Jaakko Putkonen

Accuracy of cosmogenic ages for moraines

Analyses of all published cosmogenic exposure ages for moraine boulders show an average age range of 38% between the oldest and youngest boulders from each moraine. This range conflicts with the common assumption that ages of surface boulders are the same as the age of the landform. The wide spread in boulder ages is caused by erosion of the moraine surface and consequent exhumation of fresh boulders. A diffusion model of surface degradation explains the age range and shows that a randomly sampled small set of boulders (n = 3–7) will always yield a lower age limit for the moraine. The model indicates that for identical dating accuracy, six to seven boulders are needed from old and tall moraines (40,000–100,000 yr, 50–100 m initial height) but only one to four boulders from small moraines (20,000–100,000 yr, 10–20 m). By following these guidelines the oldest obtained boulder age will be ≥90% of the moraine age (95% probability). This result is only weakly sensitive to a broad range of soil erosion rates. Our analysis of published boulder ages indicates that <3% of all moraine boulders have prior exposure, and 85% of these boulders predate the dated moraine.

Surface Dating of Dynamic Landforms: Young Boulders on Aging Moraines

The dating of landforms is crucial to understanding the evolution, history, and stability of landscapes. Cosmogenic isotope analysis has recently been used to determine quantitative exposure ages for previously undatable landform surfaces. A pioneering application of this technique to date moraines illustrated its considerable potential but suggested a chronology partially inconsistent with existing geological data. Consideration of the dynamic nature of landforms and of the ever-present processes of erosion, deposition, and weathering leads to a resolution of this inconsistency and, more generally, offers guidance for realistic interpretation of exposure ages.

A study of the 1999 monsoon rainfall in a mountainous region in central Nepal using TRMM products and rain gauge observations

Raingauge data from the 1999 monsoon were compared with precipitation derived from the precipitation radar (PR) and the microwave imager instruments on board the Tropical Rainfall Measuring Mission (TRMM) satellite. The raingauges are part of a new hydrometeorological network installed in the Marsyandi river basin, which extends from the edge of the Tibetan Plateau to the Gangetic basin. TRMM-derived precipitation showed better detection of rain at low altitude stations as compared with high elevation stations, with good scores for the PR product for rain rates >0.5 mm/hr. The 3D PR rain rates suggest strong interaction between mesoscale convective systems and steep terrain at elevations of 1–2 km, which is consistent with the very high rainfall measured at those locations. Analysis of the raingauge data shows that even at altitudes as high as 4,000 m the cumulative monsoon rainfall is comparable to the highest amount recorded in the Indian subcontinent.

Plio-Quaternary exhumation history of the central Nepalese Himalaya: 1. Apatite and zircon fission track and apatite (U-Th)/He analyses

Received 5 May 2006; revised 19 October 2006; accepted 22 December 2006; published 4 May 2007. [1] New apatite and zircon fission track and (U-Th)/He analyses serve to document the bedrock cooling history of the central Nepalese Himalaya near the Annapurna Range. We have obtained 82 apatite fission track (AFT), 7z ircon fission track (ZFT), and 7a patite (U-Th)/He (AHe) ages from samples collected along the Marsyandi drainage, including eight vertical relief profiles from ridges on either side of the river averaging more than 2 km in elevation range. In addition, three profiles were sampled along ridge crests that also lie � 2 km above the adjacent valleys, and a transect of >20 valley bottom samples spans from the Lesser Himalaya across the GreaterHimalayaandintotheTethyanstrata.Asaconsequence, these data provide one of the more comprehensive low-temperature thermochronologic studies within the Himalaya. Conversely, the youthfulness of this orogen is pushing the limits of these dating techniques. AFTages range from >3.8 to 0 Ma, ZFTages from 1.9 to 0.8 Ma, and AHe ages from 0.9 to 0.3 Ma. Most ridges have maximum ages of 1.3–0.8 Ma at 2 km above the valley bottom. Only one ridge crest (in the southcentralzoneofthefieldarea)yieldedsignificantly older ZFTand AFTagesof � 2 Ma; we infer that a splay of the Main Central Thrust separates this ridge from the rest of the Greater Himalaya. ZFTand AFTages from a vertical transect along this ridge indicate exhumation rates of � 1.5 km Myr � 1 (r 2 > 0.7) from � 2 to 0.6– 0.8 Ma, whereas AHe ages indicate a faster exhumation rate of � 2.6 km Myr � 1 (r 2 = 0.9) over the last 0.8 Myr. Exhumation rates calculated for six of the remaining seven vertical profiles ranged from 1.5 to 12 km Myr � 1 (all with low r 2 values of <0.6) for the time period from � 1.2 to 0.3 Ma, with no discernible patterns in south to north exhumation rates evident. The absence of a trend in exhumation rates, despite a strong spatial gradient in rainfall, argues against a correlation of long-term exhumation rates with modern patterns of rainfall. AFT ages in the Tethyan strata are, on average, older than intheGreater Himalaya andmay bearesponse toa drier climate, slip on the South Tibetan Detachment, or a gentler dip of the underlying thrust ramp. These data arefurtherevaluatedwiththermokinematic modelingin the companion paper by Whipp et al. Citation: Blythe, A. E., D. W. Burbank, A. Carter, K. Schmidt, and J. Putkonen (2007), Plio-Quaternary exhumation history of the central Nepalese Himalaya: 1. Apatite and zircon fission track and apatite [U-Th]/He analyses, Tectonics, 26, TC3002, doi:10.1029/2006TC001990.

