Jason A. Rech

Neogene climate change and uplift in the Atacama Desert, Chile

The relationship between Andean uplift and extreme desiccation of the west coast of South America is important for understanding the interplay between climate and tectonics in the Central Andes, yet it is poorly understood. Here we use soil morphological char- acteristics, salt chemistry, and mass independent fractionation anomalies ( 17 O values) in dated paleosols to reconstruct a middle Miocene climatic transition from semiaridity to extreme hyperaridity in the Atacama Desert. Paleosols along the southeastern margin of the Calama Basin change from calcic Vertisols with root traces, slickensides, and gleyed horizons to an extremely mature salic Gypsisol with pedogenic nitrate. We interpret this transition, which occurred between 19 and 13 Ma, to represent a change in precipitation from 200 mm/yr to 20 mm/yr. This drastic reduction in precipitation likely resulted from uplift of the Central Andes to elevations 2 km; the uplift blocked moisture from the South American summer monsoon from entering the Atacama. The mid-Miocene Gyp- sisol with pedogenic nitrate is located at elevations between 2900 and 3400 m in the Cal- ama Basin, significantly higher than modern nitrate soils, which occur below 2500 m. Modern and Quaternary soils in this elevation zone contain soil carbonate and lack ped- ogenic gypsum and nitrate. We infer that 900 m of local surface uplift over the past 10 m.y. displaced these nitrate paleosols relative to modern nitrate soils and caused a return to wetter conditions in the Calama Basin by decreasing local air temperatures and creating an orographic barrier to Pacific air masses.

Isotopic evidence for the source of Ca and S in soil gypsum, anhydrite and calcite in the Atacama Desert, Chile

Abstract The origin of pedogenic salts in the Atacama Desert has long been debated. Possible salt sources include in situ weathering at the soil site, local sources such as aerosols from the adjacent Pacific Ocean or salt-encrusted playas (salars), and extra-local atmospheric dust. To identify the origin of Ca and S in Atacama soil salts, we determined δ 34 S and 87 Sr/ 86 Sr values of soil gypsum/anhydrite and 87 Sr/ 86 Sr values of soil calcite along three east-west trending transects. Our results demonstrate the strong influence of marine aerosols on soil gypsum/anhydrite development in areas where marine fog penetrates inland. Results from an east-west transect located along a breach in the Coastal Cordillera show that most soils within 90 km of the coast, and below 1300 m in elevation, are influenced by marine aerosols and that soils within 50 km, and below 800 m in elevation, receive >50% of Ca and S from marine aerosols (δ 34 S values > 14‰ and 87 Sr/ 86 Sr values >0.7083). In areas where the Coastal Cordillera is >1200 m in elevation, however, coastal fog cannot penetrate inland and the contribution of marine aerosols to soils is greatly reduced. Most pedogenic salts from inland soils have δ 34 S values between +5.0 to +8.0‰ and 87 Sr/ 86 Sr ratios between 0.7070 and 0.7076. These values are similar to average δ 34 S and 87 Sr/ 86 Sr values of salts from local streams, lakes, and salars (+5.4 ±2‰ δ 34 S and 0.70749 ± 0.00045 87 Sr/ 86 Sr) in the Andes and Atacama, suggesting extensive eolian reworking of salar salts onto the surrounding landscape. Ultimately, salar salts are precipitated from evaporated ground water, which has acquired its dissolved solutes from water-rock interactions (both high and low-temperature) along flowpaths from recharge areas in the Andes. Therefore, the main source for Ca and S in gypsum/anhydrite in non-coastal soils is indirect and involves bedrock alteration, not surficially on the hyperarid landscape, but in the subsurface by ground water, followed by eolian redistribution of ground-water derived salar salts to soils. The spatial distribution of high-grade nitrate deposits appears to correspond with areas that receive the lowest fluxes of local marine and salar salt, supporting arguments for tropospheric nitrogen as the main source for soil nitrate.

