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Early footsteps in the Americas Despite a plethora of archaeological research over the past century, the timing of human migration into the Americas is still far from resolved. In a study of exposed outcrops of Lake Otero in White Sands National Park in New Mexico, Bennett et al . reveal numerous human footprints dating to about 23,000 to 21,000 years ago. These finds indicate the presence of humans in North America for approximately two millennia during the Last Glacial Maximum south of the migratory barrier created by the ice sheets to the north. This timing coincided with a Northern Hemispheric abrupt warming event, Dansgaard-Oeschger event 2, which drew down lake levels and allowed humans and megafauna to walk on newly exposed surfaces, creating tracks that became preserved in the geologic record. —AMS

During the termination of the last glacial period the western U.S. experienced exceptionally wet conditions, driven by changes in location and strength of the mid-latitude winter storm track. The distribution of modern winter precipitation is frequently characterized by a north-south wet/dry dipole pattern, controlled by interaction of the storm track with ocean-atmosphere conditions over the Pacific and Atlantic Oceans. Here we show that a dipole pattern of similar geographic extent persisted and switched sign during millennial-scale abrupt climate changes of the last deglaciation, based on a new lake level reconstruction for pluvial Lake Chewaucan (northwestern U.S.), and a compilation of regional paleoclimate records. This suggests the dipole pattern is robust, and one mode may be favored for centuries, thereby creating persistent contrasting wet/dry conditions across the western U.S. The TraCE-21k climate model simulation shows an equatorward enhancement of winter storm track activity in the northeastern Pacific, favoring wet conditions in southwestern U.S. during the second half of Heinrich Stadial 1 (16.1–14.6 ka) and consistent with paleoclimate evidence. During the Bølling/Allerød (14.6–12.8 ka), the northeastern Pacific storm track contracted poleward, consistent with wetter conditions concentrated poleward toward the northwest U.S.

The nature of multidecadal to centennial variability of the Asian monsoon is largely unknown, hindering our understanding of how the modern precipitation regime compares to preindustrial variability, as well as how the Asian monsoon may change in coming decades. Here we use the sediment record from a closed-basin lake in southern Tibet, Ngamring Tso, along with other published paleoclimatic records, to assess summer monsoon precipitation variability over the 20th century and during the late Holocene (4100 cal yr BP to present). The first principal component of the Ngamring Tso grain size record, which is highly correlated with median grain size, correlates significantly with observed summer (June–September) precipitation. From CE 1940 to 2007, grain size decreased with increasing summer precipitation and increased with decreasing summer precipitation. Satellite images of Ngamring Tso reveal larger lake area in the 1970s and 2000s and smaller lake area in the 1980s and 1990s, suggesting monsoon precipitation-induced changes in lake depth or area likely govern grain size variability. In the context of the last 4100 yr, prolonged periods of relatively weak summer monsoon precipitation occurred from 2800–2600 cal yr BP, 2500–2300 cal yr BP, and 1600–400 cal yr BP. A trend toward increased summer precipitation began around 1000 cal yr BP, with above-average summer precipitation from 400 cal yr BP to present, peaking between 200–100 cal yr BP (CE 1750–1850). Dry and wet periods are coincident with dry and wet periods in other south-central Tibetan lake sediment records that reflect precipitation or moisture balance, and with regional proxies of the ISM and EASM, indicating south-central Tibet is influenced by both monsoon subsystems. However, western Tibet lake sediment records do not covary with the Ngamring Tso record, suggesting the spatial variability in Tibet precipitation that occurs on interannual timescales today may also characterize precipitation on multidecadal and centennial timescales during the late Holocene. 20th century precipitation variability in southern Tibet falls within the range of natural variability in the last 4100 yr, and does not show a clear trend of increasing precipitation as projected by models. Instead, it appears that poorly understood multidecadal to centennial internal modes of monsoon variability remained influential throughout the last 4100 yr. Thus, substantial multidecadal to centennial-scale hydroclimatic variability will complicate our ability to project and prepare for anthropogenic changes in the region's monsoon precipitation.

