TCS Daily

Welcome to the Ordovician

By Sallie Baliunas - April 23, 2004 12:00 AM

An ice age strode the world 440 million years ago, when the globally-averaged surface temperature was approximately 25 degrees Fahrenheit cooler than today. Welcome to the late Ordovician ice age, the first of four ice ages during the last 500 million years.

The three more recent ice ages -- the lengthy Permian ice age of approximately 300 million years ago, the Jurassic ice age of approximately 170 million years ago and the present icehouse called the Pleistocene that set in 2 to 3 million years ago -- all were accompanied by very low concentrations of atmospheric carbon dioxide. Yet estimates from rock chemistry, models and fossil leaf structures indicate that the carbon dioxide content of the air during the Ordovician ice age was 2,000 parts per million by volume or higher, roughly five to ten times larger than the air's pre-industrial content.

And the high concentration of carbon dioxide in the air during the Ordovician ice age is a reminder that the climate is extraordinarily complex and affected by more factors than just the air's carbon dioxide content. Solving the Ordovician puzzle could improve understanding of the complicated processes that store carbon in and exchange it between the land, ocean, sea floor, biota and atmosphere on many different time scales, and how that carbon cycle affects -- and is affected by -- climate.

A factor common to all four great ice ages has been that a continent-size land mass crowning one of the polar regions as plate tectonics continually shift the position of the continents by a few centimeters per year. In the Ordovician, Africa was nearly the center of the supercontinent called Gondwana that had settled over the south polar region, with Africa's continental margins crowded by South America, Australia, India, Madagascar and Antarctica. Gondwana's eastern portion stretched to the equator. A thick ice sheet draped northern and western Africa all the way to present day Arabia.

The Ordovoician world was very different from today's. The moon was closer to the earth and would have been seen as slightly larger in size. Owing to the smaller distance between the earth and the moon, tidal forces were enhanced. The day was only about 20 to 21 hours long, and the year lasted just over 400 of those shortened days. The sun's luminosity was about 5 percent less than now, because the sun's fusion reactions run steadily hotter over time. The high greenhouse gas content of the air half a billion years ago -- a remnant from the earth's earlier atmospheric composition and enhanced by vigorous emissions from volcanoes and sea floor ridges -- should have provided warmth in compensation for a dimmer sun.

Life, too, was very different then. Some 600 million years ago inland seas covered continental land areas and provided a warm marine environment, as seen in the impressions left in the now hardened rock of the anciently soft seafloors by early metazoans -- the new, soft-bodied, multicellular organisms accompanying the early, single-celled life that had existed on earth for three billion years.

As the Ordovician rolled in, the fossil record writes of rich array of established marine life with protective shells, tubes or plates, including the phyla of arthropods (for example, trilobites), echinoderms (starfish), brachiopods and coral cnidaria (corals). The oldest armored, jawless fishes scooting along shallow sea shores and, sharing the chordate phylum with later Homo sapiens, appear in the early Ordovician. Meanwhile, plants had begun to invade the dry land along the shores of the inland seas.

The specific factors that triggered the Ordovician ice age once Gondwana shifted to the south polar region remain speculative. But once started, the southern ice sheet grew at the expense of the continental inland seas, which were drawn down by 50 to 70 meters -- 150 to 230-plus feet -- or more, even in the low-latitude continents.

Another puzzle of the Ordovician ice age is that the ice sheet may have grown relatively quickly, persisted for only a brief period -- half a million years -- compared to later ice ages, and speedily retreated.

The apparent rapid fall and then rise in sea levels and temperature associated with the growth and retreat of the ice sheet coincided with extinction of about one-fourth of families in the kingdom of life, and perhaps 85 percent of all species. The Ordovician's extinction severity was second only to that of the Permian extinction of 250 million years ago during which perhaps one-half of families and 90 percent of species died out.

The complexity of climate and the interaction of the ocean, land and vegetation in amplifying, stabilizing and reducing carbon dioxide and other greenhouse gases in their environmental pools make explanation of the triggering and retreat of the Ordovician ice age 440 millions years ago difficult. But explaining it would be an important step in demystifying not only the Ordovician's rapid extinctions but also today's carbon budget and tomorrow's climate.


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