The variability of Holocene climatic change; evidence from varved lake sediments

Abstract. Varved sediments from a lake near the present forest-prairie border in northwestern Minnesota provide an annual record of climate change for the last 10,400 years. Climate-sensitive mineral, chemical, and biological components show that the mid-Holocene dry interval between 8500 and 4000 years ago is asymmetrical and actually consists of two distinct drier pulses separated by a moister interval that lasted about 600 years. Cyclic jluctuations with periods of several hundred years were abrupt and persistent throughout the Holocene and are most clearly recorded within the two drier pulses. The mid-Holocene is recognized as a time of unusually warm or dry climate (or both) in Europe and North America (1, 2). In Minnesota, prairie vegetation such as grasses (Gramineae) and sage-brush (Artemisia) expanded eastward into forested regions of the state betwee