HOLOCENE ISOTOPIC VARIATION AND PERIODICITIES IN CARBONATE SEDIMENTS OF A GROUNDWATER-DRIVEN LAKE IN WESTERN MONTANA

Decadal (15-year) resolution records of Holocene lacustrine carbonate δ¹⁸O and δ¹³C from a groundwater-dominated lake in west-central Montana show significant spectral power in frequency ranges associated with both Pacific Northwest and Northern Great Plains (NGP) climatic provinces. Highly resolved sediment core reflectance (here a proxy for carbonate mineral concentration) shows similar spectral features in multi-decadal and century periods. Changes in oxygen isotopes and core reflectance record variation in lake residence time and rates of groundwater-lake exchange. Climatic forcing of lake residence time included multi-decadal periodic components in the earliest Holocene, and century-scale spectral power similar to that seen in NGP records after 9000 cal ¹⁴C yrs BP. Decadal and century-scale oscillations are superimposed on a long-term trend toward decreasing lake residence time after 9K BP. The late Glacial-early Holocene transition is punctuated by an abrupt geochemical reversal (lake freshening) from 11.2K to 10.8K BP.

Field data and modeling show that the groundwater through-flow character of Jones Lake, which is governed by glacial and early post-glacial landforms, results in low-amplitude but rapid response of lake isotopic composition to hydroclimatic change. Our high-resolution history of isotopic variation thus provides a record of hydrologic periodicities and their changes over the Holocene. Inferred fluid-balance variations may represent changes in either groundwater inflow or evaporation rates, but in either case reflect changes in status of the shallow groundwater regime. Periodicities in shallow groundwater fluxes imply systematic change in stream baseflow, wetland permanence, and other hydrologic characteristics dependent on groundwater discharge.