Lee B. Corbett

Constraining landscape history and glacial erosivity using paired cosmogenic nuclides in Upernavik, northwest Greenland

High-latitude landscape evolution processes have the potential to preserve old, relict surfaces through burial by cold-based, nonerosive glacial ice. To investigate landscape history and age in the high Arctic, we analyzed in situ cosmogenic 10 Be and 26 Al in 33 rocks from Upernavik, northwest Greenland. We sampled adjacent bedrock-boulder pairs along a 100 km transect at elevations up to 1000 m above sea level. Bedrock samples gave significantly older apparent exposure ages than corresponding boulder samples, and minimum limiting ages increased with elevation. Two-isotope calculations ( 26 Al/ 10 Be) on 20 of the 33 samples yielded minimum limiting exposure durations up to 112 k.y., minimum limiting burial durations up to 900 k.y., and minimum limiting total histories up to 990 k.y. The prevalence of 10 Be and 26 Al inherited from previous periods of exposure, especially in bedrock samples at high elevation, indicates that these areas record long and complex surface exposure histories, including significant periods of burial with little subglacial erosion. The long total histories suggest that these high-elevation surfaces were largely preserved beneath cold-based, nonerosive ice or snowfields for at least the latter half of the Quaternary. Because of high concentrations of inherited nuclides, only the six youngest boulder samples appear to record the timing of ice retreat. These six samples suggest deglaciation of the Upernavik coast at 11.3 ± 0.5 ka (average ± 1 standard deviation). There is no difference in deglaciation age along the 100 km sample transect, indicating that the ice-marginal position retreated rapidly at rates of ∼120 m yr −1 .

Preservation of a Preglacial Landscape Under the Center of the Greenland Ice Sheet

Deep Freeze Geologists usually consider glaciers and ice sheets to be gigantic abrasives, scouring the ground beneath them and carving out relief on the underlying landscapes. Bierman et al. (p. 402, published online 17 April) show that this is not always the case. They found that the silt at the very bottom of the Greenland Ice Sheet Project 2 core contained significant amounts of beryllium-10, an isotope produced in the atmosphere by cosmic rays and which adheres to soils when it is deposited on them. Hence, the dust at the bottom of the ice sheet indicates the persistence of a landscape under 3000 meters of glacial ice that is millions of years old. Soil has been frozen to the central part of the bed of the Greenland Ice Sheet for at least 2.7 million years. Continental ice sheets typically sculpt landscapes via erosion; under certain conditions, ancient landscapes can be preserved beneath ice and can survive extensive and repeated glaciation. We used concentrations of atmospherically produced cosmogenic beryllium-10, carbon, and nitrogen to show that ancient soil has been preserved in basal ice for millions of years at the center of the ice sheet at Summit, Greenland. This finding suggests ice sheet stability through the Pleistocene (i.e., the past 2.7 million years). The preservation of this soil implies that the ice has been nonerosive and frozen to the bed for much of that time, that there was no substantial exposure of central Greenland once the ice sheet became fully established, and that preglacial landscapes can remain preserved for long periods under continental ice sheets.

A persistent and dynamic East Greenland Ice Sheet over the past 7.5 million years

Climate models show that ice-sheet melt will dominate sea-level rise over the coming centuries, but our understanding of ice-sheet variations before the last interglacial 125,000 years ago remains fragmentary. This is because terrestrial deposits of ancient glacial and interglacial periods are overrun and eroded by more recent glacial advances, and are therefore usually rare, isolated and poorly dated. In contrast, material shed almost continuously from continents is preserved as marine sediment that can be analysed to infer the time-varying state of major ice sheets. Here we show that the East Greenland Ice Sheet existed over the past 7.5 million years, as indicated by beryllium and aluminium isotopes (10Be and 26Al) in quartz sand removed by deep, ongoing glacial erosion on land and deposited offshore in the marine sedimentary record. During the early Pleistocene epoch, ice cover in East Greenland was dynamic; in contrast, East Greenland was mostly ice-covered during the mid-to-late Pleistocene. The isotope record we present is consistent with distinct signatures of changes in ice sheet behaviour coincident with major climate transitions. Although our data are continuous, they are from low-deposition-rate sites and sourced only from East Greenland. Consequently, the signal of extensive deglaciation during short, intense interglacials could be missed or blurred, and we cannot distinguish between a remnant ice sheet in the East Greenland highlands and a diminished continent-wide ice sheet. A clearer constraint on the behaviour of the ice sheet during past and, ultimately, future interglacial warmth could be produced by 10Be and 26Al records from a coring site with a higher deposition rate. Nonetheless, our analysis challenges the possibility of complete and extended deglaciation over the past several million years.

Cold-based Laurentide ice covered New England’s highest summits during the Last Glacial Maximum

To better understand glacial history and process in New England (northeastern United States), a mountainous area overrun by the Laurentide Ice Sheet, we measured three cosmogenic nuclides in nine upland samples. The concentrations of 10Be and 26Al in some samples collected near the summits of Katahdin (Maine) and Mount Washington and Little Haystack Mountain (New Hampshire) are 2–10 times higher than expected for a single exposure period, considering field evidence indicating that continental ice-covered all New England peaks during the Last Glacial Maximum. In-situ 14C exposure ages from the summits are much younger, suggesting that high-elevation sampling sites were ice-covered before and during the Last Glacial Maximum. Field and isotopic data are consistent with New England summits being covered in part by cold-based continental ice that did not erode much rock. The contrast in erosion rates between stable summits and deeply eroded valleys likely contributes to the development and maintenance of northern Appalachian topography.

