The isotopic composition of trapped oxygen molecule and nitrogen molecule in ice cores: Biogeochemical and stratigraphic implications

• Main Author: Sowers, Todd Anthony
• Format: Text
• Language: English
• Published: DigitalCommons@URI 1991
• Subjects: Geochemistry; ice core
• Online Access: https://digitalcommons.uri.edu/dissertations/AAI9217544

Climate records from ice cores and deep sea sediment cores have supplied vast amounts of information about late Pleistocene climate. Records of continental ice volume and sea surface temperature have been generated from deep sea sediment studies. Temperature records and the composition of the paleoatmosphere have been constructed from ice cores. One very important climate record which is missing from the ice core reconstructions, however, is that of continental ice volume. The lack of an indicator for continental ice volume in ice cores has made it difficult to establish the temporal relationship between climate events recorded in ice cores and other climate events recorded in deep-sea sediments. We present a proxy for continental ice volume in ice cores, a record of the $\delta\sp{18}$O of atmospheric O$\sb2$($\delta\sp{18}$O$\sb{\rm atm}$) covering the last $\sim$160,000 years. The data are based on the analysis of the $\delta\sp{18}$O of trapped O$\sb2$ in the Vostok ice core. This record shows a remarkable similarity to records of the $\delta\sp{18}$O of sea-water ($\delta\sp{18}$O$\sb{\rm sw}$) inferred from studies of the isotopic composition of foraminiferal calcite. This strong similarity suggests that past variations in $\delta\sp{18}$O$\sb{\rm sw}$ have been transmitted to the atmospheric O$\sb2$ reservoir by photosynthesizing organisms which reside in the surface waters of the world's oceans as well as within the continental biosphere. We correlated our record of $\delta\sp{18}$O$\sb{\rm atm}$ with the SPECMAP stacked $\delta\sp{18}$O record using an inverse correlation method. The results of the correlation show that 77% of the variance is shared between our $\delta\sp{18}$O$\sb{\rm atm}$ record and the SPECMAP $\delta\sp{18}$O record. We attribute the unshared variance to changes in $\sp{18}$O fractionation which are most likely associated with the hydrologic and biologic cycles involving water and oxygen. Having a record of atmospheric CO$\sb2$ along with a continental ice volume proxy ($\delta\sp{18}$O$\sb{\rm atm}$) in the same ice core allows us to compare the timing of changes in these two parameters with little uncertainty in the relative ages of important events. Our results suggest that, during the penultimate glacial termination, atmospheric CO$\sb2$ began to increase at least 3 kyr before the initial introduction of meltwater to the oceans. In order to construct accurate records of the atmosphere from the trapped gases in ice cores one needs to be able to calculate the difference in the age of the trapped air and that of the surrounding ice. We have utilized our date on the $\delta\sp{15}$N of trapped N$\sb2$ to investigate the ice age-gas age difference ($\Delta$Age) for glacial sections of the Vostok core. Our results indicate that $\Delta$Age values for Vostok are between 3,000 and 10,000 years during the last glacial period. The large range of $\Delta$Age values introduces substantial uncertainty in the construction of the composition of the paleoatmosphere from the trapped gases in this core.