Reconstruction of decadal to centennial Holocene variability of the North Atlantic Oscillation using speleothems from Morocco
Physical Geography
Final Report Abstract
The main aims of the project were 1) to improve our understanding of the processes that affect trace element variability in calcite and aragonite speleothems in order to provide a robust interpretation in terms of climate parameters; 2) to provide high resolution trace element (nearannual) and stable isotope records from speleothems from NW Morocco, and 3) to assess the temporal variability of the North Atlantic Oscillation (NAO) through the Holocene. Although not all data has been published yet, these aims have been fulfilled. Two of several papers focused on trace element variability in aragonite speleothems by determining trace element distribution coefficients from stratigraphical calcite to aragonite transitions within speleothems. The U concentration in aragonite speleothems was identified as the most sensitive proxy for effective rainfall through the process of prior aragonite precipitation. A third paper shows that an increase in calcite Mg/Ca ratio is associated with a higher Sr distribution coefficient. This has important implications for the identification of prior calcite precipitation in calcite speleothems. Prior calcite precipitation is depending on effective rainfall, and is the key process that is used to explain positive correlations between speleothem calcite Mg/Ca and Sr/Ca ratios. A proxy comparison of two speleothem records from Morocco and Germany combined with climate simulations resulted in major progress on the response of the NAO to the Early Holocene deglaciation of the North American ice sheet. In particular, the meltwater combined with the presence of the North American ice sheet affected the position of the NAO sea level pressure centers. As a result, the course of the westerlies and its associated spatial precipitation pattern were different during the Early Holocene compared to the Mid – Late Holocene. Furthermore, a new high resolution speleothem record from SW Morocco allowed to assess the regional impact of the NAO during the last millennium. Frequency analysis showed that the NAO, but also the Atlantic Multi-decadal Oscillation were recorded in the oxygen isotope composition of the speleothem. In SW Morocco the Saharan low plays a more important role than in NW Morocco. The Sr-isotope composition of speleothems can provide information on water residence time and changes in the source of the trace elements incorporated into speleothems. In the context of this project a method was set up to measure Sr isotopes in-situ at very high resolution with the Laser Ablation MC-ICPMS technique. This method is extremely fast compared to conventional solution based analyses of Sr-isotopes. However, cave monitoring data of trace element to Ca ratios in drip water that fed stalagmite GP5 indicated that both prior calcite precipitation as well as prior aragonite precipitation are identified. This complicates the interpretation of the U/Ca ratios in stalagmite GP5 because PCP is associated with an inverse relation between drip water U/Ca and drip rate, whereas PAP induces a positive correlation. Extreme dry and wet conditions affecting drip rate in opposite directions may thus result in similar drip water U/Ca ratios. Nevertheless, the U/Ca variability of aragonite speleothem GP2 is highly similar to that from aragonite speleothem GP5, indicating the dominance of one of the two processes over multi-centennial timescales during the Holocene. The project drew attention in the public media with two press releases: http://www.uni-mainz.de/presse/aktuell/320_ENG_HTML.php; http://aktuell.ruhr-uni-bochum.de/pm2016/pm00101.html.en
Publications
- 2016. Climate variability of central Asia on orbital to millennial timescales. Scientific Reports. 6
Cheng, H., Spötl, C., Breitenbach, S.F.M., Sinha, A., Wassenburg, J.A., Jochum, K.P., Scholz, D., Li, X., Yi, L., Peng, Y., Lv, Y., Zhang, P., Votintseva, A., Loginov, V., Ning, Y., Kathayat, G., Edwards, R.L.
(See online at https://doi.org/10.1038/srep36975) - 2016. Determination of aragonite trace element partition coefficients from speleothem calcite-aragonite transitions. Geochim. Cosmochim. Acta. 190, 347-367
Wassenburg, J.A., Scholz, D., Jochum, K.P., Cheng, H., Oster, J., Immenhauser, A., Richter, D.K., Häger, T., Hoffmann, D.L., Breitenbach, S.F.M.
(See online at https://doi.org/10.1016/j.gca.2016.06.036) - 2016. Intra- and inter-annual uranium concentration variability in a Belizean stalagmite controlled by prior aragonite precipitation: a new tool for reconstructing hydro-climate using aragonitic speleothems. Geochim. Cosmochim. Acta. 190, 332-346
Jamieson, R.A., Baldini, J.U.L., Brett, M.J., Taylor, J., Ridley, H.E., Ottley, C.J., Prufer, K.M., Wassenburg, J.A., Scholz, D., Breitenbach, S.F.M.
(See online at https://doi.org/10.1016/j.gca.2016.06.037) - 2016. Major reorganization of the North Atlantic Oscillation during Early Holocene deglaciation. Nat. Geosci. 9, 602-605
Wassenburg, J.A., Dietrich, S., Fietzke, J., Fohlmeister, J., Jochum, K.P., Scholz, D., Richter, D.K., Sabaoui, A., Spötl, C., Lohmann, G., Wei, W., Andreae, M.O., Immenhauser, A.
(See online at https://doi.org/10.1038/NGEO2767) - 2017. Speleothem record of decadal to multidecadal hydroclimate variations in southwestern Morocco during the last millennium. Earth Planet. Sci. Let. 476, 1 – 10
Ait Brahim, Y., Cheng, H., Sifeddine, A., Wassenburg, J.A., Da Cruz Junior, F.W., Khodri, M., Sha, L., Pérez-Zanón, N., Beraaouz, E.H., Apaéstegui, J., Guyot, J.-L., Jochum, K.P., Bouchaou, L.
(See online at https://doi.org/10.1016/j.epsl.2017.07.045) - 2017. Sr-isotope analysis of speleothems by LA-MC-ICP-MS: high temporal resolution and fast data acquisition. Chem. Geology 468, 63 – 74
Weber, M., Wassenburg, J.A., Jochum, K.P., Breitenbach, S.F.M., Oster, J., Scholz, D.
(See online at https://doi.org/10.1016/j.chemgeo.2017.08.012)
