Modeling the oxygen-isotopic composition of the North American Ice Sheet and its effect on the isotopic composition of the ocean during the last glacial cycle

Abstract

We used a 2.5-dimensional thermomechanical icesheet model including the oxygen-isotope ratio 18O/16O as a passive tracer to simulate the isotopic composition (d18O) of the North American Ice Sheet (NAIS) during the last glacial cycle. This model allowed us to estimate the NAIS contribution to the change of seawater d18O (dw) between the Last Glacial Maximum (LGM) and the Holocene and to evaluate the effect of nonequilibrium isotopic composition of the NAIS on the relationship between ice-volume variations and the ocean isotopic enrichment. The enrichment due to the NAIS at the LGM was 0.63%, corresponding to 60% of the LGM sea-level lowstand and to a mean d18O of the NAIS of approximately 31%. The modeled NAIS volume variations and the induced dw changes over the past 120,000 years indicated no significant time lag. The inaccuracy associated with linearly inferring ice-volume variations from dw changes was generally less than 10%.