The Huancane II moraines deposited by the Quelccaya Ice Cap in southern Peru were selected by the CRONUS-Earth Project as a primary site for evaluating cosmogenic-nuclide scaling methods and for calibrating production rates. The CRONUS-Earth Project is an effort to improve the state of the art for applications of cosmogenic nuclides to earth-surface chronology and processes. The Huancane II moraines are situated in the southern Peruvian Andes at about 4850 m and ∼13.9°S, 70.9°W. They are favorable for cosmogenic-nuclide calibration because of their low-latitude and high-elevation setting, because their age is very well constrained to 12.3 ± 0.1 ka by 34 radiocarbon ages on peat bracketing the moraines, and because boulder coverage by snow or soil is thought to be very unlikely. However, boulder-surface erosion by granular disintegration is observed and a ∼4% correction was applied to measured concentrations to compensate. Samples from 10 boulders were analyzed for 10Be, 26Al, and 36Cl. Interlaboratory bias at the ∼5% level was the largest contributor to variability of the 10Be samples, which were prepared by three laboratories (the other two nuclides were only prepared by one laboratory). Other than this issue, variability for all three nuclides was very low, with standard deviations of the analyses only slightly larger than the analytical uncertainties. The site production rates (corrected for topographic shielding, erosion, and radionuclide decay) at the mean site elevation of 4857 m were 45.5 ± 1.6 atoms 10Be (g quartz)−1 yr−1, 303 ± 15 atoms 26Al (g quartz)−1 yr−1, and 1690 ± 100 atoms 36Cl (g K)−1 yr−1. The nuclide data from this site, along with data from other primary sites, were used to calibrate the production rates of these three nuclides using seven global scaling methods. The traditional Lal formulation and the new Lifton-Sato-Dunai calibrations yield average ages for the Huancane samples that are in excellent-to-good agreement with the radiocarbon age control (within 0.7% for 10Be and 36Cl and 6% for 26Al). However, all of the neutron-monitor-based methods yielded ages that were too young by about 20%. The nuclide production ratios at this site are 6.74 ± 0.34 for 26Al/10Be in quartz and 37.8 ± 2.3 (atoms 36Cl (g K)−1) (atom 10Be (g SiO2)−1)−1 for 36Cl/10Be, in sanidine and quartz, respectively.

The CRONUS-Earth and CRONUS-EU Projects have recently examined the reproducibility of cosmogenic nuclide measurements and the ability of cosmogenic-production models to fit high-quality global calibration data sets. In both cases, although results are adequate for present geochronological needs, they must be improved to meet future demands. We recommend that the cosmogenic-nuclide community embark on a continuing effort to understand the sources of the remaining model-versus-data discrepancy, and that in the meanwhile all cosmogenic ages be benchmarked against reference ages calculated in a consistent fashion. We also recommend that interlaboratory reference materials be routinely analyzed along with samples and blanks, that the results be tracked, and that methods of achieving consistency and precision in sample preparation and analysis be shared within the community. We also recommend additional organization of the community to facilitate the use of intercomparison materials, foster analytical improvements, oversee the evolution of the community age calculator, and synthesize ongoing calibration efforts.

