Centennial-scale variability of past and water mass exchange across the eastern equatorial Atlantic basin
Final Report Abstract
The goal of this project was the determination of the linkages between high latitude climate changes and tropical water masses and climate during the past 156,000 years. A set of selected proxies (εNd, REE, element/calcium ratios, Mg/Ca, Ba/Ca, δ18O, δ13C) was applied to different authigenic phases (planktonic and benthic foraminifera, sediment coatings, detrital sediments) of core top sediments for a detailed calibration to the present day properties. For this purpose, two different cleaning methods were analyzed to obtain seawater neodymium isotope compositions from foraminiferal shells. Following reductive cleaning it is shown that foraminiferal carbonates can be used to reconstruct changes in bottom water mass mixing using Nd isotopes despite the influence of nearby rivers and preformed coatings. Bulks sediment leachates in the study area contain river signatures originating from preformed coatings from nearby rivers. Subsequently the proxies were applied to the high resolution sediment core MD03-2707 to reconstruct the evolution of riverine input into the Gulf of Guinea as a function of variations in monsoonal precipitation of the past 156,000 years. More radiogenic εNd values during the interglacials and less radiogenic values during glacial periods were caused by latitudinal migration of the rain belt and the ITCZ over the different catchment areas of the different rivers with their distinct isotopic signatures and changes in supply of the dissolved and particulate load of these rivers. The most southerly locations of the rainbelt were found during glacial stages 6 and 2 and detrital signatures lead precipitation maxima demonstrating influence by supply from the shelves during sea level lowstands. A multi-species Mg/Ca-temperature calibration for the Gulf of Guinea was developed, based on which changes in physical parameters, such as temperature and salinity, as well as changes of the sources of subsurface water masses (< 500 m) in the eastern tropical Atlantic were reconstructed at high time resolution (100 to 200 years sampling interval). A clear link between the tropical Atlantic Ocean and the northern and southern high latitude regions could be demonstrated. Based on the records of the oxygen isotope compositions of N. dutertrei and G. crassaformis the changes in the thermocline waters resemble the patterns of Greenland ice cores, whereas the SACW variability is comparable to the Antarctic ice core records. Between 60,000 and 32,000 years ago the records of the thermocline and the SACW follow similar trends suggesting enhanced influence of southern source waters and enhanced mixing processes as a result of upwelling probably caused by enhanced trade winds. Since 32,000 years ago the records of the foraminiferal species N. dutertrei and G. crassaformis have diverged. This was particularly pronounced during Heinrich Events, when the thermocline cooled and the SACW warmed. The salinity reconstructions have been anti-correlated implying a reduction of the influence of southern water masses.
Publications
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(2010) Influence of different cleaning methods on seawater εNd extracted from planktonic foraminifera, 10th International Conference on Paleoceanography, ICP 2010, San Diego, CA, USA, 29.08.-03.09.2010
Kraft, S., Hathorne, E., Frank, M., and Weldeab, S.
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(2011) Influence of different cleaning methods on seawater εNd extracted from planktonic foraminifera, Goldschmidt Conference 2011, Prague, Czech Republic, 14.08.- 19.08.2011
Kraft, S., Hathorne, E., Frank, M., and Weldeab, S.
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(2012) Past changes in riverine input and ocean circulation in the Gulf of Guinea, Goldschmidt Conference 2012, Montreal, Canada, 24.06.-29.06.2012
Kraft, S., Weldeab, S., Hathorne, E., and Frank, M.
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2013. The Late Quaternary evolution of subsurface water masses in the Gulf of Guinea: High-latitude versus monsoonal control. Dissertation, Christian-Albrechts-Universität, Kiel, Germany, 211 pp.
Kraft, S.,
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Assessment of seawater Nd isotope signatures extracted from foraminiferal shells and authigenic phases from Gulf of Guinea sediments, Geochimica et Cosmochimica Acta (2013)
Kraft, S., Frank, M., Hathorne, E. C. & Weldeab, S.
