Eastern Mediterranean water outflow during the Younger Dryas was twice that of the present day

The Mediterranean Sea's thermohaline circulation (Med-THC) is characterized by a negative net precipitation-evaporation balance. Inflowing Atlantic surface waters transform into saltier waters, sinking at the Levantine Sea as Levantine Intermediate Water (LIW). LIW contributes to deep water convection in the Adriatic and Aegean Seas, forming Eastern Mediterranean Deep Water (EMDW). EMDW and LIW outflow through the Strait of Sicily (Eastern Mediterranean Source Waters, EMSW), influencing Western Mediterranean Deep Water (WMDW) formation. Both WMDW and EMSW outflow into the Atlantic as Mediterranean Outflow Water (MOW). The Mediterranean and Atlantic are a coupled system, with changes in one impacting the other. Past climate changes likely modified MOW properties, influencing the Atlantic Meridional Overturning Circulation (AMOC). The Med-THC is highly sensitive to climate oscillations. Last deglacial sea level rise and increased Atlantic freshwater inflow caused surface water stratification, weakening deep water convection at the Gulf of Lions. This stratification also contributed to poorly-oxygenated conditions and the formation of an organic-rich layer (15–8.9 kyr BP). This layer coincided with the African Humid Period (AHP), driven by intensified African Monsoon and increased runoff into the Eastern Mediterranean. Freshening of surface waters likely weakened convection prior to Sapropel formation. The Younger Dryas (YD; 12.95–11.65 kyr BP) involved global precipitation pattern changes. Weakened monsoon precipitation in North Africa decreased freshwater supply to the Mediterranean, while cold and arid conditions prevailed in the Eastern Mediterranean. Some studies suggest that these cold/arid conditions may have favored partial reactivation of deep water convection. The maximum reduction in Eastern Mediterranean convection occurred during the early Holocene, with the deposition of the last Sapropel (S1; 10.8–6.1 kyr BP). S1 formation is attributed to strong surface stratification from increased freshwater influx and increased export productivity. The impact of S1 on water exchange between the Eastern and Western Mediterranean is debated. This study aimed to provide quantitative estimates of Eastern-Western water exchange since the last deglaciation, focusing on the YD and S1 events. The study will utilize neodymium isotope (εNd) measurements on planktic foraminifera Fe-Mn crusts as a tracer of past changes in Med-THC.

Numerous studies have explored the relationship between the Mediterranean Sea and the Atlantic Ocean through the Strait of Gibraltar. Research suggests that past climate changes significantly influenced the Mediterranean thermohaline circulation (Med-THC) and, consequently, the properties of Mediterranean Outflow Water (MOW). Several proxies indicate the high sensitivity of Med-THC intensity to past climate oscillations at various timescales. The African Humid Period (AHP) and the Younger Dryas (YD) periods have been particularly highlighted in this research area, with studies focusing on the changes in freshwater fluxes, surface water stratification, and the resulting effects on deep water convection and the formation of sapropels. However, a quantitative evaluation of the changes in eastern-western water exchange through the Strait of Sicily during these periods was lacking before this research. Previous work focused on qualitative assessments or utilized limited proxies. This study bridges the gap by presenting a comprehensive quantitative analysis of water exchange, encompassing the YD and S1 events.

This study uses a continuous record of neodymium isotope (εNd) measurements from planktic foraminifera Fe-Mn crusts in gravity core NDT-6-2016, collected at the transition area between the W-Sicily channel and the southern Tyrrhenian Sea at 1066 m water depth. The core location is situated below the interface between WMDW and EMSW outflowing through the Strait of Sicily, making it suitable for evaluating changes in intermediate-deep water exchange. Results are complemented by other published εNd records from different Mediterranean regions. Present-day E-Med seawater εNd values are higher than W-Med values, establishing the basis for using Nd isotopes as a tracer of Med-THC changes. A three-end-member isotopic mixing model is used to estimate the mixing proportions between EMSW and WMDW during the late deglaciation and Holocene. The model incorporates εNd data from the W-Sicily record and previously published records for WMDW, EMDW, and LIW. To account for uncertainties, a Monte-Carlo approach is implemented, considering analytical errors and various scenarios of LIW and EMDW mixing proportions in EMSW. The study also conducted additional εNd analysis on the detrital fraction (<63 µm) to ensure that the foraminiferal εNd values accurately reflect changes in local hydrography, excluding external contributions of Nd from terrigenous sediments or atmospheric dust. The radiocarbon dating and age modeling were performed using 20 ¹⁴C dates analyzed in planktic foraminifera samples, calibrated using the MARINE20 calibration curve, along with three tie points based on an adjusted alignment with the NGRIP isotope record. The Bayesian software Bacon was used to construct the age model. Detailed procedures for neodymium isotope measurements, including sample processing, purification, and mass spectrometry analysis, are described in the Materials and Methods section. The accuracy of the method was validated using the AMES II standard.

The εNd record from the W-Sicily core exhibits significant changes during the late deglaciation and Holocene. Highest εNd values are recorded during the Younger Dryas (YD), while the lowest values are predominant during the S1 interval. Pre-S1 and post-S1 periods show intermediate values. Analysis of the detrital fraction (<63 µm) confirmed that the measured foraminiferal εNd values reflect changes in local hydrography without significant external Nd contributions. The remarkably high εNd values during the YD are interpreted as the result of enhanced EMSW contribution through the Strait of Sicily, and/or a shift towards a more radiogenic composition in the EMSW end-members (EMDW and LIW). A three-end-member isotopic mixing model, incorporating various scenarios for LIW/EMDW proportions, estimated EMSW percentages. The model indicates a significantly larger proportion of EMSW reaching the W-Sicily during the YD compared to pre- and post-YD periods. These findings are consistent with previous results suggesting partial re-ventilation of the E-Med basin during the YD. The high εNd values during the YD concur with maximum values in sediment grain-size from the eastern flank of Corsica and the Alboran Sea, suggesting increased current velocities at intermediate depths in the W-Med and a connection between EMSW flow and MOW. Enhanced YD EMSW outflow appears linked to re-oxygenation at intermediate depths in the Alboran Sea and intensification of MOW currents. In contrast to the high εNd values during the YD, the S1 period shows relatively low εNd values, indicating a weaker outflow of EMSW into W-Sicily. Mixing model estimates support a substantial reduction of EMSW outflow during S1, approximately half of the pre- and post-S1 estimates. The end of the S1 period is marked by a transition in εNd values, indicating an increase in EMSW contribution. This increase, around 7.5-7 kyr BP, coincides with an earlier convection enhancement in the Adriatic and Aegean Seas and the end of stagnant conditions at intermediate depths in the E-Med. The observed evolution of EMSW flow agrees with changes in MOW current intensities at the Gulf of Cadiz.

The findings demonstrate a strong link between Eastern Mediterranean deep-water convection and the intensity of Mediterranean Outflow Water (MOW). The enhanced EMSW outflow during the Younger Dryas (YD) resulted from a combination of intensified convection in the Aegean and Levantine basins due to regional aridification and weaker Western Mediterranean Deep Water (WMDW) formation. This led to a deeper expansion of EMSW into the Western Mediterranean, influencing intermediate-depth currents and potentially impacting the Atlantic Meridional Overturning Circulation (AMOC). Conversely, the reduced EMSW outflow during the last Sapropel (S1) period highlights the role of increased freshwater influx and strong surface stratification in suppressing deep convection and reducing water exchange between the Eastern and Western Mediterranean basins. The observed re-establishment of EMSW outflow towards the end of S1 underscores the complex interplay between climate forcing, freshwater input, and thermohaline circulation dynamics in the Mediterranean Sea. This study provides robust quantitative evidence for the significant impact of Eastern Mediterranean deep-water formation on the larger-scale Atlantic circulation.

This study presents a quantitative assessment of Eastern Mediterranean Source Water (EMSW) outflow into the Western Mediterranean during the last deglaciation and Holocene, using neodymium isotopes as a tracer. The results highlight the significant increase in EMSW outflow during the Younger Dryas and the substantial decrease during the last Sapropel. This dynamic interplay between deep-water convection in the Eastern Mediterranean and the strength of the Mediterranean Outflow Water (MOW) is crucial for understanding regional and global climate dynamics. Future research could investigate the precise mechanisms driving the changes in deep-water convection within the Eastern Mediterranean and further explore the feedback mechanisms between the Mediterranean and Atlantic systems. High-resolution studies across multiple locations within the Mediterranean are needed to fully resolve the spatial and temporal complexity of these oceanographic changes.

While this study provides robust quantitative evidence, some limitations exist. The three-end-member mixing model relies on the accuracy of previously published Nd isotope records and assumptions about past seawater Nd concentrations. Although the study addressed potential diagenetic effects by analyzing the detrital fraction, it cannot entirely rule out the influence of rare diagenetic processes. The temporal resolution of the εNd record may also limit the precise identification of short-lived events. Future research incorporating additional proxies and improved temporal resolution would further refine our understanding of the complex interplay of factors influencing EMSW outflow.