Modeling Mediterranean ocean biogeochemistry of the Last Glacial Maximum

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Abstract. We present results of simulations with a physical-biogeochemical ocean model of the Mediterranean Sea for the last glacial maximum (LGM) and analyse the difference in physical and biochemical states between the present day and the past. Long-term simulations with an Earth system model based on ice sheet reconstructions provide the necessary atmospheric forcing data, oceanic boundary conditions at the entrance to the Mediterranean Sea, and river discharge to the entire basin. Our regional model accounts for changes in bathymetry due to ice-sheet volume changes, reduction in atmospheric carbon content, and an adjusted aeolian dust and iron deposition. The physical ocean state of the Mediterranean during the LGM shows a reduced baroclinic water exchange at the Strait of Gibraltar, a more sluggish zonal overturning circulation, and the relocation of intermediate and deep water formation areas – all in line with estimates from paleo sediment records or previous modelling efforts. Most striking features of the biogeochemical realm are a reduction of net primary production, an accumulation of nutrients below the euphotic zone, and an increase of organic matter deposition at the sea floor. This seeming contradiction of increased organic matter deposition and decreased net primary production challenges our view of possible changes in surface biological processes during the LGM. We attribute the origin of a reduced net primary production to the interplay of increased stability of the upper water column, changed zonal water transport at intermediate depths, and colder water temperatures, which slow down all biological processes during the LGM. The cold water temperatures also affect the remineralisation rates of organic material which explains the simulated increase of organic matter deposition, in good agreement with sediment proxy records. In addition, we discuss changes of an artificial tracer which captures the surface ocean temperature signal during organic matter production. A shifted seasonality of biological production in the LGM leads to a difference in the recording of the climate signal by this artificial tracer of up to 1 K. This could be of relevance for the interpretation of proxy records like e.g. alkenones. Our study does not only provide the first consistent insights into the biogeochemistry of the glacial Mediterranean Sea, it will also serve as the starting point for long-term simulations over the entire last deglaciation.

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