Coeval stalagmite records from the Rocky Mountains record Holocene climate change

Tree ring records show cool-season droughts in the western US have been characterized by three spatial patterns over the past 500 years: “western-wide drought”, “wet north/dry south”, and “dry north/wet south”. Previous work shows that these drought patterns can persist on timescales of decades to centuries and are driven by internal climate variability, with secondary influence by sea surface temperatures. However very few high-resolution records of western US precipitation extend beyond the tree ring record (~1400 CE), limiting our understanding of the occurrence and persistence of these patterns of natural climate variability on longer timescales and further in the past. Here we use trace element (Mg/Ca, Sr/Ca, Ba/Ca, P/Ca) and stable isotope (δ¹⁸O, δ¹³C) data from two coeval stalagmites to construct a Holocene paleoclimate record for Titan Cave (TC), northern Wyoming, extending the hydroclimate record of the northern Rockies and providing the opportunity to assess longer-term natural climate variability in the northern Rockies. The TC-2 and TC-7 speleothems grew over the past ~5.7 ka and ~3.1 ka, respectively. Proxies from both stalagmites exhibit strong correlations with several coeval climate records, including regional snowpack as recorded by tree rings during the past ~600 years, suggesting the speleothems record winter precipitation patterns in the northern Rockies. Decreased snowpack and dry conditions at TC correlate to warmer sea surface temperatures in the Gulf of Alaska and the positive phase of the Pacific Decadal Oscillation. Comparison of speleothem δ¹⁸O with the δ¹⁸O of Bison Lake sediments from central Colorado suggests all three patterns of western US drought occurred during intervals of the late Holocene. Specifically, the wet north/dry south and dry north/wet south winter precipitation dipoles in the Rocky Mountains with a boundary near 40° N latitude, were established by approximately 3 ka. Multiple, centuries-long “western-wide” droughts occurred throughout the record, most notably from 2.2 – 2 ka during the Roman Warm Period. Other dated TC stalagmites grew during the mid-Holocene and Last Interglacial, providing records of winter precipitation in the northern Rockies during past warm intervals, which may serve as analogs for future anthropogenically-warmed climate states.