The mystery interval: An enigma in Earth’s history 

green pattern

While dramatic transformations in the environment are seen as novel, Earth’s history includes extensive environment change, including a period known as ‘The Mystery Interval’. Photo credit: Iris Campbell-Lange, The Oxford Scientist


…a three-thousand-year shift in atmospheric and hydrological systems, and has puzzled scientists since its discovery…

Twenty thousand years ago, our planet left its last ice age and started to warm. Records of the Earth’s climate show this steady rise was interrupted twice when temperatures briefly returned to ice age conditions. One abrupt shift in climate, known as the Younger Dryas, took place between 12,900 and 11,500 years ago. The other caused a three-thousand-year shift in atmospheric and hydrological systems, and has puzzled scientists since its discovery—this event is known as ‘The Mystery Interval’.  

Studying the Earth’s climate since the last glacial maximum (when global ice sheets were at their fullest extent) is relevant for understanding our current climate and how it responds to changes in the Earth’s system. Scientists have been able to determine how changes in aspects such as global ice cover and atmospheric composition have impacted global climate. 

Studying the Earth’s climate since the last glacial maximum…is relevant for understanding our current climate and how it responds to changes in the Earth’s system.

The Younger Dryas has previously been the subject of extensive research, and scientists now have a relatively good hypothesis as to what occurred. Melt from the North American ice sheets added freshwater to the North Atlantic Ocean, impacting ocean currents and causing global temperatures to drop. This is similar to what happened during previous glacial maximums, however this time the meltwater followed a different path, meaning more freshwater entered the North Atlantic so the extent of global cooling was greater. A smaller drop in temperature also occurred between 17,500 and 14,500 years ago during The Mystery Interval. 

This time, however, mountain glaciers continued to melt and retreat, while atmospheric carbon dioxide levels continued to rise despite the cold air and sea surface temperatures. Scientists are rightly prompted to question what factors could have caused such environmental reactions.

A simple answer could be that this period was characterized by extreme seasons. Cold ocean temperatures and extensive northern sea ice resulted in cold winters, while hot summers caused mountain glacier retreat as a result of rising carbon dioxide levels in the atmosphere. The start of The Mystery Interval coincides with an event known as Heinrich Stadial One, where large groups of icebergs broke off from glaciers and traveled across the North Atlantic Ocean. When this iceberg armada melted, it reduced ocean salinity which affected major ocean currents. 

The reduction of ocean salinity at the beginning of The Mystery Interval prevented climate regulation by the meridional overturning circulation (MOC), a current that carries warm water from the tropics northwards. The resulting abundance of cold water in the North Atlantic Ocean allowed ice sheets to spread as far south as Paris during the Northern Hemisphere winter. This change in the MOC also triggered a southern shift of the inter tropical convergence zone (ITCZ), a narrow zone of low atmospheric pressure near the equator, weakening the summer monsoon in Asia. It has been suggested that the magnification of events during The Mystery Interval is due to a random alignment of maximum ice sheet volume and the first Heinrich Stadial. 

The last piece of the puzzle of The Mystery Interval is found in the sediments of the North American Great Basin Lakes. Geologists have found evidence of extreme aridity during the first half of the interval, immediately followed by extensive freshwater deposits. This has caused scientists to split the interval into two stages: the Big Dry and the Big Wet. But the change in ocean circulation does not appear to correlate with the abrupt hydrological shift. The Big Dry–Big Wet shift does, however, coincide with the change in Asian monsoon strength, and scientists propose this could have been caused by a southward shift of the planet’s thermal equator, the Earth’s belt of lowest annual mean temperature. 

Scientists need a precise chronology of events to conduct further research into the interval and determine its main cause.  Until then, it remains a mystery.