Holocene diatom-derived climate history of Medicine Lake, northern California, USA
The Medicine Lake record is unusual because it responds not only to local and regional climate signals, but changes in conditions on Medicine Lake volcano during the Holocene. Ice retreated within the Medicine Lake volcano occurred around 11,400 years ago, followed by filling of two sub-basins. The absence of Cyclotella indicates that the early lake was probably less than 5 m deep. The low Abies/Artemisia ratio suggests that the climate was relatively dry. Over the next 4000 years, the level of the lake rose as relatively organic-rich fine-grained sediments filled the basin. The increase in abundance of Cyclotella also suggests that the lake gradually deepened. The abundance of Abies in the basin also increased, suggesting the presence of a deeper snowpack that existed into the late spring and summer. The increased snowpack was likely the primary water source that filled the lake during this period. About 5500 years ago, the lake flooded the shallow shelf area surrounding the two sub-basins. Variations in the abundance of Cyclotella and benthic taxa, dominated by Navicula, indicate that the area of the flooded shelf fluctuated during this interval. The abundance of Isoetes and Abies responded similarly to changes in the basin, both suggesting an increase in effective moisture. Their increase corresponds to an increase in Sequoia pollen observed at ODP Site 1019, which records the establishment of modern climatic conditions along the northern California coast (relatively warm wet winters and cool, foggy summers). A connection between coastal and inland 6 climates appears to have strengthened at about this time. These fluctuations are in part due to these changes in moisture availability, but may also be due to changes in the shape of the lake basin brought about by the movement of magma within the Medicine Lake volcano.