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Testing the role of Mediterranean thermohaline circulation as a sensor of transient climate events and shaker of North Atlantic Circulation

Periodic Reporting for period 3 - TIMED (Testing the role of Mediterranean thermohaline circulation as a sensor of transient climate events and shaker of North Atlantic Circulation)

Reporting period: 2020-01-01 to 2021-06-30

TIMED project aims to assets the sensitivity of the Mediterranean thermohaline circulation (MTHC) system to past climate variability and its ultimately impact into the North Atlantic Ocean. TIMED focuses in particular past climate transitions triggered by very different forcings to identify their impact in the individual Mediterranean convection cells. The purpose of this approach is to better understand the interplay between these different cells and characterise its relative contribution to the exporting waters into the Atlantic Ocean, which ultimately impacts the global climate system. Understanding the natural range of variability of this oceanographic system, and the major forces controlling it become critical for our society under the current situation of climate change. The available future predictions for the end of this century indicate an overall weakening of the MTHC, but, in the absence of direct measurements to assess the evolution of this system and the doubts on the role of different climate forcings, it becomes critical the detailed analysis of the past record. TIMED project aims to cover this gap in base to the study of a wide array of sediment cores from the Eastern and Western Mediterranean Sea and complemented with some key locations in the North Atlantic Ocean. The big value of the planned approach resides in the combined application of several different geochemical, sedimentological and also micropalontological tools sensitive of different properties that reflect the state of the MTHC system.
An important effort of the TIMED project relays in the validation of some of the applied tools for past oceanographic reconstructions with direct oceanographic measurements. In this direction, a relevant achievement during the first phase of the project comes from a validation exercise for a proxy of deep water convection intensity. This exercise consisted in the integrated analysis of direct measurements of deep current intensities, with grain-size distribution analysis on sediment trap collected particles and on the sedimentary record. The results of this exercise have allowed to directly link a particular mode of coarse size particles to fast current speeds associated to deep convection events in the Gulf of Lion. After this proxy validation, its application in the sedimentary record has allowed, by the first time, to reconstruct changes in the deep convection intensity for the last 2000yr, 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. On another hand, a strong focus of the TIMED project is the application of Nd isotopes to reconstruct changes in the mixing rates of Mediterranean water masses formed in different water cells. A big part of the efforts during the first phase of the project have been dedicated to the implementation of this analytical technique in the University of Barcelona with the acquisition and installation of a new analytical instrument (MC-ICPMS). The first application of this infrastructure has been in a proxy validation exercise focussed in the central Mediterranean region through the measurement of Nd isotopes and also Rare Earth Element concentrations in water samples collected through an oceanographic transect from the Ionian to the Tyrrhenian sea. These results have allowed to trace the export rate of Eastern Mediterranean source waters into the Tyrrhenian sea, providing a new insight into the relative contribution between the Eastern Mediterranean Deep Water mass and the Levantine Intermediate Water mass into the Western Mediterranean Sea. This detailed characterization of current Mediterranean oceanography is going to be key for the interpretation of all the on going Nd isotopes measurements within the TIMED project that target past climate transitions by means of the sedimentary record. Another relevant achievement of this first phase of the TIMED project has been the identification of the appearance of a distinctive intermediate water mass in the Alboran Sea, the westernmost basin in the Mediterranean Sea, right before the onset of the current Interglacial (Holocene period). This is a particular intriguing period in the evolution of the MTHC system since major changes occurred in both Mediterranean basins, but not in the same direction and their interplay is not yet understood. This new described water mass has been identified in base to its relatively high oxygen content, but further work to characterise its physical and chemical properties will allow to give an insight into its source area. An additional effort during this first TIMED phase has been to characterise surface oceanography conditions of some key Mediterranean locations. This is the case of the westernmost Alboran Sea, where sea surface temperatures for the whole Holocene and part of the deglaciation have been reconstructed at high resolution. These new results have allowed to identify a series of major changes in the main trends and also in the intensity of millennial scale events, this evolution is proposed to be associated to changes in the North Atlantic Oceanography and transmitted into the Mediterranean through the inflowing surface waters. Additional effort has been dedicated to the surface changes during the last 2000 years, a particular complex period due to the relatively weak climate forcings that lead a strong regional heterogeneity in the climate evolution. In this line, changes in surface temperatures have been analysed for the Balearic sector, the Sicily channel and also the Adriatic Sea and compared to pervious published records from other Mediterranean locations. One of the most solid features that have emerged from these studies is the overall warm conditions of the Roman period for the whole Mediterranean which was the warmest period of the last two millennia.
Currently, the TIMED project is in an intensive phase of data production. An important effort of the project is dedicated to the period of the Last Sapropel formation in the Eastern Mediterranean Sea, analysing the role of the Adriatic Sea convection cell in the development of this stagnation event and the associated changes in the chemical and physical water properties exported into the western Mediterranean Sea. TIMED aims to establish, by he first time, a semi-quantitative picture of changes in the mixing rate of the water masses through the Sicily channel during this period. Intensive work in the western Mediterranean Sea is addressed to the changes in deep convection prior to this Sapropel formation period in the eastern, when the weakest circulation mode occurred in the western basin that led the formation of the last organic rich layer in the Alboran Sea. We are characterising the properties and source of a newly formed water mass that filled the intermediate depths right prior to the reventilation phase of the Western basin and the major stagnation phase of the eastern. In addition, the short variability of the last 2000 years is going to be further explored along the basin. A important effort of TIMED is going to be dedicated to characterise the impact of all these described changes into the outflowing water mass through the analysis of deep sea corals and also sediments that track the path of the Mediterranean outflow waters.
TIMED results applying Nd isotopes to characterize current Mediterranean oceanography