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198 CHAPTER 6. FORSCHUNGSSTELLE “RADIOMETRIE” 6.1.8 Isochron dating of fossil reef corals and the reconstruction of past sea level fluctuations Participating scientist Denis Scholz, A. Mangini, K. Bohn, T. Felis (Bremen), D. Meischner (Göttingen) Abstract We developed an isochron dating approach that enables to derive reliable ages for fossil reef corals that cannot be dated by the conventional Th/U-method. Isochron dating was applied to Porites corals from Aqaba and Baja California. In addition, we reconstructed magnitude,timing, and duration of the Marine Isotope Substage (MIS) 6.5 ( 175,000 years before present) sea level peak by U-series dating of fossil Acropora palmata corals from Barbados. Figure 6.7: Isochron dating of coral AQB 3A from Aqaba. The isochron age is calculated from the intersection point, the age error is estimated from the intersection points of the confidence bands. Background Reconstructions of past sea level fluctuations allow to determine the magnitude, timing and duration of glacial/interglacial periods. This is essential for the understanding of the causal relationships and the complex interplay between the forcing and feedback mechanisms in the climate system. Some reef coral genera grow in upper 5m below sea surface only and record past sea level fluctuations. Thus, the determination of accurate U-series ages provides a direct method for sea level reconstruction. Unfortunately, many fossil reef corals show elevated ( 234 U/ 238 U) activity ratios which are clear evidence for post-depositional open-system behaviour. Therefore, conventional Th/U-ages obtained from such corals cannot be considered as strictly reliable. There are two possibilities to deal with this problem: (i) to identify corals that have not been altered and (ii) to develop and apply appropriate correction techniques. Funding BMBF (DEKLIM project) Methods and results We analysed five fossil Porites corals from Aqaba, Jordan, by thermal ionisation mass spectrometry (TIMS). ( 234 U/ 238 U) and ( 230 T h/ 238 U) activity ratios of Figure 6.8: Sea level reconstruction for MIS 6.5. Black squares are our coral data from Barbados. Red and yellow symbols are coral data from other studies, straight curves are based on oxygen isotopes. different sub-samples from one coral specimen show a high linear correlation (Fig. 6.7). This can be explained by a model assuming different degree of U addition and subsequent loss for different sub-samples. The model predicts that the true coral age can be calculated from the intersection point of the isochron with the seawater evolution curve (Fig. 6.7). Isochron dating was also successfully applied to corals from Baja California, Mexico [Bohn, 2005]. Detailed investigation of Acropora palmata corals from Barbados, West Indies, showed that these corals suffered U-redistribution [Scholz and Mangini, in press]. Thus, isochron dating cannot be applied. Careful selection of reliable Th/Uages by application of strict reliability criteria suggests that MIS 6.5 sea level ranged from - 50±11 to -47±11 m relative to present sea level between 176,100±2,800 and 168,900±1,400 years before present (Fig. 6.8). At present, this reconstruction is one of the most accurate for MIS 6.5. Outlook/Future work publication of results Main publication Scholz et al. [2004], Felis et al. [2004], Scholz [2005], Bohn [2005], Scholz and Mangini [in press]
6.1. RADIOMETRIC DATING OF WATER AND SEDIMENTS 199 6.1.9 Dating and interpretation of the carbon and oxygen isotopes of two Holocene stalagmites from the South of Chile (Patagonia) Participating scientist Daniel Schimpf, C. Mühlinghaus, D. Scholz, A. Schröder-Ritzrau, A. Mangini, R. Kilian (Trier), C. Spötl (Innsbruck) Abstract Two Holocene stalagmites (MA-1 and MA-2) from the Marcelo-Arevalo cave, which is located in the South of Chile (53 ◦ S) in the vicinity of the Pacific coast, were dated with the 230 T h/ 234 U method and analyzed with respect to their oxygen and carbon isotopes. We find that the Westwind drift, which controls precipitation, is strongly influenced by the El Nino Southern Oscillation. age (ka BP) 5 4 3 2 1 Stalagmite MA-1 Akima fit 0 0 50 100 150 200 250 distance from top (mm) Background For the first time stalagmites from a cave of the southernmost Andes were analyzed at high resolution. The intensity of precipitation at this location reflects the shifts of the position of the Westwind drift. Funding Diploma thesis, therefore not applicable. Methods and results The 230 T h/ 234 U dating method was used to establish a depth-age model for the two stalagmites (MA-1 and MA-2). The measurements were performed with a Thermal Ionization Mass Spectrometer (TIMS). Figure 6.9 shows the depth-age model for stalagmite MA-1 including all reliable data points. An Akima fit was implemented to get a continuous depth-age Figure 6.9: Depth-age model of stalagmite MA-1. relation. It shows an almost linear relation between age and depth. 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 the ENSO strongly influenced the intensity of precipitation, probably causing North- South-shifts of the Southern Westerlies. Outlook/Future work Further stalagmites from the Marcelo-Arevalo cave will be analysed. The stable isotope profiles are modeled by C. Mühlinghaus. We have applied for a DFG - Project to continue this study. Main publication Schimpf [2005]
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CONTENTS 7 3.2.1 Glaciological pilo
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2.3. RADIATIVE TRANSFER 45 2.3 Radi
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2.3. RADIATIVE TRANSFER 47 Funding
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2.7. CARBON CYCLE GROUP 107 Diploma
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2.7. CARBON CYCLE GROUP 111 2.7.4 I
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