Lateglacial and holocene hydroclimate inferred from a groundwater flow-through lake, northern rocky mountains, USA

Climate-driven variations in lake-groundwater exchange are recorded by sediments in groundwater-dominated lakes. A groundwater flow-through lake in west-central Montana (USA) registers latest Pleistocene and Holocene hydroclimatic variation in fluid and solute balance, as controlled by rates and timing of groundwater recharge. Early Holocene warming occurred under conditions of relative aridity and low groundwater throughflow, punctuated by a c. 450-yr episode of lake dilution centered on 11 000 cal. yr BP. Maximum evaporative concentration of lake waters, registered in both ?¹⁸O values and mineralogy of endogenic carbonates, coincided with the early-Holocene peak in insolation seasonality at about 9750 cal. yr BP. Subsequently, progressively decreasing lake residence time drove a sustained long-term decline in salinity while having a very subdued effect on mean ?¹⁸O values. We explain this decoupling by (1) limits placed on oxygen isotope sensitivity by groundwater throughflow, and (2) a shift toward greater summer rain contribution to lake inflow after mid-Holocene time. Superimposed multidecadal- and century-scale variation in lakeĝ€"groundwater exchange generated high-frequency but low-amplitude isotopic oscillations throughout the record. High rates of groundwater throughflow maintaining low lake salinity similar to that observed today were established around 1400 cal. yr BP. We infer reduced regional stream baseflow, decrease in permanent wetlands (relative increase in ephemeral wetlands) and enhanced lake and wetland salinity prior to this time, relative to the late Holocene.