Death by Air
Published in The EarthSphere Blog. Feature image: Beachfront Property at the End of the Ordovician(by WM House & CF Lovelace; ©ArcheanArt)
Prologue
The last two posts in the Forgotten Origins series looked at the Ordovician Period. But all good things come to an end, and the end of the Ordovician was a catastrophe.
A Lesson Worth Learning
The Ordovician Period was a great time for life on Earth until it wasn’t. The biosphere was reminded of a biblical adage as it applies to mother nature “she giveth and she taketh away.” The official end of the Ordovician clocks in at 443.8 million years ago, but the catastrophe started about a million years earlier. It lasted for 4 million years, extending into the beginning of the Silurian Period. In the beginning, the changes probably seemed like minor perturbations in the environmental status quo, but by the end, 85 percent of Ordovician species had disappeared into the void of extinction. Perhaps there is a lesson for Anthropocene climate change deniers and agnostics in this ancient event.
Four million years is a long time, and the progressing extinction came in pluses. The earliest casualties appeared as many of the graptolites, brachiopods, and trilobites faded away. Dropping seal levels signaled the onset of phase two as water drained from the shallow seas, destroying the cradle of Ordovician marine life. The final phase kicked in during the early Silurian when global cooling switched to a warming trend, and sea levels began to rise again.
This long, drawn-out extinction differs from mass extinctions caused by discrete events like flood basalts or asteroid strikes. It’s worth remembering that mass extinctions come in a variety of flavors. Several billion years before the Ordovician-Silurian mass extinction, the introduction of oxygen into Earth’s biosphere wiped out large portions of chemosynthetic life. Then, during the Cryogenian crisis, it was just ‘too damn cold’ to live. Later at the end of the Cambrian, the absence of oxygen proved deadly. More recent extinctions relied on massive volcanic eruptions to bring on rapid environmental change and disrupt life. Lastly, there is death by an asteroid strike.
Cooling Off
Rising glaciers at the south pole on the Gondwana supercontinent locked away needed water into ice, and at the same time, Ordovician plants facilitated the depletion of carbon dioxide in Earth’s atmosphere. Steady cooling as CO2 levels dropped encouraged even more glaciation. Earth was locked into a feedback loop, the perfect storm to plunge the planet into a cold spell.
We don’t know exactly how cold the planet was. Paleoclimate investigations suggest atmospheric and oceanic temperatures ranging from slightly warmer to slightly cooler than today. But it is important to remember the geographic context of this period. Most landmasses were in the southern hemisphere, leaving the northern hemisphere as an open ocean.
The massive Gondwana supercontinent provided ample room for the largest ice sheets of the Paleozoic. Some researchers estimate that glaciation at the end of the Ordovician covered an area twice the size of the most recent Pleistocene ice age glaciers, and the Ordovician ice sheets were twice as thick.
A simple geometric model of these numbers says the Ordovician glaciation may have locked up four times as much water as our last ice age. Global sea levels were 120 meters lower during maximum Pleistocene glaciation, so they may have dropped by 480 meters at the end of the Ordovician. But his number doesn’t account for thermal contraction.
Ocean temperatures were dropping at the end of the Ordovician. The same physics of thermal expansion, which added to sea-level rise as the mid-Ordovician world warmed and ocean volume expanded, worked in reverse. Cooling oceans caused water volume shrinkage, further exasperating sea-level drop.
Death by Air
Debates about the exact causes of the Ordovician-Silurian mass extinction are varied and speculative. As always, science is largely determined by the weight of evidence. Science is often a world of competing theories, not absolute facts. Individual scientists are like cautious gamblers weighing all the evidence and placing their bet for a winner on the weight of supporting evidence.
I will roll the dice and mark up this particular mass extinction to a loss of habitat. Toxicity doesn’t look like an issue, although some researchers have proposed that an increase in toxic metals killed off the base of the Ordovician food chain. Likewise, the cold alone doesn’t appear severe enough to be the major factor. It did get colder than the mid-Ordovician, but with estimated temperatures close to today, and a gradually cooling environment, it seems evolutionary change would have accommodated the temperature changes.
Flood Basalts and asteroid strikes also seem unlikely, leaving us with habitat. Mid-Ordovician life thrived in vast shallow seas, and during this period, Earth’s oceans rose to the highest levels in geological history. Large portions of Earth’s continents were below shallow seas, and Ordovician life depended on this environmental combination of shallow water, ample sunlight, and good oxygen exchange.
Gondwana glaciation and cooling seas resulted in a devastating fall in ocean levels. Water drained from the continents. Shallow seas became dry land, and all of life there experienced death by air because life outside the wet, briny womb of the oceans was not yet possible. There were no more vast shallow seas, only waves lapping at the edges of steep continental slopes, and so began the Silurian Period.
Sources:
Ordovician-Silurian extinction (Source: Britannica)
International Chronostratigraphic Chart
Taking the temperature of ancient earth (by Tony Fitzpatrick; The Source)
The climatic significance of Late Ordovician-early Silurian black shales (by A. Pohl, Y. Donnadieu, G. Le Hir, D. Ferreira; AGU)
New Theory for What Caused Earth’s Second-Largest Mass Extinction (by Cheryl Katz; National Geographic)
