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Late Quaternary coastal evolution and aeolian sedimentation in the tectonically-active southern Atacama Desert, Chile
Abstract
Analyses of aeolianites and associated dune, surficial carbonate and marine terrace sediments from north-central Chile (27° 54′ S) yield a record of environmental change for the coastal southern Atacama Desert spanning at least the last glacial-interglacial cycle. Optically stimulated luminescence dating indicates phases of aeolian dune construction at around 130, 111–98, 77–69 and 41–28 ka. Thin-section and stable carbon and oxygen isotope analyses suggest a predominantly marine sediment source for the three oldest dune phases. Aeolianites appear to have accumulated mainly from tectonically-uplifted interglacial marine sediments that were deflated during windier and/or stormier intervals. Bedding orientations indicate that sand-transporting winds varied in direction from S-ESE during MIS 5e and WNW-ESE during MIS 5c-5a. Winds from the southeast quadrant are unusual today in this region of the Atacama, suggesting either major shifts in atmospheric circulation or topographic airflow modification. Thin-section evidence indicates that the aeolianites were cemented by two phases of vadose carbonate, tentatively linked to wetter periods around 70 and 45 ka. Tectonic uplift in the area has proceeded at an average rate of 305–542 mm kyr− 1. The study illustrates the complexity of understanding onshore-offshore sediment fluxes in the context of Late Quaternary sea-level fluctuations for an area undergoing rapid tectonic uplift.
... A lo anterior García et al. (2019) señala que las precipitaciones asociadas a los vientos del SWW tendrían el mismo un efecto climático y pedogenético en este segmento de la costa, pero recurrente cada miles de años dentro del último periodo glacial. Así mismo el nivel del mar condiciona la extensión de la plataforma costera y el área efectiva para la deflación, permitiendo el desplazamiento de las arenas medias y finas por saltación y suspensión (Pye y Tsoar, 1990, Lamy et al., 1999, Veit et al., 2015y Nash et al., 2018. ...
... Durante las regresiones marinas la plataforma continental queda expuesta a la acción del viento permitiendo el transporte de las arenas (Veit et al., 2015), lo que es concordante con lo expuesto por Paskoff et al. (2002citado en García et al., 2019, al señalar que en costas rocosas la formación de paleodunas están relacionados a un bajo nivel del mar durante un periodo glacial, como evidencia Veit et al. (2015) para el último máximo glaciar (UMG). Sin embargo, Veit et al. (2015) aclaran que los niveles bajos del mar no explican totalmente la ciclicidad en la formación de las paleodunas y paleosuelos, siendo una plausible explicación la presencia de terrazas marinas, ya que como indica Nash et al. (2018) las terrazas marinas aumentan las áreas disponibles para que el viento actúe, contribuyendo a la deflación eólica durante periodos más ventosos, posibilitando la depositación de arenas sobre los distintos niveles de las terrazas marinas. ...
... 2004y Sepúlveda, 2013. La FDL (Sahu, 1964) como la presencia de estratificación cruzada (Pye y Tsoar, 1990, Veit et al., 2015y Nash et al., 2018 permiten reforzar que estas unidades corresponden a un ambiente eólico. El paleohumedal se reconoce como un horizonte superficial clasificado como un suelo de tipo Mollisol (Marck et al., 1993), por su estructura granular, mayor contenido de arcillas y limo (50,3%), mayor capacidad de retención de humedad y la marcada presencia de rizolitos y poros. ...
The coast of central Chile maintains an extensive eolian sedimentary record that has remained without much attention and has potential as a paleoenvironmental record. In the Quintero Bay (32ºS), Ventanas sector, extensive paleodune fields have been described with a stratigraphy that includes sand sequences intercalated with paleosols, but details on the morphogenesis and pedogenesis of these records and their paleoenvironmental evolution remain unknown. This paper aims to determine the paleogeographic and paleoenvironmental implications of the Ventanas II stratigraphic site. By applying field, laboratory, statistical, and depositional environment descriptions we recognize sedimentary facies and pedogenetic intensity. In Ventanas II site, 13 sedimentary units were identified, which we classified into four facies: paleodune, paleosol, paleowetland, and paleobeach. The lower units of the sequence correspond to a coastal wetland and beach and the upper units (Units 10 to Unit 1) to five paleosols intercalated with paleodunes, denoting a change in the paleogeography within the stratigraphic sequence. The Unit 13 towards the base of the section is a paleosol developed on reworked sediments from the Horcón Formation. The paleosols and the paleodunes suggest similar and recurrent humid and dry conditions, respectively, caused by the latitudinal oscillation of the southern westerly winds during the last glacial period in central Chile.
... As shown in Fig. 7b, there are also fluctuations in the intensity of winter monsoon that may be accompanied by fluctuations in relative humidity. Coincidentally, there is also an example of this in Chile (Nash et al., 2018;Garcia et al., 2019), where the accumulation period of red dune sands may have occurred during the LGM for the same reason as it occurred in China. As shown in Fig. 6, the colors of the sections varies in different accumulation periods. ...
... The study of coast dunes of the Pacific coast of the Shimokita Penisula, Northeast Japan shown that seaward shoreline movement during eustatic regression, combined with strong winds, enhanced aeolian grain transport on the beach, resulting in the onset of dune growth and affected largely by monsoonal circulation (Minoura et al., 2020). The studies of the coastal dunes of central Chile shown that the development of coastal dunes related to the northward movement of the westerlies (Nash et al., 2018;Garcia et al., 2019). These studies suggested that the development of coastal dunes might be related to the transition from the period of high sea level to low sea level or from the period of strong winter monsoon to weak winter monsoon. ...
... Although the red dune sands do not have records with as high a resolution as those of the caves, the multiple periods of rapid accumulation during MIS 3 may show some vague relationship with the North Atlantic climate and the Greenland stadial-interstadial cycles. Interestingly, coastal sand dunes in Chile were also accumulated during MIS 5 and MIS 3 (Nash et al., 2018;Garcia et al., 2019). The accumulation periods of coastal sand dunes in both the northern and the southern hemispheres corresponded roughly to the peak of the insolation. ...
The study of the chronology of the red dune sands in tropical coastal areas of China, and the environmental significance thereof, is a weak link in studies of Quaternary geology. Therefore, in this study, optically stimulated luminescence (OSL) dating was carried out on two red dune sand profiles (YRS and PQR2) from Wenchang coast in the north-eastern area of Hainan Island. The results showed that the periods of red dune sands accumulation during Marine Isotope Stages MIS 5 and 3 with interglacial period and associated high sea-level stands, probably occurred by similar to, or more humid than, present conditions. The red dune sands in southern of China are controlled by global background factors such as insolation, the East Asian monsoon, and sea level changes on the million-year scale.
... Note that luminescence ages in italics are interpreted as possible maximum ages due to overestimation of the luminescence signals (cf., Bartz et al., 2020b) (figure from Bartz et al., 2020a) 93 Fig. 7-8 Compilation of age results (pIR-IRSL dating as filled circles, see also Bartz et al., 2020b, and 10 Be cosmogenic nuclide dating as unfilled circle) of alluvial fans along the coastal zone in northern Chile. Note that chronological data from GUA and BOT are derived from intercalated aeolian deposits (figure from Bartz et al., 2020a) xv Fig. 7-9 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 after Cutler et al. (2003) and modified after Siddall et al. (2007) (figure from Bartz et al., 2020a) 101 Fig. 7-10 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). ...
... The steep flank of the Coastal Cordillera exceeds over long segments 1000 m in height ( Fig. 3-1B), and is also referred to as the (great) Coastal Cliff (e.g. Mortimer and Saric, 1972;Melnick, 2016;Nash et al., 2018) or coastal escarpment (e.g. Ortlieb et al., 1996;Starke et al., 2017). ...
... Between 20.5°S and 25.5°S, numerous alluvial fans emanate from the western flank of the Coastal Cordillera, often referred to as the great 'coastal cliff' (e.g. Mortimer and Saric, 1972;Melnick, 2016;Nash et al., 2018) or 'coastal escarpment' (e.g. Ortlieb et al., 1996;Starke et al., 2017), onto the gently dipping coastal rasa and usually reach the Pacific Ocean ( Fig. 6-1A). ...
Alluvial fans represent the dominant sedimentary systems within and along the margins of mountain regions where laterally confined, sediment-rich flows enter open plains or broader valleys. Successive aggradation creates a fan-shaped depositional landform featuring a complex internal architecture. Due to the direct coupling of alluvial fans to the source area, they are principally simple sediment routing systems and ideal archives to study local fluvial morphodynamics but also long-term landscape responses to (palaeo) climatic variability, tectonic activity, and base level changes. In most settings, however, it remained challenging to decipher these allogenic environmental controls.
Along the coast of the hyperarid Atacama Desert in northern Chile (20.5°S-25.5°S), alluvial fans radiate from the steep western flank of the Coastal Cordillera, presenting the source area, over the narrow Coastal Plain, the sink. In the distal part, the coastal alluvial fans (CAF) interact with the marine environment, which forms an additional external control. In contrast to the last major alluvial activity in the interior Atacama Desert, dating back to the Miocene to Pliocene, the onset of CAF formation is much younger and CAF show a higher recent activity. However, the timing of their activity beyond the Holocene has barely received any attention yet. A major influence of climate on CAF formation has been proposed for a small part of the coastal Atacama; however, the interplay of climate, lithology, and tectonics in conjunction with strong, marine-driven base-level changes is still insufficiently explored.
Hence, the CAF of the Atacama Desert are investigated in this comprehensive study as archives for recent to past sediment transport dynamics under coupled allogenic forcing. For this purpose, the CAF morphology as well as sedimentology, primary processes constructing the alluvial fans, relationships between the CAF systems’ hydromorphometry and the prevailing environmental characteristics (climate, tectonics, and source-area lithology), and the timing of their Quaternary evolution were assessed. While detailed field studies were concentrated on five key sites, overarching large-scale analyses of regional climatic and geologic geodata as well as geomorphometric analyses included 123 CAF systems.
Results reveal that alluvial fans along the coastal Atacama Desert are in an overall advanced evolutionary state featuring voluminous aggradation and in approximately half of all cases also significant to deep entrenchment. Detailed geomorphological mapping and morphometric terrain analysis at the CAF complex Guanillos (21.97°S), enabled - besides the introduction of a multi-phase morphostratigraphic model comprising the fan’s prograde evolution and successive incision - to infer debris flows as the main depositional process in this hyperarid coastal setting. The morphological evidence is further supported by the CAF stratigraphy described in detail at five sites. Secondarily, hyperconcentrated flows were found to contribute to a large extent to the CAF aggradation. In contrast, sheetfloods are of negligible relevance, which is in accordance with previous sedimentological observations.