Late Quaternary paleohydrology of the central Atacama Desert (lat 22°-24°S), Chile

In northern Chile, precipitation in the High Andes (>3500 m) recharges groundwater systems that flow down the Pacific slope and feed large aquifers in the hyperarid Atacama Desert. Wetlands, which are often found along the base of the Andes, mark locations where the water table intersects the land surface. At these locations, paleo–wetland deposits, which are present as terraces between 3 and 20 m above modern wetlands, record past water-table heights along the Andean front and are used to reconstruct changes in groundwater discharge. Paleo–wetland deposits in the central Atacama Desert (lat 22°–24°S) record an episode (>15.4–9 ka) of high water tables followed by an episode (8–3 ka) of moderately high water tables. Elevated water tables result from increased groundwater discharge and ultimately from enhanced recharge in the Andes. The concordance of results from three separate hydrologic systems suggests that changes in groundwater discharge and recharge are regional and reflect climatic fluctuations. This interpretation is supported by close agreement with other paleoclimatic records in the region. The periods of greater groundwater discharge were separated by episodes (9–8 and 3–0 ka) of significant groundwater lowering and stream incision, implying greatly diminished discharge. The central Atacama and Andes (lat 22°– 24°S) receive precipitation mainly from moist air masses transported from the Amazon Basin by the South American Summer Monsoon (SASM). Increases in groundwater recharge are therefore thought to reflect an increase in the frequency and/or moisture content of SASM air masses crossing the Andes. Fluctuations in SASM precipitation have previously been linked to summer insolation in the Southern Hemisphere. The wettest period in the central Atacama (>15.4–9 ka), however, coincides with a minimum in austral-summer insolation at 10 ka, suggesting that regional summer insolation is not a dominant influence on the SASM. Instead, intensification of the SASM may be linked to extraregional forcings such as the Walker Circulation.

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.

Landscape modification in response to repeated onset of hyperarid paleoclimate states since 14 Ma, Atacama Desert, Chile

The landscape of the hyperarid Atacama Desert in northern Chile records extremely slow change on Earth’s surface. Disputed ages for the onset of hyperaridity range from the late Paleogene through the Pleistocene. A long-term paleoclimate record is recorded in a nonmarine basin whose fill is primarily alluvial strata. For this setting, the primary proxies for climate state are the mineralogical and chemical composition of soil, which varies across a precipitation gradient, and the landforms and deposits of alluvial fans. During the most recent ∼15 million years, five climate-related landscape stages are resolved for the Pampa del Tamarugal sedimentary basin, with each successively younger stage inset lower in the local topography than its predecessor. The earliest landscape stage is expressed as a set of alluvial strata inherited from a time of arid or semi-arid climate, ca. 14–12 Ma. The younger four landscape stages generated a composite long-lasting exposure surface. Predominantly hyperarid conditions have persisted since ca. 12 ± 1 Ma, during which four intervals of arid to semi-arid climate occurred. Each wet interval was short lived, a million years or less, whereas some of the hyperarid periods were lengthy, 1–5 m.y. The hyperarid intervals were roughly 11–5.5 Ma, 4.5–4 Ma, 3.6–2.6 Ma, 2.2–1 Ma, and repeated intervals during the last 1 m.y. The onset of hyperaridity ca. 12 Ma likely reflects the growth of the Andes Mountains above a climate threshold. In contrast, sea surface temperature variability likely has controlled Atacama paleoclimate changes since the late Miocene.

