A new technique for identifying rock avalanche-sourced sediment in moraines and some paleoclimatic implications

Moraine chronologies are widely used to infer local climate change events and to correlate these events globally, based on the assumption that moraines always reflect climatic drivers. However, this assumption is unreliable in tectonically active terrain because moraines can also be formed by large landslide (rock avalanche) deposits on glaciers. These can affect glacier motion and cause moraines to form while requiring no climate variation, and can thus cause significant errors in climatic signals extracted from moraine chronologies. To eliminate such errors requires a method for identifying moraines that have been influenced by rock avalanches. Herein we present and test a new diagnostic technique that unambiguously identifies rock avalanche sediments using newly discovered fine-sediment signatures characteristic of rapid, high-stress comminution. We test this technique on Holocene moraines in the Southern Alps, New Zealand, which have previously been interpreted as climatic indicators, and demonstrate that some of them unambiguously contain rock avalanche material; so their climatic significance is questionable.