Functional relationships between hydromorphometric catchment characteristics, the frequency of extreme precipitation events, fault density, and source-area lithology suggest a primarily climatic control on the CAF morphodynamics. Mostly restricted to catchments draining the western flank of the Coastal Cordillera, a higher frequency of precipitation extremes correlates positively with morphometric parameters indicating an increased susceptibility to debris flows (mean catchment gradient and basin relief ratio) as well as geomorphic catchment maturity. Spatial trends in catchment hydromorphometry and maturity reflect the major climatic source of rainfall events capable of creating effective sediment mobilization: frontal systems and cut-off lows which originate from the extratropical austral Westerlies and become increasingly rare towards the north. Furthermore, an increasing density of Loma vegetation can be observed towards higher latitudes so that possible feedback mechanisms of biota on sediment supply need to be considered for the southern CAF catchments. The relationships between climate and CAF morphodynamics are only seen in the geomorphometry of catchments characterized by a high degree of sediment connectivity and generally do not cut back far into the hinterlands of the Coastal Cordillera. Reflecting the strongly reduced influence of the Pacific precipitation source to the E, this sharp longitudinal climatic transition is governed by the orographic effect of the Coastal Cordillera’s steep topography. In contrast, source-area lithology could be excluded as a major controlling factor for CAF depositional processes along the coastal Atacama Desert. An important indirect influence of tectonics, however, can be seen in the long-term (neo)tectonic activity within the Coastal Cordillera, which in turn shapes catchments and controls the topography.
CAF aggradation - those in the Late Pleistocene dated within the context of this study - shows peaks during the marine isotope stage (MIS) 5 (95-80 ka), MIS 3 (60-45 ka), Last Glacial Maximum (LGM) (35-20 ka), and Holocene (⪅7 ka). Those phases all coincide with more humid periods related to (i) warmer sea-surface temperatures (SST) of the SE Pacific provoking a southward shift of the Inter-Tropical Convergence Zone (ITCZ) in MIS 5 and parts of MIS 3, (ii) a weakening of the SE Pacific anticyclone and equatorward shift of the austral Westerlies during the LGM, and (iii) the onset of the modern El Niño-Southern Oscillation (ENSO) in the Middle Holocene characterized by positive SST during El Niño, which in turn favours convection of northward reaching atmospheric humidity and thus torrential rainfall. The coupling of CAF palaeoactivity to wetter phases implies that alluvial fan aggradation in the hyperarid Atacama Desert is primarily controlled by climate and is strongly transport-limited. At the CAF key site Botija (24.57°S), incision of the main channel occurred since the late MIS 3 to early LGM and can be interpreted as either a climate-driven effect due to the northward shift of the Southern Westerlies in MIS 2, the result of sudden base-level fall due to short-term megathrust earthquakes, or simply internal alluvial fan dynamics governed by autogenic controls. Regarding the onset of Pleistocene CAF formation, it cannot be excluded that remnants older than MIS 5 are buried under the younger, dated deposits. Further geochronological data along the coast is thus required to check, whether the widespread geomorphic changes of the CAF are a systematic response to an allogenic environmental control. Nonetheless, the CAF represent crucial archives for the Late Quaternary palaeoclimate of the hyperarid Atacama Desert, which preserve the climatic changes driven by the SE Pacific Ocean to a much higher degree than the interior alluvial fans. Besides the dominant control of climate on CAF evolution, tectonic impulses exert a significant short-term influence by sudden large-magnitude earthquakes changing the CAF base-level independently of eustatic sea-level changes. Against this background, mean Late Quaternary coastal uplift rates between ~0.06 and ~0.57 m/ka were derived from the datings of the elevated marine terraces.
... Besides the total amount of annual rainfall, a key aspect for the formation of coastal dunes is a dry summer season. Glacio-eustatic sea-level variations can also be a critical factor for sediment supply affecting dune formation (Paskoff et al., 2002;Paskoff and Manríquez, 2004;Londoño et al., 2012;Nash et al., 2018). The same is thought to be true for the rates of fluvial discharge and associated sediment transport from the hinterland (e.g. ...
... Interchange between sand dune accumulation and pedogenesis can be explained by changes in the precipitation regime governed by latitudinal migration of the southern westerlies. The dune accumulation phases from Veit et al (2015) and Nash et al. (2018) are represented by horizontal triangle lines. the stratigraphic sequence compared with Veit et al.'s (2015) Ventanas site. ...
... Nonetheless, other factors seem to be important for accumulation of eolian sand dunes (e.g. Paskoff, 1970;Paskoff et al., 2002;Londoño et al., 2012;Nash et al., 2018). For instance, Londoño et al. (2012) linked sand dune formation during the last glacial period to episodes of increased Andean river discharge and low eustatic sealevel stands, both factors contributing to an increased sediment supply deflated into dune fields in the southern hyperarid Peru (17˚S). ...
The eolian morphostratigraphic archive along the coast of central Chile (∼37–30°S) includes sand dune and buried paleosol sequences that despite their potential as a proxy for past environmental change, remain mostly unstudied north of Patagonia and south of the Atacama Desert core. Here, we present results of a stratigraphic and geochronologic study using the post‐infrared infrared stimulated luminescence signal stimulated at 225 °C (pIRIR225) of potassium‐rich feldspar that show the timing of past climate variability as evidenced from sand dune/buried paleosol couplets at 32°S. Our record shows that periods of sand dune accumulation during Marine Isotope Stages MIS 6, 4, 3 and 2 occurred within ice ages and associated low sea‐level stands, probably affected by similar to, or drier than, present conditions. Intercalated wetter periods inferred from pedogenesis occurred between 53 ± 4 and 52 ± 3 ka, at some time between 52 ± 3 and 36 ± 2 ka, and at some time between 31 ± 2 and 22 ± 2 ka, and reflect times when the southern westerlies rain belt shifted north. Our data reveal that central Chile experienced multi‐millennial periodicity of westerly precipitation throughout the last glacial period. We provide evidence for a humid local Last Glacial Maximum.
... Overall, aeolian deposits date back to~93-84 ka as well as 50-27 ka (Fig. 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. ...
... 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 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 after Cutler et al. (2003) and modified after Siddall et al. (2007). ...
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 ¹⁰Be 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 from CAF morphodynamics.
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 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 were derived. While tectonic activity induces base-level changes, long-term tectonic activity rather indirectly controls alluvial fan activity. We suggest that alluvial fan activity is mainly controlled by atmospheric changes from the Pacific Ocean.
Based on our observations, CAF in the hyper-arid Atacama Desert serve as suitable geo-archives for reconstructing climate changes during the Quaternary. In particular, the usefulness of alluvial fan systems in a water-limited environment is important for understanding the palaeoenvironmental evolution in a coastal desert.
... Pollen from rodent middens collected at Quebrada del Chaco I [60] (25 S) also show the presence of taxa that today occur 700 m higher in elevation and their presence has been attributed to an increase in winter precipitation before 32 ka BP, although the ages are reported as >33.8 ka BP and are thus infinite dates and were not considered here in our review. Increased dune formation at Llano Agua de los Burros [65] (27 S) in the SA between 41 and 28 ka BP suggests an increase in the source of fluvial sediments, as a result of increase rainfall in the Andes (Nash et al., 2018). ...
The long-term climate dynamics of the central Andes are part of an ongoing international research effort to reconstruct past climatic variations and sensitivity to different regional and global drivers during the last 50,000 years. The large number of diverse records, however, makes it difficult to compare results without an integrated spatial analysis that considers the nature of the record and whether they are integrating environmental conditions across a large basin (i.e., a lake record) or at a very local scale (such as a rodent midden). We compiled 92 records from the southern sector of the central Andes (SCA, 18-35 S). Recalibrated records were further compared by converting the original author's interpretation into a scale of relative moisture anomalies (compared to the present) that ranges from À2 (very dry) to very wet (þ2). Moisture anomaly maps were generated for intervals at 4, 6, 9.5, 14, 17, 21 and 32 ka BP (10 3 calibrated 14 C years before present) using records within a 5% age uncertainty. Our compilations show a surprising degree of agreement in the extent and magnitude of past climate changes during late Pleis-tocene, but less spatial agreement during the Holocene. The TRACE21 transient climate model shows similar results, with better agreement during the Pleistocene compared to the Holocene. Our analyses not only reveal discrepancies between proxy record interpretations at sites from the same region but show which regions in the SCA require more study.
... Somewhat smaller proportions of cement and pore space (5-10%) were reported by Hodge (2013) for aeolianite samples west of Cape Agulhas, but the proportions of cement/ pore space at Wilderness are actually quite comparable with aeolianites from a variety of contexts. For example, McLaren (1993) reported a mean of ∼25% cement from a range of last interglacial aeolianite sites, including the Mediterranean and the Bahamas (see also McLaren & Gardner, 2004;Nash et al., 2018). Helm et al. (2018) reported a reasonably comparable porosity (∼30%) for the Brenton-on-Sea site (Site 7 in the present study). ...
Seven hominin ichnosites in aeolianites on the Cape south coast of South Africa have been dated using Optically Stimulated Luminescence, yielding age estimates from Marine Isotope 6 through Marine Isotope Stage 4. All rock outcrops containing these sites are situated on the modern coastline. The new ages are consistent with geomorphological expectations, and with other numerical dating results from the wider southern Cape coastline. Seen in a global ichnological context, the cluster of South African sites (including two previously dated sites) contains nine of the twenty-three sites older than 70 ka from which hominin tracks have been reported. With a single exception they are also the only sites older than 40 ka that have been attributed to Homo sapiens, and include the oldest tracks (153 ± 10 ka) thus far attributed to our species. The South African coastline contains an archaeological and palaeoanthropological record of global significance, to which the hominin ichnological record, preserved on aeolianite palaeosurfaces and now chronologically constrained, can make a substantial contribution.
... Coastal barriers represent spatially extensive sand accumulations along wave-dominated coasts. With ample accommodation space and sufficient supply of calcareous and/or siliciclastic sands, some of them have been built by coastal and wind processes over multiple glacialinterglacial sea-level cycles to result in spectacular coastal landscapes in a range of climatic and geotectonic settings around the world (Tomazelli et al., 2000;Brooke, 2001;Murray-Wallace et al., 2001;Armitage et al., 2006;Rhodes et al., 2006;Porat and Botha, 2008;Andreucci et al., 2010;Bateman et al., 2011;Tamura et al., 2011;Mauz et al., 2013;Muhs et al., 2014;Rowe et al., 2014;Nash et al., 2018;Oliver et al., 2018;Jin et al., 2019). Coastal barriers provide important records of past coastal environments, notably sea-level variations (Mauz et al., 2013;Murray-Wallace and Woodroffe, 2014;Murray-Wallace, 2018). ...
Having developed in the later Quaternary Period, the Mui Ne dunefield, representing the upper-most portion of a major coastal barrier complex in SE Vietnam, formed by wind regimes associated with the Asian monsoon climate. The barrier complex is 100 km long and higher than 150 m in elevation but despite its large dimensions, research on its geomorphological and geological characterization is limited. Several active dune fields extend 2–10 km landwards from the coast while the rest of the barrier surface is stabilized. We applied ground-penetrating radar (GPR) and quartz optically-stimulated luminescence (OSL) dating to the transverse dunes of the Mui Ne dunefield to characterize the morphostratigraphy and recent aeolian sedimentation history. Currently the dunes have a relief of typically 5–10 m and a wavelength of 50–100 m. They are driven by the northeasterly winter monsoon winds and reversed with minor relief in response to the southwesterly summer monsoon. GPR profiles and OSL ages define three morphostratigraphic units: the modern, Holocene, and Pleistocene dune units. The GPR data of the modern dune unit is dominated by a series of steep reflections that dip exclusively towards the west reflecting the winter monsoon and is punctuated by reactivation surfaces. Evidence for the wind reversal is not preserved in the sediment successions but is represented by reactivation surfaces. Thus a subunit bounded by the reactivation surfaces defines annual deposition. The preservation potential of a subunit varies with the migration rate of the transverse dune, and in the fastest migrating dunes, approximately one subunit was preserved every year (1.1 ± 0.6 subunit/yr). The modern dunes are generally younger than 100 years and have migrated WSW at net rates of 0.6–2.5 m/yr. The Holocene dune unit is as thick as the modern unit although it spans a period, ten times longer, reflecting the lower preservation potential of the transverse dunes as sedimentary records, due to erosion associated with their migration. The rate of dune migration suggests that an aeolian contribution of coastal sands has occurred following the sea level highstand of the Middle Holocene, accounting for the extent of the active dunefields. This mode of the barrier accumulation is considered as an analogue for earlier highstand events. However, the dunefields only cover a fraction of the entire barrier. It is thus inferred that barrier formation occurred over a long period, probably encompassing several glacial-interglacial cycles. Sand accumulation during the lowstand, either by transport of sand from the emerged shelf and/or reworking of sand emplaced during the prior highstand, may contribute significantly to the barrier development.
