Transfer-function approach to temperature and salinity reconstructions in the Red Sea: Constraining sea-level estimates and monitoring ITCZ variability during the last four glacial cycles
Final Report Abstract
The Red Sea represents a unique marine environment, characterized by the narrow shape of the basin, its land-locked position, extremely dry climate and a limited connection with the Indian Ocean trough the narrow Strait of Bab-al-Mandab in the south. These factors combined make the Red Sea extremely sensitive to changes in global sea-level and regional climate. When sea-level rises, the Red Sea connection with the Indian Ocean becomes deeper and the salinity in the basin decreases. Thus, indicators of past salinity preserved in Red Sea sediments can be used to reconstruct past sea-level changes. The exchange of water between the Indian Ocean and the Red Sea also depends on the strength of Monsoon winds. Therefore, at times of stable high sea level, paleoenvironmental indicators in Red Sea sediments can be used to reconstruct changes in the Monsoon. This proposal was based on these premises. Its aim was to develop new methods for extraction of paleoenvironmental information from Red Sea sediments and to apply these methods in the fossil record to make it possible to produce better sea-level reconstructions and to determine how sensitively do winds and ocean currents in the Red Sea react to global change. To this end, we investigated the abundances of certain microfossils (shells of planktonic foraminifera) in Red Sea sediments and found out that their species composition can be used to reconstruct the productivity of the surface ocean. In the Red Sea, productivity is tightly linked with the speed and direction of ocean currents and the amount of water exchange with the Indian Ocean. The microfossils thus provide a means to reconstruct past circulation and determine the strength and direction of the winds that have driven these surface currents. In addition, we have measured the abundance in the sediment of certain organic chemicals, which are produced by planktonic microbes proportionately to water temperature. To our surprise, we found that the Red Sea microbes produced these chemicals in a proportion that has never been observed elsewhere. This unique chemical signature could be used not only to reconstruct past temperatures, but also to measure the amount of water exchange with the Indian Ocean. The inflowing water carries the global signature, which is gradually towards the north mixed with the unique Red Sea signal. We used a numerical model of Red Sea circulation to simulate this process and found out that the water exchange signal based on the microbial chemicals is consistent with the microfossil data. Subsequently, we have generated data from warm intervals in several sediment cores in the Red Sea, covering the current warm period and two other older warm intervals. In collaboration with colleagues from the UK (NOC Southampton), the same cores have been used to generate the longest existing continuous sea-level record, spanning the last 530,000 years. This record has been used to determine how quickly sea level rose above the present level during the last interglacial. The value obtained from our data was 1.6 m/100 years, which is almost twice as high as the estimates projected by climate models for the next century. Further, we found out that past sea-level has been strongly correlated with global temperature and carbon dioxide and that the sea level corresponding to the present-day carbon dioxide concentration lies more than 20 m higher than at present. These findings have been widely publicized in international media and are being used by policy makers in debates on impacts of future climate change. Finally, we used the abundances of microfossil species and microbial chemicals in the sediment to show that the environment of the Red Sea during each of the past warm intervals has followed the evolution of the Monsoon, which reflects the seasonal amount of sunlight following changes in Earth’s orbit around the sun. During the last 10,000 years, the Monsoon trend was interrupted by a distinct event around 2,400 years ago, which we were able to interpret as a reversal of the dominant wind pattern. We showed that this event corresponded to significant draughts recorded in numerous climatic archives in the Middle East and North Africa. The present-day Red Sea circulation pattern was found to be remarkably young, originating only 1,700 years ago. Taken together, our results will help predict how the Red Sea environment may react to the projected future climate change and the methods we have develop can be used to produce improved climatic reconstructions in the region and beyond.
Publications
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2008. High rates of interglacial sea-level rise. Nature Geoscience, 1: 38-42
Rohling, E.J., Grant, K., Hemleben, Ch., Siddall, M., Hoogakker, B.A.A., Bolshaw, M., Kucera, M.
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2008. New constraints on the timing of sea level fluctuations during early to middle marine isotope stage 3. Paleoceanography, 23(3), PA3219
Rohling, E. J., K. Grant, C. Hemleben, M. Kucera, A. P. Roberts, I. Schmelzer, H. Schulz, M. Siccha, M. Siddall, and G. Trommer
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2009. Antarctic temperature and global sea level closely coupled over the last five glacial cycles. Nature Geoscience, 2(7): 500-504
Rohling, E.J., Grant, K., Bolshaw, M., Roberts, A.P., Siddall, M., Hemleben, Ch., Kucera, M.
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2009. Distribution of Crenarchaeota tetraether membrane lipids in surface sediments from the Red Sea. Organic Geochemistry, 40, 724-731
Trommer, G., M. Siccha, M.T.J. van der Meer, S. Schouten, J.S. Sinninghe Damsté, H. Schulz, Ch. Hemleben, M. Kucera
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2009. Factors controlling the distribution of planktonic foraminifera in the Red Sea and implications for paleoenvironmental reconstructions by transfer functions. Marine Micropaleontology, 72, 146-156
Siccha, M., G. Trommer, H. Schulz, Ch. Hemleben, M. Kucera
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2010. Comparison between Holocene and Marine Isotope Stage-11 sea-level histories. Earth and Planetary Science Letters, 291: 97–105
Rohling, E.J., Braun, K., Grant, K., Kucera, M., Roberts, A.P., Siddall, M., Trommer, G.
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2010. Millennial scale variability in Red Sea circulation in response to Holocene insolation forcing. Paleoceanography, 25, PA3203
Trommer, G., Siccha, M., Rohling, E.J., Grant, K., van der Meer, M.T.J., Schouten, S., Hemleben, Ch., Kucera, M.
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2010. Sensitivity of Red Sea circulation to monsoonal variability during the Holocene: An integrated data and modeling study. Paleoceanography, 25, PA4209
Biton, E., Gildor, H., Trommer, G., Siccha, M., Kucera, M., van der Meer, M.T.J., Schouten, S.
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2011. Sensitivity of Red Sea circulation to sea level and insolation forcing during the last interglacial. Climate of the Past Discussions, 7, 1195-1233
Trommer, G., Siccha, M., Rohling, E. J., Grant, K., van der Meer, M. T. J., Schouten, S., Baranowski, U., Kucera, M.