Re-evaluation of mid-Holocene deposits at Quebrada Puripica, northern Chile

Duringthe middle Holocene (8^3 ka), wetland deposits accumulated in areas with emerg ent water tables in the central Atacama Desert (22^24‡S), producinga stratig raphic unit (Unit C) that can be mapped and correlated across different basins and geomorphic settings. Wherever mapped, Unit C is located between 6 and 30 m above modern wetlands, and includes thick sequences of diatomite and organic mats. The origin, depositional environment, and paleoclimatic significance of Unit C is controversial and currently under debate. Grosjean [Science 292 (2001) 2391a] suggests the unit developed under a regime of falling lake and ground-water levels, whereas evidence presented here suggests that Unit C formed during a period of rising ground-water levels and increased vegetation cover. The debate is embedded in broader discussions about geomorphic processes in ground-water-fed streams, as well as the history and forcingof climate variability in the central Andes. The central Atacama and Andes are remote, with few opportunities for different researchers to examine the same site. One exception is a sequence of mid-Holocene deposits at the confluence of Quebrada Seca and R| ¤o Puripica in northern Chile (23‡S). Grosjean et al. [Quat. Res. 48 (1997) 239^246] initially suggested the deposits accumulated in temporary lake basins, which formed when recurringdebris flows from a side canyon (Quebrada Seca) dammed the main channel (R|¤o Puripica) duringa period of drought and reduced stream flow. Upon our visit to the site, we found evidence that clearly demonstrates the deposits were not formed in temporary lakes, but rather were deposited in a wetland environment. Disagreement remains about the climatic interpretation of wetland deposits. Grosjean [Science 292 (2001) 2391a] now suggests that local ground-water levels rise and wetland deposits aggrade in deep canyon systems, such as R| ¤o Puripica, when stream power and channel erosion is reduced duringprolong ed dry spells. However, sedimentological evidence and the presence of Unit C in many depositional environments, not just deep canyons, indicate that it formed during a period of higher regional ground-water levels that were sustained by enhanced precipitation and recharge in the High Andes. ? 2003 Elsevier Science B.V. All rights reserved.

The influence of slope aspect on soil weathering processes in the Springerville volcanic field, Arizona

Abstract A comparison was made between soils on north- and south-facing slopes of six cinder cones in the Springerville volcanic field (SVF), Arizona, in order to determine the influence of slope aspect on soil weathering processes. Twenty-four soil pedons were sampled on different aspects of six cinder cones. To control for the influence of slope on pedogenesis, all sample sites possessed slopes of 17±2°. Soil weathering processes were characterized by solum depth, texture, and Ca:Zr chemical weathering indices. Quartz and mica were used to identify eolian additions to the volcanic soils. Accelerated rates of weathering and soil development were found to occur in soils on south-facing slopes while no trend with aspect was found for eolian additions. Accelerated rates of weathering and soil development may influence cinder cone degradation and cone morphology.

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.

Assessing open-system behavior of 14C in terrestrial gastropod shells

In order to assess open-system behavior of radiocarbon in fossil gastropod shells, we measured the 14C activity on 10 aliquots of shell material recovered from Illinoian (~190-130 ka) and pre-Illinoian (~800 ka) loess and lacustrine deposits in the Midwestern USA. Eight of the 10 aliquots yielded measurable 14C activities that ranged from 0.25 to 0.53 per- cent modern carbon (pMC), corresponding to apparent 14C ages between 48.2 and 42.1 ka. This small level of open-system behavior is common in many materials that are used for 14C dating (e.g. charcoal), and typically sets the upper practical limit of the technique. Two aliquots of gastropod shells from the Illinoian-aged Petersburg Silt (Petersburg Section) in central Illi- nois, USA, however, yielded elevated 14C activities of 1.26 and 1.71 pMC, which correspond to apparent 14C ages of 35.1 and 32.7 ka. Together, these results suggest that while many fossil gastropods shells may not suffer from major (>1%) open-sys- tem problems, this is not always the case. We then examined the mineralogy, trace element chemistry, and physical charac- teristics of a suite of fossil and modern gastropod shells to identify the source of contamination in the Petersburg shells and assess the effectiveness of these screening techniques at identifying samples suitable for 14C dating. Mineralogical (XRD) and trace element analyses were inconclusive, which suggests that these techniques are not suitable for assessing open-system behavior in terrestrial gastropod shells. Analysis with scanning electron microscopy (SEM), however, identified secondary mineralization (calcium carbonate) primarily within the inner whorls of the Petersburg shells. This indicates that SEM exam- ination, or possibly standard microscope examination, of the interior of gastropod shells should be used when selecting fossil gastropod shells for 14C dating.

Direct dating of plaster and mortar using AMS radiocarbon: a pilot project from Khirbet Qana, Israel

The authors demonstrate the potential for dating structures in Near Eastern archaeology by applying AMS radiocarbon to organic inclusions found in mortar and plaster. The method was successfully applied to date and sequence excavated walls and floors, and to spot-date structures exposed in surveys.