Continental-scale validation of the temperature signal in oxygen isotopes of Sphagnum cellulose and its application as a paleoclimate proxy

This study investigates the use of stable oxygen isotopes in Sphagnum moss cellulose as a paleotemperature indicator in peat deposits. We collected water and peat samples from temperate ombrotrophic and kettle peatlands across northern North America, spanning 80 degrees of longitude and nearly 20 degrees of latitude, and tested potential effects of evaporation, relative humidity and microtopography that might confound a temperature relationship. The delta18O of both bog water and Sphagnum moss cellulose showed a strong linear relationship with growing-season temperature. Evaporative effects were minimal, suggesting that only the biochemical fractionation factor needs to be considered in reconstructing the isotopic composition of meteoric water from cellulose delta18O values. Microtopographic effects introduced variability of ∼1.4‰, within the uncertainty of the biochemical fractionation factor. We used this paleotemperature proxy to assess relative changes in precipitation and temperature from two cores taken in Minden and Irwin Smith Bogs in central and northeastern Michigan. These records extend back to the middle Holocene, and reveal extreme drought events at ∼ 5000 and ∼ 1000 cal yr BP, the latter corresponding to warm temperature anomalies and synchronous with the Medieval Climate Anomaly (MCA). The warm/drought event during the MCA is consistent with positive temperature excursions in the North Atlantic Ocean and the Caribbean recorded in planktonic foraminifera. Comparison of pollen and isotope-based temperature reconstructions also reveal a likely shift in atmospheric circulation or precipitation seasonality during the MCA.