Melissa A. Headly

Oxygen-18 of O2 Records the Impact of Abrupt Climate Change on the Terrestrial Biosphere

Planted Evidence To understand the spatial patterns and consequences of past climate change requires the identification of reliable proxies that reflect specific aspects of those changes, such as temperature or rainfall in a given location. Also of interest are proxies for broader categories of change, such as methane production or the sources of carbon dioxide. Severinghaus et al. (p. 1431) present a 100,000-year-long record of the oxygen isotopic composition of atmospheric O2 (δ18Oatm) extracted from air from polar ice cores. δ18Oatm is a general measure of the strength of low-latitude terrestrial photosynthesis and thus of local rainfall because plant metabolism is controlled in large part by water availability. δ18Oatm changes were related to Heinrich and Dansgaard-Oeschger events, two modes of abrupt climate change common over that interval, and δ18Oatm was controlled mostly by the strength of the Asian and North African monsoons. The rapid changes observed should also help to synchronize ice core records. Ice core studies show that changes in low-latitude rainfall accompanied abrupt climate change over the past 100,000 years. Photosynthesis and respiration occur widely on Earth’s surface, and the 18O/16O ratio of the oxygen produced and consumed varies with climatic conditions. As a consequence, the history of climate is reflected in the deviation of the 18O/16O of air (δ18Oatm) from seawater δ18O (known as the Dole effect). We report variations in δ18Oatm over the past 60,000 years related to Heinrich and Dansgaard-Oeschger events, two modes of abrupt climate change observed during the last ice age. Correlations with cave records support the hypothesis that the Dole effect is primarily governed by the strength of the Asian and North African monsoons and confirm that widespread changes in low-latitude terrestrial rainfall accompanied abrupt climate change. The rapid δ18Oatm changes can also be used to synchronize ice records by providing global time markers.

CO 2 diffusion in polar ice: observations from naturally formed CO 2 spikes in the Siple Dome (Antarctica) ice core

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Instruments and Methods - A novel method for obtaining very large ancient air samples from ablating glacial ice for analyses of methane radiocarbon

We present techniques for obtaining large (� 100 L STP) samples of ancient air for analysis of 14 C of methane ( 14 CH4) and other trace constituents. Paleoatmospheric 14 CH4 measurements should constrain the fossil fraction of past methane budgets, as well as provide a definitive test of methane clathrate involvement in large and rapid methane concentration ((CH4)) increases that accompanied rapid warming events during the last deglaciation. Air dating to the Younger Dryas-Preboreal and Oldest Dryas-Bolling abrupt climatic transitions was obtained by melt extraction from old glacial ice outcropping at an ablation margin in West Greenland. The outcropping ice and occluded air were dated using a combination of d 15 No f N 2, d 18 Oo f O 2, d 18 Oice and (CH4) measurements. The (CH4) blank of the melt extractions was <4 ppb. Measurements of d 18 O and d 15 N indicated no significant gas isotopic fractionation from handling. Measured Ar / N2, CFC-11 and CFC-12 in the samples indicated no significant contamination from ambient air. Ar / N2, Kr /Ar and Xe /Ar ratios in the samples were used to quantify effects of gas dissolution during the melt extractions and correct the sample (CH4). Corrected (CH4) is elevated over expected values by up to 132 ppb for most samples, suggesting some in situ CH4 production in ice at this site.