... Between 20.5°S and 25.5°S, numerous alluvial fans emanate from the western flank of the Coastal Cordillera, often referred to as the great 'coastal cliff' (e.g. Melnick, 2016;Mortimer and Saric, 1972;Nash et al., 2018) or 'coastal escarpment' (e.g. Ortlieb et al., 1996;Starke et al., 2017), onto the gently dipping coastal rasa and usually reach the Pacific Ocean (Fig. 1A). ...
Along the coast of the hyperarid Atacama Desert, late Quaternary alluvial fans emerge from the Coastal Cordillera to the Pacific Ocean between 20.5°S and 25.5°S. Coastal alluvial fans (CAF) show, in comparison to the interior fans of the Atacama Desert, pronounced recent activity. However, the complex interplay between climate, lithology, and tectonics affecting the CAF morphodynamics in such hyperarid coastal settings needs to be better understood. We therefore aim at assessing the major factors driving CAF activity and evaluate their effects along gradients.
We conducted an extensive study relating climatic, lithologic, and tectonic characteristics to fan and catchment geomorphology of 123 CAF. Geomorphometric analyses are based on the 12.5 m TanDEM-X WorldDEM™, catchment lithology and faults are extracted from 13 regional geological maps, and the frequency of heavy rainfall events capable of activating CAF is derived from a Weather Research and Forecasting (WRF) regional climate model.
Our results point to a primary climatic control on CAF morphodynamics shown in functional relationships with catchment hydromorphometric characteristics reflecting a high susceptibility to debris flows – the main CAF-constructing process. Catchment properties along the latitudinal gradient reflect the source of significant precipitation events: frontal systems and cut-off lows which mainly originate in the extratropics and become increasingly rare towards the north. The frequency of precipitation extremes can only be correlated to the hydromorphometry and geomorphic maturity of catchments that feature a high degree of sediment transport potential and generally do not cut back far into the Coastal Cordillera. Related to the strongly reduced influence of the Pacific precipitation source towards the east, this additional climatic transition is governed by the orographic effect of the steep W-E gradient in topography. In contrast, source-area lithology is of negligible relevance for CAF catchment morphometry and fan activity. An important indirect influence of tectonics, however, can be seen in the long-term (neo)tectonic activity within the Coastal Cordillera, which in turn shapes catchments and controls the topography.
... Steffen et al. (2009) descubrió que el cuarzo contenido en sedimentos fluviales del Valle de Pisco, en Perú, subestimaba la edad de los sedimentos debido a la inestabilidad de las componentes OSL media y lenta, que dominaban la señal OSL. Otros trabajos realizados en el Desierto de Atacama usan este mineral para obtener edades, pero no proporcionan información fundamental acerca de las propiedades luminiscentes, poniendo en duda la calidad de las edades resultantes Nash et al., 2018). ...
The geomorphology of the Mejillones Peninsula, northern Chile, evidences a Quaternary tectonic uplifting process. Upper plate faults located close to the peninsula show recent activity. However, there is no consensus about the uplift rate of the Mejillones Peninsula in the millennial timescale and numeric ages of uplifted sediments or sediments related to fault activity are scarce. These geochronological data is crucial to establish the geologic history of the upper plate faults and improve the understanding on plate tectonics processes in Chile. The main aim of this Ph.D. thesis is to contribute to a better knowledge of the subduction process and upper plate deformation relationship through the quantification of the coastal uplift and the upper plate fault activity in the Mejillones Peninsula for the Late Quaternary applying luminescence dating techniques to quartz and potassium feldspar. To achieve this, 31 sediment samples were collected from four localities: marine-coastal sediments from the Pampa Mejillones, an alluvial deposit associated to the Mejillones Fault, alluvial and eolian deposits associated to the Naguayán Fault and a colluvial deposit associated to the Salar del Carmen Fault. Topographic profiles from the Mejillones Peninsula and the studied faults were obtained by means of a differential GPS. Trenches were excavated in the fault traces for paleoseismic analysis and their fault scarps were modelled with high-resolution 3D techniques.
Optically stimulated luminescence (OSL) signals from quartz and post-infrared infrared stimulated luminescence (pIRIR) signals from K-feldspar were analyzed to determine sediment burial ages. From the comparison between OSL and pIRIR ages, it was concluded that the OSL ages from quartz are largely underestimated. This is attributed to the fact that the quartz OSL signals lack a strong fast OSL component and that predominant medium and slow components show thermal instability. On the other hand, pIRIR signals are close to stability, showing low though variable fading rates (0.7-6.77 %/decade), and bleacheable, presenting low residual doses in modern sediments (between 2 and 6 Gy). The alluvial sediments spatially associated with the Mejillones Fault yielded fading-corrected pIRIR ages between 87.4 ± 6.6 and 163.4 ± 18.4 ka. For the sediments associated with the Naguayán fault, the fading-corrected pIRIR ages were between 10.4 ± 1.3 and 44.1 ± 4.7 ka. The fading-corrected pIRIR ages of the colluvial sediments associated with the Salar del Carmen Fault resulted between 14.7 ± 1.0 and 131.6 ± 74.2 ka. For the coastal sediments from the Mejillones Pampa, fading-corrected pIRIR ages were from 70.6 ± 5.1 to > 330 ka.
From the paleosismological study of the Naguayán and Salar del Carmen Faults, it is concluded that the activity of these faults has been continued during the Late Pleistocene and Holocene. The activity rates of both faults classify them as slow faults, with slip rates of ~0.06 m/ka and earthquakes recurrences of 10 ka. Based on stratigraphic references such as colluvial wedges, it was possible to estimate coseismic displacements of up to 2 m, translating into magnitudes of paleoearthquakes of up to Mw7.2. In the specific case of the Naguayán Fault, at least four events could be identified, two of which occurred less than 40 ka ago. According to the pIRIR ages obtained for the studied sediments, Pampa Mejillones has uplifted at rates ranging from 0.25 to 0.5 m/ka between 400 and 100 ka. For the last 70 ka, the pIRIR age obtained in a coastal marine deposit suggests an uplift acceleration to 1.01 m/ka. The activity of the Mejillones Fault, estimated in 0.26 m/ka, partially controls the disposition, formation and preservation of the paleocoastlines, in combination with the variation of relative sea level. Based on these data, an evolution model of the Mejillones Peninsula during the Late Pleistocene is proposed according to which the coastal deposits found in the Pampa Mejillones record coastline progradation and retrogradation as a consequence of the combination of tectonic uplift, eustatic sea level variation and upper plate fault activity. An excess in the uplift rate over the subsidence produced by the activity of the Mejillones Fault would elevate the Pampa Mejillones surface, generating the current coastal cliff. Therefore, coastal uplifting process and the upper plate fault activity, especially the potential reactivation of Mejillones and Naguayán faults, must be considered as a potential seismic hazard to the urban and productive infrastructure of Mejillones and industrial complexes located in the Coastal Cordillera.
... Given the lack of organic material and the ubiquity of quartz and K-feldspar in sediments from arid environments, the potential use of luminescence dating methods to determine burial ages of Quaternary sediments in the Atacama Desert is of great importance. Previous investigations about luminescence dating of alluvial and aeolian sediments from the Atacama Desert using quartz (Cortés et al., 2012;Nash et al., 2018) and K-feldspar (Veit et al., 2015) provided little information, respectively, about the OSL or post-IR IRSL signals and their dating performance to properly evaluate the accuracy and precision of the obtained ages. ...
This study examines the feasibility of applying luminescence dating methods to quartz and potassium feldspar (K-feldspar) grains from Quaternary continental deposits of the Mejillones Peninsula and Coastal Cordillera in Central Atacama, northern Chile. Sediment burial ages were calculated using single-aliquot regenerative-dose (SAR) protocols applied to the optically stimulated luminescence (OSL) signal of quartz as well as infrared stimulated luminescence (IRSL) and post-infrared infrared stimulated luminescence (post-IR IRSL) signals of K-feldspar. Considering that the target sedimentary deposits comprise the Late Pleistocene age range, K-feldspar grains were dated using fading-corrected IRSL signals measured at 50 °C (IR50) and post-IR IRSL measured at 225 °C (pIRIR225) to minimize the potential effect of residual doses on calculated ages. The results of the analytical procedures indicate that quartz grains extracted from the studied sediments present a very weak or even no fast OSL component. The combination of a low OSL sensitivity, signal instability and equivalent dose distributions with high overdispersion (>40% for most samples) hinder reliable age estimation using quartz aliquots. Bleaching test results show that the IR50 signal from K-feldspar aliquots is well reset after 20–24 h of light exposure, while the pIRIR225 signal may present residual doses corresponding to between 10 and 15% of the natural signal. IR50 fading rates are ∼6–7%/decade for most studied samples, with exception of one sample which yielded a g-value of 18.77 ± 2.06%/decade. Fading rates for the pIRIR225 signal yielded variable results among sampling sites, with g-values ranging between 0.70 ± 0.24 and 6.77 ± 1.05%/decade. The dating results point out that quartz OSL ages are largely underestimated in relation to K-feldspar fading-corrected IR50 and pIRIR225 ages. Fading-corrected pIRIR225 K-feldspar ages indicate that the alluvial sedimentation in the downthrown block of an active normal fault in Mejillones Peninsula occurred between 163.4 ± 18.4 ka and 87.4 ± 6.6 ka. K-feldspar ages also allow to constrain the age of fault scarp degradation and fault reactivation in two main branches of the Atacama Fault System. In the Naguayán Fault, pIRIR225 ages indicate that faulting occurred after 21.4 ± 3.2 ka while in the Salar del Carmen Fault, faulting is recorded after 14.7 ± 1.0 ka.
... Between 20.5°S and 25.5°S, numerous alluvial fans emanate from the western flank of the Coastal Cordillera, often referred to as the great 'coastal cliff' (e.g. Melnick, 2016;Mortimer and Saric, 1972;Nash et al., 2018) or 'coastal escarpment' (e.g. Ortlieb et al., 1996;Starke et al., 2017), onto the gently dipping coastal rasa and usually reach the Pacific Ocean (Fig. 1A). ...