Geological surface-exposure dating using cosmogenic-nuclide accumulation became a practical geochronological endeavor in 1986, when the utility of 10Be, 26Al, 36Cl, and 3He were all demonstrated. In response to the lack of a common basis for quantifying analytical consistency and calibrating cosmogenic-nuclide production, the CRONUS-Earth Project in the U.S. was started in 2005, along with a European partner project, CRONUS-EU. The goal of the CRONUS-Earth Project was to improve the accuracy and precision of terrestrial cosmogenic nuclide dating in general, focusing especially on nuclide production rates and their variation with altitude, latitude, and time, and to attempt to move from empirically based methods to ones with a stronger basis in physics. The CRONUS-Earth Project conducted extensive intercomparisons of reference materials to attempt to quantify analytical reproducibility at the community level. We found that stated analytical uncertainties nearly always underestimate the actual degree of variability, as quantified by the over-all coefficient of variation of the intercalibration data. The average amount by which the actual coefficient of variation exceeded the analytical uncertainty was a factor of two (100%), but ranged from 15% to 300% depending on the nuclide and material. Coefficients of variation ranged from 3−4% for 10Be to 6–8% for 36Cl, 14C, and 21Ne, to 5–11% for 26Al. Both interlaboratory bias and within-laboratory excess spread of the data played a role in increasing variability above the stated analytical uncertainties. The physical basis for cosmogenic nuclide production was investigated through numerical modeling and the measurement of energy-dependent neutron cross sections for nuclide interactions. We formulated new, physically based, scaling models, denoted LSD and LSDn, by generalizing global numerical simulations of cosmic-ray processes. The CRONUS-Earth Project identified new geological calibration sites, including one at low latitude and high elevation (Huancane, Peru), and replicated nuclide measurement at numerous laboratories. At many sites multiple nuclides were measured, providing much more confidence in the equivalence of surface-exposure ages calculated from differing nuclides. The data were interpreted using an original cosmogenic-nuclide calculator, CRONUScalc, that incorporates the new physically based scaling. The new data and model produced significantly better fits than previous efforts, but do not fully resolve apparent spatial variations in production rates. The CRONUS-Earth and CRONUS-EU Projects have provided a firm foundation for assessing the strengths and weaknesses of cosmogenic-nuclide analytical methods, adjusted the AMS standards for 10Be and consequently revised the half-life, and have provided improved calibration data sets and interpretative tools.

The Rio Grande is the fourth longest river in North America extending over 3,000 km from the Rocky Mountains to the Gulf of Mexico. The Pleistocene evolution of this river from individual subbasins into a coalesced fluvial system has been long debated. Herein, we constrain the middle Pleistocene evolution of the northernmost and largest Rio Grande basin, the San Luis basin, and the timing of incision of the Rio Grande Gorge, based on new geologic mapping, 3He surface exposure dating, and U-series dating on pedogenic carbonates. 3He dating of shoreline deposits of closed basin Lake Alamosa and fluvially scoured bedrock along the Rio Grande gorge rim indicate the San Luis basin was connected with southerly Rio Grande basins and progressively incised since ∼400 ka. Waning tectonic activity, coupled with the hydrologic response to intensified middle Pleistocene glaciation, drove southward spillover of Lake Alamosa ∼400 ka, expanding the river's drainage basin by nearly 24,000 km2, and adding recharge areas in the high-altitude, glaciated San Juan, Tusas, and Sangre de Cristo Mountains. Geologic barriers to streamflow on the Taos Plateau were breached and basin tributary drainages were integrated and inset into their present canyons ∼250-200 ka, with approximately 120 m of incision during ∼400-200 ka. Previous work demonstrates that southerly streamflow through basins south of the SLB terminated into a large, rift-related bolson complex until the middle Pleistocene, and most axial Rio Grande river incision has occurred since 630 ka. We propose that middle Pleistocene integration of the SLB was the major geomorphic event that eroded Plio-Pleistocene tectonic and volcanic barriers and re-established flow of the Rio Grande from the Rocky Mountains to the Gulf of Mexico.

International audience; Paleoaltimetric studies have characterized in detail the relationship between carbonate oxygen isotope ratios (d18Oc) and elevation in orogens with simple, single-moisture-source hydrological systems, and applied this relationship to ancient continental carbonates to provide constraints on their past elevation. However, mixing of different atmospheric moisture sources in low-elevation orogens should affect d18Oc values, but this effect has not yet been confirmed unequivocally. In the American Southwest, summer monsoonal moisture, sourced in the Equatorial Pacific and the Gulf of Mexico, and winter moisture, sourced in the East Pacific, both contribute to annual rainfall. We present stable isotope results from Quaternary carbonates within the American Southwest to characterize the regional d18Oc-elevation relationship. We then provide stable isotope results from local Eocene carbonates to reconstruct late Laramide paleoelevations. The Quaternary d18Oc-elevation relationship in the American Southwest is not as straightforward as in more simple hydrological systems. d18Oc changes with altitude are non-linear, scattered, and display an apparent isotopic lapse rate inversion above 1200 m of elevation. We speculate that decreasing surface temperatures at high altitudes limit the duration of carbonate growth to the summer months, biasing d18Oc values toward higher values typical of the summer monsoon and leading to lapse rate inversion. d18Oc-elevation relationships based on modern water isotope data or distillation models predict paleoelevations that range up to as much as 2 km higher than the modern elevations of 2000 to 2400 m for our late Eocene sites located at the southern edge of the Colorado Plateau. By contrast, our d18Oc-elevation relationship for the American Southwest yields lower paleoelevation estimates. These alternate estimates nonetheless suggest that significant elevation (at least ϳ1 km) had already been attained by the Eocene, but are also compatible with < 1 km of uplift by post-Laramide mechanisms. Our results show the limitations of standard d18Oc-elevation models in complex hydrological systems and suggest that similar mechanisms may have led to summer-biased paleoaltimetry estimates for the initial stages of other orogenies -in the American Southwest and elsewhere.