Soil Thermal and Ecological Impacts of Rain on Snow Events in the Circumpolar Arctic

Rain on snow (ROS) events are rare in most parts of the circumpolar Arctic, but have been shown to have great impact on soil surface temperatures and serve as triggers for avalanches in the midlatitudes, and they have been implicated in catastrophic die-offs of ungulates. The study of ROS is inherently challenging due to the difficulty of both measuring rain and snow in the Arctic and representing ROS events in numerical weather predictions and climate models. In this paper these challenges are addressed, and the occurrence of these events is characterized across the Arctic. Incidents of ROS in Canadian meteorological station data and in the 40-yr ECMWF Re-Analysis (ERA-40) are compared to evaluate the suitability of these datasets for characterizing ROS. The ERA-40 adequately represents the large-scale synoptic fields of ROS, but too often has a tendency toward drizzle. Using the ERA-40, a climatology of ROS events is created for thresholds that impact ungulate populations and permafrost. It is found that ROS events with the potential to harm ungulate mammals are widespread, but the large events required to impact permafrost are limited to the coastal margins of Beringia and the island of Svalbard. The synoptic conditions that led to ROS events on Banks Island in October of 2003, which killed an estimated 20 000 musk oxen, and on Svalbard, which led to significant permafrost warming in December of 1995, are examined. Compositing analyses are used to show the prevailing synoptic conditions that lead to ROS in four disparate parts of the Arctic. Analysis of ROS in the daily output of a fully coupled GCM under a future climate change scenario finds an increase in the frequency and areal extent of these events for many parts of the Arctic over the next 50 yr and that expanded regions of permafrost become vulnerable to ROS.

Continuous Snow and Rain Data at 500 to 4400 m Altitude near Annapurna, Nepal, 1999-2001

Abstract Knowledge of spatial and altitudinal variations in precipitation in high mountains is integral to quantifying alpine climates and to calibrating interactions between climate and surface processes. To date, however, few meteorological networks exist in alpine settings. A network of 14 meteorological stations was installed across the Annapurna Range in central Nepal in 1999 and expanded in subsequent years to 19 stations. In order to measure snow depths and water equivalents in high-altitude sites, a combination of look-down distance rangers and gamma-ray loggers was installed at 5 sites. The data from this network delineate a strong south-to-north gradient in monsoonal precipitation. Precipitation peaks at 5032 mm yr−1 at about 3000 m altitude on the southern side, which is also approximately the lowest altitude of winter snow in the area. Annual precipitation decreases to ∼1100 mm yr−1 in the rain shadow to the north of the Himalayan crest. Although snow depth and snow water equivalent content are strongly dependent on station altitude, snow depth shows little spatial variation at a given altitude, partly due to the surprisingly low local wind speeds. Based on extrapolation of mean monthly summer lapse rate of air temperature, only snow precipitates above 5883 m.

Rain on Snow: Little Understood Killer in the North

In October 2003, a severe rain-on-snow (ROS) event killed approximately 20,000 musk-oxen (Figure 1) on Banks Island, which is the westernmost of the Canadian Arctic islands (approximately 380 kilometers by 290 kilometers in size). The event reduced the isolated herd by 25% and significantly affected the people dependent on the herds well-being. Because of the sparsity of weather stations in the Arctic and the lack of routinely deployed weather equipment that was capable of accurately sensing the ROS event, its detection largely was based on reports from hunters who were in the affected areas at the time. Such events can significantly alter a frozen ecosystem—with changes that often persist for the remainder of a winter—by creating ice layers at the surface of, within, or below the snowpack. The water and ice layers are known to facilitate the growth of toxic fungi, significantly warm the soil surface under thick snowpack, and deter large grazing mammals. Although ROS events of the magnitude that was experienced in Banks Island in 2003 likely have reverberations throughout the entire Arctic and subarctic ecosystem, little is presently known about them and their impacts. As understanding of ROS events expands, many ROS-related aspects of the Arctic ecology and hydrology are likely to be discovered. They may include topics such as the fate of small mammals under the snowpack at the iced soil surface, the difficulty of ptarmigans to burrow into the iced snow, the limited infiltration of spring snowmelt into the iced over soil, and the changing drifting patterns of ice-crusted snowpack.

Permafrost process research in the United States since 1960

Publisher Summary This chapter provides an overview of the significant scientific advances in understanding the physical and chemical aspects of periglacial processes and the geomorphology of permafrost areas. It presents the more significant advances in terms of new approaches, methodology, or insight gained into key processes. The study of permafrost evolves from a descriptive science based mostly on field observations and limited temperature measurements in the summer to a quantitative science capitalizing on advances in understanding fundamental principles in condensed matter physics, nonlinear dynamics, soil physics, geochemistry and on technological advances that make it possible to measure soil properties and to monitor key physical and biogeochemical processes year-round, including the important times of phase transitions in the spring and autumn. Research in permafrost processes is evolving because of the recent widespread recognition of the interdependency of physical, chemical, and biological processes in the active layer, the sensitivity of the upper permafrost to ongoing climate change, and the potential for changes in the polar regions to affect the global climate.

Million year old ice found under meter thick debris layer in Antarctica

Cosmogenic nuclide measurements associated with buried glacier ice in Ong Valley, in the Transantarctic Mountains, suggest the preservation of ancient ice. There are three glacial tills on the valley floor which have formed from the concentration of regolith contained within sublimating glacier ice. Two tills are less than 1 m thick and underlain by ice. Measurements of cosmogenic 10Be, 26Al, and 21Ne show that (i) the youngest buried ice unit and corresponding till are at least 11–13 ka, (ii) another ice unit and corresponding intermediate-age till are at least 1.1 Ma old under any circumstances and most likely older than 1.78 Ma, and (iii) the oldest till is at least 1.57 Ma and most likely greater than 2.63 Ma. These observations highlight the longevity of ice under thin debris layers and the potential to sample ancient ice for paleoclimate/paleoatmosphere information close to the present land surface.