.8

3.13±0.83

93.39±6.77

65.2±6.5

86.7±11.9

B1

C-L4796

marine

1.38±0.06

3.93±0.20

1.37±0.02

2.93±0.16

12/12

15.3±3.2

2.15±0.77

148.75±9.99

50.8±4.3

61.5±7.7

B1

C-L4798

aeolian

0.99±0.05

2.36±0.13

1.03±0.02

2.28±0.12

24/24

18.1±2.7

2.07±0.39

83.56±5.22

36.7±3.0

44.0±4.1

B2

C-L4799

1.45±0.07

3.72±0.20

1.07±0.02

2.45±0.15

12/12

14.4±3.5

3.15±0.79

163.16±11.02

66.6±6.0

87.9±12.2

B2

C-L4800

1.22±0.06

2.96±0.17

0.92±0.02

2.49±0.12

34/31

22.9±3.0

1.23±0.86

122.98±8.00

49.5±4.0

55.2±6.6

B3

C-L4806

marine alluvial fan aeolian

0.71±0.04

1.62±0.10

1.13±0.02

21/24

15.5±2.5

2.05±0.92

53.23±3.24

23.0±1.8

27.4±3.4

J

r u o

l a n

2.32±0.12

ro

p e

r P

Journal Pre-proof Table 2: 10Be cosmogenic nuclide exposure data of surface samples of a wave-cut platform at VIR and an alluvial fan surface at BOT. Sample BOT10 is not an exposure age sample, but was taken from recent channel sediments in order to assess the 10Be concentration accumulated due to pre-exposure. The version 3 of the online calculator formerly known as the CRONUS-Earth calculator (http://hess.ess.washington.edu/math/v3/v3_age_in.html; Balco et al., 2008) was used for exposure age calculation using the s caling scheme of Lifton et al. (2014) (LSDn) and erosion rates of (1) 0 m/M a, (2) 2.56 m/M a, and (3) 3 m/M a interpreted here as minimum, mean, and maximum bedrock and clast erosion rates, respectively, after a statistical model (Hidy’s routine) applied by Ritz et al. (2019) for a 10Be depth profile from the M ejillones Peninsula (see text for details). h SRTM = sample elevation according to the SRTM 1’’ Global by the USGS (2014), d = sample thickness, ρ g = sample bulk density (values in italics are not measured but inferred from the petrography), ftopo = topographic shielding factor, c(10Be) = blank-corrected concentration of 10Be, at(10Be)blank/at(10Be)ix100 = percentage of total 10Be atoms measured that are subtracted by the blank correction, Interr = ‘internal’ uncertainty, Exterr = ‘external’ uncertainty. Exposure ages are not given for samples with at(10Be)blank/at(10Be)ix100 > 12%.

f o

mineral

c(10 Be) (at/g)

1σ_c (10 Be) (at/g)

at(10 Be)blank / at(10 Be)i x100 (%)

Age(1)±Interr (Exterr) (ka)

Age(2)±Interr (Exterr) (ka)

Age(3)±Interr (Exterr) (ka)

0.9337

quartz

12243

1885

14.5

-

-

-

2.83

0.8466

quartz

11290

26.7

-

-

-

4.5

2.83

0.9638

quartz

20729

4.0

2.83

0.9842

quartz

11

4.0

2.83

0.9794

quartz

-70.555263

36

4.5

2.85

0.9962

-24.574236

-70.553887

49

3.0

2.65

0.9949

BOT06

-24.573822

-70.552503

54

4.5

2.82

0.9947

BOT07

-24.573542

-70.555509

35

4.5

2.65

0.9952

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BOT09

-24.573742

-70.556334

33

4.0

2.64

BOT10

-24.573175

-70.545168

95

1.5

BOT11

-24.575900

-70.555076

33

3.0

Lon (°W)

h SRTM (m a.s.l.)

d (cm)

ρg (g/cm³)

ftopo

-22.637021

-70.262368

12

5.0

2.83

VIR08

-22.637020

-70.262340

13

3.5

VIR09

-22.636960

-70.262390

11

VIR11

-22.636361

-70.262466

9

VIR12

-22.636165

-70.262438

BOT02

-24.574575

BOT04

Sample ID

Lat (°S)

VIR07

2894 2575

12.0

9.7±1.2(1.3)

9.9±1.3(1.4)

10.0±1.3(1.4)