Der südamerikanische Kontinentalrand ist ein aktiver Kontinentalrand seit dem Spät Carboniferous. Für diesen Zeitraum (>300 Ma) werden zwei wesentliche tektonomagmatische Phasen postuliert, die spätpaläozoische und die andine Orogenese (Späte Trias-Rezent). Der Übergang zwischen diesen beiden orogenen Zyklen während des frühen Mesozoikums ist Gegenstand der Dissertation. Das Arbeitsgebiet liegt in der Cordillera de la Costa, Südchile (34°S - 37°S). Mit Hilfe petrologisch-geochemischer Methoden wurden Gesteine charakterisiert und ein geodynamisches Modell für den kritischen Zeitraum aufgestellt. Die Altersbestimmungen unterteilen die magmatische Aktivität in drei frühmesozoische Ereignisse: spättriassischer granitischer Magmatismus von ca. 225 - 220 Ma (Hualpen Stock und Constitución Granit), spättriassischer bimodaler Magmatismus von 210 - 197 Ma (Cobquecura Pluton, La Estrella Granit und Pichilemu Granit) und mitteljurassischer mafischer Magmatismus von ca. 175 - 150 Ma (Cerros del Hualve Intrusion und Montecillos Intrusion). Die drei Gruppen zeigen unterschiedliche Charakteristika und Kristallisationsbedingungen, insbesondere niedrige fO2 und aH2O des 210 - 197 Ma Magmatismus. Die Intrusionen von 225 - 220 Ma und 210 - 197 Ma sind an NW-SE bis N-S streichende Lineamente gebunden. Diese fallen mit der Richtung der permisch-jurassischen Hauptstrukturen zusammen, die am gesamten chilenischen Kontinentalrand auftreten. Der frühmesozoische Magmatismus zeigt geochemische Charakteristika subduktionsbezogener Magmen (Nb-Ta Trog, positive Pb Anomalie); zeitgleich tritt ein Akkretionskeil auf. Die 210 - 197 Ma-Plutone zeigen zusätzlich typische geochemisch-mineralogische Merkmale eines Extensions-Regimes, insbesondere der La Estrella Arfvedsonit-Granit und der Fayalit-führende Cobquecura-Pluton. Der Cobquecura-Pluton setzt sich aus komagmatisch intrudierten Gabbros und Fayalit-Granitoiden zusammen. Fayalit-Granitoide werden als extensionsbezogene Schmelzen und somit als nicht subduktionsbezogen interpretiert. Ihre Petrogenese ist umstritten. Fayalit-Granitoide gelten als Differentiate von H2O-armen Magmen mit einem fO2 unterhalb des Fayalit–Magnetit-Quarz Puffers. Das Verhalten der Haupt- und Spurenelemente der Fayalit-Granitoide (Chile) zeigt, dass sie Produkte der fraktionierten Kristallisation gabbroider Magmen in einem offenen System darstellen. Die mafische Quelle ist ähnlich der des regionalen subkontinentalen Mantels. Die Sr-Nd Isotopensignaturen unterscheiden sich von den bekannten Mantel- und Krustenquellen des regionalen paläozoischen Grundgebirges. Sowohl Gabbros als auch die Fayalit-Granitoide enthalten Fe-reiche restitische Xenolithe (Spinell-Cordierit-Quarz-Plagioklas), die aus einen anderen Niveau (mittlere oder untere Kruste) als die Rahmengesteine stammen. Vergleichbare geochemische Charakteristika und Pb-Isotopensignatur der restitischen Xenolite und der Fayalit-Granitoide weisen auf eine enge genetische Verbindung hin. Die daraus zu schließende Assimilation Fe-reicher Kruste als ein wichtiger Prozess in der Petrogenese der Fayalit-Granitoide hat in der Literatur kaum Beachtung gefunden. Weiterhin deuten die Isotopensignaturen auf einen größeren Einfluss der Mantelquelle als in den älteren Magmatiten hin. Die Fe-reiche Mineralogie und die Kristallisationsbedingungen des La Estrella Granits und des Cobquecura Plutons sind ungewöhnlich für das Phanerozoikum, jedoch häufig in proterozoischen anorogenen Gesteinen. Die geochemischen Signaturen und geologischen Charakteristika dieser Granitoide des chilenischen Kontinentalrandes beweisen jedoch, dass zeitgleich ein Subduktionsregime auftrat. Der Magmatismus von 225 - 220 Ma und 175 - 150 hat eine ähnliche Quelle wie der nordpatagonische Batolith, ein Beispiel für subduktionsbezogenen Kordillerenmagmatismus. Ein mögliches Szenario für den Übergang vom Spätpaläozoikum zur Andinen Orogenese ist ein Suprasubduktionsmilieu, im initialen Stadium mit einer schrägen Subduktion mit NW-SE gerichteten Extensionstrukturen, welche später ein steileres Abtauchen der subduzierten Platte verursacht hat. Diese steilere Subduktion könnte die Magmengenese im subkontinentalen Mantel begünstigt haben. Die Subduktionszonengeometrie in Trias-Jura ermöglichte die Platznahme von Magmen mit extensionsbezogenen Signaturen bei gleichzeitig bedeutenden Beiträgen aus Mantelquellen. Das lässt darauf schließen, dass der Beginn der andinen Orogenese durch eine Rekonfiguration der Konvergenzbedingungen während einer kontinuerlichen Subduktion vom Paläozoikum bis heute verursacht wurde.
The growth of vertically laminated calcium-sulphate wedges in the Atacama Desert is assumed to be driven by the interaction of moisture supply and salt dynamics in the subsurface. Geochronological data of these wedge laminations is yet sparse but indispensable to resolve wedge-growth phases and episodes of moisture supply and to use these deposits as a palaeoclimate archive in the hyperarid environment. Our pilot study presents a first approach of dating a calcium-sulphate wedge from the Atacama Desert using coarse-grain feldspar luminescence dating. Our results show a widespread and clustered equivalent-dose distribution of two wedge samples from ∼20 Gy up to saturation. Optically stimulated luminescence (OSL) of quartz revealed unsuitable properties for dating wedge deposits. Consequently, we applied post-infrared infrared stimulated luminescence (post-IR IRSL) to coarse-grained feldspars. Since feldspar single-grain measurements yielded a low number of luminescent grains, we used 1 mm aliquots as reliable single-grain proxies for genuine single-grain measurements. Data from energy-dispersive x-ray spectroscopy (EDX) showed that the feldspar single grains have large differences in their internal K content, resulting in an averaged internal K content of 3.9 ± 1.0% for all luminescent grains. This result was subsequently used for dose rate and age calculations. Our results of equivalent-dose distributions and palaeodoses derived from the minimum age model reveal most recent wedge-growth activities at 10.6 ± 2.2 ka and 7.9 ± 1.8 ka for the two wedge samples.
Quaternary aeolian landforms (AL) can convert into aeolian mineral deposits of economic grade through concentration of metals (Ti, Fe, Zr, Sn, REE, Au, PGE) , industrial minerals (silica) and gems (diamonds), purification (winnowing) of trash minerals (feldspar, mica) and the provision of accommodation space for uranium and base metals. Therefore, during the study of aeolian deposits (AD), emphasis is placed on the mineral assemblage, chemical composition and numerical data obtained from sedimentology to constrain the potential of accumulating mineral commodities. The physical-chemical regime of AL and AD is characterized by alkaline to neutral fluids and oxidizing conditions during the early stages of deposition. An acidic fluid regime evolves from it during late-stage supergene alteration when reducing conditions are created in fossil dunes and the likelihood of sulfide accumulation gets improved. The climate zones have a strong impact on the sorting of the AD causing different types of vegetation and degrees of wetness, more pronounced in the inland than coastal AL. This is also valid for grain sphericity, the chemical (Ti/Fe ratio) and mineralogical composition (carbonate/ silicate ratio). Chemical residues (e.g. carbocretes, sulcretes) in the fine-grained and clay minerals in coarser-grained fraction are more widespread in arid/semiarid and dry continental climate zones which are the aeolian active ones. Neighboring ones such as the wet-and-dry or humid-mid-latitude climate zones are typical of fossil inland AL and dominated by fluvial environments. With the aid of the trend analyses and composite x-y plots using HM, chemical markers, and sedimentological parameters the aeolian facies can be distinguished from fluvial and coastal environments. The source of the AD is related to the geodynamic setting and the tripartite subdivision into pre-mature, mature and supermature crustal section forms the basis for the classification of AL and AD. HM and chemical markers allow for a attribution of AD to mantle (Cu, Ni, Cr) and crustal (Zr) lithologies. In conclusion, mapping AL and investigating AD using the afore-mentioned parameters can be used as an ore guide and help delineate aeolian mineral deposits. The ore-forming aeolian processes may be subdivided into (1) concentration (light and heavy mineral placers), (2) purification (heavy mineral placers), (3) provision of accommodation space (uranium and base metal deposits).
In the past two decades, much has been learned about the late Quaternary climate history of the Atacama Desert with some details still unclear about the seasonality, timing and extent of wet and dry phases. Modern climate studies reveal that, far from exhibiting a unique pattern, seasonal precipitation originates from many sources and mechanisms. For the last 16 ka, we attempt to sort out these complexities in pollen records from four fossil rodent midden series spanning 22˚–25˚S in northern Chile. Widespread wet conditions prevailed during the late Pleistocene and early Holocene, particularly between 13 and 9 ka, evidenced by <400 m lowering of pollen zones (plant communities) compared to today. Regional differences in the timing and magnitude of this displacement may be related to the prevailing source (tropical/extra-tropical) or mode (NNW/SE) of tropical precipitation through time. Wet conditions persisted well into the early Holocene, lasting $1–1.5 ka longer than previously suggested. The pollen record suggests extreme drying $8 ka, possibly associated with a northward shift of the Inter Tropical Convergence Zone, tracking minimum insolation values at subtropical latitudes during the austral summer. The establishment of conditions similar to today happened $4 ka.
Extensive areas of active dunes are found in the Copiapó region. However, their sand supply is presently depleted. It has been proposed that deflated uplifted marine terraces of the coastal belt between the Copiapó River and Caldera were the source of the dune sand. Such a hypothesis is not supported by sedimentologic data, which suggest a fluvial origin for the aeolian sands. They could be related to Neogene alluvial accumulations built up at the mouth of the Copiapó River and the ephemeral streams located to the north of it.
Where coastal topography is low and flat, the clouds dissipate inward over broad areas with little biological impact, but where isolated mountains or steep coastal slopes intercept the clouds, a fog-zone develops. This moisture allows the development of plant communities termed lomas formations. These floristic assemblages function as islands separated by hyperarid habitat devoid of plant life. Since growth is dependent upon available moisture, an understanding of climatic patterns is essential in efforts to interpret present-day plant distributions. Topography and substrate combine to influence patterns of moisture availability. The ecological requirements and tolerances of individual species ultimately determines community composition. Species endemism exceeds 40% and suggests that the lomas formations have evolved in isolation from their nearest geographic neighbors in the Andes. While the arid environment is continuous, there appears to be a significant barrier to dispersal between 18° and 22°S latitude in extreme N Chile, Less than 7% of a total flora, estimated at nearly 1000 species, occur on both side of this region. -from Authors
The 30 000 km-long Australian open coast is located between tropical and temperate latitudes (9°–42°S), with humid north and east coasts, and generally semi-arid–arid south and west coasts. The 9000 km-long temperate arid coast is dominated by carbonate shelf and inshore sediments that have supplied contemporary carbonate beach and dune systems, as well as very extensive Pleistocene calcarenite barrier systems that dominate much of the inner shelf, coast and near interior, and which, in places, have contributed to a blanketing of the downwind interior with calcareous loess deposits. Much of the contemporary coastal landscape and, to varying degrees, the adjacent interior is therefore dominated by a legacy of carbonate marine sediments. This paper reviews the reasons for the dominance of carbonate along this section of coast. It begins by examining the sources, followed by the transport and depositional mechanisms, the nature of the onshore carbonate deposits, including their diagenesis, and their longer-term impact on the coast.
