302 research outputs found
Surface exposure geochronology using cosmogenic nuclides : applications in Antarctic glacial geology
Thesis (Ph. D.)--Massachusetts Institute of Technology, Dept. of Earth, Atmospheric, and Planetary Sciences, and the Woods Hole Oceanographic Institution, 1994.Vita.Includes bibliographical references (leaves 224-227).by Edward Jeremy Brook.Ph.D
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Atmospheric COā and climate from 65 to 30 ka B.P.
Using new and existing ice core COā data from 65 ā¼ 30 ka a new chronology for COā is established and synchronized with Greenland ice core records to study how high latitude climate change and the carbon cycle were linked during the last glacial period. Atmospheric COā rose several thousand years before abrupt warming in Greenland associated with Dansgaard-Oeschger events, 8, 12, 14, 17, four large warm events that follow Heinrich events. The COā rise terminated at the onset of Greenland warming for each of these events. Atmospheric COā is strongly correlated with the Antarctic isotopic temperature proxy with an average time lag of 720 Ā± 370 yr (mean Ā± 1Ļ) during the time interval studied. The new data and chronology should provide a better target for models attempting to explain COā variability and abrupt climate change
CO(2) Diffusion in Polar Ice: Observations from Naturally Formed CO(2) Spikes in the Siple Dome (Antarctica) Ice Core
One common assumption in interpreting ice-core CO(2) records is that diffusion in the ice does not affect the concentration profile. However, this assumption remains untested because the extremely small CO(2) diffusion coefficient in ice has not been accurately determined in the laboratory. In this study we take advantage of high levels of CO(2) associated with refrozen layers in an ice core from Siple Dome, Antarctica, to study CO(2) diffusion rates. We use noble gases (Xe/Ar and Kr/Ar), electrical conductivity and Ca(2+) ion concentrations to show that substantial CO(2) diffusion may occur in ice on timescales of thousands of years. We estimate the permeation coefficient for CO(2) in ice is similar to 4 x 10(-21) mol m(-1) s(-1) Pa(-1) at -23 degrees C in the top 287 m (corresponding to 2.74 kyr). Smoothing of the CO(2) record by diffusion at this depth/age is one or two orders of magnitude smaller than the smoothing in the firn. However, simulations for depths of similar to 930-950m (similar to 60-70 kyr) indicate that smoothing of the CO(2) record by diffusion in deep ice is comparable to smoothing in the firn. Other types of diffusion (e.g. via liquid in ice grain boundaries or veins) may also be important but their influence has not been quantified
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Gases in ice cores
Air trapped in glacial ice offers a means of
reconstructing variations in the concentrations of atmospheric
gases over time scales ranging from anthropogenic
(last 200 yr) to glacial/interglacial (hundreds of thousands of
years). In this paper, we review the glaciological processes by
which air is trapped in the ice and discuss processes that
fractionate gases in ice cores relative to the contemporaneous
atmosphere. We then summarize concentrationātime records
for COā and CHā over the last 200 yr. Finally, we summarize
concentrationātime records for COā and CHā during the last
two glacialāinterglacial cycles, and their relation to records of
global climate change.This is the publisherās final pdf. The published article is copyrighted by the National Academy of Sciences of the United States of America and can be found at: http://www.pnas.org
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Controls on MillennialāScale Atmospheric CO2 Variability During the Last Glacial Period
Changes in atmospheric CO2 on millennialātoācentennial timescales are key components of past climate variability during the last glacial and deglacial periods (70ā10ka) yet the sources and mechanisms responsible for the CO2 fluctuations remain largely obscure. Here we report the 13C/12C ratio of atmospheric CO2 during a key interval of the last glacial period at subāmillennial resolution, with coeval histories of atmospheric CO2, CH4 and N2O concentrations. The carbon isotope data suggest that the millennialāscale CO2 variability in MIS3 is driven largely by changes in the organic carbon cycle, most likely by sequestration of respired carbon in the deep ocean. Centennialāscale CO2 variations, distinguished by carbon isotope signatures, are associated with both abrupt hydrological change in the tropics (e.g. Heinrich Events) and rapid increases in northern hemisphere temperature (DO events). These events can be linked to modes of variability during the last deglaciation, thus suggesting that drivers of millennial and centennial CO2 variability during both periods are intimately linked to abrupt climate variability.National Science Foundatio
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High-precision dual-inlet IRMS measurements of the stable isotopes of COā and the NāO/COā ratio from polar ice core samples
An important constraint on mechanisms of past
carbon cycle variability is provided by the stable isotopic
composition of carbon in atmospheric carbon dioxide (Ī“Ā¹Ā³C-COā) trapped in polar ice cores, but obtaining very precise
measurements has proven to be a significant analytical challenge.
Here we describe a new technique to determine the
Ī“Ā¹Ā³C of COā at very high precision, as well as measuring the
COā and NāO mixing ratios. In this method, ancient air is extracted
from relatively large ice samples (~400 g) with a dry-extraction
āice graterā device. The liberated air is cryogenically
purified to a COā and NāO mixture and analyzed with
a microvolume-equipped dual-inlet IRMS (Thermo MAT
253). The reproducibility of the method, based on replicate
analysis of ice core samples, is 0.02ā° for Ī“Ā¹Ā³C-COā and
2 ppm and 4 ppb for the COā and NāO mixing ratios, respectively
(1Ļ pooled standard deviation). Our experiments show
that minimizing water vapor pressure in the extraction vessel
by housing the grating apparatus in a ultralow-temperature
freezer (-60Ā°C) improves the precision and decreases the
experimental blank of the method to -0.07 Ā± 0.04ā°. We
describe techniques for accurate calibration of small samples
and the application of a mass-spectrometric method based on
source fragmentation for reconstructing the NāO history of
the atmosphere. The oxygen isotopic composition of COā is
also investigated, confirming previous observations of oxygen
exchange between gaseous COā and solid HāO within
the ice archive. These data offer a possible constraint on oxygen
isotopic fractionation during HāO and COā exchange below
the HāO bulk melting temperature
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