Cosmogenic Radionuclides In Ice Cores From West Antarctica

Cosmogenic nuclides such as 10Be and 26Al are formed in the atmosphere by cosmic rays and come down to the ground through snow which became ice in Antarctica. The concentrations of 10Be and 26Al in ice cores can reveal important information about climate change, solar activity and geomagnetic change in the past. They can also be used to date very old ice. Since there is very little 26Al in the ice, its actual concentration is poorly known and the measured results don’t agree with each other. My research is focused on the measurement of the concentration of 26Al as well as 10Be in an ice core from Antarctica. The ice samples undergo several chemical and physical processes to be ready for measurement. Some key process includes separating different ions using ion chromatography, oxidizing the samples at high temperature, and loading the final sample holders. Finally the samples are measured by accelerator mass spectrometry (AMS). After the AMS measurement, we calculated the average concentrations of 10Be for our sample to be 40,000 atoms g-1 and the concentration for 26Al is around 76 atoms g-1. The average ratio of 26Al/10Be is 2.40 * 10-3. This study will contribute to our knowledge of using 26Al/10Be to date very old ice. Combined with other similar studies at different ice core depths, we can also have a full picture of the change of concentration of cosmogenic nuclides through time

Estimation of stratospheric input to the Arctic troposphere: 7Be and 10Be in aerosols at Alert, Canada

Concentrations of 7Be and 210Pb in 2 years of weekly high-volume aerosol samples collected at Alert, Northwest Territories, Canada, showed pronounced seasonal variations. We observed a broad winter peak in 210Pb concentration and a spring peak in 7Be. These peaks were similar in magnitude and duration to previously reported results for a number of stations in the Arctic Basin. Beryllium 10 concentrations (determined only during the first year of this study) were well correlated with those of 7Be; the atom ratio 10Be/7Be was nearly constant at 2.2 throughout the year. This relatively high value of 10Be/7Be indicates that the stratosphere must constitute an important source of both Be isotopes in the Arctic troposphere throughout the year. A simple mixing model based on the small seasonal variations of 10Be/7Be indicates an approximately twofold increase of stratospheric influence in the free troposphere in late summer. The spring maxima in concentrations of both Be isotopes at the surface apparently reflect vertical mixing in rather than stratospheric injections into the troposphere. We have merged the results of the Be-based mixing model with weekly O3 soundings to assess Arctic stratospheric impact on the surface O3 budget at Alert. The resulting estimates indicate that stratospheric inputs can account for a maximum of 10-15% of the 03 at the surface in spring and for less during the rest of the year. These estimates are most uncertain during the winter. The combination of Be isotopic measurements and O3 vertical profiles could allow quantification of the contributions of O3 from the Arctic stratosphere and lower latitude regions to the O3 budget in the Arctic troposphere. Although at present the lack of a quantitative understanding of the temporal variation of O3 lifetime in the Arctic troposphere precludes making definitive calculations, qualitative examples of the power of this approach are given

Terrestrial ages and exposure ages of Antarctic H-chondrites from Frontier Mountain, North Victoria Land

We measured the isotopic compositions and concentrations of He, Ne and Ar as well as the concentrations of cosmogenic ^Be, ^Al and ^Cl in 26 H-chondrites and 1 L-chondrite from a meteorite stranding area near the Frontier Mountain Range, East Antarctica. Based on the radionuclide concentrations and the noble gas signatures we conclude the 26 H-chondrite samples represent at least 13 different falls. The exposure ages of most H-chondrites are in the range of 4-10 million years (My). This age range encompasses the well-established exposure age peak at ∿7 My and an additional feature at ∿4 My. We determined the terrestrial ages on the basis of the ^Cl concentration as well as using the relation between the ^Cl/^Be ratio and the ^Be concentration. This relation also corrects for shielding effects and reduces the uncertainty in the age by ∿25% compared to simple ^Cl terrestrial ages. About 40% of the meteorites are older than 100 thousand years (ky), but none are older than 200ky. The relatively short terrestrial ages suggest that Frontier Mountain is a young meteorite stranding area. This seems to be supported by the bedrock exposure history, which shows a recent surface exposure≤70ky

