Hydroclimate and Ecosystem Changes in the Colorado Rocky Mountains Since the Last Glacial Maximum

Abstract During the Last Glacial Maximum, wetter and cooler conditions drove expanded pluvial lake systems throughout the southwestern United States. Here, we aim to understand the drivers of these hydroclimate changes using new biomarker records from Cumbres Bog, a high‐elevation fen (3,050 m) in the San Juan Mountain Range in the southern Rocky Mountains of Colorado. The leaf wax hydrogen isotope record, which we interpret as reflecting changes in the proportions of seasonal precipitation sources reaching Cumbres Bog, indicates that winter moisture delivery was reduced at the LGM, increased suddenly during the deglaciation at ca. 14.8 kyr, and then declined rapidly at ca. 11.5 kyr in association with exceptional early Holocene drought. Winter precipitation abruptly recovered at ca. 8.2 ka and remained high throughout the mid‐ to late‐Holocene. BrGDGT‐derived temperatures indicate that the southern Rockies were ca. 6.5°C colder during the LGM. Deglacial warming commenced at ca. 18.1 ka, reaching peak temperatures ∼2°C greater than present in conjunction with the peak mid‐Holocene drought. Deglacial warming was interrupted by small (∼1–2°C) abrupt cooling events coincident with the YD and HS1. These deglacial and Holocene hydroclimate changes were accompanied by changes in terrestrial vegetation, lake evolution, and microbial community changes. Indices based on isoGDGT abundances reveal microbial community changes associated with primary succession and the expansion of methanogenic Archaea with deglacial warming. Past temperature variations significantly altered carbon cycling at the Cumbres Bog site and suggest that high alpine lacustrine ecosystems could be particularly vulnerable to future anthropogenic warming.