Millennial-Scale Variations in Oxygen-18 of Atmospheric Molecular Oxygen
Abstract
The oxygen-18 content of atmospheric dioxygen (δ18Oatm) is known from ice core trapped air records to vary on orbital timescales, primarily the ~21,000 year periodicity. The cause is thought to be a combination of 1) ice sheet growth and consequent variation in O-18 of ocean water, the original source of all substrate H2O used for photosynthetic O2, and 2) changes in the biogeochemical fractionation of O2 relative to ocean water, known as the Dole Effect. Because the turnover time of O2 in the atmosphere is ~1000 years, variations on shorter timescales are not expected and previous studies have not searched for them. Here we present a new record of δ18Oatm at approximately 100-year resolution from the Siple Dome ice core, Antarctica, covering the past 95 kyr. The record shows evidence of artifactual fractionation during gas loss, in the form of an inverse correlation of O2/N2 and δ18Oatm in pair differences with slope of -0.01, probably due to poor core quality (low O2/N2 is associated with highly fractured pieces of ice). The light isotope O-16 appears to escape more readily from the bubbles during leakage of gas out of the ice, leaving the remaining gas enriched in O-18. An empirical correction for gas loss fractionation is made using the measured δO2/N2 and a slope of -0.01. Both raw and corrected records show distinct millennial-duration positive anomalies in δ18Oatm, suggesting that the anomalies are real features of the paleo-atmosphere. They occur at times of Antarctic warmth (designated A1, A2, etc.), which are also times of weak Asian monsoons and Heinrich events or `mystery intervals' in the northern hemisphere. The anomalies typically have an amplitude of +0.15‰, which compares with analytical error of ±0.02‰. We speculate that the anomalies are caused by the increase in O-18 of terrestrial precipitation associated with the weak monsoons, as seen in Asian and Brazilian speleothem records, in addition to evaporative enrichment of leaf water O-18 in the low relative humidity of the weak-monsoon regime. Regardless of origin, the anomalies may be useful as interpolar stratigraphic markers for synchronizing Antarctic and Greenland ice core records during times when methane variations are absent.
- Publication:
-
AGU Fall Meeting Abstracts
- Pub Date:
- December 2006
- Bibcode:
- 2006AGUFM.U34B..01S
- Keywords:
-
- 0315 Biosphere/atmosphere interactions (0426;
- 1610);
- 0724 Ice cores (4932);
- 1631 Land/atmosphere interactions (1218;
- 1843;
- 3322);
- 4853 Photosynthesis;
- 4932 Ice cores (0724)