Sedimentology, geochronology, oxygen isotope and grain size stratigraphy of a core from Pretty Lake, Indiana: Exploring Midwestern hydroclimate during the last 2000 years

Abstract:

Isotope-based hydroclimate records and grain size records from the mid-continental United States that span the late Holocene with sub-decadal resolution are rare, making the relationship between temperature and hydroclimate for this region not well constrained. Pollen-based temperature reconstructions from North America suggest that the Medieval Climate Anomaly (MCA; 650 to 1050 CE) was warmer than the Little Ice Age (LIA; 1550 to 1850 CE). It has further been suggested that much of the mid-continental US experienced drought during the MCA and pluvial conditions during the LIA. This is supported by modern correlations between seasonal temperatures and precipitation, which are anti-correlated for much of the Midwest; however, for portions of the continental core the opposite relationship exists between temperature and precipitation (i.e., warmer is wetter and colder is drier). Within this context and previous paleoclimate work in the Midwest we present geochronology, sedimentology oxygen isotopes and grain size results from a 12 m composite core from Pretty Lake, a 25 m deep kettle lake in LaGrange County, northeastern Indiana. Here we focus on the last 2,000 years of the 16,000-year record in order to explore hydroclimate variability in response to temperature anomalies during the MCA and LIA. Pretty Lake is well suited for this type of investigation because the closed surface hydrology of the lake basin renders it sensitive to evaporation. This is reflected in oxygen isotope (δ¹⁸O) measurements of surface waters from the lake, which show that it is approximately 4‰ higher than meteoric precipitation. High-resolution down core δ¹⁸O measurements hold tremendous potential for reconstructing regional hydroclimate during the last 2,000 years, particularly when combined with both the grain size record and the present surface water oxygen isotope measurements. Integrated, the three records from Pretty Lake reveal synoptic hydroclimate responses at unique spatial and temporal resolutions; the sediment oxygen isotopes represent the greater continental scale and overall climate during the past 7,500 years; the grain size represents the watershed scale during the last 2,000 years; and the surface water oxygen isotopes represent the smallest spatial scale of insitu lake processes during present conditions.