Claudio Latorre

Vegetation invasions into absolute desert: A 45;th000 yr rodent midden record from the Calama–Salar de Atacama basins, northern Chile (lat 22°–24°S)

Plant macrofossils, percentage abundance of grass taxa, fecal-pellet d13 C, and plant-cuticle contents from 49 fossil rodent middens dated by 14 C record changes in local vegetation and precipitation since 45 ka (calibrated or measured to thousands of calendar years before present) in the central Atacama Desert (lat 228‐248S) of northern Chile. The midden sites are along the hyperarid upper margin (2400‐3100 m) of the ‘‘absolute desert,’’ in an extreme environment sparsely vegetated by annual herbs and halophytic shrubs. Conditions between 40 and 22 ka may have been at least intermittently dry, and possibly cooler, as implied by four middens with low species richness. We infer a large increase in summer rainfall between 16.2 and 10.5 ka on the basis of the lowering of steppe grasses by as much as 1000 m, prominence of C4 grasses and summer annuals, high species richness, and displacement of northern species at least 50 km south of their modern ranges. The precipitation increase was greatest for a cluster of middens between 11.8 and 10.5 ka. Abrupt drying, evident in a dramatic decrease in grass abundance, occurred after 10.5 ka at all four midden localities. Increased percentages of

A vegetation history from the arid prepuna of northern Chile (22^23‡S) over the last 13 500 years

The Quaternary paleoclimate of the central Andes is poorly understood due to numerous discrepancies among the diverse proxy records that span this geographically and climatically complex region. The exact timing,duration and magnitude of wet and dry phases are seldom duplicated from one proxy type to another,and there have been few opportunities to compare climatic records from the same proxy along environmental gradients. Vegetation histories from fossil rodent middens provide one such opportunity on the Pacific slope of the Andes. We previously reported a vegetation history from the upper margin (2400^3000 m) of the absolute desert in the central Atacama Desert of northern Chile. That record identified a distinct wet phase that peaked between 11.8 and 10.5 ka,when steppe grasses and other upland elements expanded as much as 1000 m downslope,and a secondary wet period during the middle to late Holocene (7.1^3.5 ka). The latter wet phase remains controversial and is not as readily apparent in our lowelevation midden record. We thus sought to replicate both phases in a midden record from the mid-elevations (3100^ 3300 m) of the arid prepuna,where slight precipitation increases would be amplified. Midden records from these elevations identify conditions wetter than today at 13.5^9.6,7.6^6.3,4.4^3.2 and possibly 1.8^1.2 ka. Dry phases occurred at 9.4^8.4 ka and possibly at ca. 5.1 ka. Present floras and modern hyperarid conditions were established after 3.2 ka. The records from the two elevational bands generally match with some important differences. These differences could reflect both the discontinuous aspect of the midden record and the episodic nature of precipitation and plant establishment in this hyperarid desert. : 2003 Elsevier Science B.V. All rights reserved.

Perennial stream discharge in the hyperarid Atacama Desert of northern Chile during the latest Pleistocene

A large fraction of the vital groundwater in the Atacama Desert of northern Chile is likely composed of “fossil” or “ancient” reserves that receive little or no recharge in todays hyperarid climate. Here, we present evidence for latest Pleistocene perennial streamflow in canyons from the hyperarid core of the Atacama Desert in northern Chile. Fluvial terraces in the Pampa del Tamarugal (PdT) basin (21°S) contain widespread fossil wood, in situ roots, and well preserved leaf litter deposits indicative of perennial surface flow currently absent in these channels. Nineteen radiocarbon dates on these deposits from four separate drainages within this endorheic basin indicate ages from 16,380 to 13,740 cal yr BP, synchronous with paleolake Tauca on the Bolivian Altiplano and other regional evidence for wetter conditions during the latest Pleistocene. Groundwater-fed riparian ecosystems and associated fluvial deposits abound today in the absence of direct rainfall in northern Atacama canyons with perennial discharge. Our relict riparian ecosystems from the PdT basin are indicative of conditions similar to these northern canyons. Given that discharge was higher than present during this time, we propose that these deposits represent the most important groundwater recharge events of the last 18,000 years. A lesser recharge event occurred during the Holocene, when phreatophytic trees also grew in these drainages between 1,070 and 700 cal yr BP, during the Medieval Climatic Anomaly. Taken together, our evidence lends further support for gradient changes in the equatorial Pacific as a major driver of hydrologic change in the Atacama on both centennial and millennial time scales.

