Benthic forams /fish-teeth ( ) record -9.4 ε Nd

1. Reconstructing Mediterranean-Atlantic exchange: Can Nd isotopes tell us about the Messinian Salinity Crisis? Ruža F. Ivanović, Rachel Flecker, Marcus Gutjahr, Paul J. Valdes School of Geographical Sciences, University of Bristol, UK : Ruza.Ivanovic@bristol.ac.uk 1. INTRODUCTION 2. METHODS • Mediterranean water (38 g/l,) is denser than Atlantic water (36 g/l) [e.g. 12]. Nd isotopes are used to distinguish between these water masses. • Two way-flow through the gateway(s): • Occurs today & thought to be the normal state of exchange 8-5 Ma • Benthic forams[15] & fish-teeth[16] record the Med. water signal = -9.4 εNd • Flow from Atlantic to Mediterranean: • Thought to have occurred periodically during the MSC (Fig. 2) • Brings salt into the Med., but no salt is exported, resulting in salt precipitation • Benthic forams[15] & fish-teeth[16] record the Atlantic water signal = -11.8 εNd Nd is used to trace water masses [13-20] We aim: to understand what caused the Messinian Salinity Crisis. To do this, we use Nd isotopes to reconstruct the exchange of water between the Mediterranean Sea & Atlantic Ocean 8-5 Ma. Nd in water comes from weathered crust Distinct εNd per water mass -11.8 εNd ↓εNd = older crust -9.4 εNd Nd recorded in forams& fish-teeth Neodymium (Nd) = rare earth element Benthic forams/fish-teeth ( ) record -9.4 εNd -11.8 εNd CHUR = present day bulk earth[18] Measure εNd on mass spectrometer Physical, reductive & oxidative cleaning of sample[17] Benthic forams/fish-teeth ( ) record -11.8 εNd 3. RESULTS: Reconstructing bottom water flow direction 4. CONCLUSIONS • We present a reconstruction of bottom water flow-direction between the Mediterranean & Atlantic • Our new data tests the hypotheses proposed to explain the older sedimentary formations in the Messinian stratigraphy, as shown in Figure 2 • For the periods 7.7-7.1 Ma & 6.4-5.6 Ma bottom water flow was predominantly east to west (Med. Outflow) • The very radiogenic signal recorded by Zobzit samples from 7.20 Ma reflect the local tectonic and volcanic changes leading to closure of the Taza-Guercif basin • Following this methodology, we believe that a more definitive chronology of changes in Mediterranean-Atlantic water exchange can be constructed We compare our measured εNd (coloured circles & rectangles) to εNd estimated from the hypotheses shown in Fig. 2 (shaded grey). • This gap in the data spans the Mediterranean’s period of halite precipitation (when salinity reached ~ 380 g/l) and near freshening. • Samples have been obtained from the Loulja A & B sections to cover this. • Because of this gap in the data we cannot test Benson’s[11] Siphon Event hypothesis (Figure 2), where the εNd signal should be Atlantic. • To get this data, sample is needed from the Messadit section. • This is one of the focuses for future work and part of the purpose of fieldwork being undertaken in May 2011. Figure 1. During the MSC, Mediterranean-Atlantic water exchange occurred through the Betic (S. Spain) and Rifian (N. Morocco) Corridors[1-6]. Sample site locations are indicated. Future work will target current gaps in the data Hypotheses 5. FUTURE WORK • These data support the occurrence of small-scale outflow during gypsum deposition rather than total cessation of Mediterranean Outflow (Figure 2). • Establish whether or not shallow surface flow can be detected in planktic forams. • Determine the reliability of the results[22,23] by comparing elemental ratios (including Fe:Ca & Mn:Ca) • Complete the time-series of Rifian bottom water εNd, spanning 8-5 Ma, with improved temporal resolution & covering the current gaps in the time-series • Carry out fieldwork (N. Morocco) to collect the samples (e.g. from Messadit) needed for these further analyses Messinian Salinity Crisis extremely radiogenic εNd at Zobzit ~7.20 Ma Messinian Salinity Crisis • The extremely radiogenic εNd values measured in Zobzit samples from ~ 7.20 Ma coincide with a rapid shallowing of the Taza-Guercif basin, which preceded the ultimate closure of this part of the corridor[21]. • This shallowing has been attributed to tectonic activity (uplift) and falling glacio-eustatic sea levels[21] . • It could have isolated the basin from the sites of the other sections (e.g. OuedAkrech), or at least restricted their water exchange. • Localised volcanism associated with this tectonic activity & an increase in the influence of fluvial discharge could explain the extremely radiogenic εNd signal recorded in only the Zobzit samples ~ 7.20 Ma. • It could have been this event that initiated the Messinian Salinity Crisis21]. Acknowledgements This work was supported by the University of Bristol Centenary Scholarship, the Daniel Pidgeon Fund & the Phyllis Mary Morris Fund. We thank Dr. WoutKrijgsman & Dr. Fritz Hilgen (Universiteit Utrecht, Netherlands) for providing samples. All Mass-spectroscopic analyses were carried out in the Bristol Isotope Group’s laboratories, University of Bristol, UK. Figure 2. Composite core from sections in the Mediterranean basin. Changes in Mediterranean-Atlantic water exchange have been evoked to explain the extreme salinity fluctuations of the MSC. Our εNd reconstruction tests these. [1-11] References Hilgen, F.J. et al. (2000), EPSL,182, 237-251. van derLann, E. et al. (2006), Paleoceanography, 21, PA3011. Krijgsman, W. et al. (2004), Stratigraphy, 1 (1), 87-101. Carnevale, G et al. (2006), J. Geol. Soc., 163, 75-80. Roveri, M. et al., (2008) Terra Nova, 20, 483-488. Krijgsman, W. et al., (2008) Marine Geology, 400, 652-655. Flecker, R. & Ellam, R.M., 2006, Sed. 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