Cosmogenic nuclides indicate that boulder fields are dynamic, ancient, multigenerational features

Boulder fields are found throughout the world; yet, the history of these features, as well as the processes that form them, remain poorly understood. In high and mid-latitudes, boulder fields are thought to form and be active during glacial periods; however, few quantitative data support this assertion. Here, we use in situ cosmogenic 10Be and 26Al to quantify the near-surface history of 52 samples in and around the largest boulder field in North America, Hickory Run, in central Pennsylvania, USA. Boulder surface 10Be concentrations (n = 43) increase downslope, indicate minimum near-surface histories of 70-600 k.y., and are not correlated with lithology or boulder size. Measurements of samples from the top and bottom of one boulder and three underlying clasts as well as 26Al/10Be ratios (n = 25) suggest that at least some boulders have complex exposure histories caused by flipping and/or cover by other rocks, soil, or ice. Cosmogenic nuclide data demonstrate that Hickory Run, and likely other boulder fields, are dynamic features that persist through multiple glacial-interglacial cycles because of boulder resistance to weathering and erosion. Long and complex boulder histories suggest that climatic interpretations based on the presence of these rocky landforms are likely over simplifications

Be-10 age constraints on latest Pleistocene and Holocene cirque glaciation across the western United States

Paleoclimate: A rocky reworking of Holocene glaciology New dating of glacially-deposited rocks substantially revises our understanding of the waxing and waning of ice since the last glacial maximum. Glaciologists have long thought that moraines throughout the western United States represent ‘neoglacial’ advances about 6,000 years ago. Now, a multi-institution team led by Shaun Marcott at the University of Wisconsin-Madison has found — using cosmogenic isotopes — that these terminal deposits left by advancing glaciers are instead 9,000 to 15,000 years old. The research advances prior work by using absolute, not relative ages, and documents that glaciers retreated after the last glacial maximum ~ 21,000 years ago, fluctuated locally throughout much of the Holocene, and re-advanced during the Little Ice Age of a few hundred years ago. Glacial advances that might have occurred during the neoglacial were wiped away by the more extensive glaciations of the Little Ice Age

Timing and nature of alluvial fan and strath terrace formation in the Eastern Precordillera of Argentina

Sixty-eight 10Be terrestrial cosmogenic nuclide (TCN) surface exposure ages are presented to define the timing of alluvial fan and strath terrace formation in the hyper-arid San Juan region of the Argentine Precordillera. This region is tectonically active, and numerous fault scarps traverse Quaternary landforms. The three study sites, Marquesado strath complex, Loma Negra alluvial fan and Carpintería strath complex reveal a history of alluvial fan and strath terrace development over the past w225 ka. The Marquesado complex Q3m surface dates to w17 3 ka, whereas the Loma Negra Q1ln, Q2ln, Q3ln, Q4ln, and Q5ln surfaces date to w24 3 ka, w48 2 ka, w65 13 ka, w105 21 ka, and w181 29 ka, respectively. The Carpintería complex comprises eight surfaces that have been dated and include the Q1c (w23 3 ka), Q2c (w5 5 ka), Q3ac (w25 12 ka), Q3bc (w29 15 ka), Q4c (w61 12 ka), Q5c (w98 18 ka), Q6c (w93 18 ka), and Q7c (w212 37 ka). 10Be TCN depth profile data for the Loma Negra alluvial fan complex and Carpintería strath terrace complex, as well as OSL ages on some Carpintería deposits, aid in refining surface ages for comparison with local and global climate proxies, and additionally offer insights into inheritance and erosion rate values for TCNs (w10 104 10Be atoms/g of SiO2 and w5 m Ma 1, respectively). Comparison with other alluvial fan studies in the region show that less dynamic and older preserved surfaces occur in the Carpintería and Loma Negra areas with only younger alluvial fan surfaces preserved both to the north and south. These data in combination with that of other studies illustrate broad regional agreement between alluvial fan and strath terrace ages, which suggests that climate is the dominant forcing agent in the timing of terrace formation in this region

