Midcontinental Hydroclimate Variability from a 1,500-yr Wisconsin Lake Sediment Record

Abstract

Lacustrine sediment archives preserve continuous records of changes in basin- to regional-scale processes that reflect broader variability in climatic conditions. Here, we present a 1,500-yr sediment record of inferred effective moisture (P/E) that spans the Current Warm Period (CWP; last 150 years), Medieval Climate Anomaly (MCA; ca. 950–1250 CE), and Little Ice Age (LIA; ca. 1300–1800 CE) from glacially-formed Pope Lake in central Wisconsin. A suite of sediment proxies constrained by 14C and 210Pb ages was developed at decadal resolution to investigate Common Era changes in Upper Midwest effective moisture in response to temperature and synoptic scale atmospheric variability, such as the Pacific North American (PNA) pattern. Lake water isotopes reconstructed with authigenic carbonate oxygen isotopes (d18Ocal) from Pope Lake compare favorably with other Midwestern closed-basin lakes, indicating that evaporation was a significant control on the isotopic composition of lake water. Relatively lower d18Ocal values during the MCA suggest that the moisture availability was greater during the MCA than LIA, despite increased air temperatures. This is supported by low carbon/nitrogen (C/N) ratios and low terrestrial lithic contributions that reflect increased lake levels during the MCA. Reversals of these trends during the cooler LIA were observed. Comparisons of the Pope Lake record to synoptic scale forcings suggest that shifts in regional P/E were consistent with high amplitude PNA variability that likely affected the source and seasonality of precipitation. The general warm/wet and cool/dry relationship noted during the MCA and LIA underscores how global temperature anomalies may alter the balance of effective moisture in the Upper Midwest in relatively short succession. The Pope Lake sediment record presented here is an important step in establishing hydroclimatic history that may inform expectations of future climate for a region sparsely populated with similar high resolution late Holocene records.