The Elements of Evolution (53-8) Dryas Stadials

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The planet was jolted from the grip of glaciation by a sudden wobble in Earth’s orbit. This changed Earth’s orientation to the Sun, increasing the sunlight reaching higher latitudes, particularly the polar regions.

Though still in a glacial period, the LGM was followed by wild oscillations in global climate. This partly reflects oceanic releases of carbon dioxide into the atmosphere, the first of which helped trigger the end of the Last Glacial Maximum. The deep Southern Ocean ventilated ancient CO2-rich water. The concentration of atmospheric CO2 rose from 180 parts per million by volume (ppmv) to 280 ppmv from the end of the LGM into the onset of the Holocene 11.7 TYA. Other large carbon releases occurred contemporaneously, perhaps outside the ocean-atmosphere gyre, from sources as yet unidentified.

In the context of European climate, gradual warming from 22 TYA was interrupted by 3 stadials before the Holocene began. A stadial is an extended cold spell of insufficient duration or intensity to be considered a glacial period.

The Oldest Dryas stadial was 18–15 TYA. The term Dryas derives from Dryas octopetala, an artic-alpine angiosperm. The Dryas stadials are so named because the flower’s pollen, found in core samples of glacial ice and peat bogs, are used for dating these periods.

The interstadial after the Oldest Dryas was the Bølling oscillation, termed after a peat sequence discovered at Bølling lake in central Jutland, the peninsula of Denmark. Sea level rose 100 meters from glacial melt during the Bølling oscillation. Temperate forests expanded.

The next stadial was the 100–150-year Older Dryas, centered at its coldest time 14.1 TYA. The Older Dryas had variable frigidity.

Then came the Allerød oscillation, which foreshadowed the climate of the 20th century. Forests prevailed in Eurasia, with more evergreens in the north, and more deciduous trees toward the south. The Allerød oscillation is termed after the town in Denmark where the soil samples used to first identify the climate period were obtained. Comparing the Bølling and the Allerød oscillations: the Bølling was warmer and came on more suddenly.

A cosmic impact ~12.8 TYA in the northern hemisphere provoked the last Dryas stadial. Debris thrown into the atmosphere cooled the climate quickly. Megafauna especially suffered from the rapid climate change.

The Younger Dryas stadial began ~12.8 TYA. Lasting 1,100 years before suddenly shifting into the interglacial Holocene, the Younger Dryas was a period of considerable climactic variability.