A layer of rotten mud
Deep beneath the surface of the Mediterranean Sea, hidden within the layers of sediment, lies a dark record of ancient climate change. These layers, known as sapropels, are black, organic-rich muds that mark periods when the deep sea became completely devoid of oxygen. The name "sapropel" originates from the Greek words sapros and pelos, meaning "putrid" and "mud," respectively. These layers form when a massive amount of organic matter sinks to the seafloor and, due to the lack of oxygen, fails to decompose. Instead, it accumulates, creating distinct black bands that can be several centimeters to over a meter thick.
The dark color comes from high concentrations of organic carbon—typically over 2% by weight—and iron sulfides like pyrite, which form in the absence of oxygen. These anoxic conditions also mean that bottom-dwelling organisms are absent, so the delicate layers of sediment are left undisturbed by burrowing animals. This exceptional preservation makes sapropels a high-resolution record of past environmental conditions. Scientists drilling sediment cores from the seafloor can read these layers, showing a history of repeated suffocation events stretching back millions of years.
The orbital pacemaker
The formation of sapropels is not random; it has a distinct rhythm, occurring roughly every 21,000 years. This timing is dictated by Earth's orbital cycles, specifically the precession of the equinoxes—the slow "wobble" of the Earth's axis. This wobble changes which hemisphere is closest to the sun during the summer, altering the amount of solar energy different parts of the planet receive.
When the Northern Hemisphere experiences hotter summers due to this cycle, the African monsoon intensifies dramatically. This leads to immense rainfall over the headwaters of the Nile River, turning vast areas of the Sahara into an area of lakes and rivers in a phenomenon known as a "Green Sahara" period. The result is a massive increase in freshwater discharge from the Nile into the eastern Mediterranean.
This influx of less-dense freshwater spreads across the saltier, denser seawater, creating a stable, stratified "lid" on the ocean surface. This stratification prevents the mixing of oxygen-rich surface waters with the deep sea. The nutrients carried by the Nile water trigger enormous blooms of phytoplankton at the surface. As these organisms die and sink, their decomposition consumes all the available oxygen in the deep water, leading to anoxia and sometimes even euxinia—the presence of toxic hydrogen sulfide. This process kills off all aerobic life on the seafloor, and their remains, along with the dead plankton, accumulate to form a layer of black sapropel. The most recent major event, Sapropel S1, occurred between about 10,000 and 6,000 years ago.