THE EFFECT OF INTERANNUAL VARIABILITY FORCED GLACIAL ADVANCES ON THE MORAINE RECORD: A CASE STUDY FROM THE COLORADO FRONT RANGE DURING THE LAST GLACIAL MAXIMUM

Valley glacier moraines are commonly used to infer mean climate conditions (annual precipitation and mean melt-season temperature) at the time of formation. However, recent research has demonstrated that even in steady climates, substantial decadal-scale fluctuations in glacier length also occur in response to stochastic, year-to-year variability in mass balance. All climates, steady or transient, include interannual variability. When interpreting moraine sequences it is therefore important to include the effect of interannual variability on glacier length because moraines can be 1) formed by interannual variability forced advances or 2) formed by advances forced by a combination of a climate change component and an interannual variability component. We address this issue for eleven LGM glaciers from the Colorado Front Range, USA. Using a linear glacier model that allows for a thorough exploration of parameter uncertainties, supplemented by a shallow-ice flowline model, our analyses suggest that i) individual LGM terminal moraines were formed by a combination of climate change and interannual variability forced advances; ii) estimates of mean climate using maximum LGM glacier geometries are ~10–15% too extreme; and iii) classic ‘recessional’ moraines may be formed by re-advances during the LGM as opposed to re-advances or standstills during deglaciation.

It is often assumed that century scale glacial standstills were required to form large (>10 m in relief) LGM terminal moraines. Our numerical model suggests that the longest standstills for the modeled glaciers lasted ~50 years. Historical records of interannual variability forced glacier advances, which formed >10 m terminal moraines provide modern validation to our conclusions. We expect interannual variability to play an important role in kilometer-scale glacier fluctuations and moraine emplacement in the past and present as well as in maritime, Alpine, and continental settings (e.g. Oerlemans, 2001).