Mediterranean thermohaline circulation sensitivity: lessens from the past for future – MEDSENS

We apply various proxies to the archive collected along the zonal transect in the Mediterranean. For example, the change in the surface hydrological cycle is reconstructed using oxygen isotopic composition (d18O) of planktonic foraminifera and detrital fraction characterisation. The deep-water circulation condition is reconstructed by foraminiferal and leachate eNd values, benthic foraminiferal stable isotopes, benthic foraminiferal assemblage, redox-sensitive element concentration. Regional model experiments (1/8º) integrating proxies are performed using the NEMO/MED12 model. In the case of seawater eNd simulation, exchange with the margins, river inputs from atmospheric dust deposition and the internal cycle with vertical export associated with particle fall are represented.

Characterisation of the detrital fraction of the Sirte Gulf core indicates the change in sediment sources in relation to rainfall over Africa during the Holocene. The result reveals that the contribution of wadi-transported sediments from central Africa was later than the Nile input suggesting the amplifying role of the contribution of freshwater input by wadis rather than the trigger for stagnant circulation (Beny et al., in prep.).
The simulation of Nd in the NEMO/PISCES model has highlighted the essential role of atmospheric dust input for the eNd distribution of seawater in the present Mediterranean (Ayache et al., in prep.). This finding will be taken into account for the simulation of the past.
Characterisation of the Nd isotopic composition of foraminiferal samples dated to the last deglaciation and the Holocene from several central Mediterranean cores selected on either side of the Siculo-Tunisian Strait has provided new constraints on the mechanisms at the origin of the hydrological changes that may have induced the establishment of the S1 sapropel (10.2-6.4 cal ka BP) (Colin et al., 2021). The eNd variations obtained during the S1 period will be compared with the analyses underway by CEREGE and GEOPS on the S5 sapropel period.

We have started to obtain planed targeted proxy records, in particular reconstructed seawater Nd isotopic composition along the zonal transect in the Mediterranean Sea. We will focus on synthesis of our new data with previously published records to clarify spatiotemporal trends. Our special attention will be paid to (1) comparison of S1 and S5 circulation state based on proxy records and (2) comparison between the proxy reconstruction and numerical simulation.

Two multi-partner articles are published in the period of the intermediate report:
1. Colin, C., Duhamel, M., Siani, G., Dubois-Dauphin, Q., Ducassou, E., Liu, Z., Wu, J., Revel, M., Dapoigny, A., Douville, E., Taviani, M., Montagna, P. (2021). Changes in the intermediate water masses of the Mediterranean Sea during the last climatic cycle - New constraints from neodymium isotopes in foraminifera, Paleoceanography and Paleoclimatology, 36(4).
2. Montagna P., Colin C., Frank M., M., Störling, T., Tanhua, T., Rijkenberg, M., Taviani, M., Schroeder, K., Chiggiato, J., Gao G., Dapoigny A., Goldstein, S.L. (in press) Dissolved neodymium isotopes in the Mediterranean Sea, Geochimica et Cosmochimica Acta.
Two papers by MedSens post-docs are in prepation (Beny et al. and Ayache et al.).
Three presentations in international conferences were realised in the period of the intermediate report.
1. Tachikawa, K., Vidal, L., Pérez-Asensio, J. N., Garcia, M., Pratiwi, A. Schulz, H., 2020. Comparison of Holocene and Last Interglacial sapropels in the Gulf of Sirte (eastern Mediterranean). EGU General Assembly, Online 4–8 May 2020 ?10.5194/egusphere-egu2020-5212?. ?hal-03541574?
2. Tachikawa Kazuyo, François Beny, Marine Cornuault, Abel Guihou, Pierre Deschamps, et al. Neodymium Isotope Budget in the Mediterranean Sea Inferred From Core-top Sediments, Seawater Data Synthesis and Box Model Calculation. AGU fall meeting, Dec 2021, New Orleans and Online, United States. ?hal-03542944?
3. François Beny, Tachikawa Kazuyo, Laurence Vidal, Abel Guihou, Pierre Deschamps, et al.. Timing and source of excess freshwater inputs to the Eastern Mediterranean Sea at the origin of water stagnation and Sapropel S1 deposition. AGU fall meeting, Dec 2021, New Orleans and Online, United States. ?hal-03544783?

The Mediterranean (Med) region will be warmer and drier with enhanced seasonal contrast in the 21st century. Warming and reduced river discharge have antagonistic effects on the Med surface water density. The future circulation is estimated to be weaker although the degree of modification is model-dependant.
The objective of MedSens project is to evaluate the Med Sea circulation sensitivity to hydrological/thermal perturbation under warm and strong seasonality condition. To tackle this issue, we will combine (1) the reconstruction of Med Sea state during the past perturbation events of strong amplitude using a series of proxies with (2) numerical simulation based on highly-resolved regional (1/8º) proxy-enabled models that can simulate localized convection in the Med Sea. Our target is organic-rich layers called sapropel that were deposited in stagnant circulation state, in particular sapropel S5 formed during the last interglacial period, the penultimate warm period comparable with near future. This strategy is supported by the success of our previous works on the reconstruction of the Med circulation during the Holocene sapropel S1 with neodymium isotopic composition recorded in the authigenic phases as well as the development of regional modelling platform that simulate the past Med circulation. Major innovative and original aspects of the MedSens project are as follows. First of all, the combination of proxy reconstruction and proxy-enabled (neodymium isotopic composition and oxygen isotopic composition) regional modelling to study the past Med circulation is unique on the international scale. It will allow direct data-model comparison to evaluate physical processes affecting proxy variability and the performance of model that will be used for future projection. Secondly, we will apply multi-proxy approach (geochemistry and micropaleontology, including new potential proxies) to a series of sediment cores along a large zonal transect in the Med Sea. This approach will obtain robust reconstruction of climate variables by minimising the effects of proxy-specific bias. Thirdly, recently developed automated tool MiSo (Microfossil Sorter prototype, within “Foraminifera Image Recognition and Sorting Tool” first.cerege.fr, patent pending) will permit foraminiferal sample preparation and automated micropaleontology, yielding high-throughput and high resolution micropaleontological and geochemical records.
The MedSens project is based on consortium of partners with complementary expertise: proxy reconstruction (CEREGE/GEOAZUR and GEOPS) and numerical modelling (LSCE/LMD). The collaboration CEREGE-GEOPS-GEOAZUR is essential to obtain sufficient spatiotemporal coverage of records. We will provide information for variability of the eastern and western Med circulation states associated with S5 formation, the relationship between surface hydrological changes and intermediate/deep circulation, new proxy-enabled regional Med models that will be open to scientific community, S1 and S5 simulation results as well as new proxy data bases of water ventilation and surface hydrological changes during S1 and S5. These expected results will definitively enhance and clarify the various factors determining Med circulation sensitivity. We will also propose dissemination strategy of the project results toward a wide audience that is in line with the concept of education and training of young generations as well as the national plan for Open Archives.