Pliocene tree-ring width and isotopic composition: Deciphering Arctic climate variability in a past analog greenhouse world

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The extreme sensitivity of high latitudes to global climate changes is the stimulus for the study of ancient Arctic ecosystems under greenhouse conditions. With an increasing number of studies, including the most recent Intergovernmental Panel on Climate Change (IPCC) report, highlighting Pliocene climate as an analogue for future warmth, the need for accurate proxy records for this period is crucial. The presence of numerous polar forest sites throughout the Arctic spanning the Pliocene (2.6-5 million years ago) provides the opportunity to study the Pliocene with interannual resolution, through the use of tree-rings. This award addresses the need to understand this critical period of Arctic climate to provide insight into the climate system and the range of variability as it existed in an earlier ‘greenhouse world’. Additionally this award will address questions relating to the evolution of Arctic climate throughout the Pliocene, whether well-known modern modes of climate variability were in operation and perhaps even hint at mechanisms of climate change throughout the Pliocene. In Objective 1, climatic information will be derived from the growth parameters and isotopic ratios of Pliocene Arctic fossil wood. These data will be used to constrain estimates of temperature and relative humidity and also to explore whether inter-annual and inter-decadal climatic oscillations were an important component of Pliocene climate. The Pliocene provides an ideal ‘testbed’ for validating global climate model (GCM) predictions of high-latitude warming, given that this is the last period when temperatures were as warm as those predicted for around the turn of the 21st Century. The discrepancy between empirical and model estimates of Pliocene warming highlights the importance of paleobotanical data to the development and testing of climate models. In Objective 2, I will use the diversity of sites and time periods represented to investigate how Pliocene climates have evolved from the early through the mid-late Pliocene. This will afford the opportunity to examine how inter-annual and seasonal variability changed at high latitudes as climate cooled approaching the onset of glaciation. The high resolution records may provide insight into how seasonality, ecosystems and the hydrologic cycle changed through this period of global climate change. Pre-Quaternary high-resolution climate records are notoriously scarce, thus the intellectual merit of this project rests in the detailed Pliocene climatic reconstructions that will result from this award. Empirical estimates of Pliocene temperature and relative humidity for the Arctic generated by this study are beneficial for the validation of climate models. This study will generate additional time series of climate proxy data for evaluating changes in the behavior of climate oscillations (i.e. Arctic Oscillation, North Atlantic Oscillation) over geologic time scales. The broader impacts of this study will be in providing annually resolved data sets that will provide a baseline for studies of the operation of the Arctic climate system in a generally warmer world. This may contribute to reconstructions of Pliocene sea ice (an important constituent of the Arctic Climate system). These data will permit further investigations into the mechanisms of Pliocene cooling. This project will also complement research currently being conducted on the response of Arctic ecosystems to past warm intervals.