The last glacial-interglacial transition (ca. 18–11 ka) was interrupted by abrupt climate events that differed in each hemisphere. During the Antarctic Cold Reversal (ca. 14.5–12.9 ka), the Southern Hemisphere high and mid latitudes cooled, while the Northern Hemisphere warmed. The pattern of change then reversed during the Younger Dryas (ca. 12.9–11.7 ka), which was characterized by cold conditions in much of the Northern Hemisphere. Well-dated paleoclimate records serve to reveal the possible mechanisms for these events. Here we present a reconstruction of the late glacial fluctuations of Quelccaya Ice Cap, located in the southern tropics, based on 38 new radiocarbon ages. Quelccaya was retreating from its Last Glacial Maximum (ca. 21 ka) extent by ca. 17.2 ka, and was located upvalley from its late glacial moraines by 13.6–12.8 ka. Quelccaya experienced a significant readvance that culminated at 12.5–12.4 ka, and then receded several kilometers to near, or within, its late Holocene extent by ca. 11.6 ka. This record provides the most detailed evidence yet of glacier fluctuations in the southern tropics during late glacial time.

Fluvial channels in metamorphic core complexes are preferentially oriented parallel and perpendicular to the direction of tectonic extension. This pattern has been variably attributed to such causes as tectonic tilting during extension, channel elongation by slip along the range-bounding detachment fault, and the exploitation of extension-related joint sets during channel incision. In this paper we use field measurements, digital elevation model analyses, and numerical modeling to test hypotheses for the tectonic and structural control of fluvial channels in metamorphic core complexes, using the Catalina-Rincon core complex in southern Arizona, USA, as a type example. Field measurements and aerial photographic analyses indicate that channels of all sizes exploit steeply dipping joint sets during fluvial incision. As a consequence, channels become preferentially aligned along those joint sets. First and second Strahler-order channels preferentially exploit a joint set oriented perpendicular to the extension direction, while higher-order channels preferentially exploit a joint set oriented parallel to the extension direction. While these observations support the joint-exploitation hypothesis for structural control of drainage architecture, numerical modeling indicates that the spatial distribution of rock uplift during the initial phase of extension plays a crucial role by determining which joint set is preferentially exploited by channels of which Strahler orders. Numerical models indicate that higher-order channels exploit the joint set that is most closely aligned with the direction of initial tectonic tilting, even if that tilting is active for only a short period of time following the initiation of uplift. We conclude that the drainage architecture in the Catalina-Rincon core complex is the result of a combination of joint exploitation and tectonic tilting mechanisms. Structure also plays an important role in controlling the longitudinal profiles of channels in metamorphic core complexes. Channels in the Catalina-Rincon core complex are characterized by structurally controlled knickpoints with a wide distribution of heights and spacings. Field observations indicate that the occurrence of structurally controlled knickpoints and the resulting variability in longitudinal profile form is related to spatial variations in joint density. Numerical models that incorporate spatial variations in joint density using a stochastic bedrock erodibility coefficient are capable of reproducing the statistical properties of longitudinal profiles in the Catalina-Rincon core complex, including the power spectrum of longitudinal profiles and the frequency size distribution of structurally controlled knickpoints. The results of this study illustrate the important roles played by both jointing and the spatial distribution of rock uplift on the geomorphic evolution of metamorphic core complexes. More broadly, the study provides a recipe for how to incorporate joint-related structural controls into landscape evolution models.