Unknown and unexplained aspects of the human condition were the focus of a television
series in the USA created by Rod Serling in the early 1960s. The term “The Twilight
Zone” used in his show is even more pertinent to the mysterious region between 100 and
1000m in the great oceans of the world, the “middle ground between light and shadow”.
It is here where the sunlight at the ocean surface is finally extinguished and replaced by
occasional flashes of biological light. This mesopelagic zone, as it is more formally
called, is a region of immense change with depth and it is here that most of the biogenic
material that settles out of the sunlit or euphotic zone is broken down and returned to the dissolved state. The gravitational downward flux of particles thus decreases with depth in
general, and the animals that traverse this great depth, some each and every day, exert a
powerful influence on the distribution of many types of materials. The extent of mixing
also declines dramatically with depth, such that the water at 1000m is isolated from the
atmosphere for many decades to centuries, and this has great significance when
considering the influence of the oceans on the overlying atmosphere.