Emergence of social complexity among coastal hunter-gatherers in the Atacama Desert of northern Chile

The emergence of complex cultural practices in simple hunter-gatherer groups poses interesting questions on what drives social complexity and what causes the emergence and disappearance of cultural innovations. Here we analyze the conditions that underlie the emergence of artificial mummification in the Chinchorro culture in the coastal Atacama Desert in northern Chile and southern Peru. We provide empirical and theoretical evidence that artificial mummification appeared during a period of increased coastal freshwater availability and marine productivity, which caused an increase in human population size and accelerated the emergence of cultural innovations, as predicted by recent models of cultural and technological evolution. Under a scenario of increasing population size and extreme aridity (with little or no decomposition of corpses) a simple demographic model shows that dead individuals may have become a significant part of the landscape, creating the conditions for the manipulation of the dead that led to the emergence of complex mortuary practices.

The structure and rate of late Miocene expansion of C4 plants: Evidence from lateral variation in stable isotopes in paleosols of the Siwalik Group, northern Pakistan

This study uses stable isotope variation within individual Mio-Pliocene paleosols to investigate subkilometer-scale phytogeography of late Miocene vegetation change in southeast Asia between ca. 8.1 and 5 Ma, a time interval that coincides with dramatic global vegetation change. We examine trends through time in the distribution of low-latitude grasses (C4 plants) and forest (C3 plants) on Indo-Gangetic floodplains using carbon (δ13C) and oxygen isotopic (δ18O) values in buried soil carbonates in Siwalik Series sediments exposed in the Rohtas Anticline, north-central Pakistan. Revised, high-resolution magnetostratigraphy and a new 40Ar/39Ar date provide improved age control for the 2020 m Rohtas section. Carbon isotope results capture lateral variability of C3 versus C4 plants at five stratigraphic levels, R11 (8.0 Ma), R15 (6.74–6.78 Ma), R23 (5.78 Ma), R29 (4.8–4.9 Ma), and upper boundary tuff (UBT; 2.4 Ma), using detailed sampling of paleosols traceable laterally over hundreds of meters. Paleosols and the contained isotopic results can be assigned to three different depositional contexts within the fluvial sediments: channel fill, crevassesplay, and floodplain environments. δ13C results show that near the beginning (8.0 Ma) and after (4.0 Ma) the period of major ecological change, vegetation was homogeneously C3 or C4, respectively, regardless of paleo-landscape position. In the intervening period, there is a wide range of values overall, with C4 grasses first invading the drier portions of the system (floodplain surfaces) and C3 plants persisting in moister settings, such as topographically lower channel swales. Although abrupt on a geologic timescale, changes in abundance of C4 plants are modest (∼2% per 100,000 yr) compared to rates of vegetation turnover in response to glacial and interglacial climate changes in the Quaternary. Earlier research documented a sharply defined C3 to C4 transition in Pakistan between 8.1 and 5.0 Ma, based on vertical sampling, but this higher-resolution study reveals a more gradual transition between 8.0 and 4.5 Ma in which C3 and C4 plants occupied different subenvironments of the Siwalik alluvial plain. δ18O values as well as δ13C values of soil carbonate increase up section at Rohtas, similar to isotope trends in other paleosol records from the region. Spatially, however, there is no correlation between δ13C and δ18O values at most stratigraphic levels. This implies that the changes in soil hydrology brought about by the shift from forest to grassland (i.e., an increase in average soil evaporation) did not produce the shift through time in δ18O values. We interpret the trend toward heavier soil carbonate δ18O values as a response to changes in external climatic factors such as a net decrease in rainfall over the past 9 Ma.

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.

Molecular analysis of a 11 700-year-old rodent midden from the Atacama Desert, Chile

DNA was extracted from an 11 700‐year‐old rodent midden from the Atacama Desert, Chile and the chloroplast and animal mitochondrial DNA (mtDNA) gene sequences were analysed to investigate the floral environment surrounding the midden, and the identity of the midden agent. The plant sequences, together with the macroscopic identifications, suggest the presence of 13 plant families and three orders that no longer exist today at the midden locality, and thus point to a much more diverse and humid climate 11 700 years ago. The mtDNA sequences suggest the presence of at least four different vertebrates, which have been putatively identified as a camelid (vicuna), two rodents (Phyllotis and Abrocoma), and a cardinal bird (Passeriformes). To identify the midden agent, DNA was extracted from pooled faecal pellets, three small overlapping fragments of the mitochondrial cytochrome b gene were ampli‐fied and multiple clones were sequenced. These results were analysed along with com‐plete cytochrome b sequences for several modern Phyllotis species to place the midden sequence phylogenetically. The results identified the midden agent as belonging to an ancestral P. limatus. Today, P. limatus is not found at the midden locality but it can be found 100 km to the north, indicating at least a small range shift. The more extensive sampling of modern Phyllotis reinforces the suggestion that P. limatus is recently derived from a peripheral isolate.