The Pacific Rim is a region where tectonic processes play a significant role in coastal landscape evolution. Coastal California, on the eastern margin of the Pacific Rim, is very active tectonically and geomorphic expressions of this include uplifted marine terraces. There have been, however, conflicting estimates of the rate of late Quaternary uplift of marine terraces in coastal California, particularly for the northern Channel Islands. In the present study, the terraces on San Miguel Island and Santa Rosa Island were mapped and new age estimates were generated using uranium-series dating of fossil corals and amino acid geochronology of fossil mollusks. Results indicate that the 2nd terrace on both islands is ~120 ka and the 1st terrace on Santa Rosa Island is ~80 ka. These ages correspond to two global high-sea stands of the Last Interglacial complex, marine isotope stages (MIS) 5.5 and 5.1, respectively. The age estimates indicate that San Miguel Island and Santa Rosa Island have been tectonically uplifted at rates of 0.12e0.20 m/ka in the late Quaternary, similar to uplift rates inferred from previous studies on neighboring Santa Cruz Island. The newly estimated uplift rates for the northern Channel Islands are, however, an order of magnitude lower than a recent study that generated uplift rates from an offshore terrace dating to the Last Glacial period. The differences between the estimated uplift rates in the present study and the offshore study are explained by the magnitude of glacial isostatic adjustment (GIA) effects that were not known at the time of the earlier study. Set in the larger context of northeastern Pacific Rim tectonics, Channel Islands uplift rates are higher than those coastal localities on the margin of the East Pacific Rise spreading center, but slightly lower than those of most localities adjacent to the Cascadia subduction zone. The uplift rates reported here for the northern Channel Islands are similar to those reported for most other localities where strike-slip tectonics are dominant, but lower than localities where restraining bends (such as the Big Bend of the San Andreas Fault) result in crustal shortening. Published by Elsevier Ltd.
Aim In deserts, past climate change (and particularly past rainfall variability) plays a large role in explaining current plant species distributions. We ask which species were most and which were least affected by changes in rainfall during the late Quaternary in northernmost Chile.LocationQuebrada La Higuera (QLH; 18° S), a shallow canyon that cuts east–west through the western Andean precordillera of northern Chile, connecting the Altiplano with the hyperarid Atacama Desert.
Methods We collected and dated 22 rodent middens from elevations of 3100–3500 m in QLH. These were analysed for identifiable plant macrofossils and pollen. We also measured chinchilla rat (Abrocoma cinerea) faecal pellets in the youngest middens to explore how they relate to past ecological and climatic change.
Results The three oldest middens dated to more than 37 ka (thousand calibrated 14C years), four middens dated to 14.4–11.6 ka, and fifteen middens spanned the last 650 years. During all the intervals examined, extralocal species (those found today at higher elevations and indicative of positive rainfall anomalies) were present at our midden sites. In the youngest interval, Parastrephia pollen (indicating increased rainfall) increased abruptly at ad 1760 and remained high until the mid-1800s. This increase was also seen in our faecal pellet record.
Main conclusions Extralocal species were prevalent in late Pleistocene middens at lower elevations when the climate was wetter. When combined with other regional midden records, we postulate that many species found today in the Altiplano were displaced to lower elevations during the late Pleistocene. The recent large-scale mortality documented among arboreal cactus populations along the present upper margins of the Atacama suggests that these are relict populations that are likely to have flourished during a wetter period in the early 1800s.
In the south of the Atacama desert it is located an Aeolian Sand Sea Dunes that is composed by six intermountain dune fields with complex typology of active, semi-active, and stable desert dunes; plus cerro Medanoso, where active dunes reach 1,643 meters of altitude. The uniqueness of desert dune forms and their large volume constitute a remarkable scenic landscape, of regional and national relevance, in the neighbourhoods of Copiapó (27° S). Besides this, the mass of sand in the dunes is an heritage that comes from morphoclimatic conditions from the past, hence their scientific relevance. The typological classification, description, and analysis of the Sand Sea, plus detailed cartography, all of them results from this research, constitute a scientific foundations base to promote protection, and sustainable use of the Sand Sea, and Atacama's geoheritage.
[Full text: http://www.sciencedirect.com/science/article/pii/S0277379114001231#] Aeolianite successions of low-gradient continental margins commonly show complex records of coastal dune deposition linked to a wide range of sea-level positions and climatic periods of the middle and late Pleistocene, recording both regional and broader-scale drivers of sediment production, coastal dune development and landform preservation. To better characterise the general pattern of sedimentation that occurs over Quaternary glacial–interglacial cycles on low-gradient, temperate carbonate continental shelves we examine the morphology, stratigraphy and age of aeolianite deposits in the Perth region, Western Australia. This includes an analysis of well-defined drowned coastal landforms preserved on the adjacent shelf. New and previously published optical ages provide a preliminary timeframe for the deposition of aeolianite in the Perth region and on Rottnest Island, 17 km offshore. An extensive aeolianite ridge near Perth, representing a former barrier, has Optically Stimulated Luminesence (OSL) ages that range from 120 ± 12 to 103 ± 10 ka (MIS 5e–5a in the context of associated age uncertainties). OSL ages for an exposure in the same ridge 2.5 km inland, record the onlap of much older aeolianite, OSL age 415 ± 70 ka, by shell-rich estuarine beds, OSL age 290 ± 30 ka. A further 5.5 km inland from the coast, two thick aeolianite units, separated by a well-developed palaeosol, have stratigraphically consistent OSL ages of 310 ± 30 and 155 ± 20 ka. In contrast, aeolianite units that form the northern coast of Rottnest Island have OSL ages of 77 ± 12 ka and 27 ± 5 ka. The new OSL ages and previously reported TL and U/Th ages indicate that the bulk of the island comprises dunes deposited around the end of the Last Interglacial sensu lato (MIS 5a–4) and during the Last Glacial (MIS 4–2), accumulating over a Last Interglacial coral reef and basal calcarenite. Drowned barrier and dune landforms preserved on the adjacent continental shelf reveal that barriers were formed during periods of intermediate sea level (e.g. MIS 3) and significant dune mobility occurred when the shelf was subaerially exposed. The pattern of shelf sedimentation discernible in the Perth region – large-scale coastal carbonate dune deposition during periods of high and intermediate sea level and reactivation during glacial lowstands – is largely consistent with published stratigraphic and age data for large-scale aeolianite deposits on other low-gradient carbonate shelves. Based on these data, a general model is proposed for the cycle of Quaternary sedimentation and landform evolution that occurs on these shelves, which are dynamic sedimentary environments with coastal landforms and sedimentary successions that are very sensitive to erosion and sediment reworking.
In north-central Chile, a wide shore platform is morphologically connected with a high fluvial terrace and a pediment. The eastward extension of Quaternary coastal uplift in the Southern Central Andes is poorly constrained since no age correlation between marine and continental landforms has been reported. We use 26Al and 10Be concentrations to constrain the geomorphic evolution of these marine and continental landforms near the Choapa valley (31.6° S). 10Be ages for the shore platform indicate that this surface was repeatedly reoccupied during sea-level highstands between ~ 800 and 500 ka and uplifted after 500 ka. While ‘zero erosion’ ages for the pediment between ~ 600 and 300 ka only partly overlap the shore platform age range, more realistic exposure ages calculated for an erosion rate of 1 m/Ma are between ~ 945 and 475 ka, fitting the age range of the correlated shore platform. 10Be concentrations of the high fluvial terrace are highly scattered evidencing vertical mixing of clasts probably due to slow lowering of the surface. Although it is not possible to determine an age for this landform, the scattering among its 10Be concentrations implies that this marker is several hundreds of thousands of years old and that the high fluvial terrace began to form at ~ 1200 ka or after. Finally, 10Be concentrations of the high fluvial terrace, the pediment and the shore platform are of the same order of magnitude, which is consistent with the clear morphologic correlation between these three types of landforms. These data suggest that the marine and continental landforms studied formed synchronously, with some local differences, during a long period of relative tectonic stability between ~(1200?) 800 and 500 ka and uplifted after 500 ka. Our results confirm recent studies showing a post-400 ± 100 ka renewal of uplift along the Pacific coast after a Lower to Middle Pleistocene period of slow uplift. Moreover, the extension of the surfaces suggests that a broad region of ~ 40 km has been uplifted ca. 150 m during the Quaternary.
Few archeological sites in South America contain uncontroversial evidence for when the first peopling of the continent occurred. Largely ignored in this debate, extreme environments are assumed either as barriers to this early wave of migration or without potential for past habitability. Here, we report on a rare 12e13 ka human occupation from Quebrada Maní (site QM12), a plantless, near rainless landscape (1240 m asl and 85 km from the Pacific Ocean) located in the hyperarid core of the Atacama Desert. This
location harbored wetlands and riparian woodlands that were fed by increased rainfall further east in the central Andes during the latest Pleistocene. Excavations at QM12 yielded a diverse cultural assemblage of lithics, burned and cut bones, marine gastropods, pigments, plant fibers, and wooden artifacts alongside a prepared fireplace. Sixteen radiocarbon dates from site QM12 on charcoal, marine shells, animal dung, plant remains and wood reveal that the occupation took place between 12.8 and 11.7 ka. These results
demonstrate that the Atacama Desert was not a barrier to early American settlement and dispersal, and provide new clues for understanding the cultural complexity and diversity of the peopling of South America during the Last Glacialeinterglacial transition.
The response of the northern extension of the Atacama Desert and the Peruvian upwelling system to climate conditions during the Last Interglacial ([LIG]; ∼125kyr ago) was tracked using molecular fossils of marine and terrestrial organisms preserved in Peruvian shelf sediments. High resolution records of ODP Site 1229 (past 145 kyr) indicated that warmer and wetter conditions (rainfall and river runoff) along the coast occurred during the LIG, when global temperatures were comparable or even higher than today. A ∼3°C warming of surface waters, enhanced water column stratification, rainfall and river runoff were associated with low primary productivity and a ∼1.5°C decrease in the temperature gradient across the Equatorial Pacific (i.e., weak Walker circulation), suggesting a prolonged El Niño-like response of the tropical Pacific during the LIG. In contrast, the late Holocene ([LH] last 3 kyr) was characterized by colder surface waters, higher export and primary productivity, and a drier climate.