3822

14.6

-

-

-

2311

8.3

10.3±1.1(1.2)

10.6±1.1(1.3)

10.6±1.1(1.3)

190367

8024

1.0

72.9±3.1(5.4)

88.5±4.7(8.1)

91.9±5.1(8.8)

quartz

r P 160409

6265

0.7

60.7±2.4(4.4)

68.5±3.2(5.7)

70.5±3.4(6.1)

quartz

77640

4218

2.2

29.2±1.6(2.4)

31.3±1.8(2.7)

31.8±1.9(2.8)

quartz

82241

3818

1.3

31.3±1.5(2.4)

34.0±1.7(2.8)

34.4±1.8(2.8)

0.9956

quartz

78870

3674

1.4

29.9±1.4(2.3)

32.2±1.6(2.6)

32.7±1.7(2.7)

2.8

0.8931

quartz

247070

12408

2.2

-

-

-

2.65

0.9960

quartz

266872

9430

0.3

100.1±3.6(7.1)

125.7±6.1(11.8)

133.1±6.9(13.5)

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p e

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quartz

27137 22644

Journal Pre-proof Table 3: Compilation of chronological data from alluvial fan deposits in the area between 21° and 25°S. (1) According to Vargas et al. (2006), 1σ-uncertainty range is presented (cal. BP), (2) weighted mean age and associated 1σ-error of sample C-L4343 and C-L4344, (3) weighted mean age of three 10Be exposure ages of the alluvial fan surface at BOT (1σ-error).

Location

Lat (°S)

Deposit

Material

Dating technique

Age (ka)

Reference

Río Seco (SEC) Río Loa mouth Guanillos (GUA) Guanillos (GUA) Guanillos (GUA) Guanillos Guanillos Guanillos Guanillos Guanillos Guanillos Caleta El Fierro (VIR) Antofagasta Antofagasta Antofagasta Antofagasta Antofagasta Coloso (COL) Coloso (COL) Coloso (COL) Coloso (COL) Botíja (BOT) Botíja (BOT) Botíja (BOT) Botíja (BOT)

20.99 21.42 21.97 21.97 21.97 21.97 21.97 21.97 21.97 21.97 21.97 22.64 23.57 23.57 23.71 23.71 23.71 23.76 23.76 23.76 23.76 24.57 24.57 24.57 24.57

alluvial alluvial aeolian aeolian aeolian alluvial alluvial alluvial alluvial alluvial alluvial alluvial alluvial alluvial alluvial alluvial alluvial alluvial aeolian aeolian aeolian aeolian alluvial aeolian alluvial

sediment mollusc shell sediment sediment sediment mollusc shell mollusc shell mollusc shell mollusc shell mollusc shell charcoal sediment terrestrial snail terrestrial snail charcoal mollusc shell mollusc shell sediment sediment sediment sediment sediment sediment sediment surface boulder

K-feldspar pIR225 14 C K-feldspar pIR225 K-feldspar pIR225 K-feldspar pIR225 14 C 14 C 14 C 14 C 14 C 14 C K-feldspar pIR225 14 C 14 C 14 C 14 C 14 C K-feldspar pIR225 K-feldspar pIR225 K-feldspar pIR225 K-feldspar pIR225 K-feldspar pIR225 K-feldspar pIR225 K-feldspar pIR225 quartz 10Be

56.6±6.5 0.52±0.04 46.9±5.01 44.6±3.2 30.1±2.1 0.36±0.14(1) 2.49±0.15(1) 2.66±0.14(1) 5.42±0.20(1) 5.55±0.13(1) 6.89±0.14(1) 13.95±1.84(2) 19.64±0.45(1) 23.56±0.57(1) 0.85±0.02(1) 5.33±0.14(1) 5.76±0.12(1) 56.3±4.3 92.5±16.4 83.8±10.9 86.7±11.9 44.0±4.1 55.2±6.6 27.4±3.4 32.5±1.1(3)

