Small alpine glaciers are sensitive to climate, and projections of future global warming indicate that many alpine glaciers throughout the world will shrink considerably, if not disappear altogether. This award will address two fundamental questions relevant to understanding the history and controls of alpine glaciation in the western U.S.: (1) What is the age of moraines deposited by glaciers that occur immediately downstream to those deposited during the Little Ice Age ~300 years ago? (2) How have changes in large-scale controls of climate through the last ~10,000 years (Holocene) affected glacier fluctuations? Methods to be employed to accomplish these goals will involve systematically dating cirque-glacier moraines throughout the western U.S. using in-situ cosmogenic radionuclides and radiocarbon techniques. A hierarchical climate and glacier modeling strategy will be employed to evaluate the effects of changing large-scale climate controls through the Holocene (ice sheets, insolation, greenhouse gases) on glacier mass balance and resulting fluctuations. This research strategy provides an outstanding approach to applying the geologic record to constrain the sensitivity of alpine glaciers to radiative forcing.
Broader Impact. Understanding the sensitivity of alpine glaciers to radiative forcing is of great interest with regard to landscape appearance, slope stability, water resources, sediment loading to rivers, and ecosystem health. The modeling approaches developed here will be applicable (outside of this project) to improve assessments of future anthropogenic impacts on alpine glaciation, with implications for sea-level, water resources, and societal concerns over the aesthetic qualities of glaciers. This study includes a strong element of inter-agency cooperation, with contributions for modeling supported by the USGS. This proposal will directly contribute to human resources by including the training, education, and research contributions of one Ph.D. student, with strong cross-training in elements of data acquisition, data analysis, and modeling.
We dated 124 samples with 10Be from 20 moraines from nine sites in the western U.S. and one site in southwestern Canada. This is the largest such data set ever generated, and will represent the most comprehensive coverage of dated cirque glaciation in the western U.S. Our results demonstrate that all the sampled moraines in the western U.S. were deposited during the latest Pleistocene or earliest Holocene, in contrast to previous interpretations which interpreted the majority of these deposits as middle or late Holocene in age. Our data from nearly all of the sampled locations clearly demonstrate that glaciers in the American West did not reach positions more extensive than the Little Ice Age (LIA) during the Neoglaciation, a conclusion that is likely true for all of western North America. From the glacial chronologies of these ten mountain ranges and the existing data from other studies a scenario of alpine glacial activity in western North America can be derived. Following the local LGM, glaciers across western North America began retreating from the large Pleistocene moraines 15-30 km downvalley from the cirque headwalls. By ~15 kyr these glacier had reached new equilibrium positions 1-2 km from the cirque peaks, and by 15-13 ka warmer and/or drier conditions across western North America caused further retreat. Cooler and/or wetter conditions beginning at ~13 kyr likely caused the alpine glaciers to begin readvancing to new equilibrium positions until 12-11 ka when a return to warmer conditions caused some glaciers to retreat. By 11-8 ka the remaining cirque glaciers across western North America began to retreat once again and likely completely disappeared until the next major glacial phase in western North America some 7-10 kyr later culminating during the LIA. While glaciers may have waxed and waned from 8 kyr to the LIA, they never advanced beyond the LIA moraines except in few isolated locations. This finding is in good agreement with our reconstruction of Holocene climate where Northern Hemisphere temperatures were generally warmer in the early and middle Holocene, and thus not conducive for major glacial growth until the Little Ice Age. In particular, our Holocene temperature reconstruction exhibits ~0.7°C of warming from the early Holocene, 11,300 yr BP, to a mid Holocene temperature plateau extending from 9,500 to 5,500 yr BP. This warm interval gives way to a long-term cooling trend of 0.8°C from 5,500 to 200 yr BP, followed by recent warming. Extratropical Northern-Hemisphere sites (30-90oN) contribute most of the variance to the global signal, whereby a strong cooling trend of ~2°C begins around 7,000 yrs BP and culminates at ~100 yr BP with temperatures ~1oC cooler than modern values. By comparison, the low latitudes (30oN-30oS) exhibit only a slight warming trend of ~0.4oC from 11,000-5,000 yr BP, with temperature leveling off thereafter, while the extratropical Southern Hemisphere (30oS-90oS) cooled ~0.4oC from about 11,000-7000 yr BP, followed by relatively stable temperatures except for some possible multi-centennial variability in the last 2.5 ka. The general pattern of high-latitude cooling in both hemispheres opposed by warming at low latitudes is consistent with mean annual insolation forcing associated with decreasing orbital obliquity of 0.78° since 9 ka. The especially pronounced cooling of the Northern Hemisphere extratropics suggests an important role for summer insolation in this region, perhaps through snow-ice albedo and vegetation feedbacks. Such a mechanism to rectify seasonal insolation is plausible at these latitudes where the fraction of continental land masses relative to the ocean is uniquely high. Additional effects likely further influenced the evolution of climate through the Holocene. In the early Holocene, the deglaciating Northern Hemisphere ice sheets would have modulated regional warming of the northern high latitudes relative to peak seasonal insolation. Radiative forcing by greenhouse gases rose 0.5 W/m2 during the mid-to-late Holocene, which would be expected to yield ~0.4°C warming for a mid-range climate sensitivity, but any response to this forcing was apparently largely offset by the opposing orbital insolation forcing that is greater by one to two orders of magnitude over the course of the Holocene. Total solar irradiance reconstructed from cosmogenic isotopes also varied by 0.5-1 W/m2, although most of this variance is at higher frequencies than those resolved by our stacked temperature records, and likely contributed little to longer term trends. Northward heat transport in the Atlantic basin by the meridional overturning circulation (MOC) may have weakened since the mid-Holocene, contributing to the strong cooling in the North Atlantic while dampening insolation-forced cooling in the mid-to-high latitude Southern Hemisphere due to the bipolar seesaw. Insofar as winter conditions influence the sources of deep water masses, a weakening MOC may partly reflect the decrease in high northern latitude winter insolation since the mid-Holocene.
