Joseph A. Graly

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.

Chemical weathering under the Greenland Ice Sheet

To contrast continental and alpine subglacial weathering regimes and thereby assess the role of large ice masses in chemical weathering, borehole and outlet water samples were collected from multiple locations on a major, land-terminating outlet of the Greenland Ice Sheet. Boreholes, reaching ice depths to 824 m, were drilled to the bed with hot-water methods in four areas of the ice sheet ablation zone along a 45 km transect extending inland from the outlet terminus. The bulk chemical composition of these samples shows substantially less influence of sulfides and carbonates than found in alpine glaciers, suggesting that the sediment under this region of the ice sheet has become depleted of accessory minerals. The waters show wide variability in chemical composition over both large and small temporal-spatial scales, suggesting large ranges in length of subglacial water storage and in rates of abrasion and comminution of subglacial earth materials. The dissolved solids concentrations found in the Greenland Ice Sheet are comparable to and in some cases exceed those of alpine glaciers, suggesting that large ice masses are capable of generating substantial dissolved loads through silicate weathering mechanisms.

Calibrating a long‐term meteoric 10Be accumulation rate in soil

[1] Using 13 samples collected from a 4.1 meter profile in a well-dated and stable New Zealand fluvial terrace, we present the first long-term accumulation rate for meteoric 10 Be in soil (1.68 to 1.72 × 10 6 at/(cM 2 ·yr)) integrated over the past ~18 ka. Site-specific accumulation data, such as these, are prerequisite to the application of meteoric 10 Be in surface process studies. Our data begin the process of calibrating long-term meteoric 10 Be delivery rates across latitude and precipitation gradients. Our integrated rate is lower than contemporary meteoric 10 Be fluxes measured in New Zealand rainfall, suggesting that long-term average precipitation, dust flux, or both, at this site were less than modern values. With accurately calibrated long-term delivery rates, such as this, meteoric 10 Be will be a powerful tool for studying rates of landscape change in environments where other cosmogenic nuclides, such as in situ 10 Be, cannot be used.

Calculating the balance between atmospheric CO2 drawdown and organic carbon oxidation in subglacial hydrochemical systems

In order to constrain CO2 fluxes from biogeochemical processes in subglacial environments, we model the evolution of pH and alkalinity over a range of subglacial weathering conditions. We show that subglacial waters reach or exceed atmospheric pCO2 levels when atmospheric gases are able to partially access the subglacial environment. Subsequently, closed system oxidation of sulfides is capable of producing pCO2 levels well in excess of atmosphere levels without any input from the decay of organic matter. We compared this model to published pH and alkalinity measurements from 21 glaciers and ice sheets. Most subglacial waters are near atmospheric pCO2 values. The assumption of an initial period of open system weathering requires substantial organic carbon oxidation in only 4 of the 21 analyzed ice bodies. If the subglacial environment is assumed to be closed from any input of atmospheric gas, large organic carbon inputs are required in nearly all cases. These closed system assumptions imply that order of 10 g m−2 y−1 of organic carbon are removed from a typical subglacial environment—a rate too high to represent soil carbon built up over previous interglacial periods and far in excess of fluxes of surface deposited organic carbon. Partial open system input of atmospheric gases is therefore likely in most subglacial environments. The decay of organic carbon is still important to subglacial inorganic chemistry where substantial reserves of ancient organic carbon are found in bedrock. In glaciers and ice sheets on silicate bedrock, substantial long-term drawdown of atmospheric CO2 occurs.

Polar desert chronologies through quantitative measurements of salt accumulation

We measured salt concentration and speciation in the top horizons of moraine sediments from the Transantarctic Mountains (Antarctica) and compared the salt data to cosmogenic-nuclide exposure ages on the same moraine. Because the salts are primarily of atmospheric origin, and their delivery to the sediment is constant over relevant time scales, a linear rate of accumulation is expected. When salts are measured in a consistent grain-size fraction and at a consistent position within the soil column, a linear correlation between salt concentration and exposure age is evident. This correlation is strongest for boron-containing salts (R2 > 0.99), but is also strong (R2 ≈ 0.9) for most other water-extracted salt species. The relative mobility of salts in the soil column does not correspond to species solubility (borate is highly soluble). Instead, the highly consistent behavior of boron within the soil column is best explained by the extremely low vapor pressure of boric acid at cold temperatures. The environment is sufficiently dry that mobility of salt species within the soil column is controlled by vapor phase effects. In other cold desert settings, topsoil salts, specifically boron, may be employed as a proxy for relative sediment exposure age.