Figure 1. Map of the Mediterranean, with names of the main geographic features. The Gulf of Lions, as mentioned frequently in the text, is the embayment to the W-SW of the Rhone delta. Click on thumbnail for full-sized figure.
It all started some 9,500 years ago. Over a period
of several centuries, all waters below about 350 meters depth in the entire
eastern Mediterranean – from Sicily to the Middle East and from the African
margin to the European continent – became starved of oxygen. In some basins,
like the Aegean Sea, the upper limit reached even shallower depths, up
to 120 meters. The oxygen-starvation (also called ‘anoxia’, meaning ‘non-oxygenated’)
wiped out the entire deep ecosystem of the eastern Mediterranean.
Except for some specialised microbes, no organisms can survive anoxia, especially when such conditions are sustained over more than a few seasons to years. On a basin-wide scale, sustained deep-water oxygenation only resumed around 6,000 years Before Present (BP), following the initial shutdown around 9,500 years BP. The 3,500-year episode of basin-wide anoxia below 350 meters depth caused an ecological catastrophe of stupendous proportions.
Ironically, our early ancestors, living around the shores of the Mediterranean, may have been completely oblivious to the environmental devastation in the greater depths of the sea that provided their fish and other seafood. We can compare this situation with that in the present-day Black Sea: the anoxic, virtually sterile depths are hidden below a fertile, productive, system in the surface layers that remain sufficiently oxygenated by exchange with the atmosphere. In today’s Black Sea, the sharp gradient in oxygen concentrations between well oxygenated surface layers and anoxic deep waters – technically known as the ‘oxycline’ – resides around 100 m depth. It sat around 350 meters depth in the eastern Mediterranean between 9,500 and 6,000 years BP.
Had our ancestors had the technology to make basin-wide inventories, they might even have noted an increase in the exploitable stocks of shallow-living (‘epi-pelagic’) marine life in the eastern Mediterranean. The surface system was enriched with nutrients vital to the growth of microscopic marine plants – ‘phytoplankton’ – due to a natural fertilisation process that consisted of nutrient up-mixing from the deep anoxic layers. The epi-pelagic organisms, living specifically in the surface waters, could therefore thrive on increased food supplies. Meanwhile, however, fundamental habitat destruction took place for all species that spent (part of) their life in intermediate to great depths – ‘meso-’ to ‘bathy-pelagic’ organisms, respectively.
Ultimately, the reduction of biodiversity, due to extermination of the meso/bathy-pelagic ecosystem, reverberated through the entire food web. It upset the precarious balance in competition for resources within the ecosystem, triggering a dramatic shift in living strategies throughout the basin. The abrupt extermination of many ‘specialist’ species, which had over time adapted to a narrow range of specific conditions, offered vacant possession of their habitats to new species with great flexibility regarding their preferred living conditions (‘opportunists’).
When conditions even marginally improved, opportunists rapidly moved to colonise the barren wastelands. Recolonisation by the specialists was much slower, following the expansion of their specific living conditions from the small patches – ‘refugia’ – where they had survived in marginal numbers. Even today, 6,000 years after the reinstatement of deep-water oxygenation, the eastern Mediterranean’s deep ecosystem has not yet fully recovered.
Figure 1. Map of the Mediterranean,
with names of the main geographic features. The Gulf of Lions, as mentioned
frequently in the text, is the embayment to the W-SW of the Rhone
delta. Click on thumbnail for full-sized figure.
What caused this catastrophe? Fresh water! Globally, the monsoons had been gaining in intensity since about 11,000 years BP, in their long-term response to changes in the position of earth relative to the sun. One distinct consequence was the severe reduction in size of the Sahara, being compressed by a dramatic northward shift in the monsoonal penetration over Africa. Many large lakes existed in the Sahara, from west to east. Fossils and paintings/carvings from central Saharan sites bear witness of a diverse African wildlife that included giraffe, elephant, rhino, hippo, antelope and ostrich. A wealth of archaeological sites suggests extensive presence of humans with pastoral lifestyles, as confirmed in the rich Saharan art. The ending of the monsoonal maximum started about 6,500 years BP, leading to the present-day type conditions by about 4,000 years BP.
During the maximum, monsoon-fed rivers drained the North African margin, and the Nile River especially was affected. Its flow-rate had swollen to triple those of historical times before the completion of the Aswan dam. At the same time, much more humid conditions prevailed over the lands along the northern margin of the Mediterranean. In particular, summer rainfall was strongly increased, in stark contrast with the typical winter-wet summer-dry climate of the present-day Mediterranean.
The strong increase in freshwater flux into the semi-isolated Mediterranean basin notably reduced the salt concentrations (‘salinity’) of its surface waters. This inhibited deep-water formation, depriving the deep-sea from ventilation with new, oxygenated, deep water. An anoxic catastrophe had become inevitable ….
So what is the "The dark secret of the Mediterranean?" It refers to the repeated depostion of the dark, olive green to pitch black, organic-rich sedimentary layers (‘sapropels’) that formed during such anoxic episodes. The ‘secret’ about them concerns the fact that only a small group of people is aware that they exist at all, and that their formation is repetitive in time, following the beat of an astronomical clock. It has been so for at least three – possibly nine – millions of years. The "dark secret" title sounds perhaps a bit dramatic and ominous? I consider that to be justified, since periods of sapropel deposition were periods of profound ecosystem collapse, caused by a collapse of deep ventilation and consequent oxygen starvation in the water column from the bottom to 350 meters depth.
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In this text, I explore how the Mediterranean functions and how we can unravel its recent geological history (note 1). I then evaluate in more detail the causes of widespread anoxic events in the basin, and why such catastrophes recur with a more-or-less systematic periodicity. Next, shorter-term variabilities are discussed, and within that context, I conclude with a brief speculation on the potential impacts of human activity on the oceanography of the Mediterranean.