Periodic Reporting for period 4 - TIMED (Testing the role of Mediterranean thermohaline circulation as a sensor of transient climate events and shaker of North Atlantic Circulation)
Reporting period: 2021-07-01 to 2023-06-30
The target of TIMED was to assets the sensitivity of the Mediterranean thermohaline circulation (Med-ThC) to past climate variability and its ultimately impact into the North Atlantic Ocean. The understanding of the natural range of variability and major forces controlling this system, is of particularly relevance now, when we are facing the threat of climate change. The available future projections for the end of this century indicate an overall weakening of the Med-ThC but, in the absence of direct measurements long enough to assess any long term trend on this system, the past record becomes the only source to prove the sensitivity of this system to climate change. TIMED project aimed to cover this information gap in base to the study of a wide array of sediment cores from the Eastern and Western Mediterranean Sea focusing in some key events of the last deglacial and Holocene periods. The big value of the used approach relies in the combined application of several tools that allow the parallel reconstruct of key environmental variables: oxygen content, reservoir ages, current speeds, mixing rates of water masses, physical properties (temperature and salinity) and hydrological changes on land (among others). This exercise has allowed to identify how changes have evolve between different Mediterranean sub-basins and at different depths, thus gaining an overall view of the interconnections between the different overturning cells that control Med ThC.
Overall, the project results have provided new insights on the natural range of variability of the thermohaline circulation of the Mediterranean Sea during past climate conditions and the possibility to infer which were the main controlling factors of this variability.
The most unexpected outcome has been the estimated enhancement in the outflow volume of the E Med waters during the YD based on Nd isotopes analyses. Despite previous evidence suggested enhanced energy in intermediate waters of the W-Med, none of them had proved so far for such as intensification of the E-Med ThC. However, TIMED results are indeed very consistent with the extreme arid conditions described for this period, particularly around the Aegean Sea.
Another breakthrough of TIMED has been to identify the presence of old carbon waters that would have been accumulated in the Med during water stagnation periods. Such old water masses were expected to appear at the end of S1. However, TIMED results interestingly indicate that the major anomaly occurred actually during the YD. This is consistent with previous TIMED findings on the reactivation of most of the Med-ThC system during the YD that pumped out the accumulated old waters. Surprisingly, the detected age anomaly reveals that the precedent stagnation to the YD had to be larger than previous expectations.
The healthy development followed by the disappearance of cold-water coral ecosystems in the Med has been largely described for the deglacial and Holocene periods, and many environmental factors have been proposed and discussed as the controlling factors. TIMED has used these ecosystems to produce accurate reservoir ages, but, during this process, TIMED has also identified an additional controlling factor that had not been previously considered in the region. The appearance of old water masses enriched in respired carbon might involve an acidification of the environment preventing the correct development or even the preservation of the cold-water corals.
TIMED has also identified, for the first time, the development of a very intense oxygen minimum zone at intermediate depths of the W-Med due to deglacial melting and sea level rise. These results were totally unexpected since this oxygen minimum was previously attributed to deep waters. This new perspective involves a re-thinking on the source and mechanisms behind this weakening in the Med-ThC. TIMED has also been able to accurately date the onset of the W-Med postglacial re-oxygenation at several water depths, proving that the major E-Med stagnation developed when a rapid re-oxygenation occurred in the W-Med, and thus supporting the existence of a see-saw operation mode between the two Med basins.
One of the TIMED innovative strategies has been the use of monthly resolved series of sediment traps samples available. This approach was applied by TIMED to validate the application of the grain size parameter UP10 as an indicator of changes in deep current intensity associated to the formation of W-Med deep water in the Gulf of Lions. The results gave solid bases to interpret a new produced UP10 record in terms of changes in the intensity of deep convection in the W-Med during the last two millennia, indicating that the Roman and the late Medieval periods together with the early Little Ice Age recorded the strongest intensity in the formation of Western Mediterranean Deep Water Masses. These results have revealed that strong deep convection occurred during climatic periods of relatively warm but also cold conditions, pointing out to the evaporation-precipitation valance of the whole basin as a key factor controlling deep convection intensity. TIMED results have also proved that the Roman period show overall warm temperatures in the whole Mediterraneean Sea, the maximum pre-industrial temperatures of the las 2kyr, and has hypothesized the relation between the dominant climate conditions and the Roman empire history.
TIMED has also perform a high effort setting up an innovative optimized laboratory protocol for the measurement of radiogenic isotopes in marine sediments. This technique has been then applied for the first time in the study of sediment trap samples to detect the effect of meteorological conditions on the radiogenic isotopic signal of the sediments transported towards the Mediterranean. This novel approach has provided a first quantification of the Saharan dust supply to this region demonstrating this to be a minor contributor. On the other hand, a rain regime over south Iberia has been recognized as a determinant controller in the Sr isotopic signal of the sediment supply towards the Alboran Sea. This information is key for the interpretation of this proxy along the Holocene period.
The most unexpected outcome has been the estimated enhancement in the outflow volume of the E Med waters during the YD based on Nd isotopes analyses. Despite previous evidence suggested enhanced energy in intermediate waters of the W-Med, none of them had proved so far for such as intensification of the E-Med ThC. However, TIMED results are indeed very consistent with the extreme arid conditions described for this period, particularly around the Aegean Sea.
Another breakthrough of TIMED has been to identify the presence of old carbon waters that would have been accumulated in the Med during water stagnation periods. Such old water masses were expected to appear at the end of S1. However, TIMED results interestingly indicate that the major anomaly occurred actually during the YD. This is consistent with previous TIMED findings on the reactivation of most of the Med-ThC system during the YD that pumped out the accumulated old waters. Surprisingly, the detected age anomaly reveals that the precedent stagnation to the YD had to be larger than previous expectations.
The healthy development followed by the disappearance of cold-water coral ecosystems in the Med has been largely described for the deglacial and Holocene periods, and many environmental factors have been proposed and discussed as the controlling factors. TIMED has used these ecosystems to produce accurate reservoir ages, but, during this process, TIMED has also identified an additional controlling factor that had not been previously considered in the region. The appearance of old water masses enriched in respired carbon might involve an acidification of the environment preventing the correct development or even the preservation of the cold-water corals.
TIMED has also identified, for the first time, the development of a very intense oxygen minimum zone at intermediate depths of the W-Med due to deglacial melting and sea level rise. These results were totally unexpected since this oxygen minimum was previously attributed to deep waters. This new perspective involves a re-thinking on the source and mechanisms behind this weakening in the Med-ThC. TIMED has also been able to accurately date the onset of the W-Med postglacial re-oxygenation at several water depths, proving that the major E-Med stagnation developed when a rapid re-oxygenation occurred in the W-Med, and thus supporting the existence of a see-saw operation mode between the two Med basins.
One of the TIMED innovative strategies has been the use of monthly resolved series of sediment traps samples available. This approach was applied by TIMED to validate the application of the grain size parameter UP10 as an indicator of changes in deep current intensity associated to the formation of W-Med deep water in the Gulf of Lions. The results gave solid bases to interpret a new produced UP10 record in terms of changes in the intensity of deep convection in the W-Med during the last two millennia, indicating that the Roman and the late Medieval periods together with the early Little Ice Age recorded the strongest intensity in the formation of Western Mediterranean Deep Water Masses. These results have revealed that strong deep convection occurred during climatic periods of relatively warm but also cold conditions, pointing out to the evaporation-precipitation valance of the whole basin as a key factor controlling deep convection intensity. TIMED results have also proved that the Roman period show overall warm temperatures in the whole Mediterraneean Sea, the maximum pre-industrial temperatures of the las 2kyr, and has hypothesized the relation between the dominant climate conditions and the Roman empire history.
TIMED has also perform a high effort setting up an innovative optimized laboratory protocol for the measurement of radiogenic isotopes in marine sediments. This technique has been then applied for the first time in the study of sediment trap samples to detect the effect of meteorological conditions on the radiogenic isotopic signal of the sediments transported towards the Mediterranean. This novel approach has provided a first quantification of the Saharan dust supply to this region demonstrating this to be a minor contributor. On the other hand, a rain regime over south Iberia has been recognized as a determinant controller in the Sr isotopic signal of the sediment supply towards the Alboran Sea. This information is key for the interpretation of this proxy along the Holocene period.
TIMED has reached its end of the funded period and several publications, PhD thesis and Conference presentations have already disseminated its results. But still many publications are to come, some already submitted, other almost submitted and others in preparation. Therefore, it is expected that during the coming year/s many new results from TIMED will still appear and bring new lights on the Med ThC. In addition, TIMED has two daughter projects already funded by Spanish Research Agency (TRANSMOW and MORIA) that are going to bring new information on the current and past paths of the Mediterranean waters into the Atlantic Ocean, an objective that TIMED could not fully achieve.