Origin of sand dunes in the Copiapó region, Atacama Desert, Chile. Extensive areas of active dunes are found in the Copiapó region. However, their sand supply is presently depleted. It has been proposed that deflated uplifted marine terraces of the coastal belt between the Copiapó River and Caldera were the source of the dune sand. Such a hypothesis is not supported by sedimentologic data, which suggest a fluvial origin for the aeolian sands. They could be related to Neogene alluvial accumulations built up at the mouth of the Copiapó River and the ephemeral streams located to the north of it
The physical forcing of the upwelling along the subtropical west coasts of the continents encompasses a broad range of time scales which shape both phytoplankton biomass (Chl-a) and primary productivity (carbon fixation) changes over any given time interval. The narrow continental shelf and the steep alongshore orography off north-central Chile provide for a unique combination of year-round, upwelling-favorable winds with quasi-weekly upwelling pulses associated with atmospheric coastal-trapped disturbances (coastal lows). This variability is modulated by intraseasonal oscillations in the depth of the thermo/nutricline, produced by coastal-trapped waves in the ocean, upon which annual (seasons) and interannual (ENSO) cycles are superimposed. During coastal field experiments off Cruz Grande bay (29° S), carried on in November 1987 and 1988 (opposite extremes of the ENSO cycle), mean changes of the phytoplankton-integrated Chl-a(B) and carbon fixation rate (PP) from the active to the relaxed phases of the local upwelling forcing cycle (phyto-pattern) were characterized. Those data were contrasted against similar ones reported off Punta Lengua de Vaca (Coquimbo, 30° S) and off Mejillones peninsula (Antofagasta, 23° S), encompassing different seasons and phases of intraseasonal and interannual (ENSO) cycles from 1992 to 1997. A "warm" phyto-pattern was schematically characterized by a significant increase in B and a quasi-steady evolution of PP from the active to the relaxed phases of one complete upwelling event. Conversely, relative small changes in B and a significant increase in PP characterized a "cold" phyto-pattern. It is proposed here that the ENSO "cold/warm" signal may be offset by more than one opposite "thermal" condition (seasonal and/or intraseasonal) in defining a "warm" or "cold" phyto-pattern associated with a particular cycle of the local upwelling forcing.
Optically stimulated luminescence (OSL) dates are reported for silts and very fine sands believed to be loessic sediments from northwest England. At three sites loessic sediments were initially interpreted as primary aeolian deposits, and at two other sites as loess incorporated into the matrix of cemented scree. However, the results of OSL dating indicate a more complex pattern of accumulation than originally hypothesized and have prompted reconsideration of these materials. Whatever the process(es) and underlying cause(s), it is evident that significant amounts of soil erosion occurred on the limestone uplands earlier than previously thought. All but one of the ages fall entirely within the Holocene period and suggest that these deposits contain reworked, rather than primary loess. Four of the five sites are characterized by non-Gaussian dose distributions, and consequently equivalent doses have been estimated using a range of appropriate age models. The implications of differences in the ages derived from the fine silt and fine sand fractions of the samples are considered. Three processes, namely aeolian transport, overland flow and subsoil piping, are invoked to account for the reworking of loess, although their relative contributions cannot be quantified. At one site the inclusion of limestone clasts within the reworked loess strongly suggests that the sediment can be regarded as loess-derived colluvium. Human impacts on the landscape and climate shifts, either separately or in combination, are considered to have been the most likely mechanisms that triggered loess erosion.
The cause and geodynamic impact of flat subduction are investigated. First, the 1500 km long Peru flat slab segment is examined. Earthquake hypocenter data image two morphologic highs in the subducting Nazca Plate which correlate with the positions of subducted oceanic plateaus. Travel time tomographic images confirm the three-dimensional slab geometry and suggest a lithospheric tear may bound the NW edge of the flat slab segment, with possible slab detachment occurring down dip as well. Other flat slab regions worldwide are discussed: central Chile, Ecuador, NW Colombia, Costa Rica, Mexico, southern Alaska, SW Japan, and western New Guinea. Flat subduction is shown to be a widespread phenomenon, occurring in 10% of modern convergent margins. In nearly all these cases, as a spatial and temporal correlation is observed between subducting oceanic plateaus and flat subduction, we conclude that flat subduction is caused primarily by (1) the buoyancy of thickened oceanic crust of moderate to young age and (2) a delay in the basalt to eclogite transition due to the cool thermal structure of two overlapping lithospheres. A statistical analysis of seismicity along the entire length of the Andes demonstrates that seismic energy release in the upper plate at a distance of 250-800 km from the trench is on average 3-5 times greater above flat slab segments than for adjacent steep slab segments. We propose this is due to higher interplate coupling and the cold, strong rheology of the overriding lithosphere which thus enables stress and deformation to be transmitted hundreds of kilometers into the heart of the upper plate.
_____________________________________________________________________________________________ Abstract In the field of luminescence and electron spin resonance dating, dose rate conversion factors are widely used to convert concentrations of radioactive isotopes to dose rate values. These factors are derived from data provided by the National Nuclear Data Center of the Brookhaven National Laboratory, which are compiled in Evaluated Nuclear Structure Data Files (ENSDF) and Nuclear Wallet Cards. The recalculated dose rate conversion factors are a few percent higher than those previously published, except for beta and gamma emissions of the isotopes of the U-series decay chains. Introduction In luminescence and electron spin resonance dating, an age is obtained by dividing the palaeodose with the dose-rate that an object to be dated has been exposed to. The latter is determined by measurements of concentrations of radioelements or activity using gamma spectrometry, ICP MS, neutron activation analysis, alpha counting, beta counting or flame photospectrometry. The elemental concentrations are then converted in dose rates using conversion factors. These depend on the properties of the nuclear decays involved. The conversion factors have been calculated from time to time, for example by Nambi and Aitken (1986) or Adamiec and Aitken (1998; see also references therein) based on the ENSDF data. However, a new data set is available and an update is timely. The data used here were downloaded on 5th November 2009 on the Chart of Nuclides (http://www.nndc.bnl.gov/chart/index.jsp) and are based on Evaluated Nuclear Structure Data Files (ENSDF) and Nuclear Wallet Cards. This paper presents updated conversion factors following the approach of Adamiec and Aitken (1998).
The importance of the Norte Chico region in north-central Chile has long been recognized for the paleoclimates recorded in its soils. This area lies in an extreme climate gradient between the hyper-arid Atacama Desert in the north and a Mediterranean climate in the south, which has made it very sensitive to past climate changes. Nevertheless, few paleoclimate studies have been undertaken in the region, and these were mostly concentrated on the Holocene. We studied Pleistocene climate changes recorded in soils that formed over a series of marine terraces near Tongoy about 60 km south of La Serena. The calcrete and soil development took place on four marine terraces associated with Marine Isotope Stages MIS 11, MIS 7e, MIS 5e, and MIS 1. The different types of calcretes that developed on the three oldest terraces containing calcareous material indicate that they developed during different periods, and that climatic conditions favorable for the development of these soils existed in the area at least from MIS 11 (412 ka) until post-MIS 5e (125 ka). The calcrete horizons show well-defined development stages recording cyclic climate changes varying between arid and more humid during the late Pleistocene. These climate changes recorded in the Tongoy soils are reflected by sedimentological, geomorphological and pedogenic processes. Climate cycles have only been recorded previously for the post-MIS 5e stage in the area, this study being the first to include climate variations reaching MIS 11. (c) 2012 Elsevier B.V. All rights reserved.
Se estudia la morfología de estos ergs, su interacción con las condiciones topográficas pre-existentes y los pulsos formativos correspondientes. Estas dunas no son rasgos costeros en el presente, sino sistemas internos de cuenca y montaña, compuestos por conos de expansión dunar interconectados por cinturones dunares de valle. Los primeros son sistemas aklé y los segundos, conjuntos transverso-transgresivos y de dunas lineales complejas. En su formacion se observa cinco pulsos. Por sus propiedades morfoscópicas y mineralógicas, las arenas fueron abastecidas por antiguas playas marinas, deflaciones formativas de yardangs y el fondo del valle fluvial principal más cercano. Debido a la interacción entre la deriva de arena y las cuencas pre-existentes, los pulsos formativos están representados tanto en losacumuladores principales de cuenca como en las formas de derivación entre cuencas. De acuerdo con las edades isotópicas de las terrazas marinas cercanas, los pulsos formativos operaron durante largo tiempo, en el Pleistoceno y quizás el Holoceno, de modo que las dunas lineales de derivación son formaspolifásicas, incluyendo un pulso presente. Entre los elementos de teoría de ergs de desiertos montañosos, ha sido observado el efecto de vientos variables en dunas lineales de bypass y en asociaciones aklé, relaciones entre yardangs y procesos dunares formativos y la estructura dual (valle y cuenca) de maresde arena. Esta importante asociación morfológica es un Formenschatz de Mortensen, en el sentido de tesoro morfogenético
We have dated Huon Peninsula, Papua New Guinea and Barbados corals that formed at times since the Last Interglacial Period, applying both ^(230)Th and ^(231)Pa dating techniques as a test of age accuracy. We show that Marine Isotope Stage (MIS) 5e ended prior to 113.1 ± 0.7 kyr, when sea level was −19 m. During MIS 5b sea level was −57 m at 92.6 ± 0.5 kyr, having dropped about 40 m in approximately 10 kyr during the MIS 5c–5b transition. Sea level then rose more than 40 m during the MIS 5b–5a transition, also in about 10 kyr. MIS 5a lasted until at least 76.2 ± 0.4 kyr, at a level of −24 m at that time. Combined with earlier data that places MIS 4 sea level at −81 m at 70.8 kyr, our late MIS 5a data indicate that sea level fell almost 60 m in less than 6 kyr (10.6 m/kyr) during the MIS 5–4 transition. The magnitude of the drop is half that of the glacial–interglacial amplitude and approximately equivalent to the volume of the present-day Antarctic Ice Sheet. During this interval the minimum average rate of net continental ice accumulation was 18 cm/yr, likely facilitated by efficient moisture transport from lower latitudes. At three specific times (60.6 ± 0.3, 50.8 ± 0.3, and 36.8 + 0.2 kyr) during MIS 3, sea level was between −85 and −74 m. Sea level then dropped to −107 m at 23.7 ± 0.1 kyr early in MIS 2, before dropping further to Last Glacial Maximum (LGM) values and then rising to present values during the last deglaciation. Times of rapid sea-level drop correspond to times of high winter insolation at low northern latitudes and high winter latitudinal gradients in northern hemisphere insolation, supporting the idea that these factors may have resulted in high water-vapor pressure in moisture sources and efficient moisture transport to high-latitude glaciers, thereby contributing to glacial buildup. We combined our sea-level results with deep-sea δ^(18)O records as a means of estimating the temperature and ice-volume components in the marine δ^(18)O record. This analysis confirms large deep-ocean temperature shifts following MIS 5e and during Termination I. Deep-ocean temperatures changed by much smaller amounts between MIS 5c and 2. Maximum temperature shift in the deep Pacific is about 2°, whereas the shift at a site in the Atlantic is 4°. Under glacial conditions temperatures at both sites are near the freezing point. The shift in the Atlantic is likely caused by a combination of changing proportions of northern and southern source waters as well as changing temperature at the sites where these deep waters form.
