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164 CHAPTER 6. FORSCHUNGSSTELLE “RADIOMETRIE” 6.1.4 Ghost Resonance in Glacial Climate Holger Braun Abstract A new hypothesis was elaborated to explain the 1470-year cycle of abrupt climate changes in the Last Glacial. This hypothesis was tested with two different types of models. Figure 6.4: Illustration of the ghost resonance mechanism with a conceptual model. The forcing (black, in arbitrary units) consists of two sinusoidal cycles with periods of 1470/7 (=210) and 1470/17 (about 86.5) years. Each time the forcing crosses a certain threshold function (green), the model switches its state (negative values of the threshold function correspond to the ground state, positive values to the excited state). The response of the system (i.e. the time evolution of the threshold function) has a main spectral component of 1470 years, which is missing in the forcing. Background Paleoclimatic records (ice cores, deep sea sediments, speleothems) from the Last Glacial show repeated large-scale (10-15 Kelvin) warming events in the North Atlantic region, the so-called Dansgaard-Oeschger (DO) events. In the GISP2 ice core from Greenland, DO events are often spaced by about 1470 years or multiples thereof. This indicates that the events were probably triggered by a regular external forcing. But no such forcing with a pronounced 1470-year spectral component is known. Methods and results I elaborated a new hypothesis to explain the 1470-year cyclicity of DO events. According to this hypothesis, which I tested with a climate model and a simple conceptual model, the regularity of DO events is due to a nonlinear resonance caused by a periodic external forcing with a period of 1470 years, but without a corresponding spectral component (i.e. with spectral components corresponding only to harmonics of a missing 1470-year fundamental frequency). This resonance mechanism, which is known as ghost resonance, can occur in excitable nonlinear systems with a threshold. Since DO events probably represent threshold-like switches between two possible modes of deep water formation, the ghost resonance mechanism is indeed plausible to explain the events. The new hypothesis attributes the glacial 1470-year cycle of DO events to solar variability, since proxies of solar activity (i.e. records of cosmogenic radionuclides) exhibit pronounced spectral components near harmonics of 1470 years. Outlook/Future work In the DFG project Dansgaard-Oeschger events (DFG project number MA821/33-1) I have started to investigate with a climate model if past solar irradiance variations could indeed have been strong enough to trigger regular DO events in the Last Glacial. Funding The work was funded by the Heidelberg Academy of Sciences and by the DFG (DFG project number MA821/33-1). Main publications Braun et al. [2005] Braun [2006]
6.1. FORSCHUNGSSTELLE “RADIOMETRIE” 165 6.1.5 Dating and interpretation of climate proxies for three Holocene stalagmites from the South of Chile (Patagonia) Daniel Schimpf (Participating scientists: C. Mühlinghaus, D. Scholz, A. Schröder-Ritzrau, A. Mangini, R. Kilian (Trier), A. Kronz (Göttingen), C. Spötl (Innsbruck)) Abstract Three Holocene stalagmites (MA-1, MA-2 and MA-3) from the Marcelo-Arévalo cave, which is located in the South of Chile (53 ◦ S) at the Pacific coast, were dated with the 230 Th/ 234 U method and analysed with respect to their oxygen and carbon isotopes. Furthermore a Mg/Ca ratio record for MA-1 was measured at high resolution. It is a proxy for precipitation and correlates very well with the isotope data. We find that the West wind drift, which controls precipitation, is strongly influenced by the El Nino Southern Oscillation. δ 13 C (‰ VPDB) 0 1 2 3 4 -6 40 point smoothing 100*Mg/Ca δ 13 C MA-1 -8 -10 -12 -14 -16 0 1 2 3 4 age (kyrs) Figure 6.5: Correlation between δ 13 C data and the Mg/Ca ratio Background For the first time stalagmites from a cave of the southernmost Andes were analysed at high resolution. The intensity of precipitation at this location reflects the shifts of the position of the West wind drift. Methods and results The 230 Th/ 234 U dating method was used to establish a depth-age model for the three stalagmites (MA-1, MA-2 and MA- 3). The measurements were performed by a Thermal Ionization Mass Spectrometer (TIMS). Furthermore the Mg/Ca ratio was measured by Electron Microprobe, this ratio is a proxy for precipitation. Figure 6.5 shows a comparison between the δ 13 C profile and Mg/Ca ratio of stalagmite MA-1. The very good correlation between the two records confirms that the δ 13 C record (and in the same case the δ 18 O record) also depends on precipitation. The comparison of the dated δ 13 C and δ 18 O profiles with a record of the ENSO intensity, derived from a sediment record off the coast of Peru, reveals that ENSO strongly influenced the 1,0 0,9 0,8 0,7 0,6 0,5 0,4 0,3 0,2 0,1 100*Mg/Ca intensity of precipitation, probably causing North- South-shifts of the Southern Westerlies. An eruption of volcano Mt. Burney (4.3 kyrs BP), which occurred 100 km to the north-west of the MA cave, resulted in a higher uranium content in the stalagmites for the following 2 kyrs due to a leaching effect caused by sulpherous ashes that covered the soil above the cave. Outlook/Future work ICP-MS data for all three stalagmites have been measured, these trace element data will be analysed. In addition a sediment core of lake Tamar in the vicinity of the MA cave has been taken and will be analysed. The stable isotope profiles will be modelled by C. Mühlinghaus. Funding Stalagmite der Südanden, DFG project Main publication Schimpf [2005], Kilian et al. [2006]
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