Glacial history and landscape evolution of southern Cumberland Peninsula, Baffin Island, Canada, constrained by cosmogenic 10Be and 26Al

Since the first application of cosmogenic nuclides to the study of glacial history and processes in 1990, increasing numbers of studies have used a variety of cosmogenic isotopes to quantify the exposure age and erosion rate of glaciated landscapes. However, obtaining chronological data from glaciated landscapes once covered by cold-based, nonerosive ice is challenging because these surfaces violate assumptions associated with simple cosmogenic exposure dating. Nonerosive glacial ice fails to completely remove nuclides produced during previous periods of exposure, leaving behind rock surfaces with complex, multigenetic nuclide inventories. Here, we constrain the glacial history, landscape evolution, and efficiency of subglacial erosion in the Pangnirtung Fiord region of southern Baffin Island using over 300 paired analyses of in situ cosmogenic 10Be and 26Al. Simple exposure ages are 6.3–160 ka for 10Be ( n = 152) and 4.3–124 ka for 26Al ( n = 153). Paired bedrock-boulder samples have discordant ages, simple exposure ages generally increase with elevation, 10Be and 26Al ages for the same sample disagree, and both boulders and bedrock yield multimodal age distributions—all patterns suggestive of limited subglacial erosion. Measured 26Al/10Be ratios indicate that about one third of the samples in the data set experienced at least one period of pre-Holocene exposure followed by burial with limited erosion. Modeled two-isotope minimum-limiting exposure durations are as high as hundreds of thousands of years, and minimum-limiting burial durations range up to millions of years, implying that parts of southern Baffin Island have been preserved beneath nonerosive glacial ice for much, if not all, of the Quaternary. A subset of the samples contains few nuclides inherited from prior periods of exposure and is thus useful for constraining the chronology of the most recent deglaciation. Using these samples, we infer that deglaciation of most of the landscape occurred ca. 11.7 ka and that the Duval moraines, a prominent feature of the last deglaciation, formed ca. 11.2 ka.

Cosmogenic 26Al/10Be surface production ratio in Greenland

The assumed value for the cosmogenic 26Al/10Be surface production rate ratio in quartz is an important parameter for studies investigating the burial or subaerial erosion of long-lived surfaces and sediments. Recent models and data suggest that the production ratio is spatially variable and may be greater than originally thought. Here, we present measured 26Al/10Be ratios for 24 continuously exposed bedrock and boulder surfaces spanning ~61-77 °N in Greenland. Empirical measurements, such as ours, include nuclides produced predominately by neutron-induced spallation with percent-level contributions by muon interactions. The slope of a York regression line fit to our data is 7.3 ± 0.3 (1σ), suggesting that the 26Al/10Be surface production ratio exceeds the commonly used value of 6.75, at least in the Arctic. A higher 26Al/10Be production ratio has implications for multi-nuclide cosmogenic isotope studies because it results in greater modeled burial durations and erosion rates.

Incorporating Concept Sketching Into Teaching Undergraduate Geomorphology

ABSTRACT Constructing concept sketches (diagrams annotated with short captions in which students demonstrate their understanding of form, process, and interactions) provides a new and different way to teach Earth surface processes and assess the depth of student learning. During a semester-long course in Geomorphology, we used concept sketches as an icebreaker, as a means to help students place field observations in a spatial context, and as a catalyst for understanding complex graphical presentations of data. For the mid-term and final assessment components of the course, we required students to consider a historic aerial photograph of a local site they had not visited previously in order to strengthen their abilities in landscape interpretation based upon imagery alone; a task many of them will be required to undertake in their future endeavors. Anecdotal student response to the use of concept sketches in Geomorphology was uniformly positive with students self-reporting that the sketches helped them to synthesize large amounts of seemingly disparate information. As instructors, we found concept sketches particularly useful for motivating students and for identifying misconceptions and knowledge gaps.

Minimal East Antarctic Ice Sheet retreat onto land during the past eight million years

The East Antarctic Ice Sheet (EAIS) is the largest potential contributor to sea-level rise. However, efforts to predict the future evolution of the EAIS are hindered by uncertainty in how it responded to past warm periods, for example, during the Pliocene epoch (5.3 to 2.6 million years ago), when atmospheric carbon dioxide concentrations were last higher than 400 parts per million. Geological evidence indicates that some marine-based portions of the EAIS and the West Antarctic Ice Sheet retreated during parts of the Pliocene1,2, but it remains unclear whether ice grounded above sea level also experienced retreat. This uncertainty persists because global sea-level estimates for the Pliocene have large uncertainties and cannot be used to rule out substantial terrestrial ice loss3, and also because direct geological evidence bearing on past ice retreat on land is lacking. Here we show that land-based sectors of the EAIS that drain into the Ross Sea have been stable throughout the past eight million years. We base this conclusion on the extremely low concentrations of cosmogenic 10Be and 26Al isotopes found in quartz sand extracted from a land-proximal marine sediment core. This sediment had been eroded from the continent, and its low levels of cosmogenic nuclides indicate that it experienced only minimal exposure to cosmic radiation, suggesting that the sediment source regions were covered in ice. These findings indicate that atmospheric warming during the past eight million years was insufficient to cause widespread or long-lasting meltback of the EAIS margin onto land. We suggest that variations in Antarctic ice volume in response to the range of global temperatures experienced over this period—up to 2–3 degrees Celsius above preindustrial temperatures4, corresponding to future scenarios involving carbon dioxide concentrations of between 400 and 500 parts per million—were instead driven mostly by the retreat of marine ice margins, in agreement with the latest models5,6.Analysis of cosmogenic isotopes from a marine sediment core shows that much of the land-based East Antarctic Ice Sheet has remained stable for the past eight million years, including during the warm Pliocene epoch.