The age of the uppermost emerged marine terrace in the Caldera-Bahía Inglesa area, located at 224±6 m a.s.l. is presented herein. The methodology applied is based on the exposure ages in clasts of quartz using cosmogenic 21Ne. The age obtained was 0.86±0.11 My. Three strong sealevel highstands corresponding to isotopic stages MIS 19 (780 ky), MIS 21 (860 ky) and MIS 25 (950 ky) were developed during this age interval. In each one this marine terrace could have been formed. The average uplift rate considering these three cases is 0.28± 0.02 mm/y, that is smaller than the value determined by previous authors for the last 400 ky. No marine terraces exist in the study area between 750 and 400 ky, and this could be an indicator of small uplift rates, weak interestadials during this interval and the stronger sealevel highstand developed during MIS 11 (400 ky) that provoked a significative erosion of the littoral topography, including the marine terraces that could have been formed between 750 and 400 ky. The littoral uplift in the study area is regional and could be related to processes generated due to the interaction of the Nazca and South American plates; e.g., subduction earthquakes.
A field program in the Pampa de la Joya in June-July 1964 included the mapping of barchan dunes and the measurement of their size and shape. This work was in part repeated during June 1983. The pronounced shrinking of dunes without marked change in displacement rate during 1964-83 suggests a 30-40% decrease in sand transport. This may result from lesser duration of wind speed above a critical threshold as well as diminished sand supply.-from Author
Portions of the back-island dune fields on Padre island, Texas, undergo seasonal destructive and constructive phases in which they are reduced to a nearly planar surface during the winter and then reform during the spring and summer. Initial depositional sites for dry sand that we observed were associated with roughness elements that caused a lowering of the transport capacity of the flow. Bedforms develop through a series of morphologic and dynamic stages. Padre Island dune fields represent a type of eolian system where accumulations of dunes and interdune flats occur during periods of a water-table rise, with bypassing and deflation occurring as the water table becomes static or falls, respectively. -from Authors
Reliable surface wind information and the recently available Landsat imagery for deserts of the entire world has facilitated the comparison of wind regimes to dune forms on a worldwide basis. In this report the morphologies of dunes, both from well-studied and from relatively less-studied parts of the world, were compared with local and regional winds. Characteristic wind environments were discerned for barchanoid, linear and star dunes. -from Author
Changes in the position and intensity of the Southern Westerly Winds (SWW) and related causal processes during the Quaternary are controversial and not well understood. Here, we present a record from continental Central Chile, based on coastal aeolian sand and dunes with intercalated palaeosols, reaching back 190 ka in time. Sixteen samples for luminescence dating and additional samples for geochemical procedures were analysed from three locations in the “Norte Chico” (La Serena, Los Vilos, Las Ventanas). Besides the recent Bw-horizons, four palaeosols (Btb1, Btb2, Btb3, Btb4) are identified. They formed in periods with stable surface conditions and a relatively dense vegetation cover, whereas sand accumulation reflects increased aeolian activity under dry conditions and, in parts, glacial sea level lowering. Three of these soils are well bracketed by luminescence data to < 14 ka (Bw), 59-47 ka (Btb4) and 135-125 ka (Btb2). The formation of Btb1 and Btb3 tentatively occurred at 190-160 ka and 107-95 ka. Btb-horizons are interpreted to reflect wetter conditions than modern ones (Bw-horizons). Since the only way to bring wetter conditions to the coastal area of the Norte Chico are the SWW, the documented changes should reflect changes in paleo atmospheric circulation. The more humid periods appear to show a periodicity, dominated by the obliquity cycle. Increased Antarctic sea-ice during austral winter combined with a weak South Pacific Anticyclone at subtropical latitudes, seem to have favoured winter incursions of humid air masses from the Westerlies.
We study the erosive convergent margin of north-central Chile (at ~31°S) by using high-resolution bathymetric, wide-angle refraction, and multichannel seismic reflection data to derive a detailed tomographic 2-D velocity-depth model. In the overriding plate, our velocity model shows that the lowermost crustal velocities beneath the upper continental slope are 6.0–6.5 km/s, which are interpreted as the continental basement composed by characteristic metamorphic and igneous rocks of the Coastal Cordillera. Beneath the lower and middle continental slope, however, the presence of a zone of reduced velocities (3.5–5.0 km/s) is interpreted as the outermost fore arc composed of volcanic rocks hydrofractured as a result of frontal and basal erosion. At the landward edge of the outermost fore arc, the bathymetric and seismic data provide evidence for the presence of a prominent trenchward dipping normal scarp (~1 km offset), which overlies a strong lateral velocity contrast from ~5.0 to ~6.0 km/s. This pronounced velocity contrast propagates deep into the continental crust, and it resembles a major normal listric fault. We interpret this seismic discontinuity as the volcanic-continental basement contact of the submerged Coastal Cordillera characterized by a gravitational collapse of the outermost fore arc. Subduction erosion has, most likely, caused large-scale crustal thinning and long-term subsidence of the outermost fore arc.
The distributions of optically stimulated luminescence (OSL) ages from
dunes are affected by palaeoenvironmental changes, complex dune dynamics
and sampling strategy. Extracting the relative importance of these
factors when interpreting discontinuous OSL chronologies from sand dunes
has proven difficult, and is particularly hindered in contexts where the
internal sedimentary structures of dunes are not visible. In this study
samples for OSL dating were taken from three major dune exposures in the
Rub' al Khali, United Arab Emirates, each showing clear internal
structure, with the aim of addressing these problems. Specific
objectives were to assess how ages of individual sedimentary units
represent dune accumulation as a whole, and how the dune record in turn
reflects known past environmental changes. Final ages were calculated
using the OxCal software package, by incorporating the known relative
stratigraphy through the application of Bayesian methods. The results
show that stratigraphy alone is not sufficient at these sites to guide
OSL sampling; that is, chrono- and lithostratigraphic boundaries do not
necessarily coincide. Where chronological hiatuses are present, internal
sediment stratigraphy can be a useful tool in identifying potential
problems of under-sampling the full dune record. The implications of
these findings for reconstructing Quaternary climates from dune
chronologies are considered.
A study of the early vadose diagenesis of a wide range of late Quaternary age aeolianites from Oman, south India, the Bahamas, Tunisia and Mallorca has been undertaken to assess the wider applicability of models. The rigid application of such models to areas outside those for which they have been developed is inappropriate. In the aeolianites analysed, spatial and temporal heterogeneity is the key characteristic. All tend towards an end point of low-Mg calcite allochems and meniscus, rim and pore-filling cement types but at different rates in different places, both locally and regionally, and via several different diagenetic routes. -from Authors
A methodology for reconstructing wind direction, speed, and event
duration from aeolian dune cross-strata was developed from analysis of
crescentic dunes at White Sands, New Mexico, during wind events. Dune
lee faces were surveyed, lee-face deposits mapped, deposition rates
measured, grain size sampled by stratification type, and winds
characterized from meteorological and field data. The spatial
distribution of lee-face stratification styles is a function of the
incidence angle formed between the wind and the brinkline, with
secondary controls by wind speed and dune sinuosity and height. Sets of
wind-ripple strata form at incidence angles of 25°-40°,
grainfall/grainflow foresets over wind-ripple bottomsets at
40°-70°, and grainflow/grainfall foresets at 70°-90°.
Erosional reactivation surfaces form at incidence angles up to 15°;
bypass surfaces up to 25°. The total sediment load is fractionated
within lee-face stratification types. Wind speed can be reconstructed
from relationships between grain size, transport mode, shear velocity
and grain-settling velocity. Where the full range of grain transport
modes occurs and grain size is limited by shear stress, the shear
velocity and grain-size range in each transport mode can be estimated by
assuming the coarse fraction in grainflow strata traveled in creep, and
the coarse fraction in grainfall traveled in saltation. The minimum
duration of a wind event can be estimated using measures of shear
velocity, dune height and dune forward migration. Method limitations
arise with source-area control on grain size, extremes in wind events,
and severe truncation of sets of cross-strata.
Dose-rate conversion factors relevant to luminescence and electron spin resonance dating have been derived from values for the energy carried by radiations emitted during nuelear transformations given in the current ENSDF (Evaluated Nuelear Structure Data File). For beta and gamma radiation the factors are a few percent lower than previously used. For the effective alpha dose-rate it is more appropriate to use an approach based on particle ranges and resultant values are given.
The Alto Ilo dune field in the coastal desert of southern Peru is located at the edge of the Atacama Desert and is one of the driest landscapes on Earth. A stratigraphic sequence of eolian sands, paleosols and debris flow deposits are identified in this dune field spanning the past 55 ka. We infer that eolian deposition is preceded by an increase in sediment supply from local alluvial fans that are upwind of the Alto Ilo dune field and thus, reflects periods of relative hydrologic excess. Eolian sediment supply may increase also with the concomitant fall in sea level providing an additional foreland source area for eolian particles. The presence of paleosols indicate millennial-scale periods of locally wetter conditions, possibly associated with spread of “lomas” vegetation fed from coastal fog sources. Debris flows reflect significant rainfall events possibly associated with ENSO variability. The chronology is provided by optically stimulated luminescence using multiple aliquot regeneration protocols, which reveal four major eolian depositional episodes at ca. 55 to 45 ka, 38 to 27 ka, 22 to 16 ka, and 12 ka. Eolian deposition in the late Pleistocene dominates the record and is generally coincident with metrics of hydrologic excess from the Peruvian Andes and the Altiplano, which appears to translate into enhanced sediment availability on the coast. A significant period of soil development between 18 and 13 ka is consistent with timing of Central Andean Pluvial Event and indicates widespread wet conditions. The Holocene record of eolian deposition is sparse reflecting limited alluvial fan sources and decreasing particle availability upwind with the rise in postglacial sea level.
There is significant debate over the rates and types of fluvial activity at the Plio-Pleistocene boundary in the hyperarid Atacama Desert of Chile. To quantify fluvial processes and help resolve this debate, we measure terrestrial cosmogenic nuclide (TCN) (10Be and 21Ne) concentration depth profiles in three settings representing a chronosequence: (1) a late Pliocene alluvial fan representative of major regional deposits, (2) a modern, active channel and (3) an adjacent low terrace inset into the Pliocene alluvium. Late Pliocene deposits that are widely preserved in the region contain TCN profiles consistent with relatively rapid stripping of upland sediment at the Plio-Pleistocene boundary. Deposits inset into these Late Pliocene features record cut and fill cycles that rework sediment throughout the Quaternary. The TCN profile in the modern channel is best explained by sediment aggradation at 2.1 m Myr−1 during the last 250,000 yr. Similarly, the adjacent low terrace sediments contain TCN concentration profiles consistent with aggradation of 2.0 m Myr−1 over a period of 250,000–750,000 yr prior to the last 250,000 yr of stability. In summary, depth profiles of two TCNs provide constraints on the rates of sediment deposition, sources of sediment and transport history, as well as the subsequent exposure conditions of the sediment following deposition. Our results are consistent with early Quaternary initiation of hyperaridity for the region. During the Quaternary, winter precipitation events experienced at our sites' latitude (24°S) drive active erosion–deposition cycles. The northward migration of the subtropical front during Quaternary glacial cycles may have enhanced precipitation at 24°S, leading to more active fluvial processes during cooler periods.