THE IMPACT OF ENSO IN THE ATACAMA DESERT AND AUSTRALIAN ARID ZONE: EXPLORATORY TIME-SERIES ANALYSIS OF ARCHAEOLOGICAL RECORDS

La comparacion de datos arqueologicos del desierto de Atacama y de la zona arida de Australia muestra el impacto de El Nino-Oscilacion del Sur en los ultimos 5.000 anos. Con base en un listado de > 1.400 dataciones radiocarbonicas de ambas regiones, utilizadas como fuente indirecta de fluctuaciones de la poblacion, se realizaron graficos de densidad de las dataciones para explorar las respuestas de las poblaciones prehistoricas a las variaciones climaticas provocadas por ENSO. Bajo un regimen ENSO las precipitaciones se encuentran en antefase entre Australia y la costa del norte de Chile. Dado que ENSO impacta en la productividad de los recursos marinos y suprime el ingreso de humedad de la cuenca del Amazonas, su efecto en las economias de subsistencia de ambos lados del Pacifico Tropical deberia correlacionarse positivamente. Esto se confirma a traves de analisis cruzados de espectros de graficos de densidad de radiocarbonos, lo que muestra fluctuaciones sincronicas de la poblacion en ambos lados del Pacifico Tropical (r = > 0,82). Las bases de datos de Australia y del desierto de Atacama muestran un aumento gradual de la poblacion desde los 13 mil cal anos a.p., y un incremento importante durante el optimo climatico del Holoceno medio. La intensificacion de ENSO a partir de los 3,7 cal anos a.p. se correlaciona con ciclos de florecimiento y declinacion de la ocupacion humana en ambos lados del Pacifico, incluyendo el colapso del sistema economico y cultural en la costa del desierto de Atacama alrededor de 3 mil anos cal a.p. y la declinacion de la poblacion en las zonas interiores aridas del Atacama y del centro de Australia, entre 3-2 mil anos cal a.p. Posterior a 2 mil anos cal a.p. las respuestas adaptativas varian entre ambas regiones, aunque se produce una recuperacion general de las poblaciones en sus zonas aridas interiores.

The stable isotope altimeter: Do Quaternary pedogenic carbonates predict modern elevations?

Stable isotope altimetry is a useful tool for estimating paleoelevation in sedimentary records. Yet questions remain regarding how source moisture, climate, and local topography can influence these estimates. Here we present stable isotope altimetry results on late Quaternary pedogenic carbonates of known elevation on both flanks of the Andean orogen at 33°S. We measured δ 18 O values of pedogenic carbonates and river water samples from small drainages at regular elevation increments within the Rio Aconcagua (Chile) and Rio Mendoza (Argentina) catchments. The δ 18 O values of river waters correlate well with elevation and show similar isotopic gradients between the Chilean (−3.7‰/km) and Argentine (−4.8‰/km) sides of the range. Uncertainties associated with scatter in the river water data and assumptions about the temperature of carbonate formation indicate that elevation estimates have 1σ errors of 350–450 m. We estimate the isotopic composition of soil water from pedogenic carbonates on both sides of the range by assuming mean annual temperatures based the modern temperature lapse rate from meteorological station data. Combined, our data show that stable isotope altimetry produces reasonable estimates of modern elevation, with the majority of our samples (60%) within the 1σ uncertainties and 77% within 2σ.

Accumulation of impact markers in desert wetlands and implications for the Younger Dryas impact hypothesis

The Younger Dryas impact hypothesis contends that an extraterrestrial object exploded over North America at 12.9 ka, initiating the Younger Dryas cold event, the extinction of many North American megafauna, and the demise of the Clovis archeological culture. Although the exact nature and location of the proposed impact or explosion remain unclear, alleged evidence for the fallout comes from multiple sites across North America and a site in Belgium. At 6 of the 10 original sites (excluding the Carolina Bays), elevated concentrations of various “impact markers” were found in association with black mats that date to the onset of the Younger Dryas. Black mats are common features in paleowetland deposits and typically represent shallow marsh environments. In this study, we investigated black mats ranging in age from approximately 6 to more than 40 ka in the southwestern United States and the Atacama Desert of northern Chile. At 10 of 13 sites, we found elevated concentrations of iridium in bulk and magnetic sediments, magnetic spherules, and/or titanomagnetite grains within or at the base of black mats, regardless of their age or location, suggesting that elevated concentrations of these markers arise from processes common to wetland systems, and not a catastrophic extraterrestrial impact event.