Valerie Sloan

Asynchronous glaciation at Nanga Parbat, northwestern Himalaya Mountains, Pakistan

We present a new glacial chronology demonstrating asynchroneity between advances of Himalayan glaciers and Northern Hemisphere ice-sheet volumes. Glaciers at Nanga Parbat expanded during the early to middle Holocene ca. 9.0–5.5 ka. No major advances at Nanga Parbat during the last global glacial stage of marine oxygen isotope stage 2 (MIS-2) between 24 and 11 ka were identified. Preliminary evidence also indicates advances between ca. 60 and 30 ka. These periods of high ice volume coincide with warm, wet regional climates dominated by a strong southwest Asian summer monsoon. The general lack of deposits dating from MIS-2 suggests that Nanga Parbat was too arid to support expanded ice during this period of low monsoon intensity. Advances during warm, wet periods are possible for the high-altitude summer accumulation glaciers typical of the Himalayas, and explain asynchronous behavior. However, the Holocene advances at Nanga Parbat appear to have been forced by an abrupt drop in temperature ca. 8.4–8.0 ka and an increase in winter precipitation ca. 7–5.5 ka. These results highlight the overall sensitivity of Himalayan glaciation to orbital forcing of monsoon intensity, and on millennial or shorter time scales, to changes in North Atlantic circulation.

Debris‐covered Glaciers and Rock Glaciers in the Nanga Parbat Himalaya, Pakistan

The origin and mobilization of the extensive debris cover associated with the glaciers of the Nanga Parbat Himalaya is complex. In this paper we propose a mechanism by which glaciers can form rock glaciers through inefficiency of sediment transfer from glacier ice to meltwater. Inefficient transfer is caused by various processes that promote plentiful sediment supply and decrease sediment transfer potential. Most debris‐covered glaciers on Nanga Parbat with higher velocities of movement and/ or efficient debris transfer mechanisms do not form rock glaciers, perhaps because debris is mobilized quickly and removed from such glacier systems. Those whose ice movement activity is lower and those where inefficient sediment transfer mechanisms allow plentiful debris to accumulate, can form classic rock glaciers. We document here with maps, satellite images, and field observations the probable evolution of part of a slow and inefficient ice glacier into a rock glacier at the margins of Sachen Glacier in c. 50 years, as well as several other examples that formed in a longer period of time. Sachen Glacier receives all of its nourishment from ice and snow avalanches from surrounding areas of high relief, but has low ice velocities and no efficient system of debris removal. Consequently it has a pronounced digitate terminus with four lobes that have moved outward from the lateral moraines as rock glaciers with prounced transverse ridges and furrows and steep fronts at the angle of repose. Raikot Glacier has a velocity five times higher than Sachen Glacier and a thick cover of rock debris at its terminus that is efficienctly removed. During the advance stage of the glacier since 1994, ice cliffs were exposed at the terminus, and an outbreak flood swept away much debris from its margins and terminus. Like the Sachen Glacier that it resembles, Shaigiri Glacier receives all its nourishment from ice and snow avalanches and has an extensive debris cover with steep margins close to the angle of repose. It has a high velocity similar to Raikot Glacier and catastrophic breakout floods have removed debris from its terminus twice in the recent past. In addition, the Shaigiri terminus blocked the Rupal River during the Little Ice Age and is presently being undercut and steepened by the river. With higher velocities and more efficient sediment transfer systems, neither the Raikot nor the Shaigiri form classic rock‐glacier morphologies.

Ca/Sr and Sr isotope systematics of a Himalayan glacial chronosequence: Carbonate versus silicate weathering rates as a function of landscape surface age

We explored changes in the relative importance of carbonate vs. silicate weathering as a function of landscape surface age by examining the Ca/Sr and Sr isotope systematics of a glacial soil chronosequence located in the Raikhot watershed within the Himalaya of northern Pakistan. Bedrock in the Raikhot watershed primarily consists of silicate rock (Ca/Sr 0.20 mol/nmol, 87 Sr/ 86 Sr 0.77 to 1.2) with minor amounts of disseminated calcite (Ca/Sr 0.98 to 5.3 mol/nmol, 87 Sr/ 86 Sr 0.79 to 0.93) and metasedimentary carbonate (Ca/Sr 1.0 to 2.8 mol/nmol, 87 Sr/ 86 Sr 0.72 to 0.82). Analysis of the exchangeable, carbonate, and silicate fractions of seven soil profiles ranging in age from 0.5 to 55 kyr revealed that carbonate dissolution provides more than 90% of the weathering-derived Ca and Sr for at least 55 kyr after the exposure of rock surfaces, even though carbonate represents only 1.0 wt% of fresh glacial till. The accumulation of carbonate-bearing dust deposited on the surfaces of older landforms partly sustains the longevity of the carbonate weathering flux. As the average landscape surface age in the Raikhot watershed increases, the Ca/Sr and 87 Sr/ 86 Sr ratios released by carbonate weathering decrease from 3.6 to 0.20 mol/nmol and 0.84 to 0.72, respectively. The transition from high to low Ca/Sr ratios during weathering appears to reflect the greater solubility of high Ca/Sr ratio carbonate relative to low Ca/Sr ratio carbonate. These findings suggest that carbonate weathering controls the dissolved flux of Sr emanating from stable Himalayan landforms comprising mixed silicate and carbonate rock for tens of thousands of years after the mechanical exposure of rock surfaces to the weathering environment. Copyright

Long-Term Impacts of a Career Development Workshop for Undergraduates

AbstractFor 15 years, the Undergraduate Leadership Workshop (ULW) has connected undergraduate students with scientists, engineers, educators, and leadership at the National Center for Atmospheric Research (NCAR) in Boulder, Colorado. The week-long program gives students exposure to research areas and to graduate school and career options in the atmospheric sciences. The ULW provides mentoring, training in leadership and professional skills, and a chance to meet peers in this field. An evaluation of the program, including surveys and focus groups of alumni from the past 15 years, has yielded insights into the program’s short- and long-term impacts. The main themes that have emerged are that the ULW provides 1) exposure to a whole new world of careers in the atmospheric sciences; 2) a sense of belonging to a group of students who are also interested in weather and climate; 3) long-term support and a professional network of peers; and 4) an increase in interest in pursuing a science, technology, engineering,...