Bartz et al., in review Vásquez et al., 2018 Bartz et al., in review Bartz et al., in review Bartz et al., in review Vargas et al., 2006 Vargas et al., 2006 Vargas et al., 2006 Vargas et al., 2006 Vargas et al., 2006 Vargas et al., 2006 Bartz et al., in review Vargas et al., 2006 Vargas et al., 2006 Vargas et al., 2006 Vargas et al., 2006 Vargas et al., 2006 This study This study This study This study This study This study This study This study

fan fan fan fan fan fan fan fan fan fan fan fan fan

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fan fan

fan fan

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Journal Pre-proof Table 4: Compilation of chronological data and uplift rate estimates derived from marine terraces in the area between 20° and 25°S. (1) Sampling elevation or marine terrace height. SA=terminal shoreline angle, which has been used for calculating uplift rates including ±5% uncertainty, (2) own interpretations of marine isotope stages (M IS) based on absolute dating results (this study and references), (3) according to Dura et al. (2016); Pedoja et al. (2014); Schellmann and Radtke (2003), (4) age uncertainties are ±15% according to Radtke (1989), (5) based on own DGPS measurements.

Dating technique

Elevation (1) (m a.s.l.) (SA)

MIS(2)

Shoreline age (ka)(3)

Palaeo-sea level to present datum (m)(3)

Uplift rate (m/ka)

96.4-118(4) 117(4) 90-110 119(4) 95.3-100(4) 90

ESRmollusc /U-series ESRmollusc pIR225/ESRquartz ESRmollusc ESRmollusc pIR225

12-22 (22) 10-13 (20(5)) 10-22 (22) 30 (30) 12-14 (20?) 18-20 (20?)

5e 5e 5e 5e 5c 5c

122±7 122±7 122±7 122±7 100±7 100±7

3±1 3±1 3±1 3±1 -15±5 -15±5

0.16±0.05 0.14±0.05 0.16±0.05 0.22±0.08 0.35±0.12 0.35±0.12

125

AAR

43 (50)

5e

122±7

ESRmollusc /U-series AAR AAR/14C U-series/AAR ESRmollusc /U-series U-series/AAR pIR225 ESRmollusc 10 Be ESRmollusc

30-34 (34) 40 (40) 6-7 (7) 18 (?) 30-36 (36) 25 (?) 5-6 (10) 5-6 (10) 61 (?) 9-15 (?)

5c 5e 1 5e 5e 5c 5e 5e 5e 5e

Location

Latitude (°S)

Age (ka)

S Iquique (golf area) Punta Patache Rio Seco (SEC) Caleta Guanillo del Norte N Gatico Caleta El Fierro (VIR, V1)

20.33 20.82 20.99 21.23 22.44 22.64

Caleta Tames

22.66

Caleta Tames Caleta M ichilla Caleta M ichilla Caleta Yayes Caleta Hornos Caleta Hornos Coloso (COL, C1) Coloso Punta Piedras Las Losas

22.66 22.70 22.70 22.86 22.92 22.92 23.76 23.76 24.76 25.12

(4)

92-92.9 125 6.73 106-124 91-103(4) /134 105-108 98 103-112(4) 124 139(4)

J

r u o

l a n

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p e

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100±7 122±7 7±1 122±7 122±7 100±7 122±7 122±7 122±7 122±7

f o

3±1

-15±5 3±1 3±1 3±1 3±1 -15±5 3±1 3±1 3±1 3±1

0.39±0.13 0.49±0.17 0.30±0.10 0.57±0.25 0.14±0.05 0.27±0.09 0.40±0.14 0.06±0.02 0.06±0.02 0.48±0.09 0.10±0.03

Reference Radtke, 1989 Ratusny and Radtke, 1988 Bartz et al., in review Ratusny and Radtke, 1988 Radtke, 1989 Bartz et al., in review Leonard and Wehmiller, 1991 Hsu et al., 1989 Ratusny and Radtke, 1988 Leonard and Wehmiller, 1991 Leonard and Wehmiller, 1991 Ortlieb et al., 1996 Radtke, 1989 Ortlieb et al., 1996 This study Radtke, 1989 M artinod et al., 2016 Radtke, 1989

Journal Pre-proof SUPPLEMENTARY INFORMATION

Late Pleistocene alluvial fan evolution along the coastal Atacama Desert (N Chile)

Melanie Bartz1* , Janek Walk2 , Steven Binnie3 , Dominik Brill1 , Georg Stauch2 , Frank Lehmkuhl2 , Dirk Hoffmeister1 , Helmut Brückner1

Institute of Geography, University of Cologne, 50923, Cologne, Germany

2

Department of Geography, RWTH Aachen University, 52056, Aachen, Germany

3

Institute of Geology, University of Cologne, 50923, Cologne, Germany

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*corresponding author: [email protected], +492214707719

