Christa Placzek

CLIMATE IN THE DRY CENTRAL ANDES OVER GEOLOGIC, MILLENNIAL, AND INTERANNUAL TIMESCALES

Abstract Over the last eight years, we have developed several paleoenvironmental records from a broad geographic region spanning the Altiplano in Bolivia (18°S–22°S) and continuing south along the western Andean flank to ca. 26°S. These records include: cosmogenic nuclide concentrations in surface deposits, dated nitrate paleosoils, lake levels, groundwater levels from wetland deposits, and plant macrofossils from urine-encrusted rodent middens. Arid environments are often uniquely sensitive to climate perturbations, and there is evidence of significant changes in precipitation on the western flank of the central Andes and the adjacent Altiplano. In contrast, the Atacama Desert of northern Chile is hyperarid over many millions of years. This uniquely prolonged arid climate requires the isolation of the Atacama from the Amazon Basin, a situation that has existed for more than 10 million years and that resulted from the uplift of the Andes and/or formation of the Altiplano plateau. New evidence from multiple terrestrial cosmogenic nuclides, however, suggests that overall aridity is occasionally punctuated by rare rainfall events that likely originate from the Pacific. East of the hyperarid zone, climate history from multiple proxies reveals alternating wet and dry intervals where changes in precipitation originating from the Atlantic may exceed 50%. An analysis of Pleistocene climate records across the region allows reconstruction of the spatial and temporal components of climate change. These Pleistocene wet events span the modern transition between two modes of interannual precipitation variability, and regional climate history for the Central Andean Pluvial Event (CAPE; ca. 18–8 ka) points toward similar drivers of modern interannual and past millennial-scale climate variability. The north-northeast mode of climate variability is linked to El Niño–Southern Oscillation (ENSO) variability, and the southeast mode is linked to aridity in the Chaco region of Argentina.

The age of Homo naledi and associated sediments in the Rising Star Cave, South Africa

New ages for flowstone, sediments and fossil bones from the Dinaledi Chamber are presented. We combined optically stimulated luminescence dating of sediments with U-Th and palaeomagnetic analyses of flowstones to establish that all sediments containing Homo naledi fossils can be allocated to a single stratigraphic entity (sub-unit 3b), interpreted to be deposited between 236 ka and 414 ka. This result has been confirmed independently by dating three H. naledi teeth with combined U-series and electron spin resonance (US-ESR) dating. Two dating scenarios for the fossils were tested by varying the assumed levels of 222Rn loss in the encasing sediments: a maximum age scenario provides an average age for the two least altered fossil teeth of 253 +82/–70 ka, whilst a minimum age scenario yields an average age of 200 +70/–61 ka. We consider the maximum age scenario to more closely reflect conditions in the cave, and therefore, the true age of the fossils. By combining the US-ESR maximum age estimate obtained from the teeth, with the U-Th age for the oldest flowstone overlying Homo naledi fossils, we have constrained the depositional age of Homo naledi to a period between 236 ka and 335 ka. These age results demonstrate that a morphologically primitive hominin, Homo naledi, survived into the later parts of the Pleistocene in Africa, and indicate a much younger age for the Homo naledi fossils than have previously been hypothesized based on their morphology. DOI: http://dx.doi.org/10.7554/eLife.24231.001

Strategies for successful U‐Th dating of paleolake carbonates: An example from the Bolivian Altiplano

We report over 90 U-Th dates from carbonates deposited around paleolakes on the Bolivian Altiplano. Petrographic and chemical data for tufas and the siliciclastic detritus contained within them allow (1) assessment of possible diagenetic effects, (2) a development of a strategy for selection of carbonate samples with low initial Th contents, and (3) assessment of the uncertainty due to initial Th. This strategy allows us to produce precise U-Th dates from lacustrine carbonates. The principal consideration in dating such carbonates is the composition and quantity of initial Th incorporated into the carbonate, and additional uncertainty is introduced because this initial Th may have two sources in lacustrine deposits. Isochron plots, measured (230Th/232Th), and X-ray diffraction and trace element chemistry of silicic residues all favor regional soil and dust as the sole source of initial Th in carbonates from the Bolivian Altiplano, a situation that simplifies single-sample dating of Altiplano carbonates.

Seismicity and the strange rubbing boulders of the Atacama Desert, northern Chile

We found clusters of 0.5–8 t boulders worn to smoothness around their midsections in the Atacama Desert of northern Chile. We suggest that the boulder smoothing is the cumulative result of at least 1 m.y. of rubbing between boulders during earthquakes. 10Be exposure ages of boulder tops from these fields average ∼1.3 m.y., unsurprisingly old given the hyperaridity of the Atacama. During a visit to one major boulder site, we experienced an earthquake that rocked but did not tip the boulders, causing them to rub against each other for about a minute. This MW 5.2 earthquake was centered ∼100 km northeast of the site. In the seismically active Atacama, earthquakes of this energy or greater occur about once every four months, suggesting that the average boulder has undergone ∼40,000–70,000 h of abrasion over the past 1.3 m.y. This unusual evidence underscores the largely unrecognized role that seismicity probably plays in hillslope sediment transport in the nearly rainless Atacama Desert, and perhaps on other seismically active but now dry worlds like Mars.