Cosmogenic \u3csup\u3e26\u3c/sup\u3eAl/\u3csup\u3e10\u3c/sup\u3eBe 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 multinuclide cosmogenic isotope studies because it results in greater modeled burial durations and erosion rates

Deglaciation of Fennoscandia

To provide a new reconstruction of the deglaciation of the Fennoscandian Ice Sheet, in the form of calendar-year time-slices, which are particularly useful for ice sheet modelling, we have compiled and synthesized published geomorphological data for eskers, ice-marginal formations, lineations, marginal meltwater channels, striae, ice-dammed lakes, and geochronological data from radiocarbon, varve, optically-stimulated luminescence, and cosmogenic nuclide dating. This 25 is summarized as a deglaciation map of the Fennoscandian Ice Sheet with isochrons marking every 1000 years between 22 and 13 cal kyr BP and every hundred years between 11.6 and final ice decay after 9.7 cal kyr BP. Deglaciation patterns vary across the Fennoscandian Ice Sheet domain, reflecting differences in climatic and geomorphic settings as well as ice sheet basal thermal conditions and terrestrial versus marine margins. For example, the ice sheet margin in the high-precipitation coastal setting of the western sector responded sensitively to climatic variations leaving a detailed record of prominent moraines and ice-marginal deposits in many fjords and coastal valleys. Retreat rates across the southern sector differed between slow retreat of the terrestrial margin in western and southern Sweden and rapid retreat of the calving ice margin in the Baltic Basin. Our reconstruction is consistent with much of the published research. However, the synthesis of a large amount of existing and new data support refined reconstructions in some areas. For example, we locate the LGM extent of the ice sheet in northwestern Russia further east than previously suggested and conclude that it occurred at a later time than the rest of the ice sheet, at around 17-15 cal kyr BP, and propose a slightly different chronology of moraine formation over southern Sweden based on improved correlations of moraine segments using new LiDAR data and tying the timing of moraine formation to Greenland ice core cold stages. Retreat rates vary by as much as an order of magnitude in different sectors of the ice sheet, with the lowest rates on the high-elevation and maritime Norwegian margin. Retreat rates compared to the climatic information provided by the Greenland ice core record show a general correspondence between retreat rate and climatic forcing, although a close match between retreat rate and climate is unlikely because of other controls, such as topography and marine versus terrestrial margins. Overall, the time slice reconstructions of Fennoscandian Ice Sheet deglaciation from 22 to 9.7 cal kyr BP provide an important dataset for understanding the contexts that underpin spatial and temporal patterns in retreat of the Fennoscandian Ice Sheet, and are an important resource for testing and refining ice sheet models

The Northwestern Greenland Ice Sheet During The Early Pleistocene Was Similar To Today

The multi-million year history of the Greenland Ice Sheet remains poorly known. Ice-proximal glacial marine diamict provides a direct but discontinuous record of ice sheet behavior; it is underutilized as a climate archive. Here, we present a novel multiproxy analysis of an Early Pleistocene marine diamict from northwestern Greenland. Low cosmogenic nuclide concentrations indicate minimal near-surface exposure, similar to modern terrestrial sediment. Detrital apatite (U-Th-Sm)/He (AHe) ages all predate glaciation by \u3e150 million years, suggesting the northwestern Greenland Ice Sheet had, by 1.9 Ma, not yet incised fjords of sufficient depth to excavate grains with young AHe ages. The diamict contains terrestrial plant leaf wax, likely from land surfaces surrounding the ice sheet. These data indicate that a persistent, dynamic ice sheet existed in northwestern Greenland by 1.9 Ma and that diamict is a useful archive of ice sheet history and process