LATE-GLACIAL TO HOLOCENE CLIMATE OF THE NORTHERN ROCKY MOUNTAINS INFERRED FROM STABLE ISOTOPE ANALYSIS OF SEDIMENTS FROM MORRISON LAKE, MT

Hydroclimatic variability in high elevation, headwater environments of the northern Rocky Mountains has significant implications for downstream ecosystems. Current knowledge of precipitation variability is limited to data collections over the last several decades by SNOTEL, weather stations, and precipitation-sensitive tree ring records. Paleoclimatic proxies that span the Holocene are necessary to better develop our understanding of multidecadal to centennial scale hydroclimatic variability, and to frame modern weather trends. Isotopic analysis has been conducted on both the inorganic carbonate mineral fraction and the bulk organic fraction of sediment cores collected at Morrison Lake, Montana. This headwater lake is characterized by significant authigenic carbonate mineral production that records oxygen isotopic variability of the lake water, which is strongly modified by evaporation. Isotopic values from both the inorganic and organic fractions, over the past 15,000 yrs. BP, show millennial, centennial and multidecadal scale variability. Nitrogen isotopes show significant multi-centennial scale variability with increasing δ¹⁵N values in the early Holocene, shifting to near atmospheric values throughout the mid Holocene, and then shifting back to enriched values in the late Holocene. δ¹⁸O values are lower in the mid-late Holocene (6,000 cal yrs. BP – modern) than the late Pleistocene-early Holocene transition. δ¹⁸O data indicates that glacial meltwater likely influenced the lake water balance during the late Pleistocene (15,500-14,800 cal yrs. BP). After glacial retreat, δ¹⁸O data supports a dry late Pleistocene – early Holocene transition (14,800-11,000 cal yrs. BP), a brief but significant increase in effective precipitation during the early Holocene (11,000-10,000 cal yrs. BP), an extremely dry transition into the middle Holocene (10,000-6,000 cal yrs. BP), and a more consistently wet climate during the mid to late Holocene. Spectral analysis of δ¹⁸O data from the past 2,300 yrs. shows that effective precipitation displayed periodicities of 25, 40, 52, 55, and 78 years, showing the presence of multidecadal climatic variability, possibly related to the Pacific Decadal Oscillation (PDO).