Geomorphic process rates in the central atacama desert, Chile: Insights from cosmogenic nuclides and implications for the onset of hyperaridity

Water plays a critical role in erosion and sediment transport and this relationship is most evident in the hyperarid Atacama Desert of Northern Chile, a region characterized by erosion rates that fall to near zero and cobbles and boulders with cosmogenic nuclide concentrations indicative of exposure for many millions of years. Cosmogenic nuclide concentrations from the Atacama are used to both determine the age of hyperaridity onset and to place important constraints on rates of geomorphic processes in this uniquely arid environment. Prior determinations of cosmogenic nuclide concentrations from the Atacama Desert focus primarily on exposure ages from boulders/cobbles or erosion rates from bedrock. However, recent determinations of cosmogenic nuclide concentrations from boulders, bedrock, and sediment at the same location suggests that material from diverse sample types have different cosmogenic nuclide concentrations. Therefore, it is critical to determine which concentrations of cosmogenic nuclides are most representative of overall erosion rates from the Atacama. Here, concentrations of cosmogenic nuclides in more than 100 samples across two east-west transects within the central Atacama Desert (22-26°S) of Northern Chile are considered. Concentrations of 10Be and 26Al were measured in samples of bedrock, alluvial sediment, active stream sediment, and boulders within several sub-regions of the Atacama Desert, including: the western and eastern Coastal Cordillera, the inner absolute desert (including the Cerro de los Tetas), the Cordillera Domeyko, and the western flank of the Andes. This data allows detailed comparisons of cosmogenic nuclides concentrations both within diverse sample types at a given site and between major geomorphic sub-regions of the Atacama. The general pattern of cosmogenic nuclide concentrations in hyperarid environments is characterized by concentrations that are higher in boulders, moderately high in bedrock, lower in hillslope sediment, and lowest in channels that flow across the desert. At many locations in the central Atacama, boulders and bedrock have erosion rates as much as an order of magnitude slower than that of finer grained sediment at the same location, a relationship that is attributed to the fact that boulders sit above thick gypcrete soils. The hillslope to basin concentrations of cosmogenic nuclides within each sub-region, along with 26Al/10Be ratios, suggests that concentrations of 10Be in most bedrock and sediments reflects erosion rates. However, in the western Coastal Cordillera 10Be concentrations in sediment also reflects transport time. The overarching trend in this data set is that inferred erosion rates are lower to both the east and the west, corresponding with increases in both precipitation and erosion rates towards both the Andean Flank and in the Coastal Cordillera. This trend is previously noted for the Atacama; however, this large data set allows the first observation of the influence of other variables upon erosion rates. Most notably, another clear influence on erosion rates in the Atacama is slope. In some cases, differences in slope are enough to overcome the influence of aridity. For example, erosion rates on the flanks of the Cordillera Domeyko are faster (>1 m/Ma) than that at the crest (<0.5 m/Ma), despite the fact that the crest of this mountain range receives more moisture. Moreover, erosion rates in boulders are up to an order of magnitude slower than that of finer grained sediment. Taken together, these patterns suggest that in the extreme environment of the central Atacama erosion rates are sensitive not only to direct changes in precipitation but also to variables such as slope and soil cover. The relationship between decreased erosion and either low slope or subaerial exposure in the Atacama is potentially stronger than similar relationships documented elsewhere. These results are part of a growing body of research suggesting that even in extreme environments erosion is a complex process controlled by a multitude of variables, and where erosion rates are strongly limited by one variable, any other variable that acts to increase erosion may also have a significant effect. These new insights invoke reexamination of the global relationship between erosion and precipitation, and it is suggested that precipitation has an increasingly greater impact on erosion rates at lower values, but with some influence beginning at rates of ∼1000 mm/year. Understanding process rates in the central Atacama is essential to interpretation of cosmogenic nuclide concentrations as the timing of the onset of hyperaridity. Previous cosmogenic nuclide studies in the Atacama have specifically targeted remnant boulders to determine the age of initial aridification and this new data set shows that the boulders do not have the same cosmogenic nuclide concentration as the sediment on which they rest. Thus, the use of boulder ages or hillslope ages for the purpose of topographic reconstruction can be problematic despite the fact that, broadly, exposure of boulders and cobbles over million-year timescales implies lack of erosion over this same period. Several aspects of these results suggest that the onset of hyperaridity in the Atacama Desert predates the Pliocene. First, a number of ages from boulders predate the Pliocene. Second, new Miocene age constraints from a surface previously identified as Pliocene was shown; the age of this surface previously formed a critical component in arguments for decreased incision rates in the late Pliocene. Third, it is shown that a site that is today very sensitive to sediment fluxes from modern storm events is at least 3 Ma in age. Finally, it is shown that most of the hillslopes in the central Atacama were exposed during the Quaternary and do not have complex exposure history; this indicates that the Atacama is not and probably never has been a frozen or static landscape.