INVESTIGATING THE UPLIFT CHRONOLOGY OF THE CASCADE MOUNTAINS WITH THE ISOTOPIC COMPOSITION OF LIPID BIOMARKERS

Trending north to south for 1,100 km and rising to elevations of 3000 meters, the Cascade Mountains form a significant orographic barrier to precipitation between western and eastern Washington State. The uplift of the North Cascades had a dramatic impact on regional ecosystems and global climate by enhancing regional aridity and providing an anchor to the north American ice-sheet during glacial ages. Despite their importance, however, the chronology of the North Cascades uplift and its origin remain poorly understood, with estimates ranging from the Eocene to the Pliocene.

Here, we study the impact of the Cascades orographic effect on the hydrogen isotopic composition (2H/1H) of lipid biomarkers in surficial soils across a 270 km transect over the mountains. Preliminary results indicate ca. 100 per mil of 2H-depletion over our transect, more than predicted by regional isotopic models. We propose to track this well-marked isotopic depletion in geological units on both sides of the Cascades, to reconstruct the evolution of rain shadow effects in deep time.