Temperature is one of the most critical climate parameters to understand, yet remains difficult to reconstruct. Earth's surface temperature has a tremendous impact on other climate factors, such as hydrological variability. Furthermore, the recent trend of global warming illustrates our need to understand temperature, and its influence on climate change for future generations. A recent report by the IPCC stated that the Earth warmed by 0.6 degrees C during the 20th century, and is projected to warm by an additional ~2-6 degrees C during the 21st century. In addition, a recent NRC Committee has concluded that the recent warming observed in many records is real, but that there is less confidence in temperature reconstructions prior to A.D. 1600, and much less prior to A.D. 900. The primary reason for a lack of confidence in temperature reconstructions, according to the NRC committee, is the relative scarcity of precisely dated proxy evidence. They propose that the generation of additional temperature records would improve our confidence in global temperature reconstructions during the period from 2,000 years before present to today. Furthermore, the NRC committee indicates that efforts to improve the reliability of new temperature reconstructions through use of new developing proxies are also of great interest.
Paleotemperature is difficult to measure in continental systems, particularly paleoarchives such as lake sediments. Recent work has highlighted the potential for new, microbial lipid based proxies for temperature to be applied in lake sediments. Bacterial-sourced branched GDGT lipids have been identified in soils, peats, and lake sediments and it appears that the degree of branching in these GDGTs is related to temperature, which can be quantified in the "MBT" index of Weijers et al. (2007). The goal of this proposal is to validate the use of the MBT paleotemperature proxy in lake sediments by (a) determining whether the branched GDGT distribution identified in lake sediments do indeed reflect watershed mean annual air temperature, and (b) to what extent other factors such as seasonal bias and sub-environments within a given watershed are influencing the apparent temperature signal preserved by these bacterial membrane lipids.
I propose a series of analyses designed to test the validity of the MBT and CBT proxies as tools to reconstruct temperature and pH from lacustrine sediments, following an approach involving three different avenues of investigation. The combination of these three analytical approaches will target some of the major questions related to reconstructing temperature with the MBT index, including issues related to accuracy of the reconstruction, biases from seasonality and environmental variability, and coherence with the instrumental record. Specifically, the proposed study will be comprised of three major objectives. 1) Development of a global calibration of CBT and MBT between lake surface sediment and watershed mean annual air temperature (MAT). 2) Analysis of MBT in soils from different sub-environments from three selected sites over an annual cycle to identify environmental factors affecting MBT and potential seasonal biases. 3) Comparison of instrumental temperature data from three selected sites with MBT temperatures reconstructed from lake sediments over the past ~100 years.
Broader impacts include training graduate and undergraduate students and disseminating information on this research to the public through displays at state parks. Broader impacts to the scientific community include a much-needed validation of the application of the MBT paleotemperature proxy in lacustrine systems. Increasing temperatures predicted by modern global climate change will significantly impact natural systems as well as human populations in the coming century. Gaining a better understanding of modern system processes and how they reflect modern climatic conditions can provide a useful tool for understanding past climate and modeling future trends. Appropriate application of rigorously investigated proxies such as the MBT provides the best information about paleotemperature estimates currently available on timescales beyond the existence if instrumental records, but the use of such proxies is only valid if we fully understand all of the caveats associated with them and apply them appropriately.
