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

We used a 2.5-dimensional thermomechanical ice-sheet model including the oxygen-isotope ratio O-18/O-16 as a passive tracer to simulate the isotopic composition (delta O-18) 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 delta O-18 (delta(w)) 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 parts per thousand, corresponding to similar to 60% of the LGM sea-level lowstand and to a mean delta O-18 of the NAIS of approximately - 31 parts per thousand. The modeled NAIS volume variations and the induced delta(w) changes over the past 120,000 years indicated no significant time lag. The inaccuracy associated with linearly inferring ice-volume variations from delta(w) changes was generally less than 10%.