Late Holocene climate change for the eastern interior United States: evidence from high-resolution δ18O values of sagittal otoliths

Abstract

Stable oxygen isotope values were determined for freshwater drum (Aplodinotus grunniens) sagittal otoliths recovered from the Eastman rockshelter archaeological site in northeast Tennessee to evaluate climate change for the eastern interior United States from 5500 calendar years ago to the present. Micromilled samples representing less than six days of otolith growth were extracted to acquire high-resolution intra-otolith variation in δ18O values. Freshwater drum sagittal otoliths form annual bands due to thermally induced growth cessation below 10°C. δ18O(H2O) values can be calculated once a high-resolution carbonate sample representing the beginning of the growing season is isolated. Maximum summer temperature can be calculated using the seasonal minimum δ18O(CaCO3) value and the δ18O(H2O) value.Maximum summer temperatures calculated from the freshwater drum sagittae suggest that summer temperatures generally decrease from 29°C at ∼5.5ka to 22°C at ∼0.3ka. However, warmer climates at 2.9, 1.7–1.6, and 1.2–1.0ka punctuate this trend. A more complete picture of the climate is reconstructed, because δ18O(H2O) values, which are a function of the ratio of summer to winter precipitation, are also calculated. A relatively low average δ18O(H2O) value of −8.1‰ VSMOW was calculated at 1.0ka, suggesting cold winters, dry summers, and/or wet winters may have prevailed during part of the Medieval Warm Period in Tennessee. Contrary to studies suggesting that the Holocene was extremely stable and the Hypsithermal was invariably warm and dry, additional evidence suggesting both significant climate variability and evidence for a wetter mid-Holocene is presented.