Investigations of temporal changes in climate and the geomagnetic field via high-resolution radiocarbon dating

Geological archives have shown periods of abrupt climate change in the relatively stable Holocene epoch (last ca. 11 700 years). One of these periods was around 2800 cal BP. Several records, mainly from Europe, reveal a shift towards wetter, cooler and windier conditions. There are, however, indications for a global extent of the climate change. The climate change coincides with a distinct increase in the atmospheric radiocarbon (14C) concentration, which has been interpreted to be a result of decreased solar activity. Therefore, a solar-induced climate change has been suggested. In addition to changes in solar activity, geomagnetic field records also show prominent variations around 3000-2000 cal BP. In order to investigate the temporal development of past changes in climate and the geomagnetic field it is crucial to establish accurate and precise chronologies for such reconstructions. Well-constrained chronologies are crucial to determine leads and lags in the climate system as well as to investigate climate forcing factors, such as the potential influence of changes in solar activity. Further¬more, highly-resolved and well-dated geomagnetic field records are needed to improve our understanding of geomagnetic field variations and to evaluate suggested linkages between geomagnetic field and climate change. The general aim of this thesis is to improve the dating of changes in climate and the geomagnetic field via linking 14C results of closely spaced samples to the 14C calibration curve – the 14C wiggle-match dating technique – with focus on the period around 3000-2000 cal BP. The 14C wiggle-match dating technique was successfully applied to construct an accurate and precise chronology of the annually laminated (varved) sediments of Lake Gyltigesjon in south-west Sweden for the period around 3000-2000 cal BP. With this technique, it was possible to estimate the old-carbon effect (14C reservoir age) in the lake sediments. The bulk sediment based chronology was validated by comparison to the 14C ages of plant macrofossils. Similarly, the 14C wiggle-match dating technique was used to test the accuracy of a varve chronology of Lake Kalksjon in west-central Sweden for the same time period. The constructed chronologies of Lake Gyltigesjon and Lake Kalksjon have uncertainties of only ±30 and ±20 years (95.4% probability range), respectively. The well-constrained chronologies enabled investigations of the post-depositional remanent magnetisation lock-in effect and its influence on records of palaeomagnetic secular variation. In addition, the hypothesis of a solar-induced climate change in southern Sweden around 2800 cal BP was investigated. The peat record of Undarsmosse in south-west Sweden has previously indicated a change to increased storminess and wet¬ter conditions around this period. The 14C wiggle-match dating technique was applied to constrain the timing of these events. Within the age model uncertainties, the shift towards wetter and windier conditions in southern Sweden can be regarded synchronous with the climate changes inferred from other sites in Europe. The recorded changes, therefore, support the hypothesis of a shift in the larger-scale atmospheric circulation, possibly induced by decreased solar activity.