New efforts to link climate reconstructions from shoreline deposits and sediment cores yield an improved and more detailed lake history from the Bolivian Altiplano. On the Southern Altiplano, 10 lake oscillations have been identified from this new unified chronology, each coincident with North Atlantic cold events such as Heinrich Events H5, H2, H1, and the Younger Dryas. By coupling this new lake history to a hydrologic budget model we are able to evaluate precipitation variability on the Southern Bolivian Altiplano over the last 130 ka. These modeling efforts underscore the relative aridity of the Altiplano during the rare and small lake cycles occurring between 80 and 20 ka, when colder temperatures combined with little or no change in rainfall produced smaller paleolakes. Relative aridity between 80 and 20 ka contrasts with the immense Tauca lake cycle (18.1–14.1 ka), which was six times larger than modern Lake Titicaca and coincided with Heinrich Event 1. This improved paleolake record from the Southern Altiplano reveals a strong link between central Andean climate and Atlantic sea-surface temperature gradients during the late Pleistocene, even though today rainfall variability is driven mostly by Pacific sea-surface temperature anomalies associated with El Niño/Southern Oscillation. However, not all Heinrich Events appear to result in lake expansions, most conspicuously during the global cold interval between 80 and 20 ka when the Altiplano and Amazon Basin were relatively arid.
A bar on the Brazos River near Calvert, Texas, has been analyzed in order to determine the geologic meaning of certain grain size parameters and to study the behavior of the size fractions with transport. The bar consists of a strongly bimodal mixture of pebble gravel and medium to fine sand; there is a lack of material in the range of 0.5 to 2 mm, because the source does not supply particles of this size. The size distributions of the two modes, which were established in the parent deposits, are nearly invariant over the bar because the present environment of deposition only affects the relative proportions of the two modes, not the grain size properties of the modes themselves. Two proportions are most common; the sediment either contains no gravel or else contains about 60% gravel. Three sediment types with characteristic bedding features occur on the bar in constant stratigraphic order, with the coarsest at the base. Statistical analysis of the data is based on a series of grain size parameters modified from those of Inman (1952) to provide a more detailed coverage of non-normal size curves. Unimodal sediments have nearly normal curves as defined by their skewness and kurtosis. Non-normal kurtosis and skewness values are held to be the identifying characteristics of bimodal sediments even where such modes are not evident in frequency curves. The relative proportions of each mode define a systematic series of changes in numerical properties; mean size, standard deviation and skewness are shown to be linked in a helical trend, which is believed to be applicable to many other sedimentary suites. The equations of the helix may be characteristic of certain environments. Kurtosis values show rhythmic pulsations along the helix and are diagnostic of two-generation sediments.
The raised beach sediments revealed in the cliffs at Sewerby, East Yorkshire are widely regarded as Ipswichian in age. Previously reported evidence for this dating is reviewed and new results from a range of luminescence dating techniques applied to the blown sand deposit overlying the raised beach are reported. These provide the first absolute date for the Sewerby site of 120.84 ± 11.82 ka, which places the blown sand at the boundary between oxygen isotope stages (OIS) 5e and 5d. As the underlying raised beach is little older than the blown sand, it probably formed during OIS Stage 5e as suggested by the faunal evidence.
This chapter provides a sampling of the geology and hydrogeology of carbonate islands. Information about thirty islands and island groups is covered in the chapter. Among these are islands of atolls and other modern reefs, islands composed of uplifted reef deposits, islands composed of reefs stranded by earlier highstands of sea level, and islands composed of Quaternary eolianites. The chapter also describes the “composite islands,” islands of “mixed geology” where underlying noncarbonate rocks are exposed. The science of carbonate islands is about the same age as the science of geology. The two hundred years of study of carbonate islands have made important contributions to geology in areas such as the deposition and diagenesis of carbonate rocks, the history and tectonics of ocean basins, the history of Cenozoic sea level, and the occurrence and behavior of island groundwater.
The crescentic sand-dune formation known as the "barchan" occurs extensively in the desert coast of southern Peru. Quantitative measurements were made on 75 of these to determine relationships between height, width, and length of horns. A consistently longer west horn was measured in most of the barchans, but this distortion cannot be readily explained. Mechanical analyses of the dune sand were performed on samples taken from the Pampa de Clemesi and the principal barchan deposits in the Pampa de la Joya, as well as from deposits further downwind from La Joya. There is a consistent increase in the proportion of the fine material in the samples taken farther from the Pampa de Clemesi. The bulk specific gravity also decreases with distance from Clemesi. Bulk specific gravity varied with dune height. Mineralogical analyses of these same samples indicate a decrease in the heavier minerals with increasing distance from the Pampa de Clemesi, and an increase in the volcanic, fibrous glass. This evidence indicate...
Post-depositional mixing or exhumation is common in surficial sediments, yet may be unobservable from field evidence. However, any disturbance may have significant consquences in terms of establishing a reliable luminescence age determination. Optically stimulated luminescence (OSL) measurements, particularly measurements at the single grain level, can be used to gain an insight into both contemporary and past post-depositional processes.This paper examines sites from Texas and Florida (USA) with independent chronological control to demonstrate the potential effects of varying degrees of bioturbation on OSL. Results show that contemporary soil forming processes clearly impact on the palaeodose (De) replicate distributions which are measured in order to derive an OSL age. Significant levels of scatter and apparently zero dose grains are observed in the upper-most sediments; declining with depth from the surface. De replicates from undisturbed and fully bleached sediments are unskewed, show low overdispersion (OD) and comparable single grain and single aliquot OSL ages. Bioturbated sediments, however, may show highly skewed multi-model De distributions with higher OD values, zero dose grains at depth, and significant diffences between single grain and single aliquot results. True burial ages may be derived from minimally bioturbated sediments through the application of statistical analysis such as finite mixture modelling to isolate De components. However, for significantly bioturbated sediments, the latter approach, even at the single grain level, produces inaccurate ages. In such cases we argue that additional evidence (both dating and contexual) may be required to identify with confidence the burial De population.
High-resolution (∼≤100 yr sampling interval) Chilean pollen data between ∼10,000 and 60,000 cal yr B.P. exhibit systematic fluctuations in Subantarctic Parkland development. These variations are dominated by a 30,000–40,000-yr cycle similar to that in the Northern Hemisphere GISP2 δ18O data and other climatic records. Both Chilean and GISP2 data show oscillations in the 5000–12,000 and 1000–3000-yr period bands. The coherence is, however, generally low, and distribution of power spectra differs, with the dynamics of the pollen assemblage dominated by a combination of 12,000- and 5000-yr cycles. We suggest a preferential nonlinear response of Chilean vegetation to climatic forcing and interhemispheric differences as possible mechanisms underlying the resemblances and dissimilarities between both records.
Significant studies of sand dunes and sand movement made in coastal southern Peru in 1959–1961 [Gay, S.P., 1962. Origen, distribución y movimiento de las arenas eólicas en el área de Yauca a Palpa. Boletin de la Sociedad Geologica del Perú 37, 37–58] have never been published in the English language and consequently have never been referred to in the standard literature. These studies contain valuable information, not developed by later workers in this field, that may be of broad general interest. For example, using airphotos of barchan dunes and plotting the rates of movement vs. dune widths, the author quantified the deduction of Bagnold [Bagnold, R.A., 1941. The Physics of Blown Sand and Desert Dunes. Methuen, London.] that the speed of barchan movement is inversely proportional to barchan size (as characterized by height or width). This led to the conclusion that all barchans in a given dune field, regardless of size, sweep out approximately equal areas in equal times. Another conclusion was that collisions between smaller, overtaking dunes and larger dunes in front of them do not result in destruction or absorption of the smaller dunes if the collision is a `sideswipe'. The dunes simply merge into a compound dune for a time, and the smaller dune then moves on intact, i.e., passes, the larger dune, whilst retaining its approximate original size and shape. Another result of the 1959–1961 studies was a map that documents the Pacific coast beaches as the source of the sand (Fig. 1), which is then blown inland through extensive dune fields of barchans and other dune forms in great clockwise-sweeping paths, to its final resting place in huge sand masses, sometimes called `sand seas' [Lancaster, N., 1995. Geomorphology of Desert Dunes. Routledge, London], at higher elevations 20 to 60 km from the coast. A minor, but nevertheless interesting, discovery was a small heavy mineral dune located directly in the lee of a large barchan, evidently formed by the winnowing action of turbulent airflow streaming off the crest of the 20 m high slipface.
The interpretation of palaeodune sequences is an important component in palaeoenvironmental analyses. Studies of the diagenetic alteration of palaeodunes is one of the main ways of determining post-depositional environmental conditions. This paper questions the long-held and generally accepted ideas that cement types/fabrics are a 'major method of recognising the original mineralogy of the cements and by inference their origin under marine, vadose or phreatic conditions' (Arthur et al. 1982). A detailed study of last interglacial aeolianites from Mallorca, Tunisia and the Bahamas has shown that although many cement types may be characteristic of the meteoric vadose zone, none are unique. No single cement type is diagnostic of the vadose zone.
We explore the relationships of the stable carbon and oxygen isotopic
composition of modem soil carbonates in relation to ecologic and
climatic variables. The carbon system can be modeled using diffusion
theory and the carbon isotopic composition of pedogenic carbonate is
well correlated with the proportion of C3 and C4
photosynthesis in the local ecosystem. The oxygen isotopic composition
of pedogenic carbonate is well correlated with the isotopic composition
of local meteoric water, even if the latter is modified by differential
filtration or evaporation. These properties make the isotopic
composition of pedogenic carbonate a very powerful indicator of ecologic
and climatic change on continents.
The "Fryberger Method" provides a useful and accessible way of evaluating potential sediment transport by wind. In this method an equation is used to estimate relative rates of sand transport (expressed as drift potential) from measurements of wind velocity. However, the values of drift potential obtained through the calculations are dependent upon the units in which wind velocities are expressed. This paper highlights some of the confusion this dependence can cause and aims to clarify the ways in which values of drift potential can be interpreted.
North central Chile is located at an important climatic transition zone between humid conditions under the influence of the Southern Hemisphere Westerly Winds in the south and the extremely dry climate of the Atacama Desert in the north. Offshore, equatorward flowing cold waters of the Peru–Chile Current converge with relatively warmer waters coming from the low latitudes. Based on terrestrial and marine proxies from the same archive (GeoB 7139-2) (30°12′S; 71°59′W), we show that during the Glacial (between ∼33 and 19kyr) cold sea surface temperatures paralleled enhanced humidity with high terrestrial input and abundant vegetation on the adjacent land, suggesting a stronger influence of cold waters from the Antarctic Circumpolar Current and the Westerly Winds. While the deglacial sea surface temperature warming started at ∼19kyr, the humidity decrease occurred around 17–16kyrBP. The early to mid-Holocene is characterized by extremely warm and dry conditions. We suggest that climate changes were driven by fluctuations in Antarctic sea-ice extent and the circulation of the Hadley cell, both ultimately linked to insolation changes. Our records further imply warm and dry conditions in north central Chile during the Northern Hemisphere Heinrich events.
This paper examines the effects of optically stimulated luminescence (OSL) components, other than that usually termed the fast component, on dose determination by the single-aliquot regenerative-dose (SAR) protocol. Results are presented for “dose recovery tests”, in which a known laboratory dose, delivered after optical bleaching at room temperature, is measured using the SAR protocol. Data obtained using either the initial OSL signal or the fast component, derived by curve fitting, are compared. Dose recovery tests are also carried out when an additional step is added to the SAR protocol, aimed at reducing recuperation, i.e. the residual signal observed in a SAR cycle when no regenerative dose is applied. The results for quartz from various sources indicate much improved dose recovery when only a well-separated fast component is analysed.