Modeling Ocean Biogeochemistry of the Mediterranean Sea at the Last Glacial Maximum

Present day Mediterranean Sea (MedSea) is an oligotrophic semi-enclosed basin.  Nutrient supply by rivers, net nutrient export through the Strait of the Gibraltar and the basin-wide circulation create a biological desert. Sediment core records, however, indicate periods of higher production during paleo times.

To gain insight into biogeochemical conditions during the Last Glacial Maximum (LGM), we apply a regional ocean-biogeochemistry model of the Mediterranean Sea.  A consistent forcing is available from a transient simulation with an Earth System Model (ESM) over 22,000 years based, among others, on an ice sheet reconstruction.  The ESM run provides atmospheric forcing fields, being downscaled to the regional setup, temperature and salinity conditions at the open western boundary in the Atlantic, and river runoff.  Nutrient concentrations of river discharge and at the Atlantic boundary are set to present-day estimates.  The automatic bathymetry adjustment to account for sea level variations due to meltwater fluxes is adopted from the ESM simulation. The LGM simulation starts at 22 ka to allow for a 1000 yr spinup run with transient forcing. The LGM period results are compared to a present-day simulation based on the same consistent ESM forcing.  

Colder temperatures and thus lower basin-wide evaporation, as well as a shallower sill depth at the Strait of Gibraltar, lead to lower baroclinic watermass exchange between the MedSea and the Atlantic. The zonal overturning circulation is more sluggish during the LGM than present day.  River discharge to the MedSea increases by 35% during the LGM, causing an increased net primary production near the river mouths.  Despite a higher nutrient inventory of the MedSea at the LGM, net primary production of the entire MedSea is lower than present day. Colder LGM temperatures reduce phytoplankton growth rates and increase the remineralisation length scale.

More characteristics of LGM biogeochemistry are presented and their drivers will be disentangled, also including additional sensitivity studies on changes in bathymetry.