Enhanced Mediterranean Sea circulation associated with a past weaker circulation of the North Atlantic ocean

Thirteen thousand years ago, during a cold interval known as Younger Dryas, an outflow of eastern Mediterranean waters twice than the present-day exited through the Strait of Sicily, likely promoting intensified high-salinity Mediterranean water-outflow into the Atlantic Ocean when its circulation was weak.
Published in Earth & Environment
Enhanced Mediterranean Sea circulation associated with a past weaker circulation of the North Atlantic ocean
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Oceanography of the Mediterranean Sea  

The Mediterranean Sea and the Atlantic Ocean behave as a coupled system where Atlantic surface waters enter through the Strait of Gibraltar and progressively transform into saltier surface waters, that eventually sink to intermediate depths at the Levantine Sea as Levantine Intermediate Water (LIW). The LIW contributes to the formation of deep waters in the Adriatic Sea and the Aegean Sea, the so-called Eastern Mediterranean Deep Water (EMDW). Both EMDW and LIW outflow through the Strait of Sicily, hereafter called Eastern Mediterranean Source Waters (EMSW), contributing also to the convection of deep waters at the Gulf of Lions to produce Western Mediterranean Deep Water (WMDW). Both WMDW and EMSW eventually outflow into the Atlantic Ocean through the Strait of Gibraltar forming the so-called Mediterranean Outflow Water (MOW) and thus, closing the Mediterranean circulation system 1–3 (Fig. 1). 

Fig. 1. Map of the study area in the Mediterranean Sea. Modern surface and deep hydrology of western and eastern Mediterranean Sea represented in the map is based on 1–3. Map of the Mediterranean Sea obtained from Ocean Data View. The green star represents the studied core NDT-6-2016. 


Paleoceanography of the Mediterranean Sea

In this work we investigate changes in the water-outflow from the Eastern Mediterranean Sea since the last deglaciation, that correspond to the last fifteen thousand years. During this time, it has been well documented a major hydrological perturbation in the Eastern Mediterranean Sea related to an increase in the north African rivers system runoff, as a result of intensified African Monsoon during the so-called African Humid Period (AHP; from ~15 to 6 kyr Before Present). Many proxy evidences indicate that enhanced fresh-water runoff into the E-Med caused significant surface water stratification, which resulted in the collapse of the E-Med deep/intermediate convection and with the deposition of an organic-rich sediment layer, the so-called last sapropel or S1 4–7


Neodymium isotopes as indicators of past Mediterranean Sea circulation

To investigate changes in the water-outflow from the Eastern Mediterranean Sea we used neodymium isotope ratios. This proxy allows to discriminate well between the eastern and the western originated waters masses, since εNd seawater values from the E-Med are typically higher than W-Med εNd 8-12 (Fig. 2). 

Fig. 2. Map of the seawater neodymium distribution in the Mediterranean Sea based on previous acquired data10,12,13,14. The green star represents the studied core NDT-6-2016. 


Changes in the past Mediterranean Sea circulation

Our results show enhanced EMSW outflow during the Younger Dryas, more than twice than the present-day. Arguably, this is the result to the combined effect of enhanced convection in the Aegean and Levantine basins in response to a regional aridification phase and weaker western Mediterranean deep water formation that led to a deeper expansion of the EMSW into the western basin. During the S1 a substantial reduction of the EMSW outflow through the Strait of Sicily occurred, being four times lower than during the YD and half than at present-day (Fig. 3). 

Fig. 3. Panel on the left) Percentages of Eastern Mediterranean Source Water (EMSW) expressed as % contribution. Shading represents the range of uncertainty in the reconstruction (95%), based on analytical errors. Grey-blue bar represents the YD (Younger Dryas) and light-brown bar the S1 (Sapropel 1). Panel on the right) Average values of EMSW export in the studied location for each period discussed in this work.

The described evolution of EMSW flow at W-Sicily, vigorous during the Younger Dryas and weak during the Sapropel 1, agrees well with changes in MOW current intensities at the Gulf of Cadiz 6. These results highlight the relevance of the E-Med intermediate/deep water convection in controlling the intensity of MOW (Fig. 4).

Fig. 4. Comparison between NDT-6-2016 records of Nd isotopes in planktic foraminifera coatings (this study, orange) and records of Zr/Al from Bahr et al., 2015 (blue). Grey-blue bar represents the YD (Younger Dryas) and light-brown bar the S1 (Sapropel 1).


References

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  3. Lascaratos, A., Williams, R. G. & Tragou, E. A mixed-layer study of the formation of Levantine Intermediate Water. J. Geophys. Res. 98, (1993).
  4. Toucanne, S. et al. Tracking rainfall in the northern Mediterranean borderlands during sapropel deposition. Quat. Sci. Rev. 129, 178–195 (2015).
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  7. Wu, J. et al. North-African paleodrainage discharges to the central Mediterranean during the last 18,000 years: A multiproxy characterization. Quat. Sci. Rev. 163, 95–113 (2017).
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  9. Ayache, M. et al. High-resolution neodymium characterization along the Mediterranean margins and modelling of Nd distribution in the Mediterranean basins. Biogeosciences 13, 5259–5276 (2016).
  10. Tachikawa, K. et al. Neodymium isotopes in the Mediterranean Sea: Comparison between seawater and sediment signals. Geochim. Cosmochim. Acta 68, 3095–3106 (2004).
  11. Henry, F., Jeandel, C., Dupré, B. & Minster, J. F. Particulate and dissolved Nd in the western Mediterranean Sea: Sources, fate and budget. Mar. Chem. 45, 283–305 (1994).
  12. Garcia-Solsona, E. et al. Rare earth elements and Nd isotopes as tracers of modern ocean circulation in the central Mediterranean Sea. Prog. Oceanogr. 185, 102340 (2020).
  13. Vance, D. et al. The use of foraminifera as a record of the past neodymium isotope composition of seawater. Paleoceanography 19, 1–17 (2004).
  14. Garcia-Solsona, E. & Jeandel, C. Balancing Rare Earth Element distributions in the Northwestern Mediterranean Sea. Chem. Geol. 532, (2020).

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