Joint Seminar: Death in the deep sea: What caused anoxia in the Mediterranean Sea during the last deglaciation?

Death in the deep sea: What caused anoxia in the Mediterranean Sea during the last deglaciation?
At present day, the eastern Mediterranean is characterized by relative low net primary production, a
well-ventilated water column and minimal preservation of organic matter in the sediment due to the
abundance of an active ground-dwelling community. During the period from 10,000 and 6,000 years
before present, climatic conditions must have been quite different, as evidenced by the extinction of
the majority of ground-dwelling organisms due to a lack of oxygen and the deposition of large
amounts of organic matter and the formation of organic-rich sapropel layers in the sediment. The
presence of sapropel layers in the eastern Mediterranean have been first documented almost a
century ago; yet the drivers that led to their formations remain unclear. The prevailing classical
hypotheses focus on an enhanced freshwater input due to changes in the strength of African
monsoon. However, time-slice simulations have shown that the timing of these climate fluctuations
does not match with the ocean response time scales and the onset of sapropel formation.
We present here results from transient simulations with a regional physical-biogeochemical ocean
model of the Mediterranean Sea from the Last Glacial Maximum to present day. This is the first
comprehensive modelling study with a consistent transient forcing which simulates anoxic conditions
in the eastern Mediterranean Sea. Our results enable the disentangling of physical and
biogeochemical drivers of sapropel formation. We examine the classical hypotheses on sapropel
formation and discuss their plausibility. Major findings are that a climate-induced stagnation of deep
water in the Mediterranean, which already started with the onset of the deglaciation, is a
prerequisite for reaching anoxia and thus sapropel formation. With the onset of the African Humid
Period, enhanced freshwater and nutrient supply lead to increased net primary production and
eventually to anoxia in the bottom water.

Datum

21.01.2025

Uhrzeit

15:15 h

Ort

Bundesstr. 53, room 022/023
Seminar Room 022/023, Ground Floor, Bundesstrasse 53, 20146 Hamburg, Hamburg

Chair

Ulrike Niemeier

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