Journal Pre-proof 1 Methods 1.1 Laboratory preparation Luminescence samples in this study were prepared under subdued red light conditions at the Cologne Luminescence Laboratory (CLL, University of Cologne). After wet sieving, coarsegrained (>100 µm) sediments were treated with HCl (10 %), H2 O2 (10 %), and sodium oxalate to remove carbonates, organic material, and clay remains. Density separation with sodium polytungstate solution was used to isolate K-feldspar (ρ <2.58 g/cm3 ). Finally, the samples were sieved to grain sizes of 150-200 µm (except for sample C-L4796, where 250300 µm were used due to the coarser grain size fraction) and mounted on stainless-steel discs 10

Be exposure dating were also prepared and processed at the

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Samples for cosmogenic

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for luminescence measurements.

University of Cologne. All samples were crushed, ground, and dry sieved to retain the 250-

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710 µm grain size fraction. In order to isolate the quartz (ρ = 2.65 g/cm3 ) from the polymineralic rock samples, we applied induced roll magnetic separation and density

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separation using sodium polytungstate solutions with densities of ρ = 2.62 g/cm3 and ρ = 2.68

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g/cm3 . Samples were then shaken in a concentrated HCl/H2 SiF6 mixture and etched several times overnight in dilute HF/HNO 3 in an ultrasonic bath following Kohl and Nishiizumi (1992). Assays of the subsequent quartz separates were tested for purity using in-house ICP-

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OES (inductively coupled plasma – optical emission spectrometry). Samples deemed sufficiently pure were prepared as AMS (accelerator mass spectrometry) targets. After spiking

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samples with ~300 μg of Be using a commercially available standard solution (Scharlab, 1000

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mg/l), between ~29 and 5 g of clean quartz separate was dissolved in concentrated HF/HNO 3 . Be was separated from Al and other interfering elements using the single-step column approach described in Binnie et al. (2015). A reagent blank was prepared in tandem with the samples, which were co-precipitated with silver using the procedure of Stone et al. (2004) and measured on CologneAMS (Dewald et al., 2013), normalized to the standards of Nishiizumi et al. (2007).

1.2 Luminescence dating procedure All luminescence measurements were carried out on automated Risø TL/OSL-DA-20 readers at the CLL. IR stimulation (880±80 nm) and signal detection through a LOT D410/30x interference filter (410 nm) was used for measuring the coarse-grained K-feldspar samples. The first 7.7 s of stimulation minus a late background of the last 38.5 s were used. An elevated temperature pIR-IRSL225 single-aliquot regenerative-dose (SAR) protocol was applied (Buylaert et al., 2009). Sample specific fading rates (g-values) were measured following Auclair et al. (2003) and corresponding ages were corrected following Huntley and

Journal Pre-proof Lamothe (2001) when g-values exceed 1%/decade. Samples were exposed to light for 24 h in a Höhnle SOL2 solar simulator for dose recovery experiments of all samples where an artificial dose (approximately equal to natural De) was given to the bleached samples. The total environmental dose rate was derived from a combination of in situ field measurements and laboratory HRGS analysis. The external gamma dose rate was measured in situ using a portable gamma spectrometre (Ortec NaI(Tl) Scintillation Probe) and calculated with the “threshold” technique (Duval and Arnold, 2013). The software DRAC v1.2 (Durcan et al., 2015) was applied for dose rate and age calculation using the conversion factors of Guérin et al. (2011) and the alpha and beta attenuation factors of Bell (1980) and Guérin et al.

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(2012), respectively. Water contents of 1±0.3% were used for samples with alluvial fan and

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aeolian origin, while a water content of 13±7% was used for marine samples (Bartz et al., in review). The cosmic dose rate contribution was assessed following the approach of Prescott

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and Hutton (1994), taking into account the altitude, latitude, and longitude of the section as well as the thickness and density of overlying sediments. The latter was assumed to be

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1.90±0.05 g/cm3 . An α-efficiency of 0.11±0.03 (Balescu and Lamothe, 1993) and an internal

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potassium content of 12.5±0.5% (Huntley and Baril, 1997) were assumed.

1.3 Cosmogenic nuclide (10 Be) dating procedure

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The reagent blank corrected concentrations of

10

Be were calculated following the approach

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described in Binnie et al. (2019). Uncertainties in the concentrations include the propagated uncertainties in the AMS ratios of the samples and blank and an estimated 1% (1σ)

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uncertainty in the mass of 9 Be added to the samples and blank during spiking. Exposure ages were derived using version 3 (wrapper 3.0.2, constants 3.0.4) of the online calculator formerly known

as

the

CRONUS-Earth

(http://hess.ess.washington.edu/math/v3/v3_age_in.html; Balco et al., 2008).

calculator

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2 Results

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Fig. S1: K-feldspar luminescence data. a) Decay and dose response curves (DRC, insert) of an aeolian sample (C-L4794). DRC of the samples have been best fitted by a single saturating exponential function. The open circle denote the sensitivity-corrected natural pIR225 signal (Ln /Tn ), while filled circles denote the sensitivity-corrected regenerated pIR225 signals; b) Dose recovery results of all samples. The grey band indicates the ±10%-threshold for the measured dose-to-given dose ratio.

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References

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Fig. S2: 10 Be exposure ages of the alluvial fan surface generation at BOT (n = 6). Shown in black are the exposure ages of BOT06, 07, and 09 with their internal uncertainties and the corresponding kernel density estimation (KDE) plots as thin lines. The thick black continuous and dashed lines represent the KDE and weighted mean age with its 1σ internal uncertainty of the alluvial fan surface, respectively, after removal of outliers (BOT02, 04, and 11) using a generalised extreme Studentized deviate test (significance level of 0.01). Exposure ages and KDE of the outliers are shown separately in grey. The online iceTEA tools by Jones et al. (2019) were used for identification of outliers and creation of KDE plots.

Auclair, M., Lamothe, M., Huot, S., 2003. Measurement of anomalous fading for feldspar IRSL using SAR. Radiation Measurements 37, 487-492. https://doi.org/10.1016/S13504487(03)00018-0 Balco, G., Stone, J.O., Lifton, N.A., Dunai, T.J., 2008. A complete and easily accessible means of calculating surface exposure ages or erosion rates from 10Be and 26Al measurements. Quaternary Geochronology 3, 174-195. https://doi.org/10.1016/j.quageo.2007.12.001 Balescu, S., Lamothe, M., 1993. Thermoluminescence dating of the holsteinian marine formation of Herzeele, northern France. Journal of Quaternary Science 8 (2), 117–124. https://doi.org/10.1002/jqs.3390080204 Bartz, M., Duval, M., Brill, D., Zander, A., King, G.E., Rhein, A., Walk, J., Stauch, G., Lehmkuhl, F., Brückner, H., in review. Testing the potential of K-feldspar pIR-IRSL and quartz ESR for dating coastal alluvial fan complexes in arid environments. Quaternary International. Bell, W.T., 1980. Alpha dose attenuation in quartz grains for thermoluminescence dating. Anc. TL 12, 4–8.

Journal Pre-proof Binnie, S.A., Dewald, A., Heinze, S., Voronina, E., Hein, A., Wittmann, H., von Blanckenburg, F., Hetzel, R., Christl, M., Schaller, M., Léanni, L., ASTER Team, Hippe, K., Vockenhuber, C., Ivy-Ochs, S., Maden, C., Fülöp, R.H., Fink, D., Wilcken, K.M., Fujioka, T., Fabel, D., Freeman, S.P.H.T., Xu, S., Fifield, L.K., Akçar, N., Spiegel, C., Dunai, T.J., 2019. Preliminary results of CoQtz-N: A quartz reference material for terrestrial in-situ cosmogenic 10 Be and 26 Al measurements. Nuclear Instruments and Methods in Physics Research, Section B: Beam Interactions with Materials and Atoms 456, 203–212. https://doi.org/10.1016/j.nimb.2019.04.073 Binnie, S.A., Dunai, T.J., Voronina, E., Goral, T., Heinze, S., Dewald, A., 2015. Separation of Be and Al for AMS using single-step column chromatography. Nucl. Instruments Methods Phys. Res. Sect. B Beam Interact. with Mater. Atoms. 361, 397-401. https://doi.org/10.1016/j.nimb.2015.03.069

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Buylaert, J.P., Murray, A.S., Thomsen, K.J., Jain, M., 2009. Testing the potential of an elevated temperature IRSL signal from K-feldspar. Radiat. Meas. 44, 560-565. https://doi.org/10.1016/j.radmeas.2009.02.007

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Dewald, A., Heinze, S., Jolie, J., Zilges, A., Dunai, T., Rethemeyer, J., Melles, M., Staubwasser, M., Kuczewski, B., Richter, J., Radtke, U., Von Blanckenburg, F., Klein, M., 2013. CologneAMS, a dedicated center for accelerator mass spectrometry in Germany. Nuclear Instruments and Methods in Physics Research, Section B: Beam Interactions with Materials and Atoms 294, 18-23. https://doi.org/10.1016/j.nimb.2012.04.030

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Durcan, J.A., King, G.E., Duller, G.A.T., 2015. DRAC: Dose Rate and Age Calculator for trapped charge dating. Quaternary Geochronoly 28, 54–61. https://doi.org/10.1016/j.quageo.2015.03.012

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Duval, M., Arnold, L.J., 2013. Field gamma dose-rate assessment in natural sedimentary contexts using LaBr3(Ce) and NaI(Tl)probes: A comparison between the ‘‘threshold’’ and ‘‘windows’’ techniques. Applied Radiation and Isotopes 74, 36-45. https://doi.org/10.1016/j.apradiso.2012.12.006

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Guérin, G., Mericier, N., Adamiec, G., 2011. Dose-rate conversion factors: update. Ancient TL 29, 5–8. Guérin, G., Mercier, N., Nathan, R., Adamiec, G., Lefrais, Y., 2012. On the use of the infinite matrix assumption and associated concepts: A critical review. Radiation Measurements 47, 778–785. https://doi.org/10.1016/j.radmeas.2012.04.004 Huntley, D.J., Baril, M.R., 1997. The K content of the K-feldspars being measured in optical dating or in thermoluminescence dating. Ancient TL 15, 11–13. Huntley, D.J., Lamothe, M., 2001. Ubiquity of anomalous fading in K-feldspars and the measurement and correction for it in optical dating. Canadian Journal of Earth Sciences 38(7), 1093-1106. https://doi.org/10.1139/e01-013 Jones, R.S., Small, D., Cahill, N., Bentley, M.J., Whitehouse, P.L., 2019. iceTEA: Tools for plotting and analysing cosmogenic-nuclide surface-exposure data from former ice margins. Quaternary Geochronology 51, 72-86. https://doi.org/10.1016/j.quageo.2019.01.001 Kohl, C.P., Nishiizumi, K., 1992. Chemical isolation of quartz for measurement of in-situ produced cosmogenic nuclides. Geochimica et Cosmochimica Acta 56(9), 3583-3587. https://doi.org/10.1016/0016-7037(92)90401-4

Journal Pre-proof Nishiizumi, K., Imamura, M., Caffee, M.W., Southon, J.R., Finkel, R.C., McAninch, J., 2007. Absolute calibration of 10Be AMS standards. Nuclear Instruments and Methods in Physics Research Section B: Beam Interactions with Materials and Atoms 258(2), 403413. https://doi.org/10.1016/j.nimb.2007.01.297

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Stone, J., Fifield, K., Beer, J., Vonmoos, M., Obrist, C., Grajcar, M., Kubik, P., Muscheler, R., Finkel, R., Caffee, M., 2004. Co-precipitated silver-metal oxide aggregates for accelerator mass spectrometry of 10 Be and 26 Al, in: Nuclear Instruments and Methods in Physics Research, Section B: Beam Interactions with Materials and Atoms 223-224, 272277. https://doi.org/10.1016/j.nimb.2004.04.055

Journal Pre-proof Declaration of interests

☒ The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

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☐The authors declare the following financial interests/personal relationships which may be consi dered as potential competing interests:

Journal Pre-proof Late Pleistocene alluvial fan evolution along the coastal Atacama Desert (N Chile)

Melanie Bartz1 , Janek Walk2 , Steven Binnie3 , Dominik Brill1 , Georg Stauch2 , Frank Lehmkuhl2 , Dirk Hoffmeister1 , Helmut Brückner1

Highlights: The alluvial fan evolution along the northern coast of Chile between 20° and 25°S in the Atacama Desert is presented for the last 100 ka.



Activity of alluvial fans could be documented during time spans 95-80 ka, 60-45 ka, 3520 ka, as well as the Holocene.



Estimated uplift rates between ~0.06 and ~0.57 m/ka have been derived from marine formations for the Late Quaternary period.



Long-term tectonic activity only indirectly controls alluvial fan activity (base-level changes).



Alluvial fan activity is mainly controlled by atmospheric changes from the Pacific Ocean.

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