Reconstructing Mediterranean Overflow Plume Properties

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The ocean plays a critical role in regulating climate, redistributing heat from the equator to poles and absorbing atmospheric CO2. These processes are partly controlled by the temperature and salinity differences across the ocean that drive ocean circulation. Seas like the Mediterranean that lose more fresh water by evaporation than they gain from rivers flowing into them, are an important source of dense, salty water. Mediterranean water overflows a narrow, shallow sill in the Gibraltar Strait, forming a plume that follows seafloor contours around Spain and Portugal, flowing northwards to the Arctic. Today, the source of the most dense water flowing into the North Atlantic is the Mediterranean. In the past however, this water was at least three times more dense. Five to six million years ago, around 6% of all the salt in the ocean precipitated on the Mediterranean sea floor as a salt giant. This raised Mediterranean salinity dramatically, changing both the chemical and physical properties of the Mediterranean's overflow water. To evaluate the impact of these changes on regional and global climate, the overflow's properties have first to be determined. Some of these properties such as the size and location of the ancient plume can be mapped from the distribution of the sediment that the overflow produced. Others, like temperature, salinity, density and the chemistry of the water cannot be measured directly, but must be reconstructed from proxies of these properties that are preserved as part of the overflow sediment.IODP Expedition 401 will recover the sediment deposited by the overflow plume in the Atlantic during salt giant formation. The size of the particles in these sediments will be measured to determine the speed it was flowing. Microscopic organisms that live at the bottom of the sea, build their shells from the water in which they dwell. These shells are preserved as fossils long after the organism has died and their chemical composition reflects, in part, the temperature, salinity and chemistry of the water. By viewing the sediments under a microscope, these tiny fossils can be picked out with a fine paintbrush and then analysed. The records generated will provide a quantitative picture of the evolution of both Mediterranean overflow water and its interaction with the Atlantic. This is a critical first step towards evaluating the climatic impact of perturbations to ocean chemistry as a consequence of evaporite formation and the resulting high salinity overflows.