Solar Activity Changes at the End of the Last Ice Age - Influences on Climate and Applications for Dating

University dissertation from Department of Geology, Quaternary Sciences, Lund University

Abstract: Throughout its history Earth experienced a variety of natural climate changes. By investigating their spatial and temporal evolution we can increase the understanding of the mechanisms and dynamics underlying natural climate change and improve our general comprehension of the climate system. Prerequisites of these investigations are reliable reconstructions of past forcing variations as well as sound and consistent chronologies of paleoclimate records. The Sun is by far Earth’s most important source of energy and variations in its irradiance have been shown to influence climate on different temporal and spatial scales. The exact mechanisms of these solar influences on climate are, however, not fully understood. Variations in solar activity also cause changes in the atmospheric production rates of cosmogenic radionuclides, such as 10Be and 14C. These radionuclides get subsequently deposited in various environments which can, hence, provide information about past solar activity levels. Furthermore, these records can be synchronized to each other by identifying coherent production rate related patterns in their radionuclide records. This project aims to extend solar activity reconstructions back into the late glacial and investigate potential sun-climate relationships. Furthermore, the consistency of the time scales underlying different records is tested by comparing their cosmogenic radionuclide records. In addition, it aims to improve radiocarbon dating calibration by extending its tree-ring based section further back in time. We present the first solar activity reconstruction for the late glacial based new and published 10Be data from the GRIP and GISP2 ice cores, supported by published 14C data. We infer that late glacial and Holocene solar activity variations have been comparable in both patterns and amplitudes. We find a persistent influence of solar activity changes on Greenland climate during the Last Glacial Maximum which appears coherent with modern day observations and climate model results. This suggests that a similar solar forcing mechanism may have been operating under otherwise very different climate regimes. We propose a time scale transfer function between Greenland ice core and radiocarbon dated records by synchronizing the temporal variations of ice core 10Be and tree-ring 14C records. We outline a statistical framework that allows time scale differences and uncertainties to be inferred. We find that there is a continuously growing difference between Greenland ice core and radiocarbon based chronologies throughout the Holocene. Furthermore, we identify a rapid shift in this time scale difference around 12,500 years ago, that cannot be explained with ice core layer counting uncertainties alone. Instead, we propose that this effect may arise from uncertainties in the absolute dating of tree-ring records. We present new 14C data on floating tree-ring chronologies that can improve radiocarbon dating calibration between 14,000 to 14,700 years ago. We introduce a method of how combined information from 14C and 10Be records can aid us to infer absolute ages for these chronologies. These new records add substantial structure to the calibration curve and we note that missing this structure can lead to erroneous calibration of 14C dates by up to 500 years.

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