Solar variability over the Holocene period : disentangling geomagnetic and solar influences on a new continuous 10Be record from Little Dome C, Antarctica

Sammanfattning: Reliable information on solar activity over the Holocene period is important to predict the Sun in the future and to understand the mechanism behind the Sun-climate linkage. Presently, there are discrepancies in the proxy data of solar activity due to reasons that are not yet understood. This leads to major uncertainties in Holocene solar reconstructions. This PhD project aims to improve the Holocene solar reconstructions with a new dataset and a better reconstruction method. The results are be important for s lar and Sun-climate studies as well as for a better understanding of past changes in the carbon cycle.Cosmogenic radionuclides such as 10Be in ice cores and 14C in tree rings are the best-known proxies for solar activity far back in time. The radionuclide records reflect a combination of production, transportation and deposition processes (atmospheric circulation (10Be) and carbon cycle (14C)). Presently, the different radionuclide records show disagreements regarding their long-term (millennial-scales) changes for unknown reasons. Moreover, the radionuclide records are also influenced by long term changes in the geomagnetic field that are not well constrained. This leads to major uncertainties in reconstructions of past changes in solar activity.This thesis is based on new 10Be measurements from 759 ice chip samples drilled at the East Antarctic site called Little Dome C (LDC). The new 10Be record continuously covers the entire Holocene. A Bayesian model was also developed to disentangle solar activity and geomagnetic field influences on the radionuclide records. The model was applied on the new LDC 10Be data and also on the existing 10Be data from other ice cores and tree-ring 14C data. The reconstructions show consistent short-term variations (decadal- and centennial-scale) of solar activity while longterm variations (multi-millennial-scale) are still uncertain. Long-term discrepancies are present among the 10Be records and the 14C data, especially for the last 4 ka. We also found hints of a polar bias effect that dampens the geomagnetic field influence on Antarctic 10Be records but the effect is absent in a Greenland 10Be record. These results point to a difference in the transportation mode of 10Be toward the different ice core sites and an influence of the carbon cycle changes on the 14C data.