Combined application of new proxies (231Pa/230Th and Nd isotopes) to reconstruct the deep ocean overturning over the last 30,000 years.
Final Report Abstract
The Atlantic Ocean holds a key role in the climatic system of our planet by controlling the transport of global heat and CO2. Past variations in the Atlantic Meridional Overturning Circulation (AMOC) have had a profound impact on the global climate system, as indicated by paleoclimate records. Therefore, assessing the variability of ocean circulation is a major challenge for paleoceanography and paleoclimatology in order to provide possible scenarios of future climate changes. Regarding the recent stability of the Atlantic circulation it is of high interest to reconstruct the strength of the North Atlantic Deep Water Export in past major climatic periods such during the Last Glacial Maximum (LGM, 19-22 thousand years before present) and the following Deglaciation. The AMOC is tightly coupled to climate on glacial–interglacial and millennial timescales, but has proved difficult to reconstruct, particularly for the Last Glacial Maximum. In this project we reconstructed strength and geometry of the AMOC for the last 25,000 years by the extensive application of the 231Pa/230Th circulation proxy and the measurement of Nd-isotopes as a proxy for water mass provenance. One main goal of this project was the combined application of these tracers for the first time on the same sediment samples improving the understanding of both isotopic systems as well as our knowledge about the past circulation. The ability of 231Pa/230Th in terms of recording past AMOC has been challenged frequently, due to the influence of biogenic opal producing phytoplankton, which preferentially removes 231Pa. However, in the first phase of the project we were able to show, that the influence of opal is subordinate. Further, we found a strong correlation of sedimentary 231Pa/230Th with water depth, which is a strong indicator of the influence of AMOC. Accordingly, we concluded, that 231Pa/230Th is not only a function of AMOC, but as well of location and water mass distribution. This explains why 231Pa/230Th from adjacent locations, but different water depths, may show very different values. The most exciting result we gained by applying all these findings to observations from the LGM, which we compared to a large number of hypothetical 231Pa/230Th distributions generated by a boxmodel as a function of AMOC geometry and strength. As expected we found, that the AMOC during the LGM was shallower, however, we also found that it was at least as strong as today (or even stronger). This surprising outcome were published in Nature Geosciene as it may have far reaching implications on the long-term interactions of atmosphere, cryosphere and hydrosphere and climate modelling. Stronger AMOC implies more heat transport to the north, but such a straightforward connection is not supportable anymore. Instead wind stress and stratification changes must have played a more significant role as well as the shift of deep water formation areas. The paper also received some attention in the public media and was the topic in a live-TV interview of the “Hessischer Rundfunk”. Our findings from Nd-isotopes imply a higher variability of deep water mass provenance all along the examined time periods. Especially the situation before the LGM was more similar to today’s situation rather than comparable to the LGM as expected. Obviously the today’s mode of deep circulation in the Atlantic Ocean prevailed during most of the last Glacial interrupted only during the LGM pointing to an influence of ocean circulation on triggering short but extreme cold events (so-called Heinrich Events). We also have observed a very tight synchrony in 231Pa/230Th with Nd-isotopes in our combined measurements documenting the bipolar see-saw between deep water circulation driven from the North or the South. With this project we demonstrated the application of a paleoceanographic tool box, which is now ready to be extended to further high interest time periods of the climatic past.
Publications
- Early arrival of Southern Source Water in the deep North Atlantic prior to Heinrich event 2, Paleoceanography, 26 (2011)
Gutjahr, M. and J. Lippold
- Testing the 231Pa/230Th paleocirculation proxy - A data versus 2D model comparison, Geophysical Research Letters, 38 (2011)
Lippold, J., J. Gherardi and Y. Luo
- Boundary scavenging at the east Atlantic margin does not negate use of Pa/Th to trace Atlantic overturning, Earth and Planetary Science Letters, 333–334, (2012)
Lippold, J., S. Mulitza, G. Mollenhauer, D. Heslop, S. Weyer and M. Christl
(See online at https://doi.org/10.1016/j.epsl.2012.04.005) - Sahel megadrought during Heinrich Stadial 1: Evidence for a three-phase evolution of the low- and mid-level West African wind system, Quaternary Science Reviews, 58 (66 − 76), (2012)
Bouimetarhan, I., M. Prange, E. Schefuß, L. Dupont, J. Lippold, S. Mulitza, K. Zonneveld
(See online at https://doi.org/10.1016/j.quascirev.2012.10.015) - Strength and Geometry of the glacial Atlantic Meridional Overturning Circulation, Nature Geoscience (2012)
Lippold, J., Y. Luo, R. Francois, S. Allen, J. Gherardi, S. Pichat, B. Hickey and H. Schulz
(See online at https://doi.org/10.1038/NGEO1608) - Towards single-foraminifera-dating with a gas ion source, Nuclear Instruments and Methods in Physics Research B, 294, 307-310, (2013)
Wacker, L., J. Lippold, M. Molnár, H. Schulz
