Siple Dome contributions

Michael Bender

Our main contribution to Siple Dome ice core studies has been to date the core for the length of its record by measuring the d18O of paleoatmospheric O2 (d18Oatm). We correlated to the GISP2 record by adjusting the Siple Dome timescale so that its curve of d18Oatm coincided with that of GISP2. Our experimental effort involved measuring the d18O of O2, d15N of N2, and the O2/N2 ratio in replicate samples, at 1 - 1.5 kyr resolution for the length of the core.

The timescale we have derived clearly constrains the age - depth curve to 100 ka. Below this age, d18Oatm of Siple Dome no longer varies with depth as at other sites. The core is inferred to be either disturbed or highly compressed.

The d18Oatm timescale matches well with the CH4 timescale of E. Brook back to 15 ka. Changes in d18Oatm define control points at 26 ka as well as between 56 - 100 ka. Throughout the core, O2/N2 ratios are highly variable and frequently very low (as low as -50 ?). d18Oatm is noisier and ofter heavier than at comparable ages in GISP2 and Vostok. These features indicate gas loss due to core relaxation and cracking at the surface. A weak negative correlation between d18O and O2/N2 of replicates indicates that there is a small fractionation of O isotopes associated with loss of O2. As a result of the core cracking artifact, Siple Dome control depths are generally less accurately constrained than at GISP2 and Vostok. As well, we are unable to identify control depths between 26 - 56 ka.

Our results nevertheless establish the general chronology of the core. (The current chronology has been distributed to all the Siple Dome PI's.) The shape of the isotopic temperature - age curve (from J. White) is similar to that of Vostok and GISP2. There are about as many interstadial events in Siple Dome as Vostok or GISP2. However, given present age uncertainties, it is not possible to link the individual Siple events with those of the other sites.

Our results make a number of ancillary contributions as well. d15N of d15N data show that, over most of the core, deposition occured at the base of "normal" firn columns of 50 - 60 m depth. Total gas content is measured by 38Ar isotope dilution; results are somewhat noisy, probably due to gas loss.

Our next work will be determining constraints on the ages of the ice core in the deepest 10 m, where the age does not appear to increase continuously with depth.