Reconstructing solar activity and geomagnetic field intensity during the Holocene

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The Sun is by far the most important energy source for the Earth's climate system. Direct observations over the last four centuries have revealed that solar activity varies over time, and these variations appear to have played a significant role in forcing periods of climate change on Earth in the pre-industrial era, such as the Medieval Warm Period (900-1400 A.D.) and the Little Ice Age (1500-1800 A.D.). The longer-term variations in solar activity over the last 11,500 years (the Holocene period), and their potential links to climate change, remain very uncertain. The production of cosmogenic nuclides, such as 10Be and 14C, in the atmosphere is controlled by solar activity, implying that records of such nuclides from natural archives may extend our knowledge of solar variation back in time. Reconstruction of past solar activity is complicated, however, by the fact that the 10Be and 14C signals embedded in natural archives are also influenced by the intensity of Earth's magnetic field and by local climatic effects. Current efforts to reconstruct past solar activity are, therefore, hampered by uncertainties in reconstructing past geomagnetic field intensity and the limited availability of high-resolution 10Be and 14C records. The long-term solar variability remains a very controversial topic and there is a great need for alternative cosmogenic nuclide records to address this issue. With this proposal I will take a new approach to improve the current understanding of solar variability during the Holocene by generating new high-resolution records from different archives of past 10Be production rates combined with analyses of past variations in the intensity of the Earth's magnetic field. The main focus of this project will be to generate a high-resolution 10Be record from annually-layered (varved) lake sediments from west-central Sweden. The annual laminations of the sediment core I propose to study provide excellent chronological control, and a unique opportunity for determining the flux of 10Be to the surface of the Earth for the past 9,500 years. I will also develop a high-resolution 10Be record for the Holocene from marine sediments in the far North Atlantic. Based on the same marine sediments, I will also generate new extremely high-resolution records of the geomagnetic field behaviour during the past 11,500 years. My previous research has shown that the sediments from this location south of Iceland are well-suited for detailed studies of variations in 10Be production rates and geomagnetic intensity. Another aspect of this project will be to explore other archives of past 10Be deposition. The novel archives I wish to concentrate on include a raised peat bog from N. Ireland and a stalagmite from a cave in central China. Combining the new 10Be records from this study with existing 10Be records from polar ice cores, will make it possible to filter out local climatic effects embedded in the individual records and produce a global record of Holocene variations in 10Be production rate. Similarly, the new high-resolution records of the geomagnetic field behaviour will be integrated with existing magnetic data to produce a global estimate of Holocene geomagnetic field intensity. By correcting the combined 10Be curve for variations in the intensity of Earth's magnetic field, it is possible to reconstruct the solar activity during the Holocene. It will also be possible to assess potential changes in the carbon cycle during the Holocene by comparing the new global